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Title: Fossil Plants: Vol. I
Author: Seward, A. C. (Albert Charles)
Language: English
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~Cambridge Natural Science Manuals.~
=BIOLOGICAL SERIES.=
GENERAL EDITOR:—ARTHUR E. SHIPLEY, M.A.
FELLOW AND TUTOR OF CHRIST’S COLLEGE, CAMBRIDGE.
FOSSIL PLANTS.
~London~: C. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE,
AND
H. K. LEWIS,
136, GOWER STREET, W.C.
[Illustration]
~Glasgow~: 263, ARGYLE STREET.
~Leipzig~: F. A. BROCKHAUS.
~New York~: THE MACMILLAN COMPANY.
~Bombay~: E. SEYMOUR HALE.
[Illustration: TREE STUMPS IN A CARBONIFEROUS FOREST. VICTORIA PARK,
GLASGOW.]
FOSSIL PLANTS
FOR STUDENTS OF BOTANY AND GEOLOGY
BY
A. C. SEWARD, M.A., F.G.S.
ST JOHN’S COLLEGE, CAMBRIDGE,
LECTURER IN BOTANY IN THE UNIVERSITY OF CAMBRIDGE.
WITH ILLUSTRATIONS.
VOL. I.
CAMBRIDGE:
AT THE UNIVERSITY PRESS.
1898
[_All Rights reserved._]
~Cambridge~:
PRINTED BY J. AND C. F. CLAY,
AT THE UNIVERSITY PRESS.
PREFACE.
In acceding to Mr Shipley’s request to write a book on Fossil Plants
for the Cambridge Natural History Series, I am well aware that I have
undertaken a work which was considered too serious a task by one who
has been called a “founder of modern Palaeobotany.” I owe more than I
am able to express to the friendship and guidance of the late Professor
Williamson; and that I have attempted a work to which he consistently
refused to commit himself, requires a word of explanation. My excuse
must be that I have endeavoured to write a book which may render more
accessible to students some of the important facts of Palaeobotany, and
suggest lines of investigation in a subject which Williamson had so
thoroughly at heart.
The subject of Palaeobotany does not readily lend itself to adequate
treatment in a work intended for both geological and botanical
students. The Botanist and Geologist are not always acquainted
with each other’s subject in a sufficient degree to appreciate the
significance of Palaeobotany in its several points of contact with
Geology and recent Botany. I have endeavoured to bear in mind the
possibility that the following pages may be read by both non-geological
and non-botanical students. It needs but a slight acquaintance with
Geology for a Botanist to estimate the value of the most important
applications of Palaeobotany; on the other hand, the bearing of fossil
plants on the problems of phylogeny and descent cannot be adequately
understood without a fairly intimate knowledge of recent Botany.
The student of elementary geology is not as a rule required to concern
himself with vegetable palaeontology, beyond a general acquaintance
with such facts as are to be found in geological text-books. The
advanced student will necessarily find in these pages much with which
he is already familiar; but this is to some extent unavoidable in a
book which is written with the dual object of appealing to Botanists
and Geologists. While considering those who may wish to extend their
botanical or geological knowledge by an acquaintance with Palaeobotany,
my aim has been to keep in view the requirements of the student who
may be induced to approach the subject from the standpoint of an
original investigator. As a possible assistance to those undertaking
research in this promising field of work, I have given more references
than may seem appropriate to an introductory treatise, and there are
certain questions dealt with in greater detail than an elementary
treatment of the subject requires. In several instances references
are given in the text or in footnotes to specimens of Coal-Measure
plants in the Williamson cabinet of microscopic sections. Now that this
invaluable collection of slides has been acquired by the Trustees of
the British Museum, the student of Palaeobotany has the opportunity of
investigating for himself the histology of Palaeozoic plants.
My plan has been to deal in some detail with certain selected types,
and to refer briefly to such others as should be studied by anyone
desirous of pursuing the subject more thoroughly, rather than to cover
a wide range or to attempt to make the list of types complete. Of late
years there has been a much wider interest evinced by Botanists in
the study of fossil plants, and this is in great measure due to the
valuable and able work of Graf zu Solms-Laubach. His _Einleitung in
die Palaeophytologie_ must long remain a constant book of reference
for those engaged in palaeobotanical work. While referring to authors
who have advanced the study of petrified plants of the Coal period,
one should not forget the valuable services that have been rendered by
such men as Butterworth, Binns, Wilde, Earnshaw, Spencer, Nield, Lomax
and Hemingway, by whose skill the specimens described by Williamson and
others were first obtained and prepared for microscopical examination.
I am indebted to many friends, both British and Continental, for help
of various kinds. I would in the first place express my thanks to
Professor T. McKenny Hughes for having originally persuaded me to begin
the study of recent and fossil plants. I am indebted to Prof. Nathorst
of Stockholm, Dr Hartz of Copenhagen, Prof. Zeiller, Dr Renault and
Prof. Munier-Chalmas of Paris, Prof. Bertrand of Lille, Prof. Stenzel
and the late Prof. Roemer of Breslau, Dr Sterzel of Chemnitz, the
late Prof. Weiss of Berlin, the late Dr Stur of Vienna, and other
continental workers, as well as to Mr Knowlton of Washington, for
facilities afforded me in the examination of fossil plant collections.
My thanks are due to the members of the Geological and Botanical
departments of the British Museum; also to Mr E. T. Newton of the
Geological Survey, and to those in charge of various provincial
museums, for their never-failing kindness in offering me every
assistance in the investigation of fossil plants under their charge.
Prof. Marshall Ward has given me the benefit of his criticism on the
section dealing with Fungi; and my friend Mr Alfred Harker has rendered
me a similar service as regards the chapter on Geological History. I
am especially grateful to my colleague, Mr Francis Darwin, for having
read through the whole of the proofs of this volume. To Mr Shipley, as
Editor, I am under a debt of obligation for suggestions and help in
various forms. I would also express my sense of the unfailing courtesy
and skill of the staff of the University Press.
My friend Mr Kidston of Stirling has always generously responded to my
requests for the loan of specimens from his private collection. Prof.
Bayley Balfour of Edinburgh, Mr Wethered of Cheltenham and others have
assisted me in a similar manner. I would also express my gratitude
to Dr Hoyle of Manchester, Mr Platnauer of York, and Mr Rowntree of
Scarborough for the loan of specimens.
To Dr Henry Woodward of the British Museum I am indebted for the loan
of the woodblocks made use of in figs. 10, 47, 60, 66, and 101, and to
Messrs Macmillan for the process-block of fig. 25.
For the photographs reproduced in figs. 15, 34, 68, 102 and 103 I owe
an acknowledgment to Mr Edwin Wilson of Cambridge, and to my friend Mr
C. A. Barber for the micro-photograph made use of in fig. 40.
In conclusion I wish more particularly to thank my wife, who has drawn
by far the greater number of the illustrations, and has in many other
ways assisted me in the preparation of this Volume.
In Volume II the Systematic treatment of Plants will be concluded,
and the last chapters will be devoted to such subjects as geological
floras, plants as rock-builders, fossil plants and evolution, and other
general questions connected with Palaeobotany.
A. C. SEWARD.
BOTANICAL LABORATORY, CAMBRIDGE.
_March_, 1898.
TABLE OF CONTENTS.
PART I. GENERAL.
CHAPTER I.
=HISTORICAL SKETCH.= Pp. 1–11.
Fossil plants and the Flood. Sternberg and Brongniart. The
internal structure of fossil plants. English Palaeobotanists.
Difficulties of identification.
CHAPTER II.
=RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY.= Pp. 12–21.
Neglect of fossils by Botanists. Fossil plants and distribution.
Fossil plants and climate. Fossil plants and phylogeny.
CHAPTER III.
=GEOLOGICAL HISTORY.= Pp. 22–53.
Rock-building. Calcareous rocks. Geological sections. Inversion of
strata. Table of Strata:
I. Archaean, 34–36. II. Cambrian, 36–37. III. Ordovician, 37–38.
IV. Silurian, 38. V. Devonian, 39. VI. Carboniferous, 39–45.
VII. Permian, 45–47. VIII. Trias., 47–48. IX. Jurassic, 48–49. X.
Cretaceous, 50–51. XI. Tertiary, 51–53. Geological Evolution.
CHAPTER IV.
=THE PRESERVATION OF PLANTS AS FOSSILS.= Pp. 54–92.
Old surface-soils. Fossil wood. Conditions of fossilisation.
Drifting of trees. Meaning of the term ‘Fossil.’ Incrustations.
Casts of trees. Fossil casts. Plants and coal. Fossils in
half-relief. Petrified trees. Petrified wood. Preservation of
tissues. Coal-balls. Fossil nuclei. Fossil plants in volcanic
ash. Conditions of preservation.
CHAPTER V.
=DIFFICULTIES AND SOURCES OF ERROR IN THE
DETERMINATION OF FOSSIL PLANTS.= Pp. 93–109.
External resemblance. Venation characters. Decorticated stems.
Imperfect casts. Mineral deposits simulating plants. Traces of
wood-borers in petrified tissue. Photography and illustration.
CHAPTER VI.
=NOMENCLATURE.= Pp. 110–115.
Rules for nomenclature. The rule of priority. Terminology and
convenience.
PART II. SYSTEMATIC.
CHAPTER VII.
=THALLOPHYTA.= Pp. 116–228.
=PAGE=
I. =PERIDINIALES= 117–118
II. =COCCOSPHERES AND RHABDOSPHERES= 118–121
III. =SCHIZOPHYTA= 121–138
A. SCHIZOPHYCEAE (CYANOPHYCEAE) 122–132
_Girvanella_ 124–126. Borings in shells 127–129.
_Zonatrichites_ 129–130.
B. SCHIZOMYCETES (BACTERIA) 132–138
_Bacillus Permicus_ 135–136. _B. Tieghemi_ and
_Micrococcus Guignardi_ 136. Fossil Bacteria 137–138.
IV. =ALGAE= 138–205
Scarcity of fossil algae. Fossils simulating Algae.
Recognition of fossil algae. _Algites &c._
A. DIATOMACEAE 150–156
Recent Diatoms. Fossil Diatoms. _Bactryllium &c._
B. CHLOROPHYCEAE 156–178
_a._ SIPHONEAE 157–177
α. =Caulerpaceae= 157–159
β. =Codiaceae= 159–164
_Codium_ 159–160. _Sphaerocodium_ 160.
_Penicillus_ 161. _Ovulites_ 161–164.
_Halimeda_ 164.
γ. =Dasycladaceae= 164–177
_Acetabularia_ 165–166. _Acicularia_ 166–169.
_Cymopolia_ 169–171. _Vermiporella_ 172–173.
_Sycidium_ 173. _Diplopora_ 174–175.
_Gyroporella_ 175. _Dactylopora_, Palaeozoic
and Mesozoic Siphoneae 175–177.
_b._ CONFERVOIDEAE 177–178
C.INCERTAE SEDIS 178–183
Boghead ‘Coal.’ _Reinschia_ 180–181. _Pila_ 181–182.
D. RHODOPHYCEAE 183–190
CORALLINACEAE 183–190
_Lithothamnion_ 185–189. _Solenopora_ 189–190.
E. PHAEOPHYCEAE 191–202
_Nematophycus_ 192–202
_Pachytheca_ 202–204
_Algites_ 204–205
V. =MYXOMYCETES (MYCETOZOA)= 205–206
_Myxomycetes Mangini_ 206.
VI. =FUNGI= 207–222
ASCOMYCETES. BASIDIOMYCETES.
Pathology of fossil tissues. _Oochytrium Lepidodendri_
216–217. _Peronosporites antiquarius_ 217–220.
_Cladosporites bipartitus_ 220. _Haplographites
cateniger_ 220. _Zygosporites_ 220–221. _Polyporus
vaporarius_ 221.
VII. =CHAROPHYTA= 222–228
CHAREAE 223–228
_Chara_ 225–228. _C. Bleicheri_ 226. _C. Knowltoni_
226–227. _C. Wrighti_ 227.
CHAPTER VIII.
=BRYOPHYTA.= Pp. 229–241.
I. =HEPATICAE= 230–236
_Marchantites_ 233–235. _M. Sezannensis_ 234–235.
II. =MUSCI= 236–241
_Muscites_ 238–241. _M. polytrichaceus_ 239–240.
Palaeozoic Mosses. _Muscites ferrugineus_ 241.
CHAPTER IX.
=PTERIDOPHYTA (VASCULAR CRYPTOGRAMS).= Pp. 242–294.
I. =EQUISETALES (RECENT)= 244–254
EQUISETACEAE 244–254
_Equisetum_ 246–254.
II. =FOSSIL EQUISETALES= 254–294
A. EQUISETITES 257–281
_Equisetites Hemingwayi_ 263–264. _E.
spatulatus_ 264–266. _E. zeaeformis_ 266. _E.
arenaceus_ 268–269. _E. columnaris_ 269–270.
_E. Beani_ 270–275. _E. lateralis_ 275–279. _E.
Burchardti_ 279–280.
B. PHYLLOTHECA 281–291
_Phyllotheca deliquescens_ 283–284. _P.
Brongniarti_ 286–287. _P. indica_ and _P.
australis_ 287–289.
C. SCHIZONEURA 291–294
_S. gondwanensis_ 292–293.
CHAPTER X.
=EQUISETALES (_continued_).= Pp. 295–388.
D. CALAMITES 295–383
I. =Historical sketch= 295–302
II. =Description of the anatomy of Calamites= 302–364
_a._ _Stems_ 304–329
_Arthropitys_, _Arthrodendron_, and
_Calamodendron_.
_b._ _Leaves_ 329–342
α. _Calamocladus_ (_Asterophyllites_) 332–336.
_C. equisetiformis_ 335–336.
β. _Annularia_ 336–342. _A. stellata_ 338–340.
_A. sphenophylloides_ 341–342.
_c._ _Roots_ 342–349
_d._ _Cones_ 349–365
_Calamostachys Binneyana_ 351–355. _C. Casheana_
355–357. _Palaeostachya vera_ 358–360.
_Calamostachys_, _Palaeostachya_ and
_Macrostachya_ 361–364.
III. =Pith-casts of Calamites= 365–380
_Calamitina_ 367–374. _Calamites_ (_Calamitina_)
_Göpperti_ 372–374. _Stylocalamites_ 374–376. _C._
(_Stylocalamites_) _Suckowi_ 374–376. _Eucalamites_
376–379. _C._ (_Eucalamites_) _cruciatus_ 378–379.
IV. =Conclusion= 381–383
E. ARCHAEOCALAMITES 383–388
_A. scrobiculatus_ 386–387.
CHAPTER XI.
SPHENOPHYLLALES. Pp. 389–414.
I. =SPHENOPHYLLUM= 389–414
A. =The anatomy of Sphenophyllum= 392–406
_a._ _Stems_ 392–398
_Sphenophyllum insigne_ and _S. plurifoliatum_
397–398.
_b._ _Roots_ 399
_c._ _Leaves_ 399
_d._ _Cones_ 401–406
_Sphenophyllostachys Dawsoni_ 402–405. _S.
Römeri_ 405–406.
B. =Types of vegetative Branches of Sphenophyllum= 407–412
_Sphenophyllum emarginatum_ 407–408. _S.
trichomatosum_ 408–409. _S. Thoni_ 410–411. _S.
speciosum_ 411–412.
C. =Affinities, Range and Habit of Sphenophyllum= 412–414
LIST OF ILLUSTRATIONS.
FRONTISPIECE. TREE STUMPS IN A CARBONIFEROUS FOREST. Drawn from a
photograph. (M. Seward.) PAGE 57.
FIG. PAGE
1. _Lepidodendron._ (M. S.) 10
2. Geological section 29
3. Table of strata 32, 33
4. Geological section (coal seam) 44
5. _Neuropteris Scheuchzeri_ Hoffm. (M. S.) 45
6. Submerged Forest at Leasowe. (M. S.) 59
7. Ammonite on coniferous wood. (M. S.) 61
8. Coniferous wood in flint. (M. S.) 62
9. Bored fossil wood. (M. S.) 62
10. Section of an old pool filled up with a mass of _Chara_. (From
block lent by Dr Woodward) 69
11. _Equisetites columnaris_ Brongn. (M. S.) 72
12. _Stigmaria ficoides_ Brongn. (M. S.) 73
13. _Cordaites etc._ in coal. (M. S.) 76
14. Crystallisation in petrified tissues 81
15. _Lepidodendron._ (From a photograph by Mr Edwin Wilson
of a specimen lent by Mr Kidston) 82
16. Cast of a fossil cell. (M. S.) 84
17. Calcareous nodule from the Coal-Measures 85
18. _Lepidodendron_ from Arran. (M. S.) 89
19. _Trigonocarpon_ seeds in a block of sandstone. (M. S.) 91
20. _Restio_, _Equisetum_, _Casuarina_ and _Ephedra_. (M. S.) 95
21. _Polygonum equisetiforme_ Sibth. and Sm. (M. S.) 96
22. _Kaulfussia æsculifolia_ Blume. (M. S.) 97
23. A branched Lepidodendroid stem (_Knorria mirabilis_ Ren.
and Zeill.). (M. S.) 102
24. Partially disorganised petrified tissue 107
25. Coccospheres and Rhabdospheres. (Lent by Messrs Macmillan) 119
26. _Girvanella problematica_ Eth. and Nich. (M. S.) 124
27. Fish-scale and shell perforated by a boring organism. (M. S.) 128
28. _Bacillus Tieghemi_ Ren. and _Micrococcus Guignardi_ Ren.
(M. S.) 135
29. _Laminaria_ sp. 140
30. Rill-mark; trail of a seaweed; tracks of a Polychaet. (M. S.) 143
31. _Chondrites verisimilis_ Salt. (M. S.) 146
32. _Lithothamnion mamillosum_ Gümb.; _Sycidium melo_ Sandb.;
_Bactryllium deplanatum_ Heer; Calcareous pebble from a
lake in Michigan. (M. S.) 155
33. _Cymopolia barbata_ (L.); _Acicularia Andrussowi_ Solms;
_Acicularia_ sp.; _A. Schencki_ (Möb.); _A. Mediterranea_
Lamx.; _Ovulites margaritula_ Lamx.; _Penicillus
pyramidalis_ (Lamx.) (M. S.) 162
34. _Acetabularia mediterranea_ Lamx. (Photograph by Mr Edwin
Wilson) 165
35. _Diplopora_; _Gyroporella_; _Penicillus_; _Ovulites
margaritula_ Lam.; _Confervites chantransioides_ (Born.) 174
36. Torbanite; _Pila bibractensis_ and _Reinschia australis_ 180
37. _Lithothamnion_ sp.; _L. suganum_ Roth.; _Sphaerocodium
Bornemanni_ Roth. 186
38. _Solenopora compacta_ (Billings). (M. S.) 189
39. _Nematophycus Logani_ (Daws.) 196
40. _Nematophycus Storriei_ Barb. (Photograph by Mr C. A. Barber) 199
41. Cells of _Cycadeoidea gigantea_ Sew., _Osmundites Dowkeri_
Carr and _Memecylon_ with vacuolated contents;
_Peronosporites antiquarius_ Smith; _Zygosporites_ 214
42. Tracheids of coniferous wood attacked by _Trametes
radiciperda_ Hart and _Agaricus melleus_ Vahl. 215
43. _Oochytrium Lepidodendri_ Ren.; _Polyporus vaporarius_ Fr.
var. _succinea_; _Cladosporites bipartitus_ Fel.;
_Haplographites cateniger_ Fel. (M. S.) 217
44. Cells of fossil plants with fungal hyphae 219
45. _Chara Knowltoni_ Sew.; _Chara foetida_ A. Br. (_A_ and _B_,
Mr Highley; _C–E_, M. S.) 224
46. _Chara Bleicheri_ Sap.; _Chara_? sp.; _C. Wrighti_ Forbes.
(M. S.) 226
47. _Chara Knowltoni_ Sew. (From block lent by Dr Woodward) 227
48. _Tristichia hypnoides_ Spreng.; _Podocarpus cupressina_ Br.
and Ben.; _Selaginella Oregana_ Eat. (M. S.) 231
49. _Marchantites erectus_ (Leck.) (M. S.) 233
50. _Marchantites Sezannensis_ Sap. (M. S.) 235
51. _Muscites polytrichaceus_ Ren. and Zeill. (M. S.) 239
52. _Equisetum maximum_ Lam.; _E. arvense_ L. 246
53. _Equisetum palustre_ L. (M. S.) 247
54. Plan of the vascular bundles in an _Equisetum_ stem; _E.
arvense_ L. 250
55. _Equisetum variegatum_ Schl.; _E. maximum_ Lam. 252
56. Calamitean leaf-sheath. (M. S.) 260
57. _Equisetites Hemingwayi_ Kidst. (Mr Highley) 262
58. _Equisetites spatulatus_ Zeill.; _E. zeaeformis_ (Schloth.);
_Equisetites lateralis_ Phill.; _Equisetites columnaris_
Brongn.; _Equisetum trachyodon_ A. Br. (M. S.) 265
59. _Equisetites platyodon_ Brongn. (M. S.) 267
60. _Equisetites Beani_ (Bunb.). (From a block lent by Dr
Woodward) 271
61. _Equisetites Beani_ (Bunb.). (M. S.) 272
62. _E. Beani_ (Bunb.). (M. S.) 274
63. _E. lateralis_ Phill. (M. S.) 275
64. _E. lateralis_ Phill. (M. S.) 278
65. _E. Burchardti_ Dunk. (M. S.) 279
66. _E. Yokoyamae_ Sew. (From a block lent by Dr Woodward) 280
67. _Phyllotheca_? sp. (From a photograph by Mr Edwin Wilson) 285
68. _Phyllotheca Brongniarti_ Zigno; _P. indica_ Bunb.;
_Calamocladus frondosus_ Grand’Eury. (M. S.) 287
69. _Schizoneura gondwanensis_ Feist. (M. S.) 293
70. Transverse section of a Calamite stem. (M. S.) 299
71. Transverse section of a young Calamite stem 305
72. Longitudinal and transverse sections of _Calamites_ 308
73. Transverse section of a Calamite stem 310
74. Transverse section of _Calamites_ (_Arthropitys_) sp. 312
75. Longitudinal section (tangential) of _Calamites_
(_Arthropitys_) sp. 313
76. Longitudinal section (tangential) of _Calamites_
(_Arthropitys_) sp. 314
77. Portion of a Calamite stem; partially restored. (M. S.) 316
78.╮ Transverse and longitudinal (radial) sections of a thick 318
79.╯ Calamite stem. (Mr Highley) 319
80. Transverse section of a Calamite showing callus wood 320
81. Longitudinal section of a young Calamite 321
82. Pith-casts of _Calamites_ (_Stylocalamites_) sp. (M. S.) 323
83. _Calamites_ (_Arthrodendron_) sp. Transverse and longitudinal
sections 327
84. Transverse section of _Calamites_ (_Calamodendron_)
_intermedius_ Ren. 328
85. Leaves of a Calamite. (M. S.) 330
86. Transverse section of a Calamite leaf 331
87. _Calamocladus equisetiformis_ (Schloth.) (Miss G. M.
Woodward) 334
88. _Annularia stellata_ (Schloth.) (M. S.) 339
89. _Annularia sphenophylloides_ Zenk. (M. S.) 340
90. Pith-cast of a Calamite, with roots. (M. S.) 343
91. Transverse sections of Calamite roots 345
92. Root given off from a Calamite stem 347
93. _Calamostachys_ sp. (M. S.) 350
94. _C. Binneyana_ (Carr.). (Mr Highley) 352
95. _C. Binneyana_ (Carr.) 354
96. _C. Casheana_ Will. 356
97. _Palaeostachya pedunculata_ Will. (M. S.) 357
98. _P. vera_ sp. nov. 359
99. _Calamites_ (_Calamitina_) Göpp. (Ett.) (M. S.) 368
100. _Calamites_ (_Calamitina_) _approximatus_ Brongn. From a
photograph by Mr Kidston 370
101. _Calamites_ (_Calamitina_) sp. (From a block lent by Dr
Woodward) 373
102. _Calamites_ (_Eucalamites_) _cruciatus_ Sternb. (From a
photograph by Mr Edwin Wilson) 377
103. _Archaeocalamites scrobiculatus_ (Schloth.). (From a
photograph by Mr Edwin Wilson) 385
104. Diagrammatic longitudinal section of _Sphenophyllum_ 393
105. Transverse and longitudinal sections of _Sphenophyllum
insigne_ (Will.) and _S. plurifoliatum_ Will. and Scott 394
106. _Sphenophyllum plurifoliatum_ Will. and Scott. (From a
photograph by Mr Highley) 398
107. _Sphenophyllum_ strobilus, stem and root 400
108. Diagrammatic longitudinal section of a _Sphenophyllum_
strobilus. (M. S.) 402
109. _Sphenophyllum emarginatum_ (Brongn.) (M. S.) 407
110. _Sphenophyllum Thoni_ Mahr.; _S. trichomatosum_ Stur.
(M. S.) 410
111. _Sphenophyllum speciosum_ (Royle). (M. S.) 411
=Note.= The references in the footnotes require a word of explanation.
The titles of the works referred to will be found in the Bibliography
at the end of the volume. In this list the authors’ names are arranged
alphabetically and the papers of each author are in chronological
order. The numbers in brackets after the author’s name in the
footnotes, and before his name in the bibliographical list, refer to
the year of publication. Except in cases where the works were published
prior to 1800, the first two figures are omitted: thus Ward (84) refers
to a paper published by L. F. Ward in 1884. This system was suggested
by Dr H. H. Field in the _Biologisches Centralblatt_, vol. XIII.
1893, p. 753. (_Ueber die Art der Abfassung naturwissenschaftlicher
Litteraturverzeichnisse._)
=PART I. GENERAL.=
CHAPTER I.
HISTORICAL SKETCH.
“But particular care ought to be had not to consult or take
relations from any but those who appear to have been both long
conversant in these affairs, and likewise persons of Sobriety,
Faithfulness and Discretion, to avoid the being misled and
imposed upon either by falsehood, or the ignorance, credulity,
and fancifulness, that some of these people are but too obnoxious
unto.” JOHN WOODWARD, 1728.
The scientific study of fossil plants dates from a comparatively recent
period, and palaeobotany has only attained a real importance in the
eyes of botanists and geologists during the last few decades of the
present century. It would be out of place, in a short treatise like
the present, to attempt a detailed historical sketch, or to give an
adequate account of the gradual rise and development of this modern
science. An excellent _Sketch of Palaeobotany_ has recently been drawn
up by Prof. Lester Ward[1], of the United States Geological Survey, and
an earlier historical retrospect may be found in the introduction to
an important work by an eminent German palaeobotanist, the late Prof.
Göppert[2]. In the well-known work by Parkinson on _The Organic Remains
of a Former World_[3] there is much interesting information as to the
early history of our knowledge of fossil plants, as well as a good
exposition of the views held at the beginning of this century.
[Sidenote: FOSSIL PLANTS AND THE FLOOD.]
As a means of bringing into relief the modern development of the
science of fossil plants, we may briefly pass in review some of the
earlier writers, who have concerned themselves in a greater or less
degree with a descriptive or speculative treatment of the records of a
past vegetation. In the early part of the present century, and still
more in the eighteenth century, the occurrence of fossil plants and
animals in the earth’s crust formed the subject of animated, not to
say acrimonious, discussion. The result was that many striking and
ingenious theories were formulated as to the exact manner of formation
of fossil remains, and the part played by the waters of the deluge in
depositing fossiliferous strata. The earlier views on fossil vegetables
are naturally bound up with the gradual evolution of geological
science. It is from Italy that we seem to have the first glimmering
of scientific views; but we are led to forget this early development
of more than three hundred years ago, when we turn to the writings
of English and other authors of the eighteenth century. “Under these
white banks by the roadside,” as a writer on Verona has expressed it,
“was born, like a poor Italian gipsy, the modern science of geology.”
Early in the sixteenth century the genius of Leonardo da Vinci[4]
compelled him to adopt a reasonable explanation of the occurrence of
fossil shells in rocks far above the present sea-level. Another Italian
writer, Fracastaro, whose attention was directed to this matter by
the discovery of numerous shells brought to light by excavations at
Verona, expressed his belief in the organic nature of the remains, and
went so far as to call in question the Mosaic deluge as a satisfactory
explanation of the deposition of fossil-bearing strata.
The partial recognition by some observers of the true nature of fossils
marks the starting point of more rational views. The admission that
fossils were not mere sports of nature, or the result of some wonderful
‘vis lapidifica,’ was naturally followed by numerous speculations
as to the manner in which the remains of animals and plants came
to be embedded in rocks above the sea-level. For a long time, the
‘universal flood’ was held responsible by nearly all writers for the
existence of fossils in ancient sediments. Dr John Woodward, in his
_Essay toward a Natural History of the Earth_, propounded the somewhat
revolutionary theory, that “the whole terrestrial globe was taken all
to pieces and dissolved at the Deluge, the particles of stone, marble,
and all solid fossils dissevered, taken up into the water, and there
sustained together with sea-shells and other animal and vegetable
bodies: and that the present earth consists, and was formed out of that
promiscuous mass of sand, earth, shells, and the rest falling down
again, and subsiding from the water[5].” In common with other writers,
he endeavoured to fix the exact date of the flood by means of fossil
plants. Speaking of some hazel-nuts, which were found in a Cheshire
moss pit, he draws attention to their unripened condition, and adds:
“The deluge came forth at the end of May, when nuts are not ripe.” As
additional evidence, he cites the occurrence of “Pine cones in their
vernal state,” and of some Coal-Measure fossils which he compares with
Virginian Maize, “tender, young, vernal, and not ripened[6].” Woodward
(1665–1728) was Professor of Physic in Gresham College; he bequeathed
his geological collections to the University of Cambridge, and founded
the Chair which bears his name.
Another writer, Mendes da Costa, in a paper in the _Philosophical
Transactions_ for 1758, speaks of the impressions of “ferns and
reed-like plants” in the coal-beds, and describes some fossils
(_Sigillaria_ and _Stigmaria_) as probably unknown forms of plant
life[7].
Here we have the suggestion that in former ages there were plants
which differed from those of the present age. Discussing the nature
of some cones (_Lepidostrobi_) from the ironstone of Coalbrookdale
in Shropshire, he concludes: “I firmly believe these bodies to be of
vegetable origin, buried in the strata of the Earth at the time of the
universal deluge recorded by Moses.” Scheuchzer of Zurich, the author
of one of the earliest works on fossil plants and a “great apostle
of the Flood Theory,” figures and describes a specimen as an ear of
corn, and refers to its size and general appearance as pointing to the
month of May as the time of the deluge[8]. Another English writer, Dr
Parsons, in giving an account of the well-known ‘fossil fruits and
other bodies found in the island of Sheppey,’ is disposed to dissent
from Woodward’s views as to the time of the flood. He suggests that the
fact of the Sheppey fruits being found in a perfectly ripe condition,
points to the autumn as the more probable time for the occurrence of
the deluge[9].
In looking through the works of the older writers, and occasionally
in the pages of latter-day contributors, we frequently find curiously
shaped stones, mineral markings on rock surfaces, or certain fossil
animals, described as fossil plants. In Plot’s _Natural History of
Oxfordshire_, published in 1705, a peculiarly shaped stone, probably a
flint, is spoken of as one of the ‘Fungi lethales non esculenti[10]’;
and again a piece of coral[11] is compared with a ‘Bryony root broken
off transversely.’ On the other hand, that we may not undervalue
the painstaking and laborious efforts of those who helped to lay
the foundations of modern science, we may note that such authors as
Scheuchzer and Woodward were not misled by the moss-like or dendritic
markings of oxide of manganese on the surface of rocks, which are not
infrequently seen to-day in the cabinets of amateurs as specimens of
fossil plants.
The oldest figures of fossil plants from English rocks which are drawn
with any degree of accuracy are those of Coal-Measure ferns and other
plants in an important work by Edward Lhwyd published at Oxford in
1760[12].
Passing beyond these prescientific speculations, brief reference may
be made to some of the more eminent pioneers of palaeobotany. The
Englishman Artis[13] deserves mention for the quality rather than the
quantity of his contributions to Palaeozoic botany; and among American
authors Steinhauer’s[14] name must hold a prominent place in the list
of those who helped to found this branch of palaeontology. Among
German writers, Schlotheim stands out prominently as one who first
published a work on fossil plants which still remains an important
book of reference. Writing in 1804, he draws attention to the neglect
of fossils from a scientific standpoint; they are simply looked upon,
he says, as “unimpeachable documents of the flood[15].” His book
contains excellent figures of many Coal-Measure plants, and we find in
its pages occasional comparisons of fossil species with recent plants
of tropical latitudes. Among the earlier authors whose writings soon
become familiar to the student of fossil plants, reference must be
made to Graf Sternberg, who was born three years before Schlotheim,
but whose work came out some years later than that of the latter.
His great contribution to Fossil Botany entitled _Versuch einer
geognostisch-botanischen Darstellung der Flora der Vorwelt_, was
published in several parts between the years 1820 and 1838; it was
drawn up with the help of the botanist Presl, and included a valuable
contribution by Corda[16]. In addition to descriptions and numerous
figures of plants from several geological horizons, this important work
includes discussions on the formation of coal, with observations on the
climates of past ages.
[Sidenote: STERNBERG AND BRONGNIART.]
Sternberg endeavoured to apply to fossil plants the same methods of
treatment as those made use of in the case of recent species. About
the same time as Sternberg’s earlier parts were published, Adolphe
Brongniart[17] of Paris began to enrich palaeobotanical science by
those splendid researches which have won for him the title of the
“Father of palaeobotany.” In Brongniart’s _Prodrome_, and _Histoire des
végétaux fossiles_, and later in his _Tableau des genres de végétaux
fossiles_, we have not merely careful descriptions and a systematic
arrangement of the known species of fossil plants, but a masterly
scientific treatise on palaeobotany in its various aspects, which
has to a large extent formed the model for the best subsequent works
on similar lines. From the same author, at a later date, there is at
least one contribution to fossil plant literature which must receive
a passing notice even in this short sketch. In 1839 he published an
exhaustive account of the minute structure of one of the well-known
Palaeozoic genera, _Sigillaria_; this is not only one of the best
of the earliest monographs on the histology of fossil species, but
it is one of the few existing accounts of the internal structure of
this common type[18]. The fragment of a Sigillarian stem which formed
the subject of Brongniart’s memoir is in the Natural History Museum
in the Jardin des Plantes, Paris. It affords a striking example of
the perfection of preservation as well as of the great beauty of the
silicified specimens from Autun, in Central France. Brongniart was
not only a remarkably gifted investigator, whose labours extend over
a period connecting the older and more crude methods of descriptive
treatment with the modern development of microscopic analysis, but
he possessed the power of inspiring a younger generation with a
determination to keep up the high standard of the palaeobotanical
achievements of the French School. In some cases, indeed, his
disciples have allowed a natural reverence for the Master to warp
their scientific judgement, where our more complete knowledge has
naturally led to the correction of some of Brongniart’s conclusions.
Without attempting to follow the history of the science to more recent
times, the names of Heer, Lesquereux, Zigno, Massalongo, Saporta and
Ettingshausen should be included among those who rendered signal
service to the science of fossil plants. The two Swiss writers,
Heer[19] and Lesquereux[20], contributed numerous books and papers
on palaeobotanical subjects, the former being especially well known
in connection with the fossil floras of Switzerland and of Arctic
lands, and the latter for his valuable writings on the fossil plants
of his adopted country, North America. Zigno[21] and Massalongo[22]
performed like services for Italy, and the Marquis of Saporta’s name
will always hold an honourable and prominent position in the list of
the pioneers of scientific palaeobotany; his work on the Tertiary and
Mesozoic floras of France being specially noteworthy among the able
investigations which we owe to his ability and enthusiasm[23]. In Baron
Ettingshausen[24] we have another representative of those students
of ancient vegetation who have done so much towards establishing the
science of fossil plants on a philosophical basis.
As in other fields of Natural Science, so also in a marked degree in
fossil botany, a new stimulus was given to scientific inquiry by the
application of the microscope to palaeobotanical investigation. In
1828 Sprengel published a work entitled _Commentatio de Psarolithis,
ligni fossilis genere_[25]; in which he dealt in some detail with
the well-known silicified fern-stems of Palaeozoic age, from Saxony,
basing his descriptions on the characteristics of anatomical structure
revealed by microscopic examination.
[Sidenote: THE INTERNAL STRUCTURE OF FOSSIL PLANTS.]
In 1833 Henry Witham of Lartington brought out a work on _The Internal
Structure of Fossil Vegetables_[26]; this book, following the much
smaller and less important work by Sprengel, at once established
palaeobotany on a firmer scientific basis, and formed the starting
point for those more accurate methods of research, which have yielded
such astonishing results in the hands of modern workers. In the
introduction Witham writes, “My principal object in presenting this
work to the public, is to impress upon geologists the advantage of
attending more particularly to the intimate organization of fossil
plants; and should I succeed in directing their efforts towards
the elucidation of this obscure subject, I shall feel a degree of
satisfaction which will amply repay my labour[27].”
On another page he writes as follows,—“From investigations made by
the most active and experienced botanical geologists, we find reason
to conclude that the first appearance of an extensive vegetation
occurred in the Carboniferous series; and from a recent examination
of the mountain-limestone groups and coal-fields of Scotland, and the
north of England, we learn that these early vegetable productions, so
far from being simple in their structure, as had been supposed, are
as complicated as the phanerogamic plants of the present day. This
discovery necessarily tends to destroy the once favourite idea, that,
from the oldest to the most recent strata, there has been a progressive
development of vegetable and animal forms, from the simplest to the
most complex[28].” Since Witham’s day we have learnt much as to the
morphology of Palaeozoic plants, and can well understand the opinions
to which he thus gives expression.
It would be difficult to overrate the immense importance of this
publication from the point of view of modern palaeobotany.
The art of making transparent sections of the tissues of fossil
plants seems to have been first employed by Sanderson, a lapidary,
and it was afterwards considerably improved by Nicol[29]. This most
important advance in methods of examination gave a new impetus to
the subject, but it is somewhat remarkable that the possibilities of
the microscopical investigation of fossil plants have been but very
imperfectly realised by botanical workers until quite recent years. As
regards such a flora as that of the Coal-Measures, we can endorse the
opinion expressed at the beginning of the century in reference to the
study of recent mosses—“Ohne das Göttergeschenk des zusammengesetzten
Mikroskops ist auf diesem Felde durchaus keine Ernte[30].” A useful
summary of the history of the study of internal structure is given
by Knowlton in a memoir published in 1889[31]. Not long after
Witham’s book was issued there appeared a work of exceptional merit
by Corda[32], in which numerous Palaeozoic plants are figured and
fully described, mainly from the standpoint of internal structure.
This author lays special stress on the importance of studying the
microscopical structure of fossil plants.
[Sidenote: ENGLISH PALAEOBOTANISTS.]
Without pausing to enumerate the contributions of such well-known
continental authors as Göppert, Cotta, Schimper, Stenzel, Schenk and
a host of others, we may glance for a moment at the services rendered
by English investigators to the study of palaeobotanical histology.
Unfortunately we cannot always extend our examination of fossil plants
beyond the characters of external form and surface markings; but in
a few districts there are preserved remnants of ancient floras in
which fragments of stems, roots, leaves and other structures have been
petrified in such a manner as to retain with wonderful completeness
the minute structure of their internal tissues. During the deposition
of the coal seams in parts of Yorkshire and Lancashire the conditions
of fossilisation were exceptionally favourable, and thus English
investigators have been fortunately placed for conducting researches on
the minute anatomy of the Coal-Measure plants. The late Mr Binney of
Manchester did excellent service by his work on the internal structure
of some of the trees of the Coal Period forests. In his introductory
remarks to a monograph on the genus _Calamites_, after speaking of
the desirability of describing our English specimens, he goes on to
say, “When this is done, we are likely to possess a literature on our
Carboniferous fossils worthy of the first coal-producing country[33].”
The continuation and extension of Binney’s work in the hands of
Carruthers, Williamson, and others, whose botanical qualifications
enabled them to produce work of greater scientific value, has gone far
towards the fulfilment of Binney’s prophecy.
[Sidenote: DIFFICULTIES OF IDENTIFICATION.]
In dealing with the structure of Palaeozoic plants, we shall be
under constant obligation to the splendid series of memoirs from
the pen of Prof. Williamson[34]. As the writer of a sympathetic
obituary notice has well said: “In his fifty-fifth year he began
the great series of memoirs which mark the culminating point of his
scientific activity, and which will assure to him, for all time, in
conjunction with Brongniart, the honourable title of a founder of
modern Palaeobotany[35].” If we look back through a few decades,
and peruse the pages of Lindley and Hutton’s classic work[36] on
the Fossil flora of Great Britain, a book which is indispensable
to fossil botanists, and read the description of such a genus as
_Sigillaria_ or _Stigmaria_; or if we extend our retrospect to an
earlier period and read Woodward’s description of an unusually good
specimen of a _Lepidodendron_, and finally take stock of our present
knowledge of such plants, we realise what enormous progress has been
made in palaeobotanical studies. Lindley and Hutton, in the preface
to the first volume of the Flora, claim to have demonstrated that
both _Sigillaria_ and _Stigmaria_ were plants with “the highest
degree of organization, such as _Cactaeae_, or _Euphorbiaceae_, or
even _Asclepiadeae_”; Woodward describes his _Lepidodendron_ (Fig. 1)
as “an ironstone, black and flat, and wrought over one surface very
finely, with a strange cancellated work[37].” Thanks largely to the
work of Binney, Carruthers, Hooker, Williamson, and to the labours
of continental botanists, we are at present almost as familiar with
_Lepidodendron_ and several other Coal-Measure genera as with the
structure of a recent forest tree. While emphasizing the value of the
microscopic methods of investigation, we are not disposed to take such
a hopeless view of the possibilities of the determination of fossil
forms, in which no internal structure is preserved, as some writers
have expressed. The preservation of minute structure is to be greatly
desired from the point of view of the modern palaeobotanist, but he
must recognise the necessity of making such use as he can of the
numberless examples of plants of all ages, which occur only in the form
of structureless casts or impressions.
[Illustration: FIG. 1. Four leaf-cushions of a _Lepidodendron_. Drawn
from a specimen in the Woodward Collection, Cambridge. (Nat. size.)]
In looking through the writings of the earlier authors we cannot help
noticing their anxiety to match all fossil plants with living species;
but by degrees it was discovered that fossils are frequently the
fragmentary samples of extinct types, which can be studied only under
very unfavourable conditions. In the absence of those characters on
which the student of living plants relies as guides to classification,
it is usually impossible to arrive at any trustworthy conclusions as
to precise botanical affinity. Brongniart and other authors recognised
this fact, and instituted several convenient generic terms of a purely
artificial and provisional nature, which are still in general use. The
dangers and risks of error which necessarily attend our attempts to
determine small and imperfect fragments of extinct species of plants,
will be briefly touched on in another place.
CHAPTER II.
RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY.
“La recherche du plan de la création, voilà le but vers lequel nos
efforts peuvent tendre aujourd’hui.” GAUDRY, 1883.
Since the greater refinements and thoroughness of scientific methods
and the enormous and ever-increasing mass of literature have inevitably
led to extreme specialisation, it is more than ever important to look
beyond the immediate limits of one’s own subject, and to note its
points of contact with other lines of research. A palaeobotanist is
primarily concerned with the determination and description of fossil
plants, but he must at the same time constantly keep in view the
bearing of his work on wider questions of botanical or geological
importance. From the nature of the case, we have in due measure to
adapt the methods of work to the particular conditions before us. It is
impossible to follow in the case of all fossil species precisely the
same treatment as with the more complete and perfect recent plants;
but it is of the utmost importance for a student of palaeobotany, by
adhering to the methods of recent botany, to preserve as far as he is
able the continuity of the past and present floras. Palaeontological
work has often been undertaken by men who are pure geologists, and
whose knowledge of zoology or botany is of the most superficial
character, with the result that biologists have not been able to avail
themselves, to any considerable extent, of the records of extinct forms
of life. They find the literature is often characterised by a special
palaeontological phraseology, and by particular methods of treatment,
which are unknown to the student of living plants and animals. From
this and other causes a purely artificial division has been made
between the science of the organic world of to-day and that of the past.
[Sidenote: NEGLECT OF FOSSILS BY BOTANISTS.]
Fossils are naturally regarded by a stratigraphical geologist
as records which enable him to determine the relative age of
fossil-bearing rocks. For such a purpose it is superfluous to inquire
into the questions of biological interest which centre round the
relics of ancient floras. Primarily concerned, therefore, with fixing
the age of strata, it is easy to understand how geologists have been
content with a special kind of palaeontology which is out of touch
with the methods of systematic zoology or botany. On the other hand,
the botanist whose observations and researches have not extended
beyond the limits of existing plants, sees in the vast majority of
fossil forms merely imperfect specimens, which it is impossible to
determine with any degree of scientific accuracy. He prefers to wait
for perfect material; or in other words, he decides that fossils must
be regarded as outside the range of taxonomic botany. It would seem,
then, that the unsatisfactory treatment or comparative neglect of
fossil plants, has been in a large measure due to the narrowness of
view which too often characterises palaeobotanical literature. This
has at once repelled those who have made a slight effort to recognise
the subject, and has resulted in a one-sided and, from a biological
standpoint, unscientific treatment of this branch of science. It must
be admitted that palaeobotanists have frequently brought the subject
into disrepute by their over-anxiety to institute specific names for
fragments which it is quite impossible to identify. This over-eagerness
to determine imperfect specimens, and the practice of drawing
conclusions as to botanical affinity without any trustworthy evidence,
have naturally given rise to considerable scepticism as to the value
of palaeobotanical records. Another point, which will be dealt with
at greater length in a later chapter, is that geologists have usually
shown a distinct prejudice against fossil plants as indices of
geological age; this again, is no doubt to a large extent the result of
imperfect and inaccurate methods of description, and of the neglect of
and consequent imperfect acquaintance with fossil plants as compared
with fossil animals.
The student of fossil plants should endeavour to keep before him the
fact that the chief object of his work is to deal with the available
material in the most natural and scientific manner; and by adopting
the methods of modern botany, he should always aim to follow such
lines as may best preserve the continuity of past and present types
of plants. Descriptions of floras of past ages and lists of fossil
species, should be so compiled that they may serve the same purpose to
a stratigraphical geologist, who is practically a geographer of former
periods of the Earth’s history, as the accounts of existing floras
to students of present day physiography. The effect of carrying out
researches on some such lines as these, should be to render available
to both botanists and geologists the results of the specialist’s work.
In some cases, palaeobotanical investigations may be of the utmost
service to botanical science, and of little or no value to geology.
The discovery of a completely preserved gametophyte of _Lepidodendron_
or _Calamites_, or of a petrified Moss plant in Palaeozoic rocks would
appeal to most botanists as a matter of primary importance, but for the
stratigraphical geologist such discoveries would possess but little
value. On the other hand the discovery of some characteristic species
of Coal-Measure plants from a deep boring through Mesozoic or Tertiary
strata might be a matter of special geological importance, but to the
botanist it would be of no scientific value. In very many instances,
however, if the palaeobotanist follows such lines as have been briefly
suggested, the results of his labours should be at once useful and
readily accessible to botanists and geologists. As Humboldt has said
in speaking of Palaeontology, “the analytical study of primitive
animal and vegetable life has taken a double direction; the one is
purely morphological, and embraces especially the natural history
and physiology of organisms, filling up the chasms in the series of
still living species by the fossil structures of the primitive world.
The second is more specially geognostic, considering fossil remains
in their relations to the superposition and relative age of the
sedimentary formations[38].”
[Sidenote: FOSSIL PLANTS AND DISTRIBUTION.]
To turn for a moment to some of the most obvious connections between
palaeobotany and the wider sciences of botany and geology. The records
of fossil species must occupy a prominent position in the data by
which we may hope to solve some at least of the problems of plant
evolution. From the point of view of distribution, palaeobotany is of
considerable value, not only to the student of geographical botany, but
to the geologist, who endeavours to map out the positions of ancient
continents with the help of palaeontological evidence. The present
distribution of plants and animals represents but one chapter in the
history of life on the Earth; and to understand or appreciate the facts
which it records, we have to look back through such pages as have been
deciphered in the earlier chapters of the volume. The distribution of
fossil plants lies at the foundation of the principles of the present
grouping of floras on the Earth’s surface. Those who have confined
their study of distribution to the plant geography of the present
age, must supplement their investigations by reference to the work of
palaeobotanical writers. If the lists of plant species drawn up by
specialists in fossil botany, have been prepared with a due sense of
the important conclusions which botanists may draw from them from the
standpoint of distribution, they will be readily accepted as sound
links in the chain of evidence. Unfortunately, however, if many of
the lists of ancient floras were made use of in such investigations,
the conclusions arrived at would frequently be of little value on
account of the untrustworthy determinations of many of the species.
In the case of particular genera the study of the distribution of
the former species both in time and space, that is geologically and
geographically, points to rational explanations of, or gives added
significance to, the facts of present day distribution. That isolated
conifer, _Ginkgo biloba_ L. now restricted to Japan and China, was in
former times abundant in Europe and in other parts of the world. It is
clearly an exceedingly ancient type, isolated not only in geographical
distribution but in botanical affinities, which has reached the last
stage in its natural life. The Mammoth trees of California (_Sequoia
sempervirens_ Endl., and _S. gigantea_ Lindl. and Gord.) afford other
examples of a parallel case. The Tulip tree of North America and China
and other allied forms are fairly common in the Tertiary plant beds
of Europe, but the living representatives are now exclusively North
American. Such differences in distribution as are illustrated by these
dicotyledonous forest trees in Tertiary times and at the present day,
have been clearly explained with the help of the geological record.
Forbes, Darwin, Asa Gray[39] and others have been able to explain many
apparent anomalies in the distribution of existing plants, and to
reconcile the differences between the past and present distribution
of many genera by taking account of the effect on plant life of the
glacial period. As the ice gradually crept down from the polar regions
and spread over the northern parts of Europe, many plants were driven
further south in search of the necessary warmth. In the American
continent such migration was rendered possible by the southern land
extension; in Europe on the other hand the southerly retreat was cut
off by impassable barriers, and the extinction of several genera was
the natural result.
The comparatively abundant information which we possess as to the
past vegetation of polar regions and the value of such knowledge to
geologists and botanists alike is in striking contrast to the absence
of similar data as regards Antarctic fossils. Darwin in an exceedingly
interesting letter to Hooker _à propos_ of a forthcoming British
Association address, referring to this subject writes as follows:—
“The extreme importance of the Arctic fossil plants is self-evident.
Take the opportunity of groaning over our ignorance of the Lignite
plants of Kerguelen Land, or any Antarctic land. It might do good[40].”
In working out any collection of fossil plants, it would be well,
therefore, to bear in mind that our aim should be rather to reproduce
an accurate fragment of botanical history, than to perform feats of
determination with hopelessly inadequate specimens. Had this principle
been generally followed, the number of fossil plant species would be
enormously reduced, but the value of the records would be considerably
raised.
[Sidenote: FOSSIL PLANTS AND CLIMATE.]
Our knowledge of plant anatomy, and of those laws of growth which
govern certain classes of plants to-day and in past time, has been very
materially widened and extended by the facts revealed to us by the
detailed study of Coal-Measure species. The modern science of Plant
Biology, refounded by Charles Darwin, has thrown considerable light
on the laws of plant life, and it enables us to correlate structural
characteristics with physiological conditions of growth. Applying the
knowledge gained from living plants to the study of such extinct types
as permit of close microscopic examination, we may obtain a glimpse
into the secrets of the botanical binomics of Palaeozoic times. The
wider questions of climatic conditions depend very largely upon the
evidence of fossil botany for a rational solution. As an instance
of the best authenticated and most striking alternation in climatic
conditions in comparatively recent times, we may cite the glacial
period or Ice-Age. The existence of Arctic conditions has been proved
by purely geological evidence, but it receives additional confirmation,
and derives a wider importance from the testimony of fossil plants. In
rocks deposited before the spread of ice from high northern latitudes,
we find indubitable proofs of a widely distributed subtropical flora in
Central and Northern Europe. Passing from these rocks to more recent
beds there are found indications of a fall in temperature, and such
northern plants as the dwarf Birch, the Arctic Willow and others reveal
the southern extension of Arctic cold to our own latitudes.
The distribution of plants in time, that is the range of classes,
families, genera and species of plants through the series of strata
which make up the crust of the earth, is a matter of primary importance
from a botanical as well as from a geological point of view.
Among the earlier writers, Brongniart recognised the marked differences
between the earlier and later floras, and attempted to correlate
the periods of maximum development of certain classes of plants
with definite epochs of geological history. He gives the following
classification in which are represented the general outlines of plant
development from Palaeozoic to Tertiary times[41].
I. Reign of Acrogens ╭ 1. Carboniferous epoch.
╰ 2. Permian epoch.
II. Reign of Gymnosperms ╭ 3. Triassic epoch.
╰ 4. Jurassic epoch (including the Wealden).
III. Reign of Angiosperms ╭ 5. Cretaceous epoch.
╰ 6. Tertiary epoch.
Since Brongniart’s time this method of classification has been
extended to many of the smaller subdivisions of the geological epochs,
and species of fossil plants are often of the greatest value in
questions of correlation. In recent years the systematic treatment
of Coal-Measure and other plants in the hands of various Continental
and English writers has clearly demonstrated their capabilities
for the purpose of subdividing a series of strata into stages and
zones[42]. The more complete becomes our knowledge of any flora, the
greater possibility there is of making use of the plants as indices of
geological age[43].
[Sidenote: FOSSIL PLANTS AND PHYLOGENY.]
Not only is it possible to derive valuable aid in the correlation of
strata from the facts of plant distribution, but we may often follow
the various stages in the history of a particular genus as we trace
the records of its occurrence through the geologic series. In studying
the march of plant life through past ages, the botanist may sometimes
follow the progress of a genus from its first appearance, through the
time of maximum development, to its decline or extinction. In the
Palaeozoic forests there was perhaps no more conspicuous or common tree
than the genus long known under the name of _Calamites_. This plant
attained a height of fifty or a hundred feet, with a proportionate
girth, and increased in thickness in a manner precisely similar to
that in which our forest trees grow in diameter. The exceptionally
favourable conditions under which specimens of calamitean plants
have been preserved, have enabled us to become almost as familiar
with the minute structure of their stems and roots, as well as with
their spore-producing organs, as with those of a living species.
In short, it is thoroughly established that _Calamites_ agrees in
most essential respects with our well known _Equisetum_, and must
be included in the same order, or at least sub-class, as the recent
genus of _Equisetaceae_. As we ascend the geologic series from the
Coal-Measures, a marked numerical decline of _Calamites_ is obvious
in the Permian period, and in the red sandstones of the Vosges, which
belong to the same series of rocks as the Triassic strata of the
Cheshire plain, the true _Calamites_ is replaced by a large _Equisetum_
apparently identical in external appearance and habit of growth with
the species living to-day. In the more recent strata the Horse-tails
are still represented, but the size of the Tertiary species agrees
more closely with the comparatively small forms which have such a
wide geographical distribution at the present time. Thus we are able
to trace out the history of a recent genus of Vascular Cryptogams,
and to follow a particular type of organisation from the time of its
maximum development, through its gradual transition to those structural
characters which are represented in the living descendants of the
arborescent Calamites of the coal-period forests. The pages of such a
history are frequently imperfect and occasionally missing, but others,
again, are written in characters as clear as those which we decipher by
a microscopical examination of the tissues of a recent plant.
As one of the most striking instances in which the microscopic study
of fossil plants has shown the way to a satisfactory solution of
the problems of development, we may mention such extinct genera as
_Lyginodendron_, _Myeloxylon_ and others. Each of these genera will be
dealt with at some length in the systematic part of the book, and we
shall afterwards discuss the importance of such types, from the point
of view of plant evolution.
The botanist who would trace out the phylogeny of any existing class or
family, makes it his chief aim to discover points of contact between
the particular type of structure which he is investigating, and that of
other more or less closely related classes or families.
Confining himself to recent forms, he may discover, here and there,
certain anatomical or embryological facts, which suggest promising
lines of inquiry in the quest after such affinities as point to a
common descent. Without recourse to the evidence afforded by the plants
of past ages, we must always admit that our existing classification
of the vegetable kingdom is an expression of real gaps which separate
the several classes of plants from one another. On the other hand our
recently acquired and more accurate knowledge of such genera as have
been alluded to, has made us acquainted with types of plant structure
which enable us to fill in some of the lacunae in our existing
classification. In certain instances we find merged in a single species
morphological characteristics which, in the case of recent plants, are
regarded as distinctive features of different subdivisions. It has
been clearly demonstrated that in _Lyginodendron_, we have anatomical
peculiarities typical of recent cycads, combined with structural
characteristics always associated with existing ferns. In rare cases,
it happens that the remarkably perfect fossilisation of the tissues of
fossil plants, enables us not only to give a complete description of
the histology of extinct forms, but also to speak with confidence as to
some of those physiological processes which governed their life.
[Sidenote: GEOLOGICAL HISTORY.]
So far, palaeobotany has been considered in its bearings on the
study of recent plants. From a geological point of view the records
of ancient floras have scarcely less importance. In recent years,
facts have been brought to light, which show that plants have played
a more conspicuous part than has usually been supposed as agents of
rock-building. As tests of geologic age, there are good grounds for
believing that the inferiority of plants to animals is more apparent
than real. This question, however, must be discussed at greater length
in a later chapter.
Enough has been said to show the many-sided nature of the science of
Fossil Plants, and the wide range of the problems which the geologist
or botanist may reasonably expect to solve, by means of trustworthy
data afforded by scientific palaeobotanical methods.
CHAPTER III.
GEOLOGICAL HISTORY.
“But how can we question dumb rocks whose speech is not clear[44]?”
In attempting to sketch in briefest outline the geological history
of the Earth, the most important object to keep in view is that of
reproducing as far as possible the broad features of the successive
stages in the building of the Earth’s crust. It is obviously
impossible to go into any details of description, or to closely
follow the evolution of the present continents; at most, we can only
refer to such facts as may serve as an introduction of the elements
of stratigraphical geology to non-geological readers. For a fuller
treatment of the subject reference must be made to special treatises on
geology.
For the sake of convenience, it is customary in stratigraphical geology
as also in biology, to make use of our imperfect knowledge as an aid
to classification. If we possessed complete records of the Earth’s
history, we should have an unbroken sequence, not merely of the various
forms of life that ever existed, but of the different kinds of rocks
formed in the successive ages of past time. As gaps exist in the chain
of life, so also do we find considerable breaks in the sequence of
strata which have been formed since the beginning of geologic time. The
danger as well as the convenience of artificial classification must be
kept in view. This has been well expressed by Freeman, in speaking of
architectural styles,—“Our minds,” he says, “are more used to definite
periods; they neglect or forget transitions which do indeed exist[45].”
The idea of definite classification is liable to narrow our view of
uniformity and the natural sequence of events.
[Sidenote: ROCK-BUILDING.]
Composing that part of the earth which is accessible to us,—or as it is
generally called the earth’s crust,—there are rocks of various kinds,
of which some have been formed by igneous agency, either as lavas or
beds of ashes, or in the form of molten magmas which gradually cooled
and became crystalline below a mass of superincumbent strata. With
these rocks we need not concern ourselves.
A large portion of the earth’s crust consists of such materials as
sandstones, limestones, shales, and similar strata which have been
formed in precisely the same manner as deposits are being accumulated
at the present day. The whole surface of the earth is continually
exposed to the action of destructive agencies, and suffers perpetual
decay; it is the products of this ceaseless wear and tear that form the
building materials of new deposits.
The operation of water in its various forms, of wind, changes of
temperature, and other agents of destruction cannot be fully dealt with
in this short summary.
A river flowing to the sea or emptying itself into an inland lake,
carries its burden of gravel, sand, and mud, and sooner or later, as
the rate of flow slackens, it deposits the materials in the river-bed
or on the floor of the sea or lake.
Fragments of rock, chipped off by wedges of ice, or detached in other
ways from the parent mass, find their way to the mountain streams,
and if not too heavy are conveyed to the main river, where the larger
pieces come to rest as more or less rounded pebbles. Such water-worn
rocks accumulate in the quieter reaches of a swiftly flowing river, or
are thrown down at the head of the river’s delta. If such a deposit
of loose water-worn material became cemented together either by the
consolidating action of some solution percolating through the general
mass, or by the pressure of overlying deposits, there would be formed
a hard rock made up of rounded fragments of various kinds of strata
derived from different sources. Such a rock is known as a CONGLOMERATE.
The same kind of rock may be formed equally well by the action of the
sea; an old sea-beach with the pebbles embedded in a cementing matrix
affords a typical example of a coarse conglomerate. Plant remains are
occasionally met with in conglomerates, but usually in a fragmentary
condition.
From a conglomerate composed of large water-worn pebbles, to a fine
homogeneous sandstone there are numerous intermediate stages. A body
of water, with a velocity too small to carry along pebbles of rock in
suspension or to roll them along the bed of the channel, is still able
to transport the finer fragments or grains of sand, but as a further
decrease in the velocity occurs, these are eventually deposited as beds
of coarse or fine sand. The stretches of sand on a gradually shelving
sea shore, or the deposits of the same material in a river’s delta,
have been formed by the gradual wearing away and disintegration of
various rocks, the detritus of which has been spread out in more or
less regular beds on the floor of a lake or sea. Such accumulations
of fine detrital material, if compacted or cemented together, become
typical SANDSTONES.
In tracing beds of sandstone across a tract of country, it is
frequently found that the character of the strata gradually alters; mud
or clay becomes associated with the sandy deposit, until finally the
sandstone is replaced by beds of dark coloured shale. Similarly the
sandy detritus on the ocean floor, or in an inland lake, when followed
further and further from the source from which the materials were
derived, passes by degrees into argillaceous sand, and finally into
sheets of dark clay or mud. The hardened beds of clay or fine grained
mud become transformed into SHALES. As a general rule, then, shales are
rocks which have been laid down in places further from the land, or at
a greater distance from the source of origin of the detrital material,
than sandstones or conglomerates. The conglomerates, or old shingle
beaches, usually occur in somewhat irregular patches, marking old
shore-lines or the head of a river delta. Coarse sandstones, or grits,
may occur in the form of regularly bedded strata stretching over a
wide area; and shales or clays may be followed through a considerable
extent of country. The finer material composing the clays and shales
has been held longer in suspension and deposited in deeper water in
widespread and fairly horizontal layers.
In some districts sandstones occur in which the individual grains
show a well marked rounding of the angles, and in which fossils
are extremely rare or entirely absent. The close resemblance of
such deposits to modern desert sands suggests a similar method of
formation; and there can be no doubt that in some instances there have
been preserved the wind-worn desert sands of former ages. Aeolian
or wind-formed accumulations, although by no means common, are of
sufficient importance to be mentioned as illustrating a certain type of
rock.
[Sidenote: CALCAREOUS ROCKS.]
The thick masses of limestone which form so prominent a feature in
parts of England and Ireland, have been formed in a manner different
from that to which sandstones and shales owe their origin. On the
floor of a clear sea, too far from land to receive any water-borne
sediment, there is usually in process of formation a mass of calcareous
material, which in a later age may rise above the surface of the water
as chalk or LIMESTONE. Those organisms living in the sea, which are
enclosed either wholly or in part by calcareous shells, are agents of
limestone-building; their shells constantly accumulating on the floor
of the sea give rise in course of time to a thick mass of sediment,
composed in great part of carbonate of lime. Some of the shells in
such a deposit may retain their original form, the calcareous body
may on the other hand be broken up into minute fragments which are
still recognisable with the help of a microscope, or the shells and
other hard parts may be dissolved or disintegrated beyond recognition,
leaving nothing in the calcareous sediment to indicate its method of
formation.
Not a few limestones consist in part of fossil corals, and owe their
origin to colonies of coral polyps which built up reefs or banks of
coral in the ancient seas.
In the white cliffs of Dover, Flamborough Head and other places, we
have a somewhat different form of calcareous rock, which in part
consists of millions of minute shells of Foraminifera, in part of
broken fragments of larger shells of extinct molluscs, and to some
extent of the remains of siliceous sponges. As a general rule,
limestones and chalk rocks are ancient sediments, formed in clear and
comparatively deep water, composed in the main of carbonate of lime,
in some cases with a certain amount of carbonate of magnesium, and
occasionally with a considerable admixture of silica.
In such rocks land-plants must necessarily be rare. There are, however,
limestones which wholly or in part owe their formation to masses of
calcareous algae, which grew in the form of submarine banks or on coral
reefs. Occasionally the remains of these algae are clearly preserved,
but frequently all signs of plant structure have been completely
obliterated. Again, there occur limestone rocks formed by chemical
means, and in a manner similar to that in which beds of travertine are
now being accumulated.
Granites, basalts, volcanic lavas, tuffs, and other igneous rocks
need not claim our attention, except in such cases as permit of plant
remains being found in association with these materials. Showers of
ashes blown from a volcano, may fall on the surface of a lake or sea
and become mixed with sand and mud of subaerial origin. Streams of lava
occasionally flow into water, or they may be poured from submarine
vents, and so spread out on the ocean bed with strata of sand or clay.
Passing from the nature and mode of origin of the sedimentary strata to
the manner of their arrangement in the Earth’s crust, we must endeavour
to sketch in the merest outline the methods of stratigraphical geology.
The surface of the Earth in some places stands out in the form of bare
masses of rock, roughly hewn or finely carved by Nature’s tools of
frost, rain or running water; in other places we have gently undulating
ground with beds of rock exposed to view here and there, but for the
most part covered with loose material such as gravel, sands, boulder
clay and surface soil.
[Sidenote: GEOLOGICAL SECTIONS.]
In the flat lands of the fen districts, the peat beds and low-lying
salt marshes form the surface features, and are the connecting links
between the rock-building now in progress and the deposits of an
earlier age. If we could remove all these surface accumulations of
sand, gravel, peat and surface soil, and take a bird’s eye view of
the bare surface of the rocky skeleton of the earth’s crust, we
should have spread before us the outlines of a geological map. In
some places fairly horizontal beds of rock stretching over a wide
extent of country, in another the upturned edges of almost vertical
strata form the surface features; or, again, irregular bosses of
crystalline igneous rock occur here and there as patches in the
midst of bedded sedimentary or volcanic strata. A map showing the
boundaries and distribution of the rocks as seen at the surface,
tells us comparatively little as to the relative positions of the
different rocks below ground, or of the relative ages of the several
strata. If we supplement this superficial view by an inspection of
the position of the strata as shown on the walls of a deep trench cut
across the country, we at once gain very important information as to
the relative position of the beds below the earth’s surface. The face
of a quarry, the side of a river bed or a railway cutting, afford
HORIZONTAL SECTIONS or PROFILES which show whether certain strata lie
above or below others, whether a series of rocks consists of parallel
and regularly stratified beds, or whether the succession of the strata
is interfered with by a greater or less divergence from a parallel
arrangement. If, for example, a section shows comparatively horizontal
strata lying across the worn down edges of a series of vertical
sedimentary rocks, we may fairly assume that some such changes as the
following have taken place in that particular area.
The underlying beds were originally laid down as more or less
horizontal deposits; these were gradually hardened and compacted,
then elevated above sea-level by a folding of the earth’s crust; the
crests of the folds were afterwards worn down by denudation, and the
eroded surface finally subsided below sea-level and formed the floor
on which newer deposits were built up. Such breaks in the continuity
of stratified deposits are known as UNCONFORMITIES; in the interval of
time which they represent great changes took place of which the records
are either entirely lost, or have to be sought elsewhere.
In certain more exceptional cases, it is possible to obtain what is
technically known as a VERTICAL SECTION; for example if a deep boring
is sunk through a series of rocks, and the core of the boring examined,
we have as it were a sample of the earth’s crust which may often teach
us valuable lessons which cannot be learnt from maps or horizontal
sections.
[Sidenote: INVERSION OF STRATA.]
It is obvious, that in a given series of beds, which are either
horizontal or more or less obliquely inclined, the underlying strata
were the first formed, and the upper beds were laid down afterwards.
If, however, we trusted solely to the order of superposition in
estimating relative age, our conclusions would sometimes be very far
from the truth. Recent geological investigations have brought to
light facts well nigh incredible as to the magnitude and extent of
rock-foldings. In regions of great earth-movements, the crust has been
broken along certain lines, and great masses of strata have been thrust
for miles along the tops of newer rocks. Thus it may be brought about
that the natural sequence of a set of beds has been entirely altered,
and older rocks have come to overlie sediments of a later geological
age. Facts such as these clearly illustrate the difficulties of correct
geological interpretation.
In the horizontal section (Fig. 2), from the summit of Büzistock on
the left to Saasterg on the right, we have a striking case of intense
rock-folding and dislocation[46]. Prof. Heim[47] of Geneva has given
numerous illustrations of the almost incredible positions assumed
in the Swiss Mountains by vast thicknesses of rocks, and in the
accompanying section taken from a recent work by Rothpletz we have a
compact example of the possibilities of earth-movements as an agent of
rock-folding. The section illustrates very clearly an exception to the
rule that the order of superposition of a set of beds indicates the
relative age of the strata. The horizontal line at the base is drawn at
a height of 1650 metres above sea-level, and the summit of Büzistock
reaches a height of 2340 m. The youngest rocks seen in the diagram are
the Eocene beds _e_, at the base and as small isolated patches on the
right-hand end of the section; the main mass of material composing the
higher ground has been bodily thrust over the Eocene rocks, and in this
process some of the beds, _b_ and _c_, have been folded repeatedly on
themselves. Similar instances of the overthrusting of a considerable
thickness of strata have been described in the North-west Highlands
of Scotland[48] and elsewhere in the British Isles. It is important
therefore to draw attention to cases of extreme folding, as such
phenomena are by no means exceptional in many parts of the world.
[Illustration: FIG. 2. Section from Büzistock to Saasterg. [After
Rothpletz, (94) Pl. II. fig. 2.]
_a_ = Sernifit or Verrucano (Permian).
_b_ = Röthidolomit etc. (Permian).
_c_ = Dogger (Jurassic).
_d_ = Malm (Jurassic).
_e_ = Eocene.]
The order of superposition of strata has afforded the key to our
knowledge of the succession of life in geologic time, and the
refinements of the stratigraphical correlation of sedimentary rocks
are based on the comparison of their fossil contents. By a careful
examination of the relics of fossil organisms obtained from rocks
of all ages and countries, it has been found possible to restore in
broken outline the past history of the Earth. By means, then, of
stratigraphical and palaeontological evidence, a classification of the
various rocks has been established, the lines of division being drawn
in such places as represent gaps in the fossil records, or striking and
widespread unconformities between different series of deposits.
It is only in a few regions that we find rocks which can reasonably be
regarded as the foundation stones of the Earth. As the globe gradually
cooled, and its molten mass became skinned over with a solid crust,
crystalline rocks must have been produced before the dawn of life, and
before water could remain in a liquid form on the rocky surface. As
soon as the temperature became sufficiently low, running water and rain
began the work of denudation and rock disintegration which has been
ceaselessly carried on ever since. In this continual breaking down and
building up of the Earth’s surface, it would be no wonder if but few
remnants were left of the first formed sediments of the earliest age.
The action of heat, pressure and chemical change accompanying
rock-foldings and crust-wrinklings, often so far alters sedimentary
deposits, that their original form is entirely lost, and sandstone,
shales and limestones become metamorphosed into crystalline quartzites,
slates and marbles.
The operation of metamorphism is therefore another serious difficulty
in the way of recognising the oldest rocks. The earliest animals and
plants which have been discovered are not such as we should expect
to find as examples of the first products of organic life. Below the
oldest known fossiliferous rocks, there must have been thousands of
feet of sedimentary material, which has either been altered beyond
recognition, or from some cause or other does not form part of our
present geological record.
As a general introduction to geological chronology, a short summary
may be given of the different formations or groups of strata, to which
certain names have been assigned to serve as convenient designations
for succeeding epochs in the world’s evolution. The following table
(Fig. 3, pp. 32, 33) represents the geological series in a convenient
form; the most characteristic rocks of each period are indicated by the
usual conventional shading, and the most important breaks or lacunae in
the records are shown by gaps and uneven lines. The relative thickness
of the rocks of each period is approximately shown; but the vertical
extent of the oldest or Archaean rocks as shown in Fig. 3 represents
what is without doubt but a fraction of their proportional thickness.
This table is taken, with certain alterations, from a paper by Prof.
T. McKenny Hughes in the Cambridge Philosophical Proceedings for 1879.
Speaking of the graphic method of showing the geological series, the
author of the paper says, “It is convenient to have a table of the
known strata, and although we cannot arrange all the rocks of the
world in parallel columns, and say that _ABC_ of one area are exactly
synchronous with _A′B′C′_ of another, still if we take any one country
and establish a grouping for it, we find so many horizons at which
equivalent formations can be identified in distant places, that we
generally make an approximation to HOMOTAXIS as Huxley called it. The
most convenient grouping is obviously to bracket together locally
continuous deposits, _i.e._ all the sediment which was formed from the
time when the land went down and accumulation began, to the time when
the sea bottom was raised and the work of destruction began. In the
accompanying table I have given the rocks of Great Britain classified
on this system, and bearing in mind that waste in one place must be
represented by deposit elsewhere, I have represented the periods of
degradation by intervals estimated where possible by the amount of
denudation known to have taken place between the periods of deposition
in the same district[49].”
[Illustration: TABLE OF STRATA.
RECENT ╮
GLACIAL ├ QUATERNARY
PLIOCENE ╯
MIOCENE ╮
│
OLIGOCENE ├ TERTIARY
│
EOCENE ╯
CRETACEOUS ╮
│
JURASSIC │
├ MESOZOIC
TRIAS │
│
PERMIAN ╯
╭ Coal-Measures ╮
│ │
│ Millstone Grit │
CARBONIFEROUS ┤ │
│ Yoredale Rocks │
│ │
╰ Mountain Limestone │
│
╭ Devonian │
│ │
│ Upper Old Red │
DEVONIAN ┤ Sandstone │
│ │
│ Lower Old Red │
╰ Sandstone │
│
╭ Ludlow │
│ │
SILURIAN ┤ Wenlock ├ PALAEOZOIC
│ │
╰ May Hill │
│
╭ Upper Bala │
│ │
│ Lower Bala │
ORDOVICIAN ┤ │
│ Llandeilo │
│ │
╰ Arenig │
│
╭ Tremadoc │
│ │
│ Lingula Flags │
CAMBRIAN ┤ │
│ Menevian │
│ │
╰ Harlech ╯
LAVAS, ╮
VOLCANIC │
ASHES, │
GRANITOID ├ ARCHAEAN
ROCKS AND │
SCHISTS OF │
ENORMOUS │
THICKNESS ╯
Fig 3.]
I. =Archaean.=
“Men can do nothing without the make-believe of a beginning.”
GEORGE ELIOT.
There is perhaps no problem at once so difficult and so full of
interest to the student of the Earth’s history, as the interpretation
of the fragmentary records of the opening stages in geological and
organic evolution. In tracing the growth and development of the human
race, it becomes increasingly difficult to discover and decipher
written documents as we penetrate farther back towards the beginning
of the historical period; the records are usually incomplete and
fragmentary, or rendered illegible by the superposed writings of a
later date. So in the records of the rocks, as we pass beyond the
oldest strata in which clearly preserved fossils are met with, we come
to older rocks which afford either no data as to the period in which
they were formed, or like the palimpsest, with its original characters
almost obliterated by a late MS., the older portions of the Earth’s
crust have been used and re-used in the rock-building of later ages.
In the first place, it is exceedingly difficult to determine with any
certainty what rocks may be regarded as trustworthy fragments of a
primaeval land. Throughout the geological eras the Earth’s surface
has been subjected to foldings and wrinklings, volcanic activity has
been almost unceasing, and there is abundant evidence to show how
the original characters of both igneous and sedimentary rocks may be
entirely effaced by the operation of chemical and physical forces. It
was formerly held that coarsely crystalline rocks such as granite are
the oldest portions of the crust, but modern geology has conclusively
proved that many of the so-called fundamental masses of rock are merely
piles of ancient sediments which have been subjected to the repeated
operation of powerful physical and chemical forces, and have undergone
a complete rearrangement of their substance. As the result of more
detailed investigations, many regions formerly supposed to consist of
the foundation stones of the Earth’s crust, are now known to have been
centres of volcanic disturbance and widespread metamorphism, and to be
made up of post-archaean rocks.
[Sidenote: THE OLDEST ROCKS.]
The first formed rocks no doubt became at once the prey of denudation
and disintegration, and on their surface would be accumulated the
products of their own destruction: newer strata would entirely cover up
portions of the original land, to be in their turn succeeded by still
later deposits. There is reason to believe that in the remotest ages of
the Earth’s history, the forces of denudation and igneous activity were
more potent than in later times, and thus the oldest rocks could hardly
retain their original structure through the long ages of geologic time.
The earliest representatives of organic life were doubtless of such
a perishable nature that their remains could not be preserved in a
fossil state even under the most favourable conditions. Such organisms,
whether plants or animals, as possessed any resistant tissues or hard
skeletons might be preserved in the oldest rocks, but as these strata
became involved in earth-foldings or were penetrated by injections of
igneous eruptions, the relics of life would be entirely destroyed.
It is, in short, practically hopeless to look for any fragments of
the primitive crust except such as have undergone very considerable
metamorphism, and equally futile to search for any recognisable remains
of primitive life.
In many parts of the world vast thicknesses of rock occur below the
oldest known fossiliferous strata; these consist largely of laminated
crystalline masses composed of quartz, felspar, and other minerals,
having in fact the same composition as granite, but differing in the
regular arrangement of the constituent parts. To such rocks the terms
gneiss and schist have been applied. Rocks of this kind are by no means
always of Archaean age, but many of the earliest known rocks consist of
gneisses of various kinds, associated with altered lavas, metamorphosed
ashes, breccias and other products of volcanic activity; with these
there may be limestones, shales, sandstones, and other strata more or
less closely resembling sedimentary deposits. Such a succession of
gneissic rocks has been described as occupying a wide area in the basin
of the St Lawrence river, and to these enormously thick and widespread
masses a late Director of the Canadian Geological Survey applied
the term Laurentian. These Laurentian rocks, with similar strata in
Scandinavia, the north-west Highlands of Scotland, in certain parts of
such mountain ranges as the Alps, Pyrenees, Carpathians, Himalayas,
Andes, Atlas, &c., have been classed together as members of the oldest
geological period, and are usually referred to under the name of
Archaean, or less frequently Azoic rocks. In some of the uppermost
Archaean rocks there have been recently discovered a few undoubted
traces of fossil animals, but with this exception no fossils are known
throughout the great mass of Archaean strata. It is true that some
authorities regard the beds of graphite and other rocks as a proof of
the abundance of plant life, but this supposition is not supported by
any convincing evidence.
The term Azoic[50] applied by some writers to these oldest rocks
suggests the absence of life during the period in which they were
formed. Life there must have been, though we are unable to discover its
records. The period of time represented by the Archaean or Pre-Cambrian
rocks must be enormous, and it was in that earliest era that the first
links in the chain of life were forged.
II. =Cambrian.=
The term Cambrian was adopted by Sedgwick for a series of sedimentary
rocks in North Wales (_Cambria_). In that district, in South Wales,
the Longmynd Hills, the Malverns, in Scotland, and other regions
there occur more or less highly folded and contorted beds of pebbly
conglomerate, sandstones, shales and slates resting on the uneven
surface of an Archaean foundation.
It is in these Cambrian rocks that trustworthy records of organic life
are first met with. Among the most constant and characteristic fossils
of this period are the extinct and aberrant members of the crustacea,
the trilobites; these with some brachiopods, sponges, and other fossils
comprise the oldest fauna, of which the ancestral types have yet to
be discovered. During the last few decades the number of Cambrian
fossils has been considerably increased, and in certain regions of
North America and China there are found many thousand feet of strata
above the typical Archaean rocks and below the newer fossiliferous beds
of Cambrian age. It is reasonable to suppose that future research may
extend the present limits of fossil-bearing rocks below the horizon,
which is marked by the occurrence of the widely distributed and oldest
known trilobite, the genus _Olenellus_.
The vast thickness of Cambrian strata was for the most part laid down
on the floor of a comparatively deep sea; other members of the series
represent the shingle beaches and coast deposits accumulated on the
slopes of Archaean islands. There have been many conjectures as to the
distribution of land and sea during the deposition of these rocks; but
the data are too imperfect to enable us to restore with any degree
of confidence the physical geography of this Palaeozoic epoch, of
which the sediments stood out as islands of Cambrian land during many
succeeding ages.
III. =Ordovician.=
Since the days when Sedgwick and Murchison first worked out the
succession of Palaeozoic strata in North Wales, there has always
existed a considerable difference of opinion as to the best method of
subdividing the Cambrian-Silurian strata. Later research has shown
that the rocks included by Sedgwick in his Cambrian system, fall
naturally into two groups; for the upper of these Prof. Lapworth has
suggested the term Ordovician, from the name of the Ordovices, who
inhabited a part of northern Wales. At the base of the system we have
a series of volcanic and sedimentary rocks to which Sedgwick gave the
name Arenig; above these there occur the Llandeilo Flags, succeeded
by a considerable thickness of rocks known as the Bala series. The
rocks making up these Ordovician sediments consist for the most part
of slates, sandstones and limestones with volcanic ashes and lavas.
Much of the typical Welsh scenery owes its character to the folded and
weathered rocks laid down on the floor of the Ordovician sea, on which
from many centres of volcanic activity lava streams and showers of ash
were spread out between sheets of marine sediment. The Arenig Hills,
Snowdonia, and many other parts of North and South Wales, parts of
Shropshire, Scotland, Sweden, Russia, Bohemia, North America and other
regions consist of great thicknesses of Ordovician strata.
IV. =Silurian.=
Passing up a stage higher in the geologic series, we have a succession
of conglomerates, sandstones, shales, and limestones; in other words,
a series of beds which represent pebbly shore deposits, the sands
and muds of deeper water, and the accumulated débris of calcareous
skeletons of animals which lived in the clear water of the Silurian
sea. The term Silurian (Siluria was the country of Caractacus and the
old Britons known as Silures[51]) was first applied by Murchison in
1835 to a more comprehensive series of rocks than are now included
in the Silurian system. The rocks of this period occur in Wales,
Shropshire, parts of Scotland, Ireland, Scandinavia, Russia, the
United States and other countries. After the accumulation of the
thick Ordovician sediments, the sea-floor was upraised and in places
converted into ridges or islands of land, of which the detritus formed
part of the material of Silurian deposits. The limestones of the
Wenlock ridge have yielded an abundant fauna, consisting of corals,
crinoids, molluscs and other invertebrates. In this period we have the
first representatives of the Vertebrata, discovered in the rocks of
Ludlow. In fact, in the Silurian period, “all the great divisions of
the Animal Kingdom were already represented[52].”
V. =Devonian.=
By the continued elevation of the Silurian sea-floor, large portions
became dry land, and during the succeeding period most of the British
area formed part of a continental mass. Over the southern part of
England, there still lay an arm of the sea, and in this were laid
down the marine sediments which now form part of Devon, and from
which the name Devonian has been taken as a convenient designation
for the strata of this period. In parts of the northern land, in the
region now occupied by Scotland, there were large inland lakes, on
the floor of which vast thicknesses of shingle beds and coarse sands
(“Old Red Sandstone”) were slowly accumulated; and it has been shown
by Sir Archibald Geikie and others that during this epoch there were
considerable outpourings of volcanic material in the Scotch area.
Farther to the West and South-west there was another large lake in
which the so-called Kiltorkan beds of Ireland were deposited. In these
Irish sediments, and others of the same age in Belgium and elsewhere
a few forms of land plants have been discovered; but it is from the
Devonian rocks of North America that most of our knowledge of the flora
of this period has been obtained.
VI. =Carboniferous.=
From the point of view of palaeobotany, the shales, sandstones, and
seams of coal included in the Carboniferous system are of special
interest. It is from the relics of this Palaeozoic vegetation that the
most important botanical lessons have been learnt.
The following classification of Carboniferous rocks shows the order of
succession of the various beds, and the nature of the rocks which were
formed at this stage in the Earth’s history.
╭ Upper Coal-Measures.
╭ Coal-Measures[53] ┤ Transition Series.
│ │ Middle Coal-Measures.
│ ╰ Lower Coal-Measures.
│ Millstone Grit.
CARBONIFEROUS ┤ ╭ Upper limestone shales
│ │ and Yoredale rocks.
│ Carboniferous limestone ┤ Carboniferous or
│ series │ Mountain limestone.
│ ╰ Lower limestone shales.
╰ Basement conglomerate.
In the classification of Carboniferous rocks adopted in Geikie’s
text-book of Geology the following arrangement is followed for the
Carboniferous limestone series[54]:—
╭ _Yoredale group_ of shales and grits passing
│ down into dark shales and limestones.
│ _Thick (Scaur or Main) limestone_ in the
│ south and centre of England and Ireland,
│ passing northwards into sandstones,
│ shales and coals with limestones.
Carboniferous ┤ _Lower limestone shale_ of the south and
limestone series │ centre of England. The Calciferous
│ sandstone group of Scotland (marine,
│ estuarine, and terrestrial organisms)
│ probably represents the Scaur limestone
│ and lower limestone shale, and graduates
│ downwards insensibly into the Upper
╰ Old Red Sandstone.
The thick beds of mountain limestone, with their characteristic marine
fossil shells and corals play an important part in English scenery.
In Derbyshire, West Yorkshire, and other places, the limestone crags
and hills are made up of the raised floor of a comparatively deep
Carboniferous sea, which covered a considerable portion of the British
Isles at the beginning of this epoch.
[Sidenote: CARBONIFEROUS ROCKS.]
The accumulation of the calcareous skeletons of marine animals,
with masses of coral, veritable shell-banks of extinct oyster-like
lamellibranchs, built up during the lapse of a long period of time,
formed widespread deposits of calcareous sediments. These were
eventually succeeded by less pure calcareous deposits, the sea became
shallower, and land detritus found its way over an area formerly
occupied by the clear waters of an open sea. The shallowing process
was gradually continued, and the sea was by some means converted into
a more confined fresh-water or brackish area, in which were laid down
many hundred feet of coarse sandy sediments derived from the waste
of granitic highlands. Finally the conditions became less constant;
the continuous deposition of sandy detritus being interrupted by the
more or less complete filling up of the area of sedimentation, and the
formation of a land surface which supported a luxuriant vegetation,
of which the débris was subsequently converted into beds of coal. By
further subsidence the land was again submerged, and the forest-covered
area became overspread with sands and muds.
Such are the imperfect outlines of the general physical conditions
which are represented by the series of sedimentary strata included in
the Carboniferous system. At the close of this period, the Earth’s
surface in Western Europe was subjected to crust-foldings on a large
scale, along lines running approximately North and South and East and
West, the two sets of movements resulting in the formation of ridges
of Carboniferous rocks. The uppermost series of grits, sandstones and
coal-seams were in great part removed by denudation from the crests
of the elevated ridges, but remained in the intervening troughs or
basins where they were less exposed to denudation. It is the direct
consequence of this, that we have our Coal-Measures preserved in the
form of detached basins of upper Carboniferous beds.
A closer examination of the comparative thickness and succession of
Carboniferous rocks in different parts of Britain shows very clearly
that in the northern area of Scotland and in the North of England the
conditions were different from those which obtained further South.
Seeing how much palaeobotanical interest attaches to these rocks, it is
important to treat a little more fully of their geology.
In parts of Devon, Cornwall and West Somerset, the Devonian strata
are succeeded by a series of folded and contorted rocks which have
yielded a comparatively small number of Carboniferous fossils. To this
succession of limestones, shales and grits the term _Culm-Measures_ was
applied by Sedgwick and Murchison in 1837. The rocks of this series
occupy a trough between the Devonian rocks of North and South Devon.
While some authorities have correlated the Culm-Measures with the
Millstone Grit, others regard them as representing a portion of the
true Coal-Measures, as well as the Carboniferous and Lower Limestone
Shale[55]. It has recently been shown that among the lower Culm strata
there occur bands of ancient deep-sea sediments, consisting of beds
of chert containing siliceous casts of various species of Radiolaria.
There can be no doubt that the discovery of deep-sea fossils in this
particular development of the British Carboniferous system leads to
the conclusion that “while the massive deposits of the Carboniferous
limestone—formed of the skeletons of calcareous organisms—were in
process of growth in the seas to the North, there existed to the
South-west a deeper ocean in which siliceous organisms predominated and
formed these siliceous radiolarian rocks[56].”
The Upper Culm-Measures consist of conglomerates, grits, sandstones
and shales with some plant remains and other fossils, and constitute a
typical set of shallow water sediments. In Westphalia, the Harz region,
Thuringia, Silesia and Moravia there are rocks corresponding to the
Culm-Measures of Devon, and some of these have also afforded evidence
of deep water conditions.
[Sidenote: COAL-MEASURES.]
_S. W. England, S. Wales, Derbyshire and Yorkshire._ In these districts
the Carboniferous limestone reaches a considerable thickness; in the
Mendips it has a thickness of 3000 feet, and in the Pennine chain of
4000 feet. At the base of this limestone series there occurs in the
southern districts the so-called lower limestone shale, consisting of
clays, shales and sandy beds. Above the limestone we have the Millstone
grit and Coal-Measures; but in the Pennine district there is a series
of rocks consisting of impure limestones and shales, intercalated
between the Millstone grit and Carboniferous limestone; for this group
of rocks the term _Yoredale series_ has been proposed. In the Isle of
Man and Derbyshire sheets of lava are interbedded with the calcareous
sediments, affording clear proof of submarine volcanic eruptions.
_N. England and Scotland._ In the Carboniferous rocks of Northumberland
we have distinct indications of a shallower sea. The regular succession
of limestone strata in West Yorkshire and other districts, gives place
to a series of thinner beds of limestones, interstratified with shales
and impure calcareous rocks. We have come within the range of land
detritus which was spread out on the floor of a shallow sea. The lowest
portion of the Mountain limestone is here represented by about 200 feet
of shales and other rocks grouped together in the _Tuedian series_.
The Upper Carboniferous limestone and Yoredale rocks of Yorkshire are
represented by sandstones, carbonaceous limestones and some seams of
coal, included in the _Bernician series_. Further north, again, another
classification has been proposed for the still more aberrant succession
of rocks; the lowest being spoken of as the _Calciferous sandstone_,
and the upper as the _Carboniferous limestone_. The calciferous
sandstone may be compared with the lower limestone shale and part of
the Carboniferous limestone of England. The Carboniferous limestone of
Scotland probably represents the upper part of the limestone of England
and the Yoredale rocks of the Pennine and other areas.
Turning to the upper members of the Carboniferous system—in the
Coal-Measures, as they were called in 1817 by William Smith,—we have a
series of coal seams, sandstones, shales, and ironstones occurring for
the most part in basin-shaped areas. As a general rule, each seam of
coal, which varies in thickness from one inch to thirty feet, rests on
a characteristic unstratified argillaceous rock known as Underclay.
The accompanying diagram (Fig. 4) illustrates the frequent
intercalation of small bands of argillaceous and sandy rocks associated
with the seams of coal.
The usual classification adopted for the British Coal-Measures is that
of Upper, Middle, and Lower Coal-Measures; between the Upper and Middle
divisions there occur certain transition or passage beds which are
known as the Transition series. Continental writers, and more recently
Mr Kidston of Stirling, have attempted with considerable success to
correlate the Coal-producing strata by means of fossil plants[57].
[Illustration:
10 in. Massive clay-shale with a few
coal films in the lower part.
10½ in. Shale full of thin streaks of
coal.
14 in. Massive shale with a few streaks
of coal and iron pyrites.
5½ in. Bastard coal; more coal than
shale.
6½ in. Good coal, with masses of iron
pyrites.
1½ in. Coal and seat-rock mixed.
5 in. Seat-rock.
Fig. 4.
Vertical section of the Bassey or Salts Coal seam, Rushton
Colliery, Blackburn (Lower Coal-Measures). From a specimen 4 feet
4 inches in height, presented by Mr P. W. Pickup to the Manchester
Museum, Owens College.]
Finally, some reference must be made to the occurrence of Carboniferous
rocks underneath more recent strata. In a geological map, or bird’s-eye
view of a country, we see such rocks as appear at the surface; by
means of deep borings, however, we are occasionally enabled to follow
the course of older beds a considerable distance below the usually
accessible part of the Earth’s crust. In the neighbourhood of London,
Dover, and other places we have Tertiary and Mesozoic strata forming
the surface of the country, but below these comparatively recent
formations, the sinking of deep wells and other borings have proved
the existence of a ridge of Palaeozoic rocks stretching from the South
Wales Coal-field through the South-east of England to northern France,
Belgium and Westphalia. It is from rocks forming part of this old
ridge that characteristic Coal-Measure plants have been obtained from
the Dover boring. In Fig. 5 is shown an almost complete pinnule of
_Neuropteris Scheuchzeri_ Hoffm., a well-known fern, marking a definite
horizon of Upper Carboniferous rocks[58]. The small hairs on the
pinnules, shown in the figure as fine lines lying more or less parallel
to the midrib and across the lateral veins, are a characteristic
feature of this species.
[Illustration: FIG. 5.
Imperfect pinnule of _Neuropteris Scheuchzeri_ Hoffm., showing the
characteristic hairs as fine lines traversing the lateral veins.
From a specimen obtained from the Dover boring and now in the
British Museum. Nat. size.]
VII. =Permian.=
Reference has already been made to the earth-foldings which marked
the close of Carboniferous times; “the open Mediterranean sea of the
Carboniferous period in Europe was converted into a large inland sea,
like the Caspian of the present day, surrounded by a rocky and hilly
continent, on which grew trees and plants of various kinds[59].” In
parts of
Lancashire, Westmoreland, the Eden Valley, and in the East of England
from Sunderland to Nottingham, there occurs a succession of limestones,
sandstones, clays and other rocks with occasional beds of rock-salt
and gypsum, which represent the various forms of sediment and chemical
precipitates formed on the floor of Permian lakes. The poverty of the
fauna and flora of Permian strata points to conditions unfavourable to
life; and there can be little doubt that the characteristic red rocks
of St Bees Head, and the creamy limestones of the Durham coast are the
upraised sediments of an inland salt-water lake. The term Dyas was
proposed by Marcou for this series of strata as represented in Germany,
where the rocks are conveniently grouped in two series, the _Magnesian
limestone_ or _Zechstein_ and the red sandstones or _Rothliegendes_.
The older and better known name of Permian was instituted by Murchison
for the rocks of this age, from their extreme development in the old
kingdom of Permia in Russia. Unfortunately considerable confusion has
arisen from the employment of different names for rocks of the same
geological period; and the grouping of the beds varies in different
parts of the world. It is of interest to note, that in the Tyrol,
Carinthia, and other places there are found patches of old marine beds
which were originally laid down in an open sea, which extended over
the site of the Mediterranean, into Russia and Asia. In Bohemia, the
Harz district, Autun in Burgundy, and other regions, there are seams
of Permian coal interstratified with the marls and sands. From these
last named beds many fossil plants have been obtained, and important
palaeobotanical facts brought to light by the investigations of
continental workers. Volcanic eruptions, accompanied by lava streams
and showers of ash, have been recognised in the Permian rocks of
Scotland, and elsewhere.
In North America, Australia, and India the term Permo-Carboniferous is
often made use of in reference to the continuous and regular sequence
of beds which were formed towards the close of the Carboniferous and
into the succeeding Permian epoch. The enormous series of freshwater
Indian rocks, to which geologists have given the name of the GONDWANA
SYSTEM, includes the sediments of more than one geological period,
some of the older members being regarded as Permo-Carboniferous in
age. These Indian beds, with others in Australia, South Africa, and
South America, are of special interest on account of the characteristic
southern hemisphere plants which they have afforded, and from the
association with the fossiliferous strata of extensive boulder beds
pointing to widespread glacial conditions.
VIII. =Trias.=
As we ascend the geologic series, and pass up to the rocks overlying
the Permian deposits, there are found many indications of a marked
change in the records of animal and plant life. Many of the
characteristic Palaeozoic fossils are no longer represented, and
in their place we meet with fresh and in many cases more highly
differentiated organisms. The threefold division of the rocks of this
period which suggested the term Trias to those who first worked out the
succession of the strata, is typically illustrated over a wide area
in Germany, in which the lowest or _Bunter_ series is followed by the
calcareous _Muschelkalk_, and this again by the clays, rock-salt, and
sandstones of the _Keuper_ series. In the Cheshire plain and in the
low ground of the Midlands, we have a succession of red sandstones,
conglomerates, and layers of rock-salt which correspond to the Bunter
and Keuper beds of German geologists. These Triassic rocks were
obviously formed in salt-water lakes, in which from time to time long
continued evaporation gave rise to extensive deposit of rock-salt
and other minerals. From the fact that it is this type of Triassic
sediments which was first made known, it is often forgotten that the
British and German rocks are not the typical representatives of this
geological period. The ‘Alpine’ Trias of the Mediterranean region,
in Asia, North America, and other countries, has a totally different
facies, and includes limestones and dolomites of deep-sea origin. “The
widespread Alpine Trias is the pelagic facies of the formation; the
more restricted German Trias, on the other hand, is a shallow shore,
bay or inland sea formation[60].”
In the Keuper beds of southern Sweden there are found workable
seams of coal, and the beds of this district have yielded numerous
well-preserved examples of the Triassic flora. A more impure coal
occurs in the lower Keuper of Thuringia and S.-W. Germany, and to this
group of rocks the term _Lettenkohle_ is occasionally applied.
In the Rhaetic Alps of Lombardy, in the Tyrol, and in England, from
Yorkshire to Lyme Regis, Devonshire, Somersetshire, and other districts
there are certain strata at the top of the Triassic system known as
the _Rhaetic_ or _Penarth_ beds. The uppermost Rhaetic beds, often
described as the White Lias, afford evidence of a change from the salt
lakes of the Trias to the open sea of the succeeding Jurassic period.
Passing beyond this period of salt lakes and wind-swept barren tracts
of land, we enter on another phase of the earth’s history.
IX. =Jurassic.=
The Jura mountains of western Switzerland consist in great part of
folded and contorted rocks which were originally deposited on the
floor of a Jurassic sea. In England the Jurassic rocks are of special
interest, both for geological and historical reasons, as it is in them
that we find a rich fauna and flora of Mesozoic age, and it was the
classification of these beds by means of their fossil contents that
gained for William Smith the title of the Father of English Geology. A
glance at a geological map of England shows a band of Jurassic rocks
stretching across from the Yorkshire coast to Dorset. These are in a
large measure calcareous, argillaceous, and arenaceous sediments of an
open sea; but towards the upper limit of the series, both freshwater
and terrestrial beds are met with. Numerous fragments of old coral
reefs, sea-urchins, crinoids, and other marine fossils are especially
abundant; in the freshwater beds and old surface-soils, as well as in
the marine sandstones and shales, we have remnants of an exceedingly
rich and apparently tropical vegetation. This was an age of Reptiles
as well as an age of Cycads. An interesting feature of these widely
distributed Jurassic strata is the evidence they afford of distinct
climatal zones; there are clear indications, according to the late Dr
Neumayr, of a Mediterranean, a middle European, and a Boreal or Russian
province[61]. The subdivisions of the English Jurassic rocks are as
follows[62]:—
╭ ╭ Purbeck beds ╮ ╮
│ │ Portland beds ├ _Upper_ │
│ │ Kimeridge clay ╯ │
│ ┤ │
│ │ Corallian beds ╮ │
JURASSIC ┤ │ Oxford clay, with ├ _Middle_ ├ Oolite.
│ ╰ Kellaways rock ╯ │
│ │
│ ╭ Great Oolite series ╮ _Lower_ │
│ ╰ Inferior Oolite series ╯ ╯
│
╰ Lias
In tracing the several groups across England, and into other parts of
Europe, their characters are naturally found to vary considerably; in
one area a series is made up of typical clear water or comparatively
deep sea sediments, and in another we have shallow water and shore
deposits of the same age. The Lias rocks have been further subdivided
into zones by means of the species of Ammonites which form so
characteristic a feature of the Jurassic fauna. In the lower Oolite
strata there are shelly limestones, clays, sandstones, and beds of
lignite and ironstone. Without discussing the other subdivisions of
the Jurassic period, we may note that in the uppermost members there
are preserved patches of old surface-soils exposed in the face of the
cliffs of the Dorset coast and of the Isle of Portland.
X. =Cretaceous.=
In the south of England, and in some other districts, it is difficult
to draw any definite line between the uppermost strata of the Jurassic
and the lowest of the Cretaceous period. The rocks of the so-called
_Wealden_ series of Kent, Surrey, Sussex, and the Isle of Wight, are
usually classed as Lower Cretaceous, but there is strong evidence
in favour of regarding them as sediments of the Jurassic period.
The Cretaceous rocks of England are generally speaking parallel to
the Jurassic strata, and occupy a stretch of country from the east
of Yorkshire and the Norfolk coast to Dorset in the south-west. The
Chalk downs and cliffs represent the most familiar type of Cretaceous
strata. In the white chalk with its numerous flints, we have part
of the elevated floor of a comparatively deep sea, which extended
in Cretaceous times over a large portion of the east and south-east
of England and other portions of the European continent. On the bed
of this sea, beyond the reach of any river-borne detritus, there
accumulated through long ages the calcareous and siliceous remains of
marine animals, to be afterwards converted into chalk and flints. At
the beginning of the period, however, other conditions obtained, and
there extended over the south-east of England, and parts of north and
north-west Germany and Belgium, a lake or estuary in which were built
up deposits of clay, sand and other material, forming the delta of
one or more large rivers. For these sediments the name _Wealden_ was
suggested in 1828. Eventually the gradual subsidence of this area led
to an incursion of the sea, and the delta became overflowed by the
waters of a large Cretaceous sea. At first the sea was shallow, and in
it were laid down coarse sands and other sediments known as the _Lower
Greensand_ rocks. By degrees, as the subsidence continued, the shallows
became deep water, and calcareous material slowly accumulated, to be
at last upraised as beds of white chalk. The distribution of fossils
in the Cretaceous rocks of north and south Europe distinctly points to
the existence of two fairly well-marked sets of organisms in the two
regions; no doubt the expression of climatal zones similar to those
recognised in Jurassic times. In North America, Cretaceous rocks are
spread over a wide area, also in North Africa, India, South Africa,
and other parts of the world. Within the Arctic Circle strata of this
age have become famous, chiefly on account of the rich flora described
from them by the Swiss palaeobotanist Heer. The fauna and flora of this
epoch are alike in their advanced state of development and in the great
variety of specific types; the highest class of plants is first met
with at the base of the Cretaceous system.
XI. =Tertiary.=
“At the close of the Chalk age a change took place both in the
distribution of land and water, and also in the development of organic
life, so great and universal, that it has scarcely been equalled at
any other period of the earth’s geological history[63].” The Tertiary
period seems to bring us suddenly to the threshold of our own times. In
England at least, the deposits of this age are of the nature of loose
sands, clays and other materials containing shells, bones, and fossil
plants bearing a close resemblance to organisms of the present era.
The chalk rocks, upheaved from the Cretaceous sea, stood out as dry
land over a large part of Britain; much of their material was in time
removed by the action of denuding agents, and the rest gradually sank
again beneath the waters of Tertiary lakes and estuaries. In the south
of England, and in north Europe generally, the Tertiary rocks have
suffered but little disturbance or folding, but in southern Europe and
other parts of the world, the Tertiary sands have been compacted and
hardened into sandstones, and involved in the gigantic crust-movements
which gave birth to many of our highest mountain chains. The Alps,
Carpathians, Apennines, Himalayas, and other ranges consist to a
large extent of piled up and strangely folded layers of old Tertiary
sediments. The volcanic activity of this age was responsible for the
basaltic lavas of the Giants’ Causeway, the Isle of Staffa, and other
parts of western Scotland.
During the succeeding phases of this period, the distribution of land
and sea was continually changing, climatic conditions varied within
wide limits; and in short wherever Tertiary fossiliferous beds occur,
we find distinct evidence of an age characterised by striking activity
both as regards the action of dynamical as well as of organic forces.
Sir Charles Lyell proposed a subdivision of the strata of this period
into Eocene, Miocene, and Pliocene, founding his classification on the
percentage of recent species of molluscs contained in the various sets
of rocks. His divisions have been generally adopted. In 1854 Prof.
Beyrich proposed to include another subdivision in the Tertiary system,
and to this he gave the name _Oligocene_.
Occupying a basin-shaped area around London and Paris there are beds of
Eocene sands and clays which were originally deposited as continuous
sheets of sediment in water at first salt, afterwards brackish and
to a certain extent fresh. In the Hampshire cliffs and in some parts
of the Isle of Wight, we have other patches of these oldest Tertiary
sediments. Across the south of Europe, North Africa, Arabia, Persia,
the Himalayas, to Java and the Philippine islands, there existed in
early Tertiary times a wide sea connecting the Atlantic and Pacific
oceans; and it may be that in the Mediterranean of to-day we have a
remnant of this large Eocene ocean. Later in the Tertiary period a
similar series of beds was deposited which we now refer to as the
Oligocene strata; such occurs in the cliffs of Headon hill in the Isle
of Wight, containing bones of crocodiles, and turtles, with the relics
of a rich flora preserved in the delta deposits of an Oligocene river.
At a still later stage the British area was probably dry land, and an
open sea existed over the Mediterranean region. In the neighbourhood
of Vienna we have beds of this age represented by a succession of
sediments, at first marine and afterwards freshwater. Miocene beds
occur over a considerable area in Switzerland and the Arctic regions,
and they have yielded a rich harvest to palaeobotanical investigators.
On the coast of Essex, Suffolk, Norfolk, the south of Cornwall, and
other districts there occur beds of shelly sand and gravel long known
under the name of ‘Crag.’ The beds have a very modern aspect; the
sands have not been converted into sandstones, and the shells have
undergone but little change. These materials were for the most part
accumulated on the bed of a shallow sea which swept over a portion of
East Anglia in Pliocene times. In the sediments of this age northern
forms of shells and other organisms make their appearance, and in the
Cromer forest-bed there occur portions of drifted trees with sands,
clays and gravels, representing in all probability the débris thrown
down on the banks of an ancient river. At this time the greater part
of the North Sea was probably a low-lying forest-covered region,
through which flowed the waters of a large river, of which part still
exists in the modern Rhine. The lowering of temperature which became
distinctly pronounced in the Pliocene age, continued until the greater
part of Britain and north Europe experienced a glacial period, and such
conditions obtained as we find to-day in ice-covered Greenland. Finally
the ice-sheet melted, the local glaciers of North Wales, the English
Lake district and other hilly regions, retreated, and after repeated
alterations in level, the land of Great Britain assumed its modern
form. The submerged forests and peat beds familiar in many parts of the
coast, the diatomaceous deposits of dried up lakes, “remain as the very
finger touches of the last geological change.”
[Sidenote: GEOLOGICAL EVOLUTION.]
The agents of change and geological evolution, which we have passed in
brief review, are still constantly at work carrying one step further
the history of the earth. A superficial review of geological history
gives us an impression of recurring and widespread convulsions,
and rapidly effected revolutions in organic life and geographical
conditions; on the other hand a closer comparison of the past and
present, with due allowance for the enormous period of time represented
by the records of the rocks, helps us to realise the continuity of
geological evolution. “So that within the whole of the immense period
indicated by the fossiliferous stratified rocks, there is assuredly
not the slightest proof of any break in the uniformity of Nature’s
operations, no indication that events have followed other than a clear
and orderly sequence[64].”
CHAPTER IV.
THE PRESERVATION OF PLANTS AS FOSSILS.
“The things, we know, are neither rich nor rare,
But wonder how the devil they got there.”
POPE, _Prologue to the Satires_.
The discovery of a fossil, whether as an impression on the surface of a
slab of rock or as a piece of petrified wood, naturally leads us back
to the living plant, and invites speculation as to the circumstances
which led to the preservation of the plant fragment. There is a certain
fascination in endeavouring, with more or less success, to picture
the exact conditions which obtained when the leaf or stem was carried
along by running water and finally sealed up in a sedimentary matrix.
Attempts to answer the question—How came the plant remains to be
preserved as fossils?—are not merely of abstract interest appealing
to the imagination, but are of considerable importance in the correct
interpretation of the facts which are to be gleaned from the records of
plant-bearing strata.
Before describing any specific examples of the commoner methods of
fossilisation; we shall do well to briefly consider how plants are
now supplying material for the fossils of a future age. In the great
majority of cases, an appreciation of the conditions of sedimentation,
and of the varied circumstances attending the transport and
accumulation of vegetable débris, supplies the solution of a problem
akin to that of the fly in amber and the manner in which it came there.
[Sidenote: OLD SURFACE-SOILS.]
Seeing that the greater part of the sedimentary strata have been formed
in the sea, and as the sea rather than the land has been for the most
part the scene of rock-building in the past, it is not surprising
that fossil plants are far less numerous than fossil animals. With
the exception of the algae and a few representatives of other classes
of plants, which live in the shallow-water belt round the coast,
or in inland lakes and seas, plants are confined to land-surfaces;
and unless their remains are swept along by streams and embedded in
sediments which are accumulating on the sea floor, the chance of their
preservation is but small. The strata richest in fossil plants are
often those which have been laid down on the floor of an inland lake
or spread out as river-borne sediment under the waters of an estuary.
Unlike the hard endo- and exo-skeletons of animals, the majority of
plants are composed of comparatively soft material, and are less likely
to be preserved or to retain their original form when exposed to the
wear and tear which must often accompany the process of fossilisation.
The Coal-Measure rocks have furnished numberless relics of a Palaeozoic
vegetation, and these occur in various forms of preservation in
rocks laid down in shallow water on the edge of a forest-covered
land. The underclays or unstratified argillaceous beds which nearly
always underlie each seam of coal have often been described as old
surface-soils, containing numerous remains of roots and creeping
underground stems of forest trees. The overlying coal has been regarded
as a mass of the carbonised and compressed débris of luxuriant forests
which grew on the actual spot now occupied by the beds of coal. There
are, however, many arguments in favour of regarding the coal seams as
beds of altered vegetable material which was spread out on the floor
of a lagoon or lake, while the underclay was an old soil covered by
shallow water or possibly a swampy surface tenanted by marsh-loving
plants[65].
The Jurassic beds of the Yorkshire Coast, long famous as some of the
richest plant-bearing strata in Britain, and the Wealden rocks of the
south coast afford examples of Mesozoic sediments which were laid
down on the floor of an estuary or large lake. Circumstances have
occasionally rendered possible the preservation of old land-surfaces
with the stumps of trees still in their position of growth. One of the
best examples of this in Britain are the so-called dirt-beds or black
bands of Portland and the Dorset Coast. On the cliffs immediately east
of Lulworth Cove, the surface of a ledge of Purbeck limestone which
juts out near the top of the cliffs, is seen to have the form here and
there of rounded projecting bosses or ‘Burrs’ several feet in diameter.
In the centre of each boss there is either an empty depression, or the
remnants of a silicified stem of a coniferous tree. Blocks of limestone
3 to 5 feet long and of about equal thickness may be found lying on
the rocky ledge presenting the appearance of massive sarcophagi in
which the central trough still contains the silicified remains of an
entombed tree. The calcareous sediment no doubt oozed up to envelope
the thick stem as it sank into the soft mud. An examination of the rock
just below the bed bearing these curious circular elevations reveals
the existence of a comparatively narrow band of softer material, which
has been worn away by denuding agents more rapidly than the overlying
limestone. This band consists of partially rounded or subangular stones
associated with carbonaceous material, and probably marks the site of
an old surface-soil. This old soil is well shown in the cliffs and
quarries of Portland, and similar dirt-beds occur at various horizons
in the Lower and Middle Purbeck Series[66]. In this case, then, we
have intercalated in a series of limestone beds containing marine and
freshwater shells two or three plant beds containing numerous and
frequently large specimens of cycadean and coniferous stems, lying
horizontally or standing in their original position of growth. These
are vestiges of an ancient forest which spread over a considerable
extent of country towards the close of the Jurassic period. The trunks
of cycads, long familiar in the Isle of Portland as fossil crows’
nests, have usually the form of round depressed stems with the central
portion somewhat hollowed out. It was supposed by the quarrymen that
they were petrified birds’ nests which had been built in the forks of
the trees which grew in the Portland forest. The beds separating the
surface-soils of the Purbeck Series, as seen in the sections exposed
on the cliffs or quarries, point to the subsidence of a forest-covered
area over which beds of water-borne sediment were gradually deposited,
until in time the area became dry land and was again taken possession
of by a subtropical vegetation, to be once more depressed and sealed up
under layers of sediment[67].
A still more striking example of the preservation of forest trees
rooted in an old surface-soil is afforded by the so-called fossil-grove
in Victoria Park, Glasgow, (Frontispiece). The stumps of several trees,
varying in diameter from about one to three feet, are fixed by long
forking ‘roots’ in a bed of shale. In some cases the spreading ‘roots,’
which bear the surface features of _Stigmaria_, extend for a distance
of more than ten feet from the base of the trunk. The stem surface is
marked by irregular wrinklings which suggest a fissured bark; but the
superficial characters are very imperfectly preserved. In one place a
flattened _Lepidodendron_ stem, about 30 feet long, lies prone on the
shale. Each of the rooted stumps is oval or elliptical in section, and
the long axes of the several stems are approximately parallel, pointing
to some cause operating in a definite direction which gave to the stems
their present form. Near one of the trees, and at a somewhat higher
level than its base, the surface of the rock is clearly ripple-marked,
and takes us back to the time when the sinking forest trees were washed
by waves which left an impress in the soft mud laid down over the
submerged area. The stumps appear to be those of Lepidodendron trees,
rooted in Lower Carboniferous rocks. From their manner of occurrence
it would seem that we have in them a corner of a Palaeozoic forest in
which Lepidodendra played a conspicuous part. The shales and sandstones
containing the fossil trees were originally overlain by a bed of
igneous rock which had been forced up as a sheet of lava into the
hardened sands and clays[68].
Other examples of old surface-soils occur in different parts of the
world and in rocks of various ages. As an instance of a land surface
preserved in a different manner, reference may be made to the thin
bands of reddish or brown material as well as clays and shale which
occasionally occur between the sheets of Tertiary lava in the Western
Isles of Scotland and the north-east of Ireland. In the intervals
between successive outpourings of basaltic lava in the north-west of
Europe during the early part of the Tertiary period, the heated rocks
became gradually cooler, and under the influence of weathering agents a
surface-soil was produced fit for the growth of plants. In some places,
too, shallow lakes were formed, and leaves, fruits and twigs became
embedded in lacustrine sediments, to be afterwards sealed up by later
streams of lava. In the face of the cliff at Ardtun Head on the coast
of Mull a leaf-bed is exposed between two masses of gravel underlying a
basaltic lava flow; the impressions of the leaves of _Gingko_ and other
plants from the Tertiary sediments of this district are exceptionally
beautiful and well preserved[69]. A large collection obtained by Mr
Starkie Gardner may be seen in the British Museum.
In 1883 the Malayan island of Krakatoa, 20 miles from Sumatra and
Java, was the scene of an exceptionally violent volcanic explosion.
Two-thirds of the island were blown away, and the remnant was left
absolutely bare of organic life. In 1886 it was found that several
plants had already established themselves on the hardened and weathered
crust of the Krakatoan rocks, the surface of the lavas having been to a
large extent prepared for the growth of the higher plants by the action
of certain blue-green algae which represent some of the lowest types
of plant life[70]. We may perhaps assume a somewhat similar state of
things to have existed in the volcanic area in north-west Europe, where
the intervals between successive outpourings of lava are represented by
the thin bands of leaf-beds and old surface-soils.
On the Cheshire Coast at Leasowe[71] and other localities, there is
exposed at low water a tract of black peaty ground studded with old
rooted stumps of conifers and other trees (fig. 6). There is little
reason to doubt that at all events the majority of the trees are in
their natural place of growth. The peaty soil on which they rest
contains numerous flattened stems of reeds and other plants, and is
penetrated by roots, probably of some aquatic or marshy plants which
spread over the site of the forest as it became gradually submerged. A
lower forest-bed rests directly on a foundation of boulder clay. Such
submerged forests are by no means uncommon around the British coast;
many of them belong to a comparatively recent period, posterior to the
glacial age. In many cases, however, the tree stumps have been drifted
from the places where they grew and eventually deposited in their
natural position, the roots of the trees, in some cases aided by stones
entangled in their branches, being heavier than the stem portion.
There is a promising field for botanical investigation in the careful
analysis of the floras of submerged forests; the work of Clement Reid,
Nathorst, Andersson and others, serves to illustrate the value of such
research in the hands of competent students.
[Illustration: FIG. 6. Part of a submerged Forest seen at low water on
the Cheshire Coast at Leasowe. Drawn from a photograph.]
The following description by Lyell, taken from his American travels,
is of interest as affording an example of the preservation of a
surface-soil:
“On our way home from Charleston, by the railway from Orangeburg, I
observed a thin black line of charred vegetable matter exposed in
the perpendicular section of the bank. The sand cast out in digging
the railway had been thrown up on the original soil, on which
the pine forest grew; and farther excavations had laid open the
junction of the rubbish and the soil. As geologists, we may learn
from this fact how a thin seam of vegetable matter, an inch or two
thick, is often the only monument to be looked for of an ancient
surface of dry land, on which a luxuriant forest may have grown
for thousands of years. Even this seam of friable matter may be
washed away when the region is submerged, and, if not, rain water
percolating freely through the sand may, in the course of ages,
gradually carry away the carbon[72].”
[Sidenote: FOSSIL WOOD.]
In addition to the remnants of ancient soils, and the preservation
of plant fragments in rocks which have been formed on the floor of
an inland lake or an estuary, it is by no means rare to find fossil
plants in obviously marine sediments. In fig. 7 we have a piece of
coniferous wood with the shell of an Ammonite (_Aegoceras planicosta_
Sow.) lying on it; the specimen was found in the Lower Lias clay at
Lyme Regis, and illustrates the accidental association of a drifted
piece of a forest tree with a shell which marks at once the age and the
marine character of the beds. Again in fig. 8 we have a block of flint
partially enclosing a piece of coniferous wood in which the internal
structure has been clearly preserved in silica. This specimen was found
in the chalk, a deposit laid down in the clear and deep water of the
Cretaceous sea. The wood must have floated for some time before it
became water-logged and sank to the sea-floor. In the light coloured
wood there occur here and there dark spots which mark the position of
siliceous plugs _b, b_ filling up clean cut holes bored by Teredos
in the woody tissue. The wood became at last enclosed by siliceous
sediment and its tissues penetrated by silica in solution, which
gradually replaced and preserved in wonderful perfection the form of
the original tissue. A similar instance of wood enclosed in flint was
figured by Mantell in 1844 in his _Medals of Creation_[73].
[Illustration: FIG. 7. _Aegoceras planicosta_ Sow. on a piece of
coniferous wood, Lower Lias, Lyme Regis. From a specimen in the
British Museum. Slightly reduced.]
[Illustration: FIG. 8. Piece of coniferous wood in flint, from the
Chalk, Croydon. Drawn from a specimen presented to the British Museum
by Mr Murton Holmes. In the side view, shown above in the figure, the
position of the wood is shown by the lighter portion, with holes, _b,
b_, bored by Teredos or some other wood-eating animal. In the end
view, below, the wood is seen as an irregular cylinder _w, w_,
embedded in a matrix of flint. ⅓ Nat. size.]
The specimen represented in fig. 9 illustrates the almost complete
destruction of a piece of wood by some boring animal. The circular and
oval dotted patches represent the filled up cavities made by a Teredo
or some similar wood-boring animal.
[Illustration: FIG. 9. Piece of wood from the Red Crag of Suffolk,
riddled with holes filled in with mud. From a specimen in the York
Museum. ⅓ Nat. size.]
[Sidenote: CONDITIONS OF FOSSILISATION.]
Before discussing a few more examples of fossils illustrating different
methods of fossilisation, it may not be out of place to quote a few
extracts from travellers’ narratives which enable us to realise more
readily the circumstances and conditions under which plant remains have
been preserved in the Earth’s crust.
In an account of a journey down the Rawas river in Sumatra, Forbes thus
describes the flooded country:—
“The whole surface of the water was covered, absolutely in a close
sheet, with petals, fruits and leaves, of innumerable species. In
placid corners sometimes I noted a collected mass nearly half a
foot deep, among which, on examination, I could scarcely find a
leaf that was perfect, or that remained attached to its rightful
neighbour, so that were they to become imbedded in some soft muddy
spot, and in after ages to reappear in a fossil form they would
afford a few difficult puzzles to the palaeontologist, both to
separate and to put together[74].”
An interesting example of the mixture of plants and animals in
sedimentary deposits is described by Hooker in his Himalayan Journals:—
“To the geologist the Jheels and Sunderbunds are a most instructive
region, as whatever may be the mean elevation of their waters,
a permanent depression of ten to fifteen feet would submerge an
immense tract, which the Ganges, Burrampooter, and Soormah would
soon cover with beds of silt and sand.
“There would be extremely few shells in the beds thus formed,
the southern and northern divisions of which would present two
very different floras and faunas, and would in all probability
be referred by future geologists to widely different epochs. To
the north, beds of peat would be formed by grasses, and in other
parts temperate and tropical forms of plants and animals would be
preserved in such equally balanced proportions as to confound the
palaeontologist; with the bones of the long-snouted alligator,
Gangetic porpoise, Indian cow, buffalo, rhinoceros, elephant,
tiger, deer, bear, and a host of other animals, he would meet with
acorns of several species of oak, pine-cones and magnolia fruits,
rose seeds, and _Cycas_ nuts, with palm nuts, screw-pines, and
other tropical productions[75].”
In another place the same author writes:
“On the 12th of January, 1848, the _Moozuffer_ was steaming amongst
the low, swampy islands of the Sunderbunds.... Every now and then
the paddles of the steamer tossed up the large fruits of _Nypa
fruticans_, Thunb., a low stemless palm that grows in the tidal
waters of the Indian Ocean, and bears a large head of nuts. It
is a plant of no interest to the common observer, but of much to
the geologist, from the nuts of a similar plant abounding in the
Tertiary formations at the mouth of the Thames, having floated
about there in as great profusion as here, till buried deep in the
silt and mud that now forms the island of Sheppey[76].”
[Sidenote: DRIFTING OF TREES.]
Of the drifting of timber, fruits, &c., we find numerous accounts in
the writings of travellers. Rodway thus describes the formation of
vegetable rafts in the rivers of Northern British Guiana:—
“Sometimes a great tree, whose timber is light enough to float,
gets entangled in the grass, and becomes the nucleus of an immense
raft, which is continually increasing in size as it gathers up
everything that comes floating down the river[77].”
The undermining of river banks in times of flood, and the transport of
the drifted trees to be eventually deposited in the delta is a familiar
occurrence in many parts of the world. The more striking instances of
such wholesale carrying along of trees are supplied by Bates, Lyell and
other writers. In his description of the Amazon the former writes:
“The currents ran with great force close to the bank, especially
when these receded to form long bays or _enseadas_, as they are
called, and then we made very little headway. In such places the
banks consist of loose earth, a rich crumbling vegetable mould,
supporting a growth of most luxuriant forest, of which the currents
almost daily carry away large portions, so that the stream for
several yards out is encumbered with fallen trees, whose branches
quiver in the current[78].”
In another place, Bates writes:
“The rainy season had now set in over the region through which the
great river flows; the sand-banks and all the lower lands were
already under water, and the tearing current, two or three miles in
breadth, bore along a continuous line of uprooted trees and islets
of floating plants[79].”
The rafts of the Mississippi and other rivers described by Lyell afford
instructive examples of the distant transport of vegetable material.
The following passage is taken from the _Principles of Geology_;
“Within the tropics there are no ice-floes; but, as if to
compensate for that mode of transportation, there are floating
islets of matted trees, which are often borne along through
considerable spaces. These are sometimes seen sailing at the
distance of fifty or one hundred miles from the mouth of the
Ganges, with living trees standing erect upon them. The Amazons,
the Orinoco, and the Congo also produce these verdant rafts[80].”
After describing the enormous natural rafts of the Atchafalaya, an arm
of the Mississippi, and of the Red river, Lyell goes on to say:
“The prodigious quantity of wood annually drifted down by the
Mississippi and its tributaries is a subject of geological
interest, not merely as illustrating the manner in which abundance
of vegetable matter becomes, in the ordinary course of nature,
imbedded in submarine and estuary deposits, but as attesting the
constant destruction of soil and transportation of matter to lower
levels by the tendency of rivers to shift their courses.... It
is also found in excavating at New Orleans, even at the depth of
several yards below the level of the sea, that the soil of the
delta contains innumerable trunks of trees, layer above layer, some
prostrate as if drifted, others broken off near the bottom, but
remaining still erect, and with their roots spreading on all sides,
as if in their natural position[81].”
The drifting of trees in the ocean is recorded by Darwin in his
description of Keeling Island, and their action as vehicles for the
transport of boulders is illustrated by the same account.
“In the channels of Tierra del Fuego large quantities of drift
timber are cast upon the beach, yet it is extremely rare to meet a
tree swimming in the water. These facts may possibly throw light
on single stones, whether angular or rounded, occasionally found
embedded in fine sedimentary masses[82].”
Fruits may often be carried long distances from land, and preserved
in beds far from their original source. Whilst cruising amongst the
Solomon Islands, the Challenger met with fruits of _Barringtonia
speciosa_ &c., 130–150 miles from the coast. Off the coast of New
Guinea long lines of drift wood were seen at right angles to the
direction of the river; uprooted trees, logs, branches, and bark, often
floating separately.
“The midribs of the leaves of a pinnate-leaved palm were abundant,
and also the stems of a large cane grass (_Saccharum_), like that
so abundant on the shores of the great river in Fiji. Various
fruits of trees and other fragments were abundant, usually floating
confined in the midst of the small aggregations into which the
floating timber was everywhere gathered.... Leaves were absent
except those of the Palm, on the midrib of which some of the
pinnæ were still present. The leaves evidently drop first to the
bottom, whilst vegetable drift is floating from a shore; thus, as
the débris sinks in the sea water, a deposit abounding in leaves,
but with few fruits and little or no wood, will be formed near
shore, whilst the wood and fruits will sink to the bottom farther
off the land. Much of the wood was floating suspended vertically
in the water, and most curiously, logs and short branch pieces
thus floating often occurred in separate groups apart from the
horizontally floating timber. The sunken ends of the wood were not
weighted by any attached masses of soil or other load of any kind;
possibly the water penetrates certain kinds of wood more easily in
one direction with regard to its growth than the other, hence one
end becomes water-logged before the other.... The wood which had
been longest in the water was bored by a _Pholas_[83].”
The bearing of this account on the manner of preservation of fossils,
and the differential sorting so frequently seen in plant beds, is
sufficiently obvious.
As another instance of the great distance to which land plants may be
carried out to sea and finally buried in marine strata, an observation
by Bates may be cited. When 400 miles from the mouth of the main
Amazons, he writes:
“We passed numerous patches of floating grass mingled with tree
trunks and withered foliage. Amongst these masses I espied many
fruits of that peculiar Amazonian tree the Ubussú Palm; this was
the last I saw of the great river[84].”
The following additional extract from the narrative of the Cruise of
H.M.S. Challenger illustrates in a striking degree the conflicting
evidence which the contents of fossiliferous beds may occasionally
afford; it describes what was observed in an excursion from Sydney to
Berowra Creek, a branch of the main estuary or inlet into which flows
the Hawkesbury river. It was impossible to say where the river came
to an end and the sea began. The Creek is described as a long tortuous
arm of the sea, 10 to 15 miles long, with the side walls covered with
orchids and _Platycerium_. The ferns and palms were abundant in the
lateral shady glens; marine and inland animals lived in close proximity.
“Here is a narrow strip of the sea water, twenty miles distant
from the open sea; on a sandy shallow flat close to its head are
to be seen basking in the sun numbers of sting-rays.... All over
these flats, and throughout the whole stretch of the creek, shoals
of Grey Mullet are to be met with; numerous other marine fish
inhabit the creek. Porpoises chase the mullet right up to the
commencement of the sand-flat. At the shores of the creek the rocks
are covered with masses of excellent oysters and mussel, and other
shell-bearing molluscs are abundant, whilst a small crab is to be
found in numbers in every crevice. On the other hand the water is
overhung by numerous species of forest trees, by orchids and ferns,
and other vegetation of all kinds; mangroves grow only in the
shallow bays. The gum trees lean over the water in which swim the
_Trygon_ and mullet, just as willows hang over a pool of carp. The
sandy bottom is full of branches and stems of trees, and is covered
in patches here and there by their leaves. Insects constantly fall
in the water, and are devoured by the mullet. Land birds of all
kinds fly to and fro across the creek, and when wounded may easily
be drowned in it. Wallabies swim across occasionally, and may add
their bones to the débris at the bottom. Hence here is being formed
a sandy deposit, in which may be found cetacean, marsupial, bird,
fish, and insect remains, together with land and sea shells, and
fragments of a vast land flora; yet how restricted is the area
occupied by this deposit, and how easily might surviving fragments
of such a record be missed by future geological explorers![85]”
[Sidenote: MEANING OF THE TERM ‘FOSSIL.’]
The term ‘fossil’ suggests to the lay mind a petrifaction or a
replacement by mineral matter of the plant tissues. In the scientific
sense, a fossil plant, that is a plant or part of a plant whether
in the form of a true petrifaction or a structureless mould or
cast, which has been buried in the earth by natural causes, may be
indistinguishable from a piece of recent wood lately fallen from the
parent tree. In the geologically recent peat beds such little altered
fossils (or sub-fossils) are common enough, and even in older rocks
the more resistant parts of plant fragments are often found in a
practically unaltered state. In the leaf impressions on an impervious
clay, the brown-walled epidermis shows scarcely any indication of
alteration since it was deposited in the soft mud of a river’s delta.
Such fossil leaves are common in the English Tertiary beds, and even in
Palaeozoic rocks it is not uncommon to find an impression of a plant
on a bed of shale from which the thin brown epidermis may be peeled
off the rock, and if microscopically examined it will be found to have
retained intact the contours of the cuticularised epidermal cells. A
striking example of a similar method of preservation is afforded by
the so-called paper-coal of Culm age from the Province of Toula in
Russia[86]. In the Russian area the Carboniferous or Permian rocks have
been subjected to little lateral pressure, and unlike the beds of the
same age in Western Europe, they have not been folded and compressed by
widespread and extensive crust-foldings. Instead of the hard seams of
coal there occur beds of a dark brown laminated material, made up very
largely of the cuticles of Lepidodendroid plants.
From such examples we may naturally pass to fossils in which the plant
structure has been converted into carbonaceous matter or even pure
coal. This form of preservation is especially common in plant-bearing
beds at various geological horizons. In other cases, again, some
mineral solution, oxide of iron, talc, and other substances, has
replaced the plant tissues. From the Coal-Measures of Switzerland Heer
has figured numerous specimens of fern fronds and other plants in which
the leaf form has been left on the dark coloured rock surface as a thin
layer of white talcose material[87]. In the Buntersandstone of the
Vosges and other districts the red imperfectly preserved impressions
of plant stems and leaves are familiar fossils[88]; the carbonaceous
substance of the tissues has been replaced by a brown or red oxide of
iron.
[Sidenote: INCRUSTATIONS.]
Plants frequently occur in the form of incrustations; and in fact
incrustations, which may assume a variety of forms, are the commonest
kind of fossil. The action of incrusting springs, or as they are
often termed petrifying springs, is illustrated at Knaresborough, in
Yorkshire, and many other places where water highly charged with
carbonate of lime readily deposits calcium carbonate on objects placed
in the path of the stream.
The travertine deposited in this manner forms an incrustation on plant
fragments, and if the vegetable substance is subsequently removed by
the action of water or decay, a mould of the embedded fragment is
left in the calcareous matrix. An instructive example of this form of
preservation was described in 1868[89] by Sharpe from an old gravel pit
near Northampton. He found in a section eight feet high (fig. 10), a
mass of incrusted plants of _Chara_ (_a_) resting on and overlain by a
calcareous paste (_c_) and (_d_) made up of the decomposed material of
the overlying rock, and this again resting on sand. The place where the
section occurred was originally the site of a pool in which Stoneworts
grew in abundance. Large blocks of these incrusted Charas may be seen
in the fossil-plant gallery of the British Museum.
[Illustration: FIG. 10. Section of an old pool filled up with a mass of
_Chara_. (From the _Geol. Mag._ vol. v. 1868, p. 563.)]
In the Natural History Museum in the Jardin des Plantes, Paris, one
of the table-cases contains what appear to be small models of flowers
in green wax. These are in reality casts in wax of the moulds or
cavities left in a mass of calcareous travertine, on the decay and
disappearance of the encrusted flowers and other plant fragments[90].
This porous calcareous rock occurs near Sézanne in Southern France,
and is of Eocene age[91]. The plants were probably blown on to the
freshly deposited carbonate of lime, or they may have simply fallen
from the tree on to the incrusting matrix; more material was afterwards
deposited and the flowers were completely enclosed. Eventually the
plant substance decayed, and as the matrix hardened moulds were left
of the vegetable fragments. Wax was artificially forced into these
cavities and the surrounding substance removed by the action of an
acid, and thus perfect casts were obtained of Tertiary flowers.
Darwin has described the preservation of trees in Van Diemen’s land
by means of calcareous substances. In speaking of beds of blown sand
containing branches and roots of trees he says:
“The whole became consolidated by the percolation of calcareous
matter; and the cylindrical cavities left by the decaying of the
wood were thus also filled up with a hard pseudo-stalactitical
stone. The weather is now wearing away the softer parts, and in
consequence the hard casts of the roots and branches of the trees
project above the surface, and, in a singularly deceptive manner,
resemble the stumps of a dead thicket[92].”
As a somewhat analogous method of preservation to that in travertine,
the occurrence of plants in amber should be mentioned. In Eocene times
there existed over a region, part of which is now the North-east
German coast, an extensive forest of conifers and other trees. Some
of the conifers were rich in resinous secretions which were poured
out from wounded surfaces or from scars left by falling branches.
As these flowed as a sticky mass over the stem or collected on the
ground, flowers, leaves, and twigs blown by the wind or falling from
the trees, became embedded in the exuded resin. Evaporation gradually
hardened the resinous substance until the plant fragments became
sealed up in a mass of amber, in precisely the same manner in which
objects are artificially preserved in Canada balsam. In many cases the
amber acts as a petrifying agent, and by penetrating the tissues of a
piece of wood it preserves the minute structural details in wonderful
perfection[93]. Dr Thomas in an account of the amber beds of East
Prussia in 1848, refers to the occurrence of large fossil trees; he
writes:
“The continuous changes to which the coast is exposed, often bring
to light enormous trunks of trees, which the common people had
long regarded as the trunks of the amber tree, before the learned
declared that they were the stems of palm trees, and in consequence
determined the position of Paradise to be on the coast of East
Prussia[94].”
[Sidenote: CASTS OF TREES.]
In 1887 an enormous fossil plant was discovered in a sandstone
quarry at Clayton near Bradford[95]. The fossil was in the form of a
sandstone cast of a large and repeatedly branched _Stigmaria_, and it
is now in the Owens College Museum, where it was placed through the
instrumentality of Prof. Williamson. The plant was found spread out
in its natural position on the surface of an arenaceous shale, and
overlain by a bed of hard sandstone identical with the material of
which the cast is composed. Williamson has thus described the manner of
formation of the fossil:
“It is obvious that the entire base of the tree became encased in a
plastic material, which was firmly moulded upon these roots whilst
the latter retained their organisation sufficiently unaltered to
enable them to resist all superincumbent pressure. This external
mould then hardened firmly, and as the organic materials decayed
they were floated out by water which entered the branching cavity;
at a still later period the same water was instrumental in
replacing the carbonaceous elements by the sand of which the entire
structure now consists[96].”
Although the branches have not been preserved for their whole length,
they extend a distance of 29 feet 6 inches from right to left, and 28
feet in the opposite direction.
The fossil represented in fig. 1 (p. 10), from the collection of Dr
John Woodward, affords a good example of a well-defined impression.
The surface of the specimen, of which a cast is represented in fig. 1,
shows very clearly the characteristic leaf-cushions and leaf-scars
of a _Lepidodendron_. The stem was embedded in soft sand, and as the
latter became hard and set, an impression was obtained of the external
markings of the _Lepidodendron_. Decay subsequently removed the
substance of the plant.
[Illustration: FIG. 11. _Equisetites columnaris_ Brongn. From a
specimen in the Woodwardian Museum, Cambridge. ⅓ nat. size.]
In fig. 11 some upright stems of a fossil Horse-tail (_Equisetites
columnaris_) from the Lower Oolite rocks near Scarborough, are seen in
a vertical position in sandstone. On the surface of the fossils there
is a thin film of carbonaceous matter, which is all that remains of the
original plant substance; the stems were probably floated into their
present position and embedded vertically in an arenaceous matrix. The
hollow pith-cavity was filled with sand, and as the tissues decayed
they became in part converted into a thin coaly layer. The vertical
position of such stems as those in fig. 11 naturally suggests their
preservation _in situ_, but in this as in many other cases the erect
manner of occurrence is due to the settling down of the drifted plants
in this particular position.
[Sidenote: FOSSIL CASTS.]
An example of _Stigmaria_ drawn in fig. 12 further illustrates the
formation of casts[97]. The outer surface with the characteristic
spirally arranged circular depressions, represents the wrinkled bark of
the dried plant; the smaller cylinder, on the left side of the upper
end (fig. 12, 2, _p_) marks the position of the pith surrounded by the
secondary wood, which has been displaced from its axial position. The
pith decayed first, and the space was filled in with mud; somewhat
later the wood and cortex were partially destroyed, and the rod of
material which had been introduced into the pith-cavity dropped towards
one side of the decaying shell of bark.
[Illustration: FIG. 12. _Stigmaria ficoides_ Brongn. 1. Side view,
showing wrinkled surface and the scars of appendages. 2. End view
(upper) showing the displaced central cylinder; _p_, pith, _x_,
xylem, _r_, medullary rays. 3. End view (lower). From a specimen in
the Woodwardian Museum. ½ nat. size.]
As the parenchymatous medullary rays readily decayed, the mud in
the pith extended outwards between the segments of wood which still
remained intact, and so spokes of argillaceous material were formed
which filled the medullary ray cavities. The cortical tissues were
decomposed, and their place taken by more argillaceous material. At one
end of the specimen (fig. 12, 3) we find the wood has decayed without
its place being afterwards filled up with foreign material. At the
opposite end of the specimen, the woody tissue has been partially
preserved by the infiltration of a solution containing carbonate of
lime (fig. 12, 2).
Numerous instances have been recorded from rocks of various geological
ages of casts of stems standing erect and at right angles to the
bedding of the surrounding rock. These vertical trees occasionally
attain a considerable length, and have been formed by the filling
in by sand or mud of a pipe left by the decay of the stem. It is
frequently a matter of some difficulty to decide how far such fossils
are in the position of growth of the tree, or whether they are merely
casts of drifted stems, which happen to have been deposited in an
erect position. The weighting of floating trees by stones held in the
roots, added to the greater density of the root wood, has no doubt
often been the cause of this vertical position. In attempting to
determine if an erect cast is in the original place of growth of the
tree, it is important to bear in mind the great length of time that
wood is able to resist decay, especially under water. The wonderful
state of preservation of old piles found in the bed of a river, and
the preservation of wooden portions of anchors of which the iron has
been completely removed by disintegration, illustrate this power of
resistance. In this connection, the following passage from Lyell’s
travels in America is of interest. In describing the site of an old
forest, he writes[98]:
“Some of the stumps, especially those of the fir tribe, take fifty
years to rot away, though exposed in the air to alternations of
rain and sunshine, a fact on which every geologist will do well
to reflect, for it is clear that the trees of a forest submerged
beneath the water, or still more, if entirely excluded from the
air, by becoming imbedded in sediment, may endure for centuries
without decay, so that there may have been ample time for the slow
petrifaction of erect fossil trees in the Carboniferous and other
formations, or for the slow accumulation around them of a great
succession of strata.”
In another place, in speaking of the trees in the Great Dismal Swamp,
Lyell writes:—“When thrown down, they are soon covered by water,
and keeping wet they never decompose, except the sap wood, which is
less than an inch thick[99].” We see, then, that trees may have
resisted decay for a sufficiently long time to allow of a considerable
deposition of sediment. It is very difficult to make any computation of
the rate of deposition of a particular set of sedimentary strata, and,
therefore, to estimate the length of time during which the fossil stems
must have resisted decay.
[Sidenote: PLANTS AND COAL.]
The protective qualities of humus acids, apart from the almost complete
absence of Bacteria[100] from the waters of Moor- or Peat-land, is a
factor of great importance in the preservation of plants against decay
for many thousands of years.
From examples of fossil stems or leaves in which the organic material
has been either wholly or in part replaced by coal, we may pass by a
gradual transition to a mass of opaque coal in which no plant structure
can be detected. It is by no means uncommon to notice on the face of a
piece of coal a distinct impression of a plant stem, and in some cases
the coal is obviously made up of a number of flattened and compressed
branches or leaves of which the original tissues have been thoroughly
carbonised. A block of French coal, represented in fig. 13, consists
very largely of laminated bands composed of the long parallel veined
leaves of the genus _Cordaites_ and of the bark of _Lepidodendron_,
_Sigillaria_, and other Coal-Measure genera. The long rhizomes and
roots below the coal are preserved as casts in the underclay.
In examining thin sections of coal, pieces of pitted tracheids or
crushed spores are frequently met with as fragments of plant structures
which have withstood decay more effectually than the bulk of the
vegetable débris from which the coal was formed.
The coaly layer on a fossil leaf is often found to be without any trace
of the plant tissues, but not infrequently such carbonised leaves, if
treated with certain reagents and examined microscopically, are seen
to retain the outlines of the epidermal cells of the leaf surface. If
a piece of the Carbonaceous film detached from a fossil leaf is left
for some days in a small quantity of nitric acid containing a crystal
of chlorate of potash, and, after washing with water, is transferred
to ammonia, transparent film often shows very clearly the outlines
of the epidermal cell and the form of the stomata. Such treatment has
been found useful in many cases as an aid to determination[101]. Prof.
Zeiller informs me that he has found it particularly satisfactory in
the case of cycadean leaves.
[Illustration: FIG. 13. Part of a coal seam largely made up of
_Cordaites_ leaves. _Stigmaria_ and _Stigmariopsis_ shown in the rock
(underclay) underlying the coal. (After Grand’Eury [82] Pl. I. fig.
3.)]
[Sidenote: FOSSILS IN HALF-RELIEF.]
It is sometimes possible to detach the thin lamina representing the
carbonised leaf or other plant fragment from the rock on which it lies
and to mount it whole on a slide. Good examples of plants treated in
this way may be seen in the Edinburgh and British Museums, especially
_Sphenopteris_ fronds from the Carboniferous oil shales of Scotland.
In the excellent collection of fossil plants in Stockholm there are
still finer examples of such specimens, obtained by Dr Nathorst from
some of the Triassic plants of Southern Sweden. In a few instances the
tissues of a plant have been converted into coal in such a manner as
to retain the form of the individual cells, which appear in section
as a black framework in a lighter coloured matrix. Examples of such
carbonised tissues were figured by some of the older writers, and
Solms-Laubach has recently[102] described sections of Palaeozoic plants
preserved in this manner. The section represented in fig. 70 is that
of a Calamite stem (8 × 9·5 cm.) in which the wood has been converted
into carbonaceous material, but the more delicate tissues have been
almost completely destroyed. The thin and irregular black line a
little distance outside the ring of wood, and forming the limit of the
drawing, probably represents the cuticle. The whole section is embedded
in a homogeneous matrix of calcareous rock, in which the more resistant
tissues of the plant have been left as black patches and faint lines.
Mention should be made of a special form of preservation which has been
described as fossilisation in half-relief. If a stem is imbedded in
sand or mud, the matrix receives an impression of the plant surface,
and if the hollow pith-cavity is filled with the surrounding sediment,
the surface of the medullary cast will exhibit markings different from
those seen on the surface in contact with the outside of the stem. The
space separating the pith-cast from the mould bearing the impression of
the stem surface may remain empty, or it may be filled with sedimentary
material. In half-relief fossils, on the other hand, we have projecting
from the under surface of a bed a more or less rounded and prominent
ridge with certain surface markings, and fitting into a corresponding
groove in the underlying rock on which the same markings have been
impressed. It is conceivable that such a cast might be obtained if
soft plant fragments were lying on a bed of sand, and were pressed into
it by the weight of superincumbent material. The plant fragment would
be squeezed into a depression, and its substance might eventually be
removed and leave no other trace than the half-relief cast and hollow
mould. A twig lying on sand would by its own weight gradually sink a
little below the surface; if it were then blown away or in some manner
removed, the depression would show the surface features of the twig.
When more sand came to be spread out over the depression, it would
find its way into the pattern of the mould, and so produce a cast. If
at a later period when the sand had hardened, the upper portion were
separated from the lower, from the former there would project a rounded
cast of the hollow mould. The preservation of soft algae as half-relief
casts has been doubted by Nathorst[103] and others as an unlikely
occurrence in nature. They prefer to regard such ridges on a rock face
as the casts of the trails or burrows of animals. This question of the
preservation of the two sides of a mould showing the same impression
of a plant has long been a difficult problem; it is discussed by
Parkinson in his _Organic Remains_. In one of the letters (No. XLVI.),
he quotes the objection of a sceptical friend, who refuses to believe
such a manner of preservation possible, “until,” says Parkinson, “I
can inform him if, by involving a guinea in plaster of Paris, I could
obtain two impressions of the king’s head, without any impression of
the reverse[104].”
It would occupy too much space to attempt even a brief reference to the
various materials in which impressions of plants have been preserved.
Carbonaceous matter is the most usual substance, and in some cases
it occurs in the form of graphite which on dark grey or black rocks
has the appearance of a plant drawn in lead pencil. The impressions
of plants on the Jurassic (Kimeridgian) slates of Solenhofen[105] in
Bavaria, like those on the Triassic sandstones of the Vosges, are
usually marked out in red iron oxide.
[Sidenote: PETRIFIED TREES.]
So far we have chiefly considered examples of plants preserved in
various ways by _incrustation_, that is, by having been enclosed in
some medium which has received an impression of the surface of the
plant in contact with it. By far the most valuable fossil specimens
from a botanical point of view are however those in which the internal
structure has been preserved; that is in which the preserving medium
has not served merely as an encasing envelope or internal cast, but has
penetrated into the body of the plant fragment and rendered permanent
the organization of the tissues. In almost every Natural History or
Geological Museum one meets with specimens of petrified trees or
polished sections of fossil palm stems and other plants, in which
the internal structure has been preserved in siliceous material, and
admits of detailed investigation in thin sections under the microscope.
Silica, calcium carbonate, with usually a certain amount of carbonate
of iron and magnesium carbonate, iron pyrites, amber, and more rarely
calcium fluoride or other substances have taken the place of the
original cell-walls. Of silicified stems, those from Antigua, Egypt,
Central France, Saxony, Brazil, Tasmania[106], and numerous other
places afford good examples. Darwin records numerous silicified stems
in Northern Chili, and the Uspallata Pass. In the central part of the
Andes range, 7000 feet high, he describes the occurrence of “Snow-white
projecting silicified columns.... They must have grown,” he adds, “in
volcanic soil, and were subsequently submerged below sea-level, and
covered with sedimentary beds and lava-flows[107].” A striking example
of the occurrence of numerous petrified plant stems has been described
by Holmes from the Tertiary forests of the Yellowstone Park. From
the face of a cliff on the north side of Amethyst mountain “rows of
upright trunks stand out on the ledges like the columns of a ruined
temple. On the more gentle slopes farther down, but where it is still
too steep to support vegetation, save a few pines, the petrified
trunks fairly cover the surface, and were at first supposed by us to
be the shattered remains of a recent forest[108].” Marsh[109] and
Conwentz[110] have described silicified trees more than fifty feet
in length from a locality in California where several large forest
trees of Tertiary age have been preserved in volcanic strata. In South
Africa on the Drakenberg hills there occur numerous silicified trunks,
occasionally erect and often lying on the ground, probably of Triassic
age[111]. In some instances the specimens measure several feet in
length and diameter. Some of the coniferous stems seen in Portland,
and occasionally met with reared up against a house side, illustrate
the silicification of plant structure on a large scale. These are of
Upper Jurassic (Purbeck) age. From Grand’Croix in France a silicified
stem of _Cordaites_ of Palaeozoic age has been recorded with a length
of twenty meters. The preservation of plants by siliceous infiltrations
has long been known. One of the earliest descriptions of this form
of petrifaction in the British Isles is that of stems found in Lough
Neagh, Ireland. In his lectures on Natural Philosophy, published at
Dublin in 1751, Barton gives several figures of Irish silicified wood,
and records the following occurrence in illustration of the peculiar
properties erroneously attributed to the waters of Lough Neagh.
Describing a certain specimen (No. XXVI), he writes:—
“This is a whetstone, which as Mr Anthony Shane, apothecary, who
was born very near the lake, and is now alive, relates, he made by
putting a piece of holly in the water of the lake near his father’s
house, and fixing it so as to withstand the motion of the water,
and marking the place so as to distinguish it, he went to Scotland
to pursue his studies, and seven years after took up a stone
instead of holly, the metamorphosis having been made in that time.
This account he gave under his handwriting. The shore thereabouts
is altogether loose sand, and two rivers discharge themselves into
the lake very near that place[112].”
The well-known petrified trees from the neighbourhood of Lough Neagh
are probably of Pliocene age, but their exact source has been a matter
of dispute[113].
[Sidenote: PETRIFIED WOOD.]
In 1836 Stokes described certain stems in which the tissues had been
partially mineralised. In describing a specimen of beech from a Roman
aqueduct at Eibsen in Lippe Bückeburg], he says:—
“The wood is, for the most part, in the state of very old dry wood,
but there are several insulated portions, in which the place of the
wood has been taken by carbonate of lime. These portions, as seen
on the surface of the horizontal section, are irregularly circular,
varying in size, but generally a little less or more than ⅛ inch in
diameter, and they run through the whole thickness of the specimen
in separate, perpendicular columns. The vessels of the wood are
distinctly visible in the carbonate of lime, and are more perfect
in their form and size in those portions of the specimen than in
that which remains unchanged[114].”
[Illustration: FIG. 14.
A. _Araucarioxylon Withami_ (L. and H.). Radiating lines of
crystallisation in secondary wood, as seen in transverse
section.
B. _Lepidodendron_ sp. Concentric lines of crystallisation, and
scalariform tracheids, as seen in longitudinal section.]
This partial petrifaction of the structure in patches is often met with
in fossil stems, and may be seriously misleading to those unfamiliar
with the appearance presented by the crystallisation of silica from
scattered centres in a mass of vegetable tissue. A good example of
this is afforded by the gigantic stems discovered in 1829 in the
Craigleith Quarry near Edinburgh[115]. Of those two large stems found
in the Sandstone rock, the longest, originally 11 meters long and
3·3–3·9 meters in girth, is now set up in the grounds of the British
Museum, and a large polished section (1 m. × 87 cm.) is exhibited in
the Fossil-plant Gallery. The other stem is in the Botanic Garden,
Edinburgh. Transverse sections of the wood of the London specimen show
scattered circular patches (fig. 14 A) in the mineralised wood in which
the tracheids are very clearly preserved; while in the other portion
the preservation is much less perfect. The patch of tissue in fig. 14
A shows a portion of the wood of the Craigleith tree [_Araucarioxylon
Withami_ (L. and H.)] in which the mineral matter, consisting of
dolomite with a little silica here and there, has crystallised in such
a manner as to produce what is practically a cone-in-cone structure
on a small scale, which has partially obliterated the structural
features. This minute cone-in-cone structure is not uncommon in
petrified tissues; it is precisely similar in appearance to that
described by Cole[116] in certain minerals. The crystallisation has
been set up along lines radiating from different centres, and the
particles of the tissue have been pushed as it were along these lines.
[Illustration: FIG. 15. Transverse section of the central cylinder of a
Carboniferous Lepidodendroid stem in the collection of Mr Kidston.
From Dalmeny, Scotland. _s._ Silica filling up the central portion of
the pith. _p._ Remains of the pith tissue. _x_¹. Primary xylem. _x_².
Secondary xylem. _c._ Innermost cortex.]
[Sidenote: PRESERVATION OF TISSUES.]
A somewhat different crystallisation phenomenon is illustrated by
the extremely fine section of a Lepidodendroid plant shown in fig.
15. The tissues of the primary and secondary wood (_x_¹ and _x_²)
are well preserved throughout in silica, but scattered through the
siliceous matrix there occur numerous circular patches, as seen in the
figure. One of these is more clearly shown in fig. 14 B drawn from
a longitudinal section through the secondary wood, _x_²; it will be
noticed that where the concentric lines of the circular patch occur,
the scalariform thickenings of the tracheids are sharply defined, but
immediately a tracheid is free of the patch these details are lost.
It would appear that in this case silicification was first completed
round definite isolated centres, and the secondary crystallisation in
the matrix partially obliterated some of the more delicate structural
features. The same phenomenon has been observed in oolitic rocks[117],
in which the oolitic grains have resisted secondary crystallisation and
so retained their original structure.
Among the most important examples of silicified plants are those from
a few localities in Central France. In the neighbourhood of Autun
there used to be found in abundance loose nodules of siliceous rock
containing numerous fragments of seeds, twigs, and leaves of different
plants. The rock of which the broken portions are found on the surface
of the ground was formed about the close of the Carboniferous period.
At the hands of French investigators the microscopic examination of
these fragments of a Palaeozoic vegetation have thrown a flood of
light on the anatomical structure of many extinct types. Sometimes the
silica has penetrated the cavities of the cells and vessels, and the
walls have decayed without their substance being replaced by mineral
material. Sections of tissues preserved in this manner, if soaked in a
coloured solution assume an appearance almost identical with that of
stained sections of recent plants. The spaces left by the decayed walls
act as fine capillaries and suck up the coloured solution[118].
[Illustration: FIG. 16. Internal cast of a sclerenchymatous cell from
the root of a Cretaceous fern (_Rhizodendron oppoliense_ Göpp.).
After Stenzel (86) Pl. III. fig. 29. × 240 and reduced to one-half.]
In the Coal-Measure sandstones of England large pieces of woody
stems are occasionally met with in which the mineralisation has been
incomplete. A brown piece of fossil stem lying in a bed of sandstone
shows on the surface a distinct woody texture, and the lines of wood
elements are clearly visible. The whole is, however, very friable and
falls to pieces if an attempt is made to cut thin sections of it;
the tracheids of the wood easily fall apart owing to the walls being
imperfectly preserved, and the absence of a connecting framework
such as would have been formed had the membranes been thoroughly
silicified. It is occasionally possible to obtain from petrified plant
stems perfect casts in silica or other substances of the cavity of
a sclerenchymatous fibre, in which the mineral has been deposited
not only in the cavity but in the fine pit-canals traversing the
lignified walls. Such a cast is represented in fig. 16, the fine
lateral projections are the delicate casts of the pit canals. Numerous
instances of minute and delicate tissues preserved in silica are
recorded in later chapters. A somewhat unusual type of silicification
is met with in some of the Gondwana rocks of India, in which cycadean
fronds occur as white porcellaneous specimens showing a certain amount
of internal structure in a siliceous matrix. Specimens of such leaves
may be seen in the British Museum.
[Sidenote: COAL-BALLS.]
In the Coal-Measures of England, especially in the neighbourhood of
Halifax in Yorkshire, and in South Lancashire, the seams of coal
occasionally contain calcareous nodules varying in size from a nut to
a man’s head, and consisting of about 70% of carbonate of calcium and
magnesium, and 30% of oxide of iron, sulphide of iron, &c.[119] The
nodules, often spoken of by English writers as ‘coal-balls,’ contain
numerous fragments of plants in which the minute cellular structure is
preserved with remarkable perfection. It should be noted that the term
coal-ball is also applied to rounded or subangular pieces of coal which
are occasionally met with in coal seams, and especially in certain
French coal fields. To avoid confusion it is better to speak of the
plant-containing nodules as calcareous nodules, restricting the term
coal-ball to true coal pebbles. A section of a calcareous nodule, when
seen under the microscope, presents the appearance of a matrix of a
crystalline calcareous substance containing a heterogeneous mixture of
all kinds of plant tissues, usually in the form of broken pieces and in
a confused mass.
[Illustration: FIG. 17. A thin section of a calcareous nodule from the
Coal-Measures. Binney collection, Woodwardian Museum, Cambridge. Very
slightly reduced.]
A large section of one of these nodules (12·5 cm. × 8·5 cm.) is
shown in fig. 17. It illustrates the manner of occurrence of various
fragments of different plants in which the structure has been
more or less perfectly preserved. In this particular example we
see sections of _Myeloxylon_ (I), _Calamites_ (II), Fern petioles
(_Rachiopteris_) (III), Stigmarian appendages (IV), Lepidodendroid
leaves (V), _Myeloxylon_ pinnules (VI), Gymnospermous seeds (VII),
Twig of a _Lepidodendron_, showing the central xylem cylinder and
large leaf-bases on the outer cortex, (VIII), Sporangia and spores
of a strobilus (IX), Tangential section of a _Myeloxylon_ petiole
(X), _Rachiopteris_ sp. (XI), _Rachiopteris_ sp. (XII), Band of
sclerenchymatous tissue (XIII), _Rachiopteris_ sp. (XIV).
The general appearance of a calcareous plant-nodule suggests a soft
pulpy mass of decaying vegetable débris, through which roots were able
to bore their way, as in a piece of peat or leafy mould. Overlying
this accumulation of soft material there was spread out a bed of muddy
sediment containing numerous calcareous shells, which supplied the
percolating water with the material which was afterwards deposited in
portions of the vegetable débris. According to this view the calcareous
nodules of the coal seams represent local patches of a widespread
mass of débris which were penetrated by a carbonated solution, and so
preserved as samples of a decaying mass of vegetation, of which by far
the greater portion became eventually converted into coal[120].
[Sidenote: FOSSIL NUCLEI.]
In such nodules, we find that not only has the framework of the
tissues been preserved, but frequently the remains of cell contents
are clearly seen. In some cases the cells of a tissue may contain in
each cavity a darker coloured spot, which is probably the mineralised
cell nucleus. (Fig. 42, _A_, 1, p. 214.) The contents of secretory
sacs, such as those containing gum or resin, are frequently found as
black rods filling up the cavity of the cell or canal. The contents
of cells in some cases closely simulate starch grains, and such may
have been actually present in the tissues of a piece of a fossil
dicotyledonous stem described by Thiselton-Dyer from the Lower Eocene
Thanet beds[121], and in the rhizome of a fossil _Osmunda_ recorded by
Carruthers[122]. (Fig. 42, _B_, p. 214.)
Schultze in 1855[123] recorded the discovery of cellulose by
microchemical tests applied to macerated tissue from Tertiary lignite
and coal. With reference to the possibility of recognising cell
contents in fossil tissue it is interesting to find that Dr Murray
of Scarborough had attempted, and apparently with success, to apply
chemical tests to the tissues of Jurassic leaves. In a letter written
to Hutton in 1833 Murray speaks of his experiments as follows:—
“Reverting to the Oolitic plants, I have again and with better
success been experimenting upon the thin transparent films of
leaves, chiefly of _Taeniopteris vittata_ and _Cyclopteris_, which
from their tenuity offer fine objects for the microscope.... By
many delicate trials I have ascertained the existence still in
these leaves of resin and of tannin.... I am seeking among the
filmy leaves of the _Fucoides_ of A. Brongniart for iodine, but
hitherto without success, and indeed can hardly expect it, as
probably did iodine exist in them, it must have long ago entered
into new combinations[124].”
Apart from this difficulty, it is not surprising that Dr Murray’s
search for iodine was unsuccessful, considering how little algal nature
most of the so-called Fucoids possess.
Some of the most perfectly preserved tissues as regards the details of
cell contents are those of gymnospermous seeds from Autun. In sections
of one of these seeds which I recently had the opportunity of examining
in Prof. Bertrand’s collection, the parenchymatous cells contained very
distinct nuclei and protoplasmic contents. In one portion of the tissue
in the nucellus of _Sphaerospermum_ the cell walls had disappeared,
but the nuclei remained in a remarkable state of preservation. The
cells shown in fig. 42 are from the ground tissue of a petiole of
_Cycadeoidea gigantea_ Sew.[125], a magnificent Cycadean stem from
Portland recently added to the British Museum collection; in the cell
_A_, 1, the nucleus is fairly distinct and in 2 and 4 the contracted
cell-contents is clearly seen. Other interesting examples of fossil
nuclei are seen in a _Lyginodendron_ leaf figured by Williamson and
Scott in a recent Memoir on that genus[126]. Each mesophyll cell
contains a single dark nucleus. The mineralisation of the most delicate
tissues and the preservation of the various forms of cell-contents
are now generally admitted by those at all conversant with the
possibilities of plant petrifaction. If we consider what these facts
mean—the microscopic investigation of not only the finest framework
but even the very life-substance of Palaeozoic plants—we feel that
the aeons since the days when these plants lived have been well-nigh
obliterated.
Occasionally the plant tissues have assumed a black and somewhat ragged
appearance, giving the impression of charred wood. A section of a
recent burnt piece of wood resembles very closely some of the fossil
twigs from the coal seam nodules. It is possible that in such cases we
have portions of mineralised tissues which were first burnt in a forest
fire or by lightning and then infiltrated with a petrifying solution.
An example of one of these black petrified plants is shown in fig. 74
B. Chap. X. In many of the fossil plants there are distinct traces of
fungus or bacterial ravages, and occasionally the section of a piece
of mineralised wood shows circular spaces or canals which have the
appearance of being the work of some wood-eating animal, and small oval
bodies sometimes occur in such spaces which may be the coprolites of
the xylophagous intruder. (Fig. 24, p. 107.)
[Sidenote: FOSSIL PLANTS IN VOLCANIC ASH.]
It is well known to geologists that during the Permian and
Carboniferous periods the southern portion of Scotland was the scene of
widespread volcanic activity. Forests were overwhelmed by lava-streams
or showers of ash, and in some districts tree stems and broken plant
fragments became sealed up in a volcanic matrix. Laggan Bay in the
north-east corner of the Isle of Arran, and Pettycur a short distance
from Burntisland on the north shore of the Firth of Forth, are two
localities where petrified plants of Carboniferous age occur in such
preservation as allows of a minute investigation of their internal
structure. The occurrence of plants in the former locality was first
discovered by Mr Wünsch of Glasgow; the fossils occur in association
with hardened shales and beds of ash, and are often exceedingly well
preserved[127]. In fig. 18 is reproduced a sketch of a hollow tree
trunk from Arran, probably a _Lepidodendron_ stem, in which only the
outer portion of the bark has been preserved, while the inner cortical
tissues have been removed and the space occupied by volcanic detritus.
[Illustration: FIG. 18. Diagrammatic sketch of a slab cut from a fossil
stem (_Lepidodendron_?) from Laggan Bay. _e_, Imperfectly preserved
bark of a large stem, extending in patches round the periphery of
the specimen; the oval and circular bodies in the interior are the
xylem portions of the central cylinders of _Lepidodendron_ stems,
_x_¹, primary wood, _x_², secondary wood. From a specimen in the
Binney collection, Woodwardian Museum, Cambridge. ⅕ nat. size.]
The smaller cylindrical structures in the interior of the hollow trunk
are the central woody cylinders of Lepidodendroid trees; each consists
of an axial pith surrounded by a band of primary wood and a broader
zone of secondary wood. One of the axes probably belonged to the stem
of which only the shell has been preserved, the others must have come
from other trees and may have been floated in by water[128]. The
microscopic details of the wood and outer cortex have in this instance
been preserved in a calcareous material, which was no doubt derived by
water percolating through the volcanic ash. It is frequently found that
in fossil trees or twigs a separation of the tissues has taken place
along such natural lines of weakness as the cambium or the phellogen,
before the petrifying medium had time to permeate the entire structure.
Tree stems recently killed by lava streams during volcanic eruptions at
the present day supply a parallel with the Palaeozoic forest trees of
Carboniferous times.
Guillemard in describing a volcanic crater in Celebes, speaks of burnt
trees still standing in the lava stream, “so charred at the base of
the trunk that we could easily push them down[129].” An interesting
case is quoted by Hooker in his _Himalayan Journals_, illustrating the
occurrence of a hollow shell of a tree, in which the outer portions of
a stem had been left while the inner portions had disappeared, the wood
being hollow and so favourable to the production of a current of air
which accelerated the destruction of the internal tissues.
On the coast near Burntisland on the Firth of Forth blocks of rock are
met with in which numerous plant fragments of Carboniferous age are
scattered in a confused mass through a calcareous volcanic matrix. The
twigs, leaves, spores, and other portions are in small fragments, and
their delicate cells are often preserved in wonderful perfection.
[Sidenote: CONDITIONS OF PRESERVATION.]
The manner of occurrence of plants in sandstones, shales or other rocks
is often of considerable importance to the botanist and geologist, as
an aid to the correct interpretation of the actual conditions which
obtained at the time when the plant remains were accumulating in beds
of sediment. To attempt to restore the conditions under which any
set of plants became preserved, we have to carefully consider each
special case. A nest of seeds preserved as internal casts in a mass
of sandstone, such as is represented by the block of Carboniferous
sandstone in fig. 19, suggests a quiet spot in an eddy where seeds
were deposited in the sandy sediment. Delicate leaf structures with
sporangia still intact, point to quietly flowing water and a transport
of no great distance. Occasionally the large number of delicate and
light plant fragments, associated it may be with insect wings, may
favour the idea of a wind storm which swept along the lighter pieces
from a forest-clad slope and deposited them in the water of a lake. In
some Tertiary plant-beds the manner of occurrence of leaves and flowers
is such as to suggest a seasonal alternation, and the different layers
of plant débris may be correlated with definite seasons of growth[130].
[Illustration: FIG. 19. Piece of Coal-Measures Sandstone with casts of
_Trigonocarpon_ seeds, from Peel Quarry near Wigan. From a specimen
in the Manchester Museum, Owens College. ½ nat. size.]
The predominance of certain classes of plants in a particular bed may
be due to purely mechanical causes and to differential sorting by
water, or it may be that the district traversed by the stream which
carried down the fragments was occupied almost exclusively by one set
of plants. The trees from higher ground may be deposited in a different
part of a river’s course to those growing in the plains or lowland
marshes. It is obviously impossible to lay down any definite rules
as to the reading of plant records, as aids to the elucidation of
past physical and botanical conditions. Each case must be separately
considered, and the various probabilities taken into account, judging
by reference to the analogy of present day conditions.
Various attempts, more or less successful, have been made to imitate
the natural processes of plant mineralisation[131]. By soaking sections
of wood for some time in different solutions, and then exposing them
to heat, the organic substance of the cell walls has been replaced by
a deposit of oxide of iron and other substances. Fern leaves heated to
redness between pieces of shale have been reduced to a condition very
similar to that of fossil fronds. Pieces of wood left for centuries
in disused mines have been found in a state closely resembling
lignite[132]. Attempts have also been made to reproduce the conditions
under which vegetable tissues were converted into coal, but as yet
these have not yielded results of much scientific value. The Geysers
of Yellowstone Park have thrown some light on the manner in which wood
may be petrified by the percolation of siliceous solutions; and it has
been suggested that the silicification of plants may have been effected
by the waters of hot springs holding silica in solution. Examples of
wood in process of petrifaction in the Geyser district of North America
have been recorded by Kuntze[133], and discussed by Schweinfurth[134],
Solms-Laubach[135] and others[136]. The latter expresses the opinion
that by a long continuance of such action as may now be observed in the
neighbourhood of hot springs, the organic substance of wood might be
replaced by siliceous material. The exact manner of replacement needs
more thorough investigation. Kuntze describes the appearance of forest
trees which have been reached by the waters of neighbouring Geysers.
The siliceous solution rises in the wood by capillarity; the leaves,
branches and bark are gradually lost, and the outer tissues of the
wood become hardened and petrified as the result of evaporation from
the exposed surface of the stem. The products of decay going on in the
plant tissues must be taken into account, and the double decomposition
which might result. There is no apparent reason why experiments
undertaken with pieces of recent wood exposed to permeation by various
calcareous and siliceous solutions under different conditions should
not furnish useful results.
CHAPTER V.
DIFFICULTIES AND SOURCES OF ERROR IN THE
DETERMINATION OF FOSSIL PLANTS.
“Robinson Crusoe did not feel bound to conclude, from the single
human footprint which he saw in the sand, that the maker of the
impression had only one leg.”
HUXLEY’S _Hume_, p. 105, 1879.
The student of palaeobotany has perhaps to face more than his due
share of difficulties and fruitful sources of error; but on the other
hand there is the compensating advantage that trustworthy conclusions
arrived at possess a special value. While always on the alert for
rational explanations of obscure phenomena by means of the analogy
supplied by existing causes, and ready to draw from a wide knowledge of
recent botany, in the interpretation of problems furnished by fossil
plants, the palaeobotanist must be constantly alive to the necessity
for cautious statement. That there is the greatest need of moderation
and safe reasoning in dealing with the botanical problems of past ages,
will be apparent to anyone possessing but a superficial acquaintance
with fossil plant literature. The necessity for a botanical and
geological training has already been referred to in a previous chapter.
It would serve no useful purpose, and would occupy no inconsiderable
space, to refer at length to the numerous mistakes which have been
committed by experienced writers on the subject of fossil plants.
Laymen might find in such a list of blunders a mere comedy of errors,
but the palaeobotanist must see in them serious warnings against
dogmatic conclusions or expressions of opinion on imperfect data and
insufficient evidence. The description of a fragment of a handle of
a Wedgwood teapot as a curious form of Calamite[137] and similar
instances of unusual determinations need not detain us as examples of
instructive errors. The late Prof. Williamson has on more than one
occasion expressed himself in no undecided manner as to the futility of
attempting to determine specific forms among fossil plants, without the
aid of internal structure[138]; and even in the case of well-preserved
petrifactions he always refused to commit himself to definite specific
diagnoses. In his remarks in this connection, Williamson no doubt
allowed himself to express a much needed warning in too sweeping
language. It is one of the most serious drawbacks in palaeobotanical
researches that in the majority of cases the specimens of plants are
both fragmentary and without any trace of internal structure. Specimens
in which the anatomical characters have been preserved necessarily
possess far greater value from the botanist’s point of view than
those in which no such petrifaction has occurred. On the other hand,
however, it is perfectly possible with due care to obtain trustworthy
and valuable results from the examination of structureless casts and
impressions. In dealing with the less promising forms of plant fossils,
there is in the first place the danger of trusting to superficial
resemblance. Hundreds of fossil plants have been described under the
names of existing genera on the strength of a supposed agreement
in external form; but such determinations are very frequently not
only valueless but dangerously misleading. Unless the evidence is
of the best, it is a serious mistake to make use of recent generic
designations. If we consider the difficulties which would attend an
attempt to determine the leaves, fragments of stems and other detached
portions of various recent genera, we can better appreciate the greater
probability of error in the case of imperfectly preserved fossil
fragments.
[Sidenote: EXTERNAL RESEMBLANCE.]
The portions of stems represented in figures 20 and 21, exhibit
a fairly close resemblance to one another; in the absence of
microscopical sections or of the reproductive organs it would be
practically impossible to discriminate with any certainty between
fossil specimens of the plants shown in the drawings. Examples such
as these, and many others which might be cited, serve to illustrate
the possibility of confusion not merely between different genera of
the same family, but even between members of different classes or
groups. The long slender branches of the _Polygonum_ represented in
(fig. 21) would naturally be referred to _Equisetum_ in the absence
of the flowers (fig. 20 _B_), or without a careful examination of the
insignificant scaly leaves borne at the nodes. The resemblance between
_Casuarina_ and _Ephedra_ and the British species of _Equisetum_, or
such a tropical form as _E. debile_, speaks for itself.
[Illustration: FIG. 20.
_A._ _Restio tetraphylla_ Labill. (Monocotyledon).
_B._ _Equisetum variegatum_ Schleich. ╮ (Vascular Cryptogam).
_C._ _Equisetum debile_ Roxb. ╯
_D._ _Casuarina stricta_ Dryand. (Dicotyledon).
_E._ _Ephedra distachya_ Linn. (Gymnosperm). (_A_–_E_ ½ nat. size).]
[Illustration: FIG. 21. _Polygonum Equisetiforme_ Sibth. and Sm. _A._
Showing habit of plant. ½ nat. size. The two flowers towards the
apex of one branch, drawn to a larger scale in _B_. _C._ Node with
small leaf and ochrea characteristic of _Polygonaceæ_. From a plant
in the Cambridge Botanic Garden.]
[Illustration: FIG. 22. _Kaulfussia æsculifolia_ Blume. From a specimen
from Java in the British Museum herbarium. ⅓ nat. size.]
Endless examples might be quoted illustrating the absolute futility,
in many cases, of relying on external features even for the purpose
of class distinction. An acquaintance with the general habit and
appearance of only the better known members of a family, frequently
leads to serious mistakes. The specimen shown in fig. 22 is a leaf of a
tropical fern _Kaulfussia_, a genus now living in South-eastern Asia,
and a member of one of the most important and interesting families
of the Filicinæ, the Marrattiaceæ; its form is widely different from
that which one is accustomed to associate with fern fronds. It is
unlikely that the impression of a sterile leaf of _Kaulfussia_ would be
recognised as a portion of a fern plant.
Similarly in another exceedingly important group of plants, the
Cycadaceæ[139], the examples usually met with in botanical gardens
are quite insufficient as standards of comparison when we are dealing
with fossil forms. Familiarity with a few commoner types leads us
to regard them as typical for the whole family. In Mesozoic times
cycadean plants were far more numerous and widely distributed than
at the present time, and to adequately study the numerous fossil
examples we need as thorough an acquaintance as possible with the
comparatively small number of surviving genera and species. The less
common and more isolated species of an existing family may often be
of far greater importance to the palaeobotanist than the common and
more typical forms. This importance of rare and little known types
will be more fully illustrated in the chapters dealing with the
Cycadaceæ and other plant groups. Among Dicotyledons, the Natural Order
Proteaceæ, at present characteristic of South Africa and Australia,
and also represented in South America and the Pacific Islands, is
of considerable interest to the student of fossil Angiosperms. In a
valuable address delivered before the Linnean Society[140] in 1870
Bentham drew attention to the marked ‘protean’ character of the members
of this family. He laid special stress on this particular division of
the Dicotyledons in view of certain far-reaching conclusions, which had
been based on the occurrence in different parts of Europe of fossil
leaves supposed to be those of Proteaceous genera[141]. Speaking of
detached leaves, Bentham says:—“I do not know of a single one which, in
outline or venation, is exclusively characteristic of the order, or of
any one of its genera.” Species of _Grevillea_, _Hakea_ and a few other
genera are more or less familiar in plant houses, but the leaf-forms
illustrated by the commoner members of the family convey no idea of the
enormous variation which is met with not only in the family as a whole,
but in the different species of the same genus. The striking diversity
of leaf within the limits of a single genus will be dealt with more
fully in volume II. under the head of Fossil Dicotyledons.
[Sidenote: VENATION CHARACTERS.]
There is a common source of danger in attempting to carry too far the
venation characters as tests of affinity. The parallel venation of
Monocotyledons is by no means a safe guide to follow in all cases as
a distinguishing feature of this class of plants. In addition to such
leaves as those of the Gymnosperm _Cordaites_ and detached pinnæ of
Cycads, there are certain species of Dicotyledons which correspond in
the character of their venation to Monocotyledonous leaves. _Eryngium
montanum_ Coult., _E. Lassauxi_ Dcne., and other species of this
genus of _Umbelliferæ_ agree closely with such a plant as _Pandanus_
or other Monocotyledons; similarly the long linear leaves of _Richea
dracophylla_, R. Br., one of the Ericaceæ, are identical in form
with many monocotyledonous leaves. Instances might also be quoted of
monocotyledonous leaves, such as species of _Smilax_ and others which
Lindley included in his family of Dictyogens which correspond closely
with some types of Dicotyledons[142]. Venation characters must be used
with care even in determining classes or groups, and with still greater
reserve if relied on as family or generic tests.
It is too frequently the case that while we are conversant with the
most detailed histological structure of a fossil plant stem, its
external form is a matter of conjecture. The conditions which have
favoured the petrifaction of plant tissues have as a rule not been
favourable for the preservation of good casts or impressions of the
external features; and, on the other hand, in the best impressions of
fern fronds or other plants, in which the finest veins are clearly
marked, there is no trace of internal structure. It is, however,
frequently the case that a knowledge of the internal structure of
a particular plant enables us to interpret certain features in a
structureless cast which could not be understood without the help of
histological facts. A particularly interesting example of anatomical
knowledge affording a key to apparently abnormal peculiarities in a
specimen preserved by incrustation, is afforded by the fructification
of the genus _Sphenophyllum_. Some few years ago Williamson described
in detail the structure of a fossil strobilus (_i.e._ cone) from the
Coal-Measures, but owing to the isolated occurrence of the specimens
he was unable to determine the plant to which the strobilus belonged.
On re-examining some strobili of _Sphenophyllum_, preserved by
incrustation, in the light of Williamson’s descriptions, Zeiller was
able to explain certain features in his specimens which had hitherto
been a puzzle, and he demonstrated that Williamson’s cone was that of a
_Sphenophyllum_. Similar examples might be quoted, but enough has been
said to emphasize the importance of dealing as far as possible with
both petrifactions and incrustations. The facts derived from a study of
a plant in one form of preservation may enable us to interpret or to
amplify the data afforded by specimens preserved in another form.
[Sidenote: DECORTICATED STEMS.]
The fact that plants usually occur in detached fragments, and that
they have often been sorted by water, and that portions of the same
plant have been embedded in sediment considerable distances apart,
is a constant source of difficulty. Deciduous leaves, cones, or
angiospermous flowers, and other portions of a plant which become
naturally separated from the parent tree, are met with as detached
specimens, and it is comparatively seldom that we have the necessary
data for reuniting the isolated members. As the result of the partial
decay and separation of portions of the same stem or branch, the wood
and bark may be separately preserved. Darwin[143] describes how the
bark often falls from _Eucalyptus_ trees, and hangs in long shreds,
which swing about in the wind, and give to the woods a desolate and
untidy appearance. In the passage already quoted from the narrative
of the voyage of the Challenger, illustrations are afforded of
the manner in which detached portions of plants are likely to be
preserved in a fossil state. The epidermal layer of a leaf or the
surface tissues of a twig may be detached from the underlying tissues
and separately preserved[144]. It is exceedingly common for a stem
to be partially decorticated before preservation, and the appearance
presented by a cast or impression of the surface of a woody cylinder,
and by the same stem with a part or the whole of its cortex intact
is strikingly different. The late Prof. Balfour[145] draws attention
to this source of error in his text-book of palaeobotany, and gives
figures illustrating the different appearance presented by a branch
of _Araucaria imbricata_ Pav. when seen with its bark intact and more
or less decorticated. Specimens that are now recognised as casts of
stems from which the cortex had been more or less completely removed
before preservation, were originally described under distinct generic
names, such as _Bergeria_, _Knorria_ and others. These are now known
to be imperfect examples of Sigillarian or Lepidodendroid plants.
Grand’Eury[146] quotes the bark of _Lepidodendron Veltheimianum_ Presl.
as a fossil which has been described under twenty-eight specific names,
and placed in several genera.
Since the microscopical examination of fossil plant-anatomy was
rendered possible, a more correct interpretation of decorticated and
incomplete specimens has been considerably facilitated. The examination
of tangential sections taken at different levels in the cortex of such
a plant as _Lepidodendron_ brings out the distribution of thin and
thick-walled tissue. Regularly placed prominences on such a stem as the
_Knorria_ shown in fig. 23 are due to the existence in the original
stem of spirally disposed areas of thin-walled and less resistant
tissue; as decay proceeded, the thinner cells would be the first to
disappear, and depressions would thus be formed in the surrounding
thicker walled and stronger tissue. If the stem became embedded in mud
or sand before the more resistant tissue had time to decay, but after
the removal of the thin-walled cells, the surrounding sediment would
fill up the depressions and finally, after the complete decay of the
stem, the impression on the mould or on the cast, formed by the filling
up of the space left by the stem, would have the form of regularly
disposed projections marking the position of the more delicate tissues.
The specimen represented in the figure is an exceedingly interesting
and well preserved example of a Coal-Measure stem combining in itself
representatives of what were formerly spoken of as distinct genera.
[Illustration: FIG. 23. A dichotomously branched Lepidodendroid stem
(_Knorria mirabilis_ Ren. and Zeill.). After Renault and
Zeiller[147]. (¼ nat. size.) The original specimen is in the
Natural History Museum, Paris.]
The surface of the fossil as seen at _e_ affords a typical example of
the Knorria type of stem; the spirally disposed peg-like projections
are the casts of cavities formed by the decay of the delicate cells
surrounding each leaf-trace bundle on its way through the cortex of
the stem. The surface _g_ exhibits a somewhat different appearance,
owing to the fact that we have the cast of the stem taken at a slightly
different level. The surface of the thick layer of coal at a shows
very clearly the outlines of the leaf-cushions; on the somewhat deeper
surfaces _b_, _c_ and _d_ the leaf-cushions are but faintly indicated,
and the long narrow lines on the coal at _c_ represent the leaf-traces
in the immediate neighbourhood of the leaf-cushions.
[Sidenote: IMPERFECT CASTS.]
It is not uncommon among the older plant-bearing rocks to find a
piece of sandstone or shale of which the surface exhibits a somewhat
irregular reticulate pattern, the long and oval meshes having the
form of slightly raised bosses. The size of such a reticulum may
vary from one in which the pattern is barely visible to the unaided
eye to one with meshes more than an inch in length. The generic
name _Lyginodendron_[148] was proposed several years ago (1843) for
a specimen having such a pattern on its surface, but without any
clue having been found as to the meaning of the elongated raised
areas separated from one another by a narrow groove. At a later date
Williamson investigated the anatomy of some petrified fragments
of a Carboniferous plant which suggested a possible explanation
of the surface features in the structureless specimens. The name
_Lyginodendron_ was applied to this newly discovered plant, of which
one characteristic was found to be the occurrence of a hypodermal band
of strong thick-walled tissue arranged in the form of a network with
the meshes occupied by thin-walled parenchyma. If such a stem were
undergoing gradual decay, the more delicate tissue of the meshes would
be destroyed first and the harder framework left. A cast of such a
partially decayed stem would take the form, therefore, of projecting
areas, corresponding to the hollowed out areas of decayed tissue, and
intervening depressions corresponding to the projecting framework of
the more resistant fibrous tissue. A precisely similar arrangement of
hypodermal strengthening tissue occurs in various Palaeozoic and other
plants, and casts presenting a corresponding appearance cannot be
referred with certainty to one special genus; such casts are of no real
scientific value[149].
The old generic terms _Artisia_ and _Sternbergia_ illustrate another
source of error which can be avoided only by means of a knowledge of
internal structure. The former name was proposed by Sternberg and the
latter by Artis for precisely similar Carboniferous fossils, having the
form of cylindrical bodies marked by numerous transverse annular ridges
and grooves. These fossils are now known to be casts of the large
discoid pith of the genus _Cordaites_, an extinct type of Palaeozoic
Gymnosperms. _Calamites_ and _Tylodendron_ afford other instances of
plants in which the supposed surface characters have been shown to be
those of the pith-cast. The former genus is described at length in
a later chapter, but the latter may be briefly referred to. A cast,
apparently of a stem, from the Permian rocks of Russia was figured in
1870 under the name _Tylodendron_; the surface being characterised by
spirally arranged lozenge-shaped projections, described as leaf-scars.
Specimens were eventually discovered in which the supposed stem was
shown to be a cast of the large pith of a plant possessing secondary
wood very like that of the recent genus _Araucaria_. The projecting
portions, instead of being leaf-cushions, were found to be the casts
of depressions in the inner face of the wood where strands of vascular
tissue bent outwards on their way to the leaves. If a cast is made of
the comparatively large pith of _Araucaria imbricata_ the features of
_Tylodendron_ are fairly closely reproduced[150].
A dried Bracken frond lying on the ground in the Autumn presents a very
different appearance as regards the form of the ultimate segments of
the frond to that of a freshly cut leaf. In the former the edges of
the pinnules are strongly recurved, and their shape is considerably
altered. Immersed in water for some time fern fronds or other leaves
undergo maceration, and the more delicate lamina of the leaf rots away
much more rapidly than the scaffolding of veins. Among fossil fern
fronds differences in the form of the pinnules and in the shape and
extent of the lamina, to which a specific value is assigned, are no
doubt in many cases merely the expression either of differences in the
state of the leaves at the time of fossilisation or of the different
conditions under which they became embedded. Differential decay and
disorganisation of plant tissues are factors of considerable importance
with regard to the fossilisation of plants. As Lindley[151] and later
writers have suggested, the absence or comparative scarcity of certain
forms of plants from a particular fossil flora may in some cases be
due to their rapid decay and non-preservation as fossils; it does not
necessarily mean that such plants were unrepresented in the vegetation
of that period. The decayed rhizomes of the Bracken fern often seen
hanging from the roadside banks on a heath or moorland, and consisting
of flat dark coloured bands of resistant sclerenchyma in a loose sheath
of the hard shrivelled tissue, are in striking contrast to the perfect
stem. A rotting Palm stem is gradually reduced to a loose stringy mass
consisting of vascular strands of which the connecting parenchymatous
tissue has been entirely removed. It must frequently have happened that
detached vascular bundles or strands and plates of hard strengthening
tissue have been preserved as fossils and mistaken for complete
portions of plants.
[Sidenote: MINERAL DEPOSITS SIMULATING PLANTS.]
Apart from the necessity of keeping in view the possible differences
in form due to the state of the plant fragments at the time of
preservation, and the marked contrast between the same species
preserved in different kinds of rock, there are numerous sources of
error which belong to an entirely different category. The so-called
moss-agates and the well-known dendritic markings of black oxide of
manganese, are among the better known instances of purely inorganic
structures simulating plant forms.
An interesting example of this striking similarity between a purely
mineral deposit and the external form of a plant is afforded by some
specimens originally described as impressions of the oldest known fern.
The frontispiece to a well-known work on fossil plants, _Le monde des
plantes avant l’apparition de l’homme_[152], represents a fern-like
fossil on the surface of a piece of Silurian slate. The supposed plant
was named _Eopteris Morierei_ Sap., and it is occasionally referred
to as the oldest land plant in books of comparatively recent date. In
the Museum of the School of Mines, Berlin, there are some specimens
of Angers slate on some of which the cleavage face shows a shallow
longitudinal groove bearing on either side somewhat irregularly
oblong and oval appendages of which the surface is traversed by fine
vein-like markings. A careful examination of the slate reveals the fact
that these apparent fern pinnules are merely films of iron pyrites
deposited from a solution which was introduced along the rachis-like
channel. Many of the extraordinary structures described as plants by
Reinsch[153] in his Memoir on the minute structure of coal have been
shown to be of purely mineral origin.
The innumerable casts of animal-burrows and trails as well as the casts
of egg-cases and various other bodies, which have been described as
fossil algae, must be included among the most fruitful sources of error.
It requires but a short experience of microscopical investigation
of fossil plant structures to discover numerous pitfalls in the
appearance presented by sections of calcareous and siliceous nodules.
The juxtaposition of tissues apparently parts of the same plant, and
the penetration by growing roots of partially decayed plant débris,
serve to mislead an unpractised observer. In sections of the English
‘calcareous nodules’ one very frequently finds the tissue of Stigmarian
appendages occupying every conceivable position, and preserved in
places admirably calculated to lead to false interpretations. The
more minute investigation of tissues is often rendered difficult by
deceptive appearances simulating original structures, but which are
in reality the result of mineralisation. It is no easy matter in
some cases to discover whether a particular cell in a fossil tissue
was originally thick-walled, or whether its sclerous appearance is
due to the deposition of mineral matter on the inside of the thin
cell-membrane. Examples of such sources of error as have been briefly
referred to, and others, will be found in various parts of the
descriptive portions of this book.
[Illustration: FIG. 24. _A._ Section of partially disorganised tissue
attacked by some boring animal. _c_, _c_, coprolites; _d_, a tunnel
made by the borer through the plant tissue.
_B._ Transverse section of a Lepidodendroid leaf, of which the
inner tissues have been destroyed and the cavity filled with
coprolites; simulating a sporangium containing spores. (A and B
from specimens in the Botanical Laboratory collection, Cambridge.)]
[Sidenote: TRACES OF WOOD-BORERS IN PETRIFIED TISSUE.]
There is one other form of pitfall which should be briefly noticed.
In sections of petrified plants one occasionally finds clean cut
canals penetrating a mass of tissue, and differing in their manner of
occurrence and in their somewhat larger size from ordinary secretory
ducts. Such tunnels or canals are probably the work of a wood-boring
animal. An example is illustrated in fig. 24 _A_. Similarly it is not
unusual to meet with groups or nests of spherical or elliptical bodies
lying among plant tissues, and having the appearance of spores. Such
spore-like bodies appear on close examination to be made up of finely
comminuted particles of tissue, and in all probability they are the
coprolites of some xylophagous animal. Examples of such coprolites are
shown in fig. 24 _A_[154], and in fig. 24 _B_ an interesting manner of
occurrence of these misleading bodies is represented. The framework
of cells enclosing the nest of coprolites in fig. 24 _B_, represents
the outer tissues of a Lepidodendroid or a Sigillarian leaf; the inner
tissues have been destroyed and the cavity is now occupied by what may
possibly be the excreta of the wood-eating animal.
Some of the oval spore-like structures met with in plant tissues
may, as Renault has suggested, be the eggs of an Arthropod[155].
In a section of a calcareous Coal-Measure nodule in the Williamson
collection (British Museum)[156] there occur several fungal spores
or possibly oogonia lying among imperfectly preserved Stigmarian
appendages. Associated with these are numerous dark coloured and larger
bodies consisting of a cavity bounded by a simple membrane; the larger
bodies may well be the eggs of some Arthropod or other animal.
[Sidenote: PHOTOGRAPHY AND ILLUSTRATION.]
In looking through the collections of Coal-Measure plants in the
Museums of Berlin, Vienna and other continental towns, one cannot fail
to be struck with the larger size of many of the specimens as compared
with those usually seen in English Museums. The facilities afforded
in the State Collieries of Germany to the scientific investigator may
account in part at least for the better specimens which he is able
to obtain. It would no doubt be a great gain to our collections of
Coal-Measure plants if arrangements could be made in some collieries
for the preservation of the finer specimens met with in the working of
the seams, instead of breaking up the slabs of shale and consigning
everything to the waste heaps. There is one more point which should be
alluded to in connection with possible sources of error, and that is
the essential importance of accuracy in the illustration of specimens,
especially as regard type-specimens. It is often impossible to inspect
the original fossils which have served as types, and it is of the
utmost importance that the published figures should be as faithful
as possible. M. Crépin[157] of Brussels, in an article on the use of
photography in illustrating, has given some examples of the confusion
and mistakes caused by imperfect drawings. It does not require a long
experience of palaeobotanical work to demonstrate the need of care in
the execution of drawings for reproduction.
CHAPTER VI.
NOMENCLATURE.
“I do not think more credit is due to a man for defining a species,
than to a carpenter for making a box.”
CHARLES DARWIN, _Life and Letters_, Vol. I., p. 371.
Any attempt to discuss at length the difficult and thorny question
of nomenclature would be entirely out of place in an elementary book
on fossil plants, but there are certain important points to which it
may be well to draw attention. When a student enters the field of
independent research, he is usually but imperfectly acquainted with the
principles of nomenclature which should be followed in palaeontological
work. After losing himself in a maze of endless synonyms and confused
terminology, he recognises the desirability of adopting some definite
and consistent plan in his method of naming genera and species. It is
extremely probable that whatever system is made use of, it will be
called in question by some critics as not being in strict conformity
with accepted rules. The opportunities for criticism in matters
relating to nomenclature are particularly numerous, and the critic who
may be but imperfectly familiar with the subject-matter of a scientific
work is not slow to avail himself of some supposed eccentricity on the
part of the author in the manner of terminology. The true value of work
may be obscured by laying too much emphasis on the imperfections of a
somewhat heterodox nomenclature. On the other hand good systematic work
is often seriously spoilt by a want of attention to generally accepted
rules in naming and defining species. It is essential that those who
take up systematic research should pay attention to the necessary
though secondary question of technical description.
[Sidenote: RULES FOR NOMENCLATURE.]
In inventing a new generic or specific name, it is well to adhere to
some definite plan as regards the form or termination of the words
used. To deal with this subject in detail, or to recapitulate a
series of rules as to the best method of constructing names whether
descriptive or personal, would take us beyond the limits of a single
chapter. The student should refer for guidance to such recognised rules
as those drawn up by the late Mr Strickland and others at the instance
of the British Association[158].
It is not infrequently the case that the same generic name has been
applied to a fossil and to a recent species. Such a double use of the
same term should always be avoided as likely to lead to confusion, and
as tending to admit a divorce between botany and palaeobotany.
In the course of describing a collection of fossil species, various
problems are bound to present themselves as regards the best method
of dealing with certain generic or specific names. A few general
suggestions may prove of use to those who are likely to be confronted
with the intricacies of scientific and pseudoscientific terminology.
In writing the name of a species, it is important to append the name,
often in an abbreviated form, of the author who first proposed the
accepted specific designation. _Stigmaria ficoides_ Brongn. written
in this form records the fact that Brongniart was the author of the
specific name _ficoides_. It means, moreover, that Brongniart not
only suggested the name, but that he was the first to give either a
figure or a diagnosis of this particular fossil. It is frequently
the case that a specific name is proposed for a new species, without
either figures or description; such a name is usually regarded as a
_nomen nudum_, and must yield priority to the name which was first
accompanied by some description or illustration sufficiently accurate
to afford a means of recognition. A practice which may be recommended
on the score of convenience is to write the name of the author of a
species in brackets if he was not the first to use the generic as well
as the specific name. _Onychiopsis Mantelli_ (Brongn.) tells us that
Brongniart founded the species, but made use of some other generic
name than that which is now accepted. This leads us to another point
of some importance. Brongniart described this characteristic Wealden
fern under the name _Sphenopteris Mantelli_; _Sphenopteris_ being one
of those extremely useful provisional generic terms which are used
in cases where we have no satisfactory proof of precise botanical
affinity. _Sphenopteris_ stands for fern fronds having a certain habit,
form of segment and venation, and in this wide sense it necessarily
includes representatives of various divisions and genera of Filices. If
an example of a sphenopteroid frond is discovered with sori or spores
sufficiently well preserved to enable us to determine its botanical
position within narrower limits, we may with advantage employ another
genus in place of the purely artificial form-genus which was originally
chosen as a consequence of imperfect knowledge. Fronds of this Wealden
fern have recently been found with well defined fertile segments
having a form apparently identical with that which characterises the
polypodiaceous genus _Onychium_. For this reason the name _Onychiopsis_
has been adopted. It is safer and more convenient to use a name which
differs in its termination from that of the recent plant with which
we believe the fossil to be closely related. A common custom is to
slightly alter the recent name by adding the termination _-opsis_ or
_-ites_. There are several other provisional generic terms that are
often used in Fossil Botany, and which might be advantageously chosen
in many cases where the misleading resemblance of external form has
often given rise to the use of a name implying affinities which cannot
be satisfactorily demonstrated.
It was the custom of some of the earliest writers, in spite of their
habit of using the names of recent Flowering plants for extinct
Palaeozoic species of Vascular Cryptogams, to adopt also general and
comprehensive terms. We find such a name as _Lithoxylon_ employed by
Lhwyd[159] in 1699 as a convenient designation for fossil wood.
[Sidenote: THE RULE OF PRIORITY.]
One of the most important and frequently disputed questions associated
with the naming of species is that of priority. No name given to a
plant in pre-Linnaean days need be considered, as our present system
of nomenclature dates from the institution of the binominal system
by Linnaeus. As a general rule, which it is advisable to follow, the
specific name which was first given to a plant, if accompanied by a
figure or diagnosis, should take priority over a name of later date.
If _A_ in 1850 describes a species under a certain name, and in 1860
_B_ proposes a new name for the same species, either in ignorance
of the older name or from disapproval of _A_’s choice of a specific
term, the later name should not be allowed to supersede _A_’s original
designation. Such a rule is not only just to the original author, but
is one which, if generally observed, would lead to less confusion and
would diminish unnecessary multiplication of specific names. Some
writers would have us conform in all cases to this rule of priority,
which they consistently adhere to apart from all considerations of
convenience or long-established custom. There are, however, cogent
reasons for maintaining a certain amount of freedom. While accepting
priority as a good rule in most cases, it is unwise to allow ourselves
to be too servile in our conformity to a principle which was framed in
the interests of convenience, if the strict application of the rule
clearly makes for confusion and inconvenience. A name may have been
in use for say eighty years, and has become perfectly familiar as
the recognised designation of a particular fossil; it is discovered,
however, that an older name was proposed for the same species ninety
years ago, and therefore according to the priority rule, we must
accustom ourselves to a new name in place of one which is thoroughly
established by long usage. From a scientific point of view, the ideal
of nomenclature is to be plain and intelligible. To prefer priority to
established usage entails obscurity and confusion. If priority is to
be the rule which we must invariably obey in the shadowy hope that by
such means finality in nomenclature[160] may be reached, it becomes
necessary for the student to devote no inconsiderable portion of
his time to antiquarian research, with a view to discover whether a
particular name may be stamped with the hallmark of ‘the very first.’
While admitting the advisability of retaining as a general principle
the original generic or specific name, the extreme subservience to
‘the priority craze’ without regard to convenience, would seem to lead
irresistibly to the view that “botanists who waste their time over
priority are like boys who, when sent on an errand, spend their time in
playing by the roadside[161].”
[Sidenote: TERMINOLOGY AND CONVENIENCE.]
There is another point which cannot be satisfactorily settled in all
cases by a rigid adherence to an arbitrary rule. How far should we
regard a generic name in the sense of a mere mark or sign to denote a
particular plant, or to what extent may we accept the literal meaning
of the generic term as an index of the affinity or character of the
plant? If we consider the etymology of many generic names, we soon find
that they are entirely inappropriate as aids in recognizing the true
taxonomic position of the plants to which they are applied. The generic
name _Calamites_ was first suggested by the supposed resemblance of
this Palaeozoic plant to recent reeds. If considered etymologically,
it is merely a record of a past mistake, but it would be absurd to
discard such a well-known name on the grounds that the genus is a
Vascular Cryptogam and far removed from reeds. On the other hand,
there often arise cases which present a real difficulty. The following
example conveniently illustrates two distinct points of view as regards
generic nomenclature. In 1875 Saporta described and figured a fragment
of a fossil plant from the Jurassic beds of France as _Cycadorachis
armata_[162]; the name being chosen in the belief that the specimen
was part of a cycadean petiole, and there were good grounds for such
a view. A few years ago Mr Rufford discovered more perfect specimens,
in the Wealden rocks of Sussex, clearly belonging to Saporta’s genus,
and these afforded definite evidence that Saporta had been deceived by
the imperfection of the specimens as to their true botanical position.
Owing to the obviously misleading name first given to this plant, I
ventured to substitute _Withamia_[163] for _Cycadorachis_, and chose
such a term in preference to one denoting affinity, on account of the
difficulty of placing the plant in a definite class or family. On
the other hand, it has been objected that the original name, despite
its meaningless meaning—if the expression may be used—should be
retained. A friendly critic[164], in writing of the proposed change
of _Cycadorachis_, urges the importance of adhering to the name which
was first applied to a genus. The same author pertinently remarks that
we can no more dispense with a nomenclature than we can dispense with
language. We may extend the comparison and point out that in language,
as in scientific nomenclature, conciseness, clearness and convenience
should be kept in view as guiding principles.
The student must judge for himself what course to follow in each
case. While adhering as far as possible to a consistent plan, he must
take care that he does not allow his own judgment to be completely
over-ridden by a blind obedience to fixed rules, which if pressed too
far may defeat their own ends.
=PART II. SYSTEMATIC.=
CHAPTER VII.
THALLOPHYTA.
The divisions of the plant kingdom dealt with in the following chapters
of Volume I. are taken in their natural sequence, beginning with the
lowest and passing gradually to the highest groups. The list of the
classes and families included in Chapters VII.–XI. is given in the
table of contents preceding Chapter I.
Thallophytes are of the simplest type, but they exhibit a very wide
range as regards both the structure and differentiation of the
vegetative body and the methods of reproduction. In some cases the
individual consists of a minute simple cell which multiplies by
cell-division; in others the body or thallus is made up of a number of
similar units, while in a great number of forms there is a well-marked
physiological division of labour, as expressed both in the external
division of the thallus into distinct organs corresponding in function
to the root, stem, and leaves of the higher plants, and further in the
high degree of histological differentiation of the tissues. In other
thallophytes, again, the thallus is a _coenocyte_ either unseptate or
incompletely septate; that is, the individual consists of a single cell
differing from a true plant-cell, in the stricter sense of the term, in
possessing several nuclei, in other words, the thallus is divided up
into compartments by transverse septa, but each division contains more
than one nucleus. Such coenocytic plants may show well-marked external
differentiation of the thallus into members or parts subserving
different functions.
A similar wide range is covered by the methods of reproduction among
thallophytes.
I. PERIDINIALES.
The organisms included under this head are of little importance from a
palaeontological point of view, but a brief reference may be made to
them as a section of the Thallophyta.
The Peridiniales include very small single-celled organisms which have
often been described as occupying a position on the borderland between
animals and plants, lying on the “shadowy boundary between animal and
vegetable life.” The individuals are rarely naked, more frequently
they are covered with a cellulose or mucilaginous investment which
has frequently the form of two or more minute armour-like plates of
a limiting membrane. The chromatophores are green, yellow, brown or
colourless. Simple division is the usual method of reproduction, but
spores have been described as occurring in some species. The motile
forms are provided with cilia. The Peridiniaceae, a section of the
Peridiniales, are regarded as nearly related to the Diatoms.
The Peridiniales play an important rôle in the Plankton flora of the
sea and freshwater lakes, and have a world-wide distribution. In the
narrative of the Challenger cruise they are described as occasionally
filling the tow-nets with a yellow coloured slime[165]. Some genera,
such as _Ceratium_, are found in enormous numbers off the British coast.
As an example of the occurrence of fossil representatives of the
Peridiniaceae reference may be made to one of two species of
_Peridinium_ described by Ehrenberg in 1836. These were found in a
siliceous rock described as Cretaceous in age from Delitzsch in Saxony.
A comparison of Ehrenberg’s figures of the fossil species _Peridinium
pyrophorum_ Ehrenb.[166], with those of the recent species _Peridinium
divergens_ Ehrenb., as given by Schütt[167] and other writers, brings
out clearly the very close resemblance if not identity of the two
forms. Bütschli[168] in his account of the Dinoflagellata in Bronn’s
_Thier-Reich_ confirms Ehrenberg’s determination of _Peridinium
pyrophorum_, and points out its striking agreement with the recent
species.
II. COCCOSPHERES AND RHABDOSPHERES.
(Organisms of doubtful affinity.)
Our knowledge of these minute calcareous organisms is derived from
Huxley’s description of coccoliths from the Atlantic in 1857, and from
the accounts of Wallich, John Murray, and other writers. In the first
volume of the narrative of the Challenger cruise[169] and in the volume
on deep-sea deposits[170] these minute forms of life are figured and
described. In the latter volume both genera are spoken of as extremely
abundant in the surface waters of the tropical and temperate regions
of the open ocean, and as forming an important constituent of the
Globigerine ooze; they are said to occur entangled in the gelatinous
substance of the Radiolarians, Diatoms, and Foraminifera, and are very
common in the stomachs of Salps, Pteropods and other pelagic animals.
Rhabdospheres are rare in regions where the temperature of the water
sinks below 65° F.; the Coccospheres occur in tropical and temperate
latitudes, and extend further north and south than the Rhabdospheres.
As regards their botanical position, John Murray expresses the view
that they are in all probability pelagic algae.
In the interesting memoir by Schütt on the _Pflanzenleben der
Hochsee_[171] there occurs a short reference to the forms described
in the Challenger Reports, but they were not obtained by the staff
of the Hensen Plankton Expedition and Schütt’s remarks are not based
therefore on personal observations. While admitting the existence of
such bodies, he points out that Zoologists have referred Coccospheres
and Rhabdospheres to the algae as organisms which cannot be included
in any group of animals, and Schütt is unable to recognise a sufficient
reason for referring them to this class of plants. It is suggested
indeed that they may be purely inorganic structures.
[Illustration: FIG. 25. (From Murray and Blackman).
_A_, Coccosphere × 1300. _B_, Rhabdosphere × 900. _C_, Portion of
the same × 1300. _D_, Rhabdosphere of another type, in optical
section × 1900. _E_, The same in surface view × 1900. _F_, End of
one of the trumpet-shaped appendages of _E_.]
The most recent account of these two genera is by Messrs G. Murray and
Blackman a short notice in _Nature_ for April 1, 1897[172]. Numerous
examples of Coccospheres and Rhabdospheres were obtained by Capt.
Milner of the R.M.S. Para during a voyage to Barbados by allowing
the sea water to enter the feed-pipe of the boiler through a fine
muslin net. All the forms described in the Challenger Reports were met
with, and an examination of the material by means of extremely high
objectives has confirmed the original account of the genera, and added
some points to our previous knowledge.
_Coccospheres_ (fig. 25 _A_). Spherical bodies of exceedingly small
size, consisting of a central protoplasmic vesicle covered with
overlapping circular calcareous scales, each of which is attached to
the minute cell by a button-like projection. The scales are frequently
found detached and are then spoken of as Coccoliths.
_Rhabdospheres_ (fig. 25 _B_–_F_). Spherical bodies, extremely minute,
consisting of a single cell, on the surface of which are embedded
numerous calcareous plates bearing long blunt spines (fig. 25, _C_)
or beautiful trumpet-like appendages (fig. 25, _D_–_F_). The detached
plates of Rhabdospheres are known as Rhabdoliths.
In addition to the text-figures of Coccospheres and Rhabdospheres in
the Challenger Reports, the same structures are shown in samples of
globigerine ooze figured in Plate XI. of the Monograph on deep-sea
deposits. In a recent number of _Nature_ Messrs Dixon and Joly[173]
have announced the discovery of Coccoliths and Coccospheres in the
coastal waters off South County Dublin. They estimate that in one
sample of water taken about three miles from the Irish coast there were
200 Coccoliths in each cubic centimetre of sea water.
The interest of these calcareous bodies from a palaeobotanical point of
view lies in the fact that similar forms have been recognized in the
Chalk and the Upper Lias. Sorby, in his memorable Address delivered
before the Geological Society in 1879, refers to the abundance of
Coccoliths in sections of chalk which he examined[174]. Rothpletz[175]
has recently recorded the occurrence of numerous Coccoliths, 5–12
µ in diameter, associated with the skeleton of a horny sponge
(_Phymatoderma_) of Liassic age.
The question of the nature of Coccospheres and Rhabdospheres cannot be
regarded as definitely settled. It has been shown by J. Murray, and
more recently by G. Murray and V. H. Blackman, that on the solution of
the calcareous material by a weak acid there remains a small gelatinous
body apparently protoplasmic in nature. We may at least express the
opinion that Schütt’s suggestion as to their being inorganic must be
ruled out of court. It would appear that they are extremely minute
unicellular organisms characterised by a delicate calcareous armour
consisting of numerous plates or scales. We know nothing as to their
life-history, and cannot attempt to determine their affinities with
any degree of certainty until further facts are before us. It is not
improbable that they are algae of an extremely minute size, and the
evidence so far obtained would lead us to regard them as complete
individuals rather than the reproductive cells of some larger organism.
Mr George Murray is of opinion that they are certainly algae, but he
considers that they cannot be included in any existing family. It is
conceivable that they may be minute eggs or reproductive cells of
animals or plants, but on the whole the balance of probability would
seem to be in favour of regarding them as autonomous organisms.
III. SCHIZOPHYTA.
A. SCHIZOPHYCEAE (CYANOPHYCEAE).
B. SCHIZOMYCETES (Bacteria).
In this group are included small single-celled plants of an extremely
low type of organisation, in which reproduction takes the form of
multiplication by simple cell-division, or the formation of spores.
The characteristic method of reproduction by division has given rise
to the general term Fission-plants for this lowest sub-class in the
vegetable kingdom. In many cases the members of this sub-class contain
chlorophyll, and associated with it a blue-green colouring matter; such
plants are classed together as the Blue-green algae, Cyanophyceae,
or Schizophyceae. Others, again, are destitute of chlorophyll, and
may be conveniently designated Schizomycetes or Fission-fungi. Seeing
how close is the resemblance and relationship between the members of
the sub-class, it has been the custom to include them as two parallel
series under the general head, Schizophyta, rather than to incorporate
them among the Algae and Fungi respectively.
A. SCHIZOPHYCEAE (CYANOPHYCEAE or Blue-green Algae).
_Chroococcaceae._ Thallus of a single cell, the cells may be either
free, or more usually joined together in colonies enveloped by a common
gelatinous matrix, formed by the mucilaginous degeneration of the outer
portion of the cell-walls. Reproduction by means of simple division or
resting cells.
_Nostocaceae._ Thallus consists of simple or branched rows of cells in
which special cells known as _heterocysts_ often occur. Reproduction
by means of germ-plants or _hormogonia_, or by resting cells specially
modified to resist unfavourable conditions.
In both families the individuals are surrounded by a gelatinous
envelope, which in some genera assumes the form of a conspicuous
and comparatively resistant sheath. Marine, freshwater, and aerial
forms are represented among recent genera. Several species occur as
endophytes, living in the tissues or mucilage-containing spaces in the
bodies of higher plants. In addition to the frequent occurrence of
blue-green algae in freshwater streams and on damp surfaces, certain
forms are particularly abundant in the open sea[176], and in lakes or
meres[177] where they are the cause of what is known in some parts
of the country as “the breaking of the meres” (“Fleurs d’eau”). From
the narrative of the cruise of the Challenger, we learn that the
Oscillariaceae are especially abundant in the surface waters of the
ocean. The “sea sawdust” so named by Cook’s sailors[178], and the same
floating scum collected by Darwin[179], affords an illustration of the
abundance of some of these blue-green algae in the sea.
Another manner of occurrence of these plants has been recorded by
different writers, which is of special importance from the point of
view of fossil algae. On the shores of the Great Salt Lake, Utah,
there are found numerous small oolitic calcareous bodies thrown up
by the waves[180]. These are coated with the cells of _Glœocapsa_
and _Glœotheca_, two genera of the Chroococcaceae. Sections of the
grains reveal the presence of the same forms in the interior of the
calcareous matrix, and it has been concluded, on good evidence that the
algae are responsible for the deposition of the carbonate of lime of
the oolitic grains. By extracting the carbonic acid which they require
as a source of food, from the waters of the lake, the solvent power of
the water is decreased and carbonate of lime is thrown down. In similar
white grains from the Red Sea[181] there is a central nucleus in the
form of a grain of sand, and cells of Chroococcaceae occur in the
surrounding carbonate of lime as in the Salt Lake oolite. Prof. Cohn of
Breslau in 1862 demonstrated the importance of low forms of plant life
in the deposition of the Carlsbad “Sprudelstein[182].” On the bottom of
Lough Belvedere, near Mullingar in Ireland[183], there occur numerous
spherical calcareous pebbles, of all sizes up to that of a filbert.
From a pond in Michigan (U.S.A.)[184] similar bodies have been obtained
varying in diameter from one to three and a half inches. In the former
pebbles a species of _Schizothrix_, one of the Nostocaceae occurs
in abundance, in the form of chains of small cells enclosed in the
characteristic and comparatively hard tubular sheath, and associated
with _Schizothrix fasciculata_ there have been found _Nostoc_ cells and
the siliceous frustules of Diatoms. In the Michigan nodules the same
_Schizothrix_ occurs, associated with _Stigonema_ and _Dichothrix_,
other genera of the Nostocaceae. One of the Michigan pebbles is shown
in section in fig. 32 _D_.
[Sidenote: OOLITIC STRUCTURE.]
The connection between the well-known oolitic structure, characteristic
of rocks of various ages in all parts of the world, and the presence
of algal cells is of the greatest interest from a geological point
of view. In recent years considerable attention has been paid to
the structure of oolitic rocks, and in many instances there have
been found in the calcareous grains tubular structures suggestive of
simple cylindrical plants, which have probably been concerned in the
deposition of the carbonate of lime of which the granules consist. In
1880 Messrs Nicholson and Etheridge[185] recorded the occurrence of
such a tubular structure in calcareous nodules obtained from a rock
of Ordovician age in the Girvan district of Scotland. These Authors
considered the tubes to be those of some Rhizopod, and proposed to
designate the fossil _Girvanella_.
_Girvanella_ (fig. 26).
Messrs Nicholson and Etheridge defined the genus as follows:—
“Microscopic tubuli, with arenaceous or calcareous (?) walls,
flexuous or contorted, circular in section, forming loosely
compacted masses. The tubes, apparently simple cylinders, without
perforations in their sides, and destitute of internal partitions
or other structures of a similar kind.”
[Illustration: FIG. 26. _Girvanella problematica_, Eth. and Nich.
Tubules of _Girvanella_ lying in various positions and surrounding
an inorganic ‘nucleus’ or centre. From a section of Wenlock
limestone, May Hill. × 65]
Since this diagnosis was published very many examples of similar
tubular fossils have been described by several writers in rocks from
widely separated geological horizons. The accompanying sketch (Fig.
26), drawn from a micro-photograph kindly lent to me by Mr Wethered
of Cheltenham, who has made oolitic grains a special subject of
careful investigation, affords a good example of the occurrence of
such tubular structures in an oolitic grain of Silurian age from
the Wenlock limestone of May Hill, Gloucestershire[186]. In the
centre is a crystalline core or nucleus round which the tubules have
grown, and presumably they had an important share in the deposition
of the calcareous substance. The nature of _Girvanella_, and still
more its exact position in the organic world, is quite uncertain;
it is mentioned rather as _à propos_ of the association of recent
Cyanophyceae with oolitic structure, than as a well-defined genus of
fossil algae.
In the typo description of the calcareous nodules from Michigan, Murray
speaks of the _Schizothrix_ filaments at the surface of the pebbles
as fairly intact, while nearer the centre only sheaths were met with.
It is conceivable that in some of the tubular structures referred to
_Girvanella_ we have the mineralised sheaths of a fossil Cyanophyceous
genus[187]. The organic nature of these tubules has been a matter
of dispute, but we may probably assume with safety that in some at
least of the fossil oolitic grains there are distinct traces of some
simple organism which was in all likelihood a plant. Some authors
have suggested that _Girvanella_ is a calcareous alga which should be
included in the family Siphoneae[188]. As a matter of fact we must
be content for the present to leave its precise nature as still _sub
judice_, and while regarding it as probably an alga, we may venture
to consider it more fittingly discussed under the Schizophyta than
elsewhere.
Wethered[189] would go so far as to refer oolitic structure in general
to an organic origin. While admitting that a Girvanella-like structure
has been very frequently met with in oolitic rocks, it would be unwise
to adopt so far-reaching a conclusion. It is at least premature to
refer the formation of all oolitic structure to algal agency, and the
evidence adduced is by no means convincing in every case. The discovery
of _Girvanella_ and allied forms in rocks from the Cambrian[190],
Ordovician, Silurian, Carboniferous, Jurassic and other systems is a
striking fact, and lends support to the view that oolitic structure
is in many cases intimately associated with the presence of a simple
tubular organism. Among recent algae we find different genera, and
representatives of different families, growing in such a manner and
under such circumstances as are favourable to the formation of a
ball-like mass of algal threads, which may or may not be encrusted
with carbonate of lime. Similarly as regards oolitic grains of various
sizes, and the occurrence in rocks of calcareous nodules, the tubular
structure is not always of precisely the same type, and cannot always
be included under the genus _Girvanella_.
Several observers have recorded the occurrence of low forms of
plant-life in the waters of thermal springs. It has been already
mentioned that Cohn described the occurrence of simple plants in the
warm Carlsbad Springs, and fission-plants of various types have been
discovered in the thermal waters of Iceland, the Azores[191], New
Zealand, the Yellowstone Park, Japan, India, and numerous other places.
A few years ago Mr Weed, of the geological survey of the United States,
published an interesting account of the formation of calcareous
travertine and siliceous sinter in the Yellowstone Park district[192].
This author emphasizes the important rôle of certain forms of plants
in the building up of the calcareous and siliceous material. Among
other forms of frequent occurrence, _Calothrix gypsophila_ and a
species _Leptothrix_ are mentioned, the former being a member of
the Nostocaceae, allied to _Rivularia_, and the latter a genus of
Schizomycetes. In many of the springs there are found masses of algal
jelly like those previously described by Cohn in the Carlsbad waters.
Sections of such dried jelly showed a number of interlaced filaments
with glassy silica between them. Weed refers to the occurrence of small
gritty particles in this mucilaginous material. These are calcareous
oolitic granules which are eventually cemented together into a compact
and firm mass of travertine by the continued deposition of carbonate
of lime. The presence of the plant filaments is often difficult to
recognise in the “leathery sheet of tough gelatinous material,” or in
“the skeins of delicate white filaments” which make up the travertine
deposits.
[Sidenote: BORINGS IN SHELLS.]
Under the head of _Cyanophyceae_, mention should be made of the recent
genus _Hyella_[193], which occurs as a perforating or boring alga in
the calcareous shells of molluscs. On dissolving the carbonate of
lime of shells perforated by this alga, the latter is isolated and
appears to consist of rows of small cells, with possibly some sporangia
containing spores. Other boring algae have been recorded among the
Chlorophyceae, and recently a member of the Rhodophyceae[194] has been
found living in the substance of calcareous shells. Such examples are
worthy of note in view of the not infrequent occurrence of fossil
corals, shells and fish-scales, which have evidently been bored by an
organism resembling in form and manner of occurrence these recent algal
borers.
The occurrence of small ramifying tubes in recent and fossil corals,
fish-scales, and bones was long ago pointed out by Quekett[195],
Kölliker[196], Rose[197] and other writers[198]. These narrow tubular
cavities have generally been attributed to the boring action of
some parasitic organism, either a fungus or an alga. In 1876 Duncan
published two important papers[199] dealing with the occurrence of
such tubes in recent corals, as well as in the calcareous skeleton
of _Calceolina_, _Goniophyllum_ and other Palaeozoic, Mesozoic and
Tertiary species of corals. This writer attributed the formation of the
cavities in the case of the fossil species to the action of a fungus
which he named _Palaeachlya perforans_, and considered as very nearly
related to _Achlya penetrans_ found in the “dense sclerenchyma” of
recent corals. In fig. 27 A. is reproduced one of the drawings given
by Rose[200] in his paper published in 1855; it shows a section of a
fish-scale from the Kimeridge clay which has been attacked by a boring
organism. Rose attributes the dichotomously branched canals to some
“infusorial parasite.”
[Illustration: FIG. 27. A, Section of a fish-scale from the Kimeridge
Clay, showing branched canals, made by a boring organism, × 85.
B, Section of a Solen shell, penetrated in all directions by the
boring thallus of _Ostracoblabe_ (a fungus?), × 330. C, Piece of
the thallus of _Ostracoblabe_ isolated by decalcification, × 745.
A, after Rose. B and C, after Bornet and Flahault.]
In the important paper by MM. Bornet and Flahault on perforating algae
a full description is given of various boring forms belonging to the
Chlorophyceae and the Cyanophyceae[201]. The canals which these algae
produce in calcareous shells and other hard substances are of the same
type as those previously described in fossil corals, fish-scales and
bones. In dealing with living perforating Thallophytes the colour and
other cell-contents often enable us to distinguish between algae and
fungi, but in fossil specimens such tests cannot be applied. The fossil
tubular borings may or may not show traces of the transverse septa
and reproductive cells; it is often the case that no trace of the
organism has been left, but only the canals by which it penetrated the
calcareous or bony skeleton. In some of the examples of _Palaeachlya_
figured by Duncan there appear to be numerous spores in some of the
sections, but it is generally a very difficult and often an impossible
task to discriminate between the borings of fungi and algae in fossil
material.
Fig. 27 B, which is copied from one of Bornet and Flahault’s drawings,
represents a piece of Solen shell riddled with small canals made by
the organism which has been named by the French authors _Ostracoblabe
implexa_, and regarded by them as a fungus. Fig. 27 C represents a
small piece of the vegetative body of _Ostracoblabe_ obtained from a
decalcified shell. In endeavouring to determine the organism which
has produced borings in fossil corals or shells, it must be borne in
mind that some forms of canals or passages may have been the work of
perforating sponges, but these are larger in diameter than those made
by algae or fungi. By some writers[202] the tubular cavities in shells
have been referred to true algae, but others consider them to be of
fungal origin.
As an example of a fossil alga referred to the Cyanophyceae, the genus
_Zonatrichites_[203] may be quoted. Bornemann, who first described the
specimens, points out the close resemblance in habit to some members of
the recent Rivulariaceae.
_Zonatrichites._
The author of the genus defines it as follows:—
“A calcareous alga, with radially arranged filaments, forming
hemispherical or kidney-shaped layers, growing on or enclosing
other bodies. Parallel or concentric zones are seen in
cross-section, formed by the periodic growth of the alga, the
older and dead layers serving as a foundation on which the young
filaments grow in radially arranged groups.”
The nodules which are apparently formed by species of this genus occur
in various sizes and shapes; Bornemann describes one hemispherical mass
8 cm. broad and 4 cm. thick. In some cases the organism has given rise
to oolitic spherules, which in radial section exhibit the branched
tubular cells spreading in fan-shaped groups from the centre of the
oolitic grain. The section parallel to the surface of a nodule presents
the appearance of a number of circular or elliptical tubes cut across
transversely or more or less obliquely. The resemblance between the
fossil and a specimen of the recent species _Zonatrichia calcivora_
Braun, is certainly very close, but it is very difficult, in the
absence of material exhibiting more detailed structure than is shown in
the specimens described by Bornemann, to decide with any certainty the
true position of the fossil. The figures do not enable us to recognise
any trace of cells in the radiating tubes. It is possible that we
have in _Zonatrichites_ an example of a Cyanophyceous genus in which
only the sheaths of the filaments have been preserved. In any case it
is probable that this Mesozoic species affords another instance of a
fossil alga which has been responsible for certain oolitic or other
structures in limestone rocks.
The species described by Bornemann was obtained from a Breccia near
Lissau in Silesia, of Keuper age.
M. Renault has recently described certain minute structures in
a Palaeozoic coprolite to which he gives the name _Gloioconis
Borneti_[204], and which he regards as a Permian gelatinous alga
similar to the well-known recent genus _Glœocapsa_. The appearances
revealed in a section of the coprolite are interpreted by this author
as a collection of small colonies of a unicellular gelatinous alga in
various stages of development. Renault’s figure shows a spherical group
of faintly outlined and cloudy bodies, most of which include one or two
small dark spots. The latter are regarded as the cells of the alga, and
the surrounding cloudy substance is described as the gelatinous sheath.
The absence of a nucleus in these extremely minute fossil cells (8–10
µ in diameter) is referred to as an argument in favour of referring
the organism to the Cyanophyceae rather than to the Chlorophyceae. It
is possible that the ill-defined structure described by Renault may be
a petrified alga, but there is not sufficient evidence to warrant a
decided opinion; the absence of nuclei can hardly be taken seriously
in such a case as this as an argument in favour of the Cyanophyceae.
[Sidenote: CYANOPHYCEAE.]
Although our exact knowledge of fossil Cyanophyceae is extremely small,
it is probable that such simple forms of plants existed in abundance
during the past ages in the earth’s history. Several writers have
expressed the opinion that the blue-green algae may be taken as the
modern representatives of those earliest plants which first existed
on an archaean land-surface. The living species possess the power of
resisting unfavourable conditions in a marked degree, and are able
to adapt themselves to very different surroundings. Their occurrence
in hot springs proves them capable of living under conditions which
are fatal to most plants, and suggests the possibility of their
occurrence in the heated waters which probably constituted the medium
in which vegetable life began. An interesting example of the growth of
blue-green algae under unfavourable conditions was recorded in 1886 by
Dr Treub[205] of the Buitenzorg Gardens, Java. In 1883 a considerable
part of the island Krakatoa, situated in the Straits of Sunda, between
Sumatra and Java, was entirely destroyed by a terrific volcanic
explosion. What remained had been reduced to a lifeless mass of hot
volcanic ashes. Three years later, Treub visited the island, and found
that several plants had already established themselves on the volcanic
rocks. Various ferns and flowering plants were recorded in Treub’s
description of this newly established flora. It seemed that the barren
rocky surface had been prepared for the more highly organised plants by
the action of certain forms of Cyanophyceae, which were able to live
under conditions which would be fatal to more complex types.
In the petrified tissues of fossil plants there are occasionally
found small spherical vesicles, with delicate limiting membranes, in
the cavities of parenchymatous cells or in the elements of vascular
tissue. Some of these spherical inclusions have been described as
possibly simple forms of endophytic algae[206], such as we are now
familiar with in species of the Cyanophyceae and other algae. So far,
however, no recorded instance of such fossil endophytic algae is
entirely satisfactory. Some of the cells figured by Williamson as
possibly algae, endophytic in the tissues of Coal-Measure plants, are
no doubt thin-walled vesicles which formed part of a highly vacuolated
cell-contents. Examples of such vesicles in living and fossil cells are
shown in fig. 42. The fact that the contents of living plant tissues
have been erroneously described as endophytic organisms, should serve
as a warning against describing fossil endophytes without the test of
good evidence to support them.
The description of a fossil _Nostoc_ by the late Prof. Heer[207] from
the Tertiary rocks of Switzerland cannot be accepted as a trustworthy
example of a fossil plant, much less of a genus of recent algae. The
application of recent generic names to fossils which are possibly not
even organic must do more harm than good.
B. SCHIZOMYCETES (Bacteria).
It is impossible to draw a sharp line between the two subdivisions of
the Schizophyta. The so-called Fission-Fungi or Bacteria differ from
the Schizophyceae or Fission-Algae in the cell-contents being either
colourless, blood-red or green, but never blue-green. We may regard
the Bacteria, generally, as the lowest forms of plants; they are
extremely simple organisms which have been derived from some primitive
types which possessed the power of independent existence and contained
chlorophyll—that important substance which enables a plant to obtain
its carbon first-hand from the carbon dioxide of the atmosphere.
Bacteria may be briefly described as single-celled plants, and as de
Bary suggested comparable in shape to a billiard ball, a lead pencil or
a corkscrew[208]. A single spherical or cylindrical cell measures about
1 µ in diameter[209]. They occur either singly or in filaments, or as
masses of various shapes consisting of numberless bacterial cells. The
nature and manner of life of Bacteria, and their extraordinary power
of successfully resisting the most unfavourable conditions, render it
probable that they constitute an extremely ancient group of organisms.
The wonderful perfection of preservation of many fossil plants enables
us to investigate the contents of petrified cells and to examine in
minutest detail the histology of extinct plants. To those who are
familiar with the possibilities of microscopical research as applied to
silicified and calcified fossil tissues, it is by no means incredible
that evidence has been detected of the existence of Bacteria as far
back in the history of the earth as the Carboniferous and Devonian
periods.
Were there no trustworthy records of the occurrence of Bacteria in
Palaeozoic times, it would still be a natural supposition that these
ubiquitous organisms must have been abundantly represented. It has
been suggested as a probable conclusion that some forms of Bacteria,
which produced chemical changes in the soil necessary for the nutrition
of plants, must have existed contemporaneously with the oldest
vegetation[210].
The paper-coal of Toula, which in some places reaches a thickness of 20
cm., is a plant-bed of exceptional interest. It differs from ordinary
coal in being made up of numberless thin brown-papery sheets associated
with a darker coloured substance largely composed of ulmic acid. Prof.
Zeiller[211], in an interesting account of the papery layers, has
shown that they consist of the cuticles of a Lepidodendroid plant,
_Bothrodendron_. An examination of a piece of one of the sheets at
once reveals the existence of a regular network of which the walls of
the meshes are the outlines of the epidermal cells, the meshes being
bridged across by a thin light brown membrane which represents the
layer of cuticularised cell-wall of each epidermal cell. At regular
intervals and disposed in a spiral arrangement, we find small gaps
in the papery cuticle which mark the position of the _Bothrodendron_
leaves. These Palaeozoic cuticles are not petrified; they are only
slightly altered, and have retained the power of swelling in water,
being able to take up stains like recent tissues. It may reasonably
be assumed that the persistent cuticles owe their preservation to a
greater power of resistance to destructive agents than was possessed
by the other tissues of the plant. It is by no means unlikely, as
Renault[212] has recently suggested, that as the _Bothrodendron_
stem-fragments lay in the swamps or marshes the tissues were gradually
eaten away by Bacteria, but the cuticles successfully resisted the
attacks of the bacterial saprophytes. The same observer has described
what he regards as the actual organism which effected this wholesale
destruction, under the name _Micrococcus Zeilleri_. He finds, after
treating the cuticles with ammonia to remove the ulmic acid, that there
occur numerous minute spherical bodies, each surrounded by a thin
envelope, either singly or in groups on the surface of the cuticular
membrane. These vary in size from ·5µ to 1µ in diameter. I have not
been able to detect any satisfactory proof of such _Micrococci_ in
specimens of the paper-coal which were treated according to Renault’s
method, but it is extremely probable that this unusual method of
preservation of stem-cuticles is the result of selective bacterial
action.
Renault believes that some of the minute spherulitic structures which
are seen in sections of decayed tissues of Palaeozoic plants owe their
origin, in part, to the ravages of bacteria. The disorganisation of
parenchymatous cells gives rise to a gelatinous substance in which
needle-like crystals of silica may be deposited, from a siliceous
solution, in a matrix which has resulted from bacterial activity. In
some of the sections of tissues figured by Renault[213] the outlines of
a few cells are still indicated by fragments of the partially decayed
wall, while in other cells the walls have been completely destroyed by
Bacteria of which some are preserved in the centre of the cell-area,
forming a kind of nucleus to the siliceous spherulites.
[Sidenote: BACILLI.]
In addition to the _Micrococcus_ described by Renault from the Toula
paper-coal, there are a host of other forms which have been minutely
diagnosed and figured by Profs. Renault and Bertrand[214]. These
authors have discovered what they believe to be well-defined species of
_Micrococcus_ and _Bacillus_ ranging in age from Devonian to Jurassic.
The material which has afforded the somewhat startling results of
their investigations consists partly of the coprolites of reptiles and
fishes, and of silicified and calcified plant tissues.
_Bacillus Permicus._ Ren. and Bert.[215] (Fig. 28 B.)
This _Bacillus_, which was discovered in sections of a Permian
coprolite from Central France, has the form of cylindrical rods
12–14µ in length, and 1·3–1·5µ broad, rounded at each end. The rods
occur either singly or occasionally, two or three individuals are
joined end to end. Fig. 28 B represents a piece of one of Renault and
Bertrand’s sections; the small rods are clearly seen lying in various
directions in the homogeneous matrix of the coprolite. Each individual
is said to be surrounded by an extremely minute empty space ·4µ in
width, originally occupied by the Bacillus membrane, the central
rod representing the mineralised cell-contents. In this example the
petrifying substance was probably derived from the phosphate of calcium
of bones which were attacked by Bacteria. I am indebted to Prof.
Renault for an opportunity of examining specimens of this and other
fossil Bacteria, and in this particular case there is undoubtedly
strong evidence in favour of the author’s determination.
[Illustration: FIG. 28. A, _Bacillus Tieghemi_ Ren. and _Micrococcus
Guignardi_ Ren. B, _Bacillus Permicus_ Ren. (After Renault.)]
_Bacillus Tieghemi_ Ren.[216] and _Micrococcus Guignardi_ Ren.[217]
(Fig. 28 A.)
Renault has given the name _Bacillus Tieghemi_ to certain minute rods
6–10µ, in length, and 2·2–3·8µ broad, often containing a dark coloured
spherical spore-like body 2µ in diameter, which have been found in the
tissues of a Coal-Measure plant.
The name _Micrococcus Guignardi_ has been applied to more or less
spherical bodies 2·2µ in diameter, also met with in silicified plants.
A portion of one of Renault’s figures is reproduced in Fig. 28 A. The
faint and broken lines mark the position of the middle lamellae of
parenchymatous cells from the pith of a Calamite. The tissue has been
almost completely destroyed, but the more resistant middle lamellae
have been partially preserved. The short and broad rods represent what
Renault terms _Bacillus Tieghemi_; the small circle in the middle of
some of these being referred to as a spore, and in one specimen shown
in the figure, the second rod at right angles to the first is described
as a small daughter-Bacillus formed by the germination of the central
spore.
The isolated circles in the figure are referred to _Micrococcus_.
[Sidenote: FOSSIL BACTERIA.]
It is unnecessary to give an account of the numerous examples of
_Micrococci_ and _Bacilli_ described by Renault from Devonian,
Carboniferous, Permian and Jurassic rocks. We may, however, in a few
words consider the general question of the existence and possible
determination of fossil Bacteria.
In 1877 Prof. Van Tieghem[218] of Paris drew attention to the method
of operation and plan of attack of _Bacillus amylobacter_ as a
destructive agent in the decay of plant débris in water. He was able
to follow the gradual disorganisation of the tissues and the various
steps in the ‘butyric fermentation’ effected by this _Bacterium_.
Similarly the same author[219] was able to detect the action of an
allied organism in some silicified tissues from the Carboniferous
nodules of Grand-Croix, a well-known locality for petrified plants near
Saint-Étienne. He recognised also the traces of the _Bacillus_ itself
in the partially destroyed plant tissues. The Palaeozoic Bacteria
made use of some cellulose-dissolving ferment of which the action is
clearly demonstrated in sections of silicified tissues. Many of the
phenomena described by Renault and Bertrand as due to similar Bacterial
action, afford additional evidence that the gradual disorganisation of
vegetable tissues was effected in precisely the same manner as at the
present day.
In some cases we have I believe trustworthy examples of the Bacteria
themselves, both in coprolites and plant-tissues, but it is more
than probable that some of the recorded examples are not of any
scientific value. The examination of petrified tissues under the
higher powers of a microscope often reveals the existence of numerous
spherical particles and rod-like bodies which agree in shape with
_Micrococci_ or _Bacilli_. Minute crystals of mineral substances may
occur in the siliceous or calcareous matrix of a petrified plant
which simulate minute organic forms. Vogelsang[220] in his important
work _die Krystalliten_ has thrown considerable light on the ontogeny
of crystals, and the minute globulites and other forms of incipient
crystallisation might well be mistaken for Bacterial cells. Granting,
however, that we have satisfactory evidence, both direct and indirect,
that some forms of Bacteria lived in the decaying tissues of Palaeozoic
plants, and in the intestines of reptiles and other animals, we cannot
safely proceed to specific diagnoses and determinations[221].
Renault has pointed out that fossil Bacteria may often be more readily
detected than living forms owing to the presence of a brown ulmic
substance which results from the carbonisation of the protoplasm. He
is forced to admit, however, that such diagnostic characters as are
obtained by Bacteriologists by means of cultures cannot be utilised
when we are dealing with fossil examples! We are told that “Partout où
nous avons cherché des Bacteriaceés, nous en avons rencontré.”[222]
This indeed is the danger; an extended examination of fossil sections
under an immersion-lens must almost inevitably lead to the discovery
of minute bodies of a more or less spherical form which _might_ be
_Micrococci_. To measure, and name such bodies as definite species of
_Micrococci_ is, I believe, but wasted energy and an attempt to compass
the impossible.
Specialists tell us that the accurate determination of species of
recent Bacteria is practically hopeless: may we not reasonably conclude
that the attempt to specifically diagnose fossil forms is absolutely
hopeless? “The imagination of man is naturally sublime, delighted with
whatever is remote and extraordinary—”, but it is to be deplored if the
fascination of fossil bacteriology is allowed to warp sound scientific
sense.
IV. ALGAE.
A. DIATOMACEAE. (Diatoms.)
B. CHLOROPHYCEAE. (Green algae.)
C. RHODOPHYCEAE. (Red algae.)
D. PHAEOPHYCEAE. (Brown algae.)
The presence of chlorophyll is one common characteristic of the
numerous plants included in the Algae. The generally adopted
classification rests in part on an artificial distinction, namely the
prevailing colour of the plant.
It must be definitely admitted, at the outset, that palaeobotany has
so far afforded extremely little trustworthy information as to the
past history of algae. Were we to measure the importance of the
geological history of these plants by the number of recorded fossil
species, we should arrive at a totally wrong and misleading estimate.
By far the greater number of the supposed fossil algae have no claim
to be regarded as authentic records of this class of Thallophytes. It
has been justly said that palaeontologists have been in the habit of
referring to algae such impressions or markings on rocks as cannot well
be included in any other group. “A fossil alga,” has often been the
_dernier ressort_ of the doubtful student.
[Sidenote: LARGE SEAWEEDS.]
Before discussing our knowledge, or rather lack of knowledge, of
fossil algae at greater length, it will be well to briefly consider
the manner of occurrence and botanical nature of existing forms. In
the sea and in fresh water, as well as in damp places and even in
situations subject to periods of drought, algae occur in abundance in
all parts of the world. We find them attaining full development and
reproducing themselves at a temperature of −1° C. in the Arctic Seas,
and again living in enormous numbers in the waters of thermal springs.
Around the coast-line of land areas, and on the floor of shallow seas
algae exhibit a remarkable wealth of form and luxuriance of growth.
As regards habit and structure, there is every gradation from algae
in which the whole individual consists of a thin-walled unseptate
vesicle, to those in which the thallus attains a length unsurpassed by
any other plant, and of which the anatomical features clearly express
a well-marked physiological division of labour such as occurs in the
highest plants.
The large and leathery seaweeds which flourish in the extreme northern
and southern seas are plants which it is reasonable to suppose might
well have left traces of their existence in ancient sediments. Sir
Joseph Hooker, in his account of the Antarctic flora[223], investigated
during Sir James Ross’s voyage in H.M. ships Erebus and Terror, has
given an exceedingly interesting description of the gigantic brown
seaweeds of southern latitudes. The trunks are described as usually
5–10 feet long, and as thick as a human thigh, dividing towards the
summit into numerous pendulous branches which are again broken up into
sprays with linear ‘leaves.’ Hooker records how a captain of a brig
employed his crew for two bitterly cold days in collecting _Lessonia_
stems which had been washed up on the beach, thinking they were trunks
of trees fit for burning. On our own coasts we are familiar with the
common _Laminaria_, the large brown seaweed with long and strap-shaped
or digitate fronds which grows on the rocks below low-tide level. The
frond passes downwards into a thick and tough stipe firmly attached to
the ground by special holdfasts. A transverse section of the stalk of
a fairly old plant presents an appearance not unlike that of a section
of a woody plant. In the centre there is a well-defined axial region
or pith consisting of thick walled, long and narrow tubes pursuing a
generally vertical though irregular course, and embedded in a matrix
of gelatinous substance derived from the mucilaginous degeneration
of the outer portions of the cell-walls. The greater part of such a
section consists, however, of regularly disposed rows of cells which
have obviously been formed by the activity of a zone of dividing or
meristematic elements. The occurrence of distinct concentric rings in
this secondary tissue clearly points to some periodicity of growth
which is expressed by the alternation of narrow and broader cells. In
the Antarctic genus _Lessonia_, the stem reaches a girth equal to that
of a man’s thigh, and in structure it agrees closely with the smaller
stem of _Laminaria_. In these large algal stems, the cells are not
lignified as in woody plants, and in longitudinal section they have
for the most part the form of somewhat elongated parenchyma, differing
widely in appearance from the tracheids or vessels of woody plants. At
the periphery of the _Laminaria_ stem, represented in fig. 29, there
occur numerous and comparatively large mucilage ducts.
[Illustration: FIG. 29. A, Transverse section of the stipe of a
_Laminaria_, slightly enlarged. B, A small piece of the tissue
between the central ‘pith’ and ‘cortex’ showing the radially
disposed secondary elements more highly magnified.]
In certain algae of different families the thallus is encrusted with
carbonate of lime, and is thus rendered much more resistant. The
Diatoms, on the other hand, possess still more durable siliceous tests
which are particularly well adapted to resist the solvent action of
water and other agents of destruction. It is these calcareous and
siliceous forms which supply the greater part of the trustworthy data
furnished by fossil algae.
[Sidenote: SCARCITY OF FOSSIL ALGAE.]
It remains to consider some of the causes to which we may attribute
the scarcity of fossil algae, and the possible sources of error which
beset any attempt to describe or assign names to impressions and casts
simulating algal forms.
In the first place, the delicate nature of algal cells is a serious
obstacle to fossilisation. Even in plants in which the woody stems have
been preserved by a siliceous or calcareous solution, we frequently
find the more delicate cells represented by a mass of crystalline
matter without any trace of the cell-walls being preserved. In such
plants as algae, where the cell-walls are not lignified, but consist of
cellulose or some special form of cellulose, which readily breaks down
into a mucilaginous product, the tissues have but a small chance of
withstanding the wear and tear of fossilisation.
The danger of relying on external form as a means of recognition is
especially patent in the case of those numerous markings or impressions
frequently met with on rocks, and which resemble in outline the
thallus of recent algae. Among animals, such as certain Polyzoa, the
flat branching body of various algae is closely simulated, and in
other plants, such as the frondose liverworts, the same thalloid and
branched form of body is again met with. Some of the much dissected
_Aphlebia_ leaves of ferns (e.g. _Rhacophyllum_ species) bear a
striking resemblance to fossil algae; and numerous other examples might
be quoted. In palaeobotanical literature we find a host of names, such
as _Chondrites_, _Fucoides_[224], _Caulerpites_ and others applied to
indefinite and indistinct surface markings which happen to resemble in
shape certain of the better known genera of recent seaweeds.
The close parallelism in outward form displayed by different genera
and families of algae is in itself sufficient argument against the use
of recent generic names for fossils of which the algal nature is often
more than doubtful. Were external form to be accepted as a trustworthy
guide, in the absence of internal structure and reproductive organs,
such a genus as _Caulerpa_[225] would afford material for numerous
generic designations. A comparison of the different species of this
Siphoneous green alga brings out very clearly the exceedingly protean
nature of this interesting genus, and serves as one instance among
many of the small taxonomic value which can be attached to external
configuration. _Caulerpa pusilla_ Mart. and Her., _C. taxifolia_
(Vahl.), _C. plumaris_ Forsk., _C. abies-marina_ J. Ag., _C.
ericifolia_ (Turn.), _C. hypnoides_ (R. Br.), _C. cactoides_ (Turn.),
_C. scalpelli formis_ (R. Br.), and others clearly illustrate the
almost endless variety of form exhibited by the species of a single
genus of algae. We constantly find in the several classes of plants
a repetition of the same form either in the whole or in the separate
members of the vegetative body, and but a slight acquaintance with
plant types should lead us to use the test of external resemblance
with the greatest possible caution. To emphasize this danger may seem
merely the needless reiteration of a self-evident fact, but there is,
perhaps, no source of error which has been more responsible for the
creation of numerous worthless species among fossil plants.
[Illustration: FIG. 30. 1. Rill-mark (after Williamson). 2. Trail made
by a seaweed dragged along a soft plaster of Paris surface (after
Nathorst). 3. Tracks made by _Goniada maculata_, a Polychaet (after
Nathorst). 4. Burrow of an insect. 4_a_. Section of the gallery
(after Zeiller).]
There is, however, another category of impressions and casts of common
occurrence in sedimentary rocks which requires a brief notice. Very
many of the fossil algae described in text-books and palaeobotanical
memoirs have been shown to be of animal origin, and to be merely
the casts of tracks and burrows. A few examples will best serve to
illustrate the identity of many of the fossils referred to algae with
animal trails and with impressions produced by inorganic agency.
Dr Nathorst of Stockholm has done more than any other worker to
demonstrate the true nature of many of the species of _Chondrites_,
_Cruziana_, _Spirophyton_, _Eophyton_, and numerous other genera. In
1867 there were discovered in certain Cambrian beds of Vestrogothia,
long convex and furrowed structures in sandstone rocks which were
described as the remains of some comparatively highly organised plant,
and described under the generic name _Eophyton_[226]. By many authors
these fossils have been referred to algae, but Nathorst has shown that
the frond of an alga trailed along the surface of soft plaster of Paris
produces a finely furrowed groove (fig. 30, 2) which would afford a
cast similar to that of _Eophyton_. The same author has also adduced
good reasons for believing that the Eophytons of Cambrian rocks may
represent the trails made by the tentacles of a _Medusa_ having a habit
similar to that of _Polydonia frondosa_ Ag. Impressions of _Medusae_
have been described by Nathorst from the beds in which _Eophyton_
occurs; and the specimens in the Stockholm Museum afford a remarkable
instance of the rare preservation of a soft-bodied organism[227]. By
allowing various animals to crawl over a soft-prepared surface it is
possible to obtain moulds and casts which suggest in a striking manner
the branched thallus of an alga. The tracks of the Polychaet, _Goniada
maculata_ Örstd.[228], one of the Glyceridae, are always branched and
very algal-like in form (fig. 30, 3). Many of the so-called fossil
algae are undoubtedly mere tracks or trails of this type. In the
fossil-plant gallery of the British Museum there are several specimens
of small branched casts, clearly marked as whitish fossils on a dark
grey rock of Lower Eocene age from Bognor; these were described by
Mantell and Brongniart[229] as an alga, but there is little doubt of
their being of the same category as the track shown in fig. 30, 3.
[Sidenote: FOSSILS SIMULATING ALGAE.]
The well-known half-relief casts met with in Cambrian, Silurian and
Carboniferous rocks, and known as _Cruziana_ or Bilobites, are probably
casts of the tracks of Crustaceans. The impression left by a King-Crab
(_Limulus_) as it walks over a soft surface affords an example of this
form of cast. It has been suggested that some of the Bilobites may
be the casts of an organism like _Balanoglossus_[230], a worm-like
animal supposed by some to have vertebrate affinities. The resemblance
between some of the lower Palaeozoic Bilobites and the external
features of a _Balanoglossus_ is very striking, and such a comparison
is worth considering in view of the fact that soft-bodied animals have
occasionally left distinct impressions on ancient sediments.
The literature on the subject of fossil algae _versus_ inorganic and
animal markings is too extensive and too wearisome to consider in
a short summary; the student will find a sufficient amount of such
controversial writing—with references to more—in the works quoted
below[231].
In the Stockholm Museum of Palaeobotany there is an exceedingly
interesting collection of plaster casts obtained by Dr Nathorst
in his experiments on the manufacture of fossil ‘algae,’ which
afford convincing proof of the value and correctness of his general
conclusions.
The pressure of the hand on a soft moist surface produces a raised
pattern like a branched and delicate thallus. The well-known _Oldhamia
antiqua_ Forbes and _Oldhamia radiata_ Forbes[232], from the Cambrian
rocks of Ireland may, in part at least, owe their origin to mechanical
causes, and we have no sufficient evidence for including them among
the select class of true fossil algae. Sollas[233] has shown that the
structure known as _Oldhamia radiata_ is not merely superficial but
that it extends across the cleavage-planes. _Oldhamia_ is recorded from
Lower Palaeozoic rocks in the Pyrenees[234] by Barrois, who agrees with
Salter, Göppert and others in classing the fossil among the algae. The
photograph accompanying Barrois’ description does not, however, add
further evidence in favour of accepting _Oldhamia_ as a genus of fossil
algae.
The burrows made by _Gryllotalpa vulgaris_ Latr., the Mole-cricket,
have been shown by Zeiller to bear a close resemblance to a branch of a
conifer in half-relief (fig. 30, 4), or to such a supposed algal genus
as _Phymatoderma_[235].
In fig. 30, 1, we have what might well be described as a fossil
alga. This is merely a cast of a miniature river-system such as one
frequently sees cut out by the small rills of water flowing over a
gently-sloping sandy beach. A cast figured and described by Newberry
as an alga, _Dendrophycus triassicus_[236], from the Trias of the
Connecticut Valley, is practically identical with the rill-marks
shown in fig. 30, 1. The cracks produced in drying and contracting
sediment may form moulds in which casts are subsequently produced by
the deposition of an overlying layer of sand, and such casts have
been erroneously referred to algal impressions[237]. Dawson[238] has
figured two good examples of Carboniferous rill-marks from Nova Scotia
in his paper on Palaeozoic burrows and tracks of invertebrate animals.
[Sidenote: RECOGNITION OF FOSSIL ALGAE.]
[Illustration: FIG. 31. _Chondrites verisimilis_ Salt. Wenlock
limestone, Dudley. From a specimen in the British Museum (V. 2550).
Slightly reduced.]
The specimen represented in fig. 31 affords an example of a fairly
well-known fossil from the Wenlock limestone, originally described by
Salter as _Chondrites verisimilis_ Salt, from Dudley[239]. He regarded
it as an alga, and the graphitic impression agrees closely in form
with the thallus of some small seaweeds. A closer examination of the
fossil reveals a curious and characteristic irregular wrinkling on the
graphite surface, which suggests an organism of more chitinous and
firmer material than that of an alga.
A similar and probably an identical fossil is described and figured
by Lapworth[240] in an appendix to a paper by Walter Keeping on the
geology of Central Wales, under the name of _Odontocaulis Keepingi_
Lap. and regarded as a dendroid graptolite. In any case we have no
satisfactory grounds for including these fossils in the plant-kingdom.
How then are we to recognise the traces of ancient algae? There is no
golden rule, and we must admit the difficulty of separating real fossil
algae from markings made by animal or mechanical agency. The presence
of a carbonaceous film is occasionally a help, but its occurrence is
no sure test of plant origin, nor is its absence a fatal objection
to an organic origin. While being fully alive to the small value of
external resemblance, and to the numerous agents which have been shown
to be capable of producing appearances indistinguishable from plant
impressions, we must not go too far in a purely negative direction.
An important contribution to the subject of fossil algae has lately
appeared by Prof. Rothpletz[241]. He deals more particularly with
the much discussed Flysch[242] Fucoids of Tertiary age, and while
refusing to accept certain examples as fossil algae, he brings forward
weighty arguments in favour of including several other forms among
the algae. He is of opinion that most of the main divisions of the
algae are represented among the Flysch Fucoids, but considers that the
Phaeophyceae are the most numerous.
Rothpletz’s work is chiefly interesting as illustrating the application
of microscopic examination and chemical analysis to the determination
of fossil algae. Although he makes out a good case in favour of
restoring many of the Tertiary fossils to the plant kingdom, the
material at his disposal does not admit of satisfactory botanical
diagnosis.
No doubt some of the fossils from the Silurian and Cambrian rocks are
true algae, and Nathorst has pointed out that such a species as Hall’s
_Sphenothallus angustifolius_[243] may well be an alga. Additional
examples might be quoted from Bornemann and other writers, but in view
of the attempts which are sometimes made to trace the development of
more recent plants to more than doubtful Lower Palaeozoic Algae, one
must agree with Nathorst’s opinion,—“Je crois que l’on rend un bien
mauvais service à la théorie de l’évolution, en essayant de baser
l’arbre généalogique des algues fossiles sur des corps aussi douteux
que les Bilobites, Crossochorda, Eophyton, etc.[244]”
There are many carbonaceous impressions on rocks of different ages
which it is reasonable to refer to algal origin, and although such
are of little or no botanical value, it may be a convenience to
refer to them under a definite term. The comprehensive generic name
_Algites_[245] has been suggested as a convenient designation for
impressions or casts which are probably those of algae.
[Sidenote: SUPPOSED FOSSIL ALGAE.]
Some of the fossils described by Mr Kidston from British Carboniferous
rocks as probably algae present an undoubted algal appearance, and
might be placed in the genus _Algites_; but in some cases—e.g.
_Chondrites plumosa_[246] Kidst. from the Calciferous Sandstone of
Eskdale, one feels much more doubtful; in this particular instance the
impressions suggest the fine roots of a water-plant.
The statement is occasionally made that the numerous fossil algae
and the absence of higher plants in the older strata justify the
description of the oldest rocks as belonging to the ‘age of algae.’
Such an assertion rests on an unsound basis, and is rather the
expression of what might be expected than what has been proved to
be the case. The oldest plants with which we are at all closely
acquainted are of such a type as to forcibly suggest that in the lowest
fossiliferous rocks we are still very far from the sediments of that
age which witnessed the dawn of plant life.
Many of the obscure markings on rock surfaces which have been referred
to existing genera of algae or described as new genera, are much
too doubtful to be included even under such a comprehensive name as
_Algites_. Space does not admit of further reference to determinations
of this type which abound in palaeontological literature.
It would be very difficult to produce satisfactory evidence for the
algal nature of many of the supposed fossil algae from Cambrian
rocks[247]; there has been a special tendency to recognise algal
remains in the oldest fossiliferous strata, due in part no doubt to the
fallacy that in that period nothing higher than Thallophytes is likely
to have existed. The so-called _Phycodes_ referred to by Credner[248]
as characteristic of the Cambrian rocks of the Fichtelgebirge
(“Phycoden-Schiefer”) is probably of inorganic origin, and comparable
to the genus _Vexillum_ of Saporta[249] and other writers, which
Solms-Laubach has described as being formed every day in the soft mud
of our ponds where local currents are checked by branches and other
obstacles[250]. There are several good specimens of _Phycodes_ in the
Bergakademie of Berlin and in the Leipzig Museum which, I believe,
clearly demonstrate the absence of all satisfactory evidence of an
algal origin.
We may next pass to a short description of a few representative types
of algae, which may reasonably be classed under definite families, and
accepted as evidence possessing some botanical value.
A. DIATOMACEAE (BACILLARIACEAE).
This family occupies a somewhat isolated position among the algae, and
is best considered as a distinct subdivision rather than as a family of
the Phaeophyceae or Brown algae, with which it possesses as a common
characteristic a brown-colouring matter.
Single-celled plants consisting of a simple protoplasmic body
containing a nucleus and brown colouring matter (diatomin) associated
with the chlorophyll. The cell-wall is in the form of two halves,
known as _valves_, which fit into one another like the two portions of
a pill-box. The cell-wall contains a large amount of silica, and the
siliceous cases of the diatoms are commonly spoken of as the valves of
the individual, or the _frustules_. Diatoms exhibit a characteristic
creeping movement, and are reproduced by division, also by the
development of spores in various forms[251].
The recent members of the family have an exceedingly wide distribution,
occurring both in freshwater and in the sea. Owing to the lightness
of the frustules, they are frequently carried along in the air, and
atmospheric dust falling on ships at sea has been found to contain
large numbers of diatoms[252]. The siliceous valves are abundant
in guano deposits, and they have been found also in association
with volcanic material. Diatomaceous deposits are now being formed
in the Yellowstone Park district; “they cover many square miles in
the vicinity of active or extinct hot spring vents of the park, and
are often three feet, four feet, and sometimes five to six feet
thick[253].” The gradual accumulation of the siliceous tests on the
floor of a fresh-water lake results in the formation of a sediment
consisting in part of pure silica. Such deposits, often spoken of as
_kieselguhr_ or _diatomite_, and used as a polishing material, occur
in many parts of Britain, marking the sites of dried-up pools or lakes.
At the northern end of the island of Skye there occurs an unusually
pure deposit of diatomite overlain by peat and turf, and extending over
an area of fifty-eight square miles. Many of the individuals in this
deposit were in all probability carried into the lake by running water,
while others lived in the lake and after death their tests contributed
to the siliceous deposit[254]. The late Dr Ehrenberg published numerous
papers on diatomaceous deposits in different parts of the world, and
in his great work, _Zur Mikrogeologie_[255], he gave numerous and
beautifully executed illustrations of such siliceous accumulations. In
many of the samples he figures one sees fragments of plant tissues,
spores of conifers and ferns, associated with the diatom tests. The
occurrence of the pollen grains of coniferous trees in lacustrine and
marine deposits is not surprising in view of their abundance in Lake
Constance and other lakes. It is stated that the pollen of conifers in
the Norwegian fiords plays an important part in the nourishment of the
Rhizopod _Saccamina_[256].
[Sidenote: DIATOMACEOUS OOZE.]
In the waters of the ocean diatoms are of frequent occurrence, and
very widely distributed. Sir Joseph Hooker records the existence of
masses of diatomaceous ooze over a wide area in Antarctic regions[257].
Along the shores of the Victoria Barrier, a perpendicular wall of ice,
between one and two hundred feet above sea-level, the soundings were
found to be invariably charged with diatom remains, and from the base
of the ice-wall there appeared to be in process of formation a bank
of these tests stretching north for a distance of 200 miles. The more
extended researches conducted during the cruise of the Challenger
have clearly proved the enormous accumulations of diatoms now being
formed on the ocean-bed[258]. South of latitude 45° S. there is now
being built up a vast deposit which may be eventually upraised as a
fairly pure siliceous rock. From extreme northern latitudes Nansen has
recently recorded the occurrence of these lowly organised plants.
He writes,—“I found a whole world of diatoms and other microscopical
organisms, both vegetable and animal, living in the fresh-water pools
on the Polar drift-ice, and constantly travelling from Siberia to the
east coast of Greenland[259].” In warmer latitudes diatoms abound in
the surface waters, but there they are associated with numerous other
forms of the Plankton vegetation. The waters of the Amazon carry with
them into the sea large numbers of fresh-water forms, which are floated
out to sea and finally added to the rock-building material which is
constantly accumulating on the ocean floor[260]. No definite results
have so far been obtained as to the geographical and bathymetrical
distribution of marine diatoms.
The enormous number of recent species precludes any attempt to give a
description of the better-known forms. It is more important for us to
realize how common and widely distributed are the living genera. The
hard and almost indestructible valves have been frequently found in a
fossil condition, often forming thick and extensive masses of siliceous
rock. From diatom-beds now forming in lakes and on the ocean-bed we
pass to deposits such as those in Skye and elsewhere, which mark the
site of recently dried-up sheets of water, and so to older rocks of
Tertiary age formed under similar conditions. Among the many examples
of diatomaceous deposits of Tertiary and Cretaceous age mention
should be made of those of Berlin, Königsberg, Bilin in Bohemia, and
Richmond in Virginia. The diatoms in the beds of Berlin are regarded
as fresh-water, and those of Richmond as marine. It has been pointed
out by Pfitzer that it is a comparatively easy matter to distinguish
between fresh-water and marine forms of diatoms. The diatomaceous rocks
of Bilin are known as polishing slates; they attain a thickness of 50
feet. In these, as in many other cases, the deposit has become cemented
together as a hard flinty or glassy rock, in which the cementing
material was formed by the solution of some of the diatom tests[261].
In many cases in which calcareous and siliceous rocks reveal no direct
evidence of organic origin it is probable that they were originally
formed by the accumulation of plants of which the structure has been
completely obliterated by secondary causes. The genus _Gallionella_
plays an important part in the composition of the Bilin beds.
Occasionally impressions of leaves and other organic remains are found
associated with the diatoms in the siliceous rocks. In the British
Museum (Botanical department) a large block of white powdery rock is
exhibited as an example of a diatomaceous deposit of Tertiary age from
Australia. It is described as being largely made up of the tests of
fresh-water diatoms, such as _Navicula_, _Gomphonema_, _Cymbella_,
_Synedra_, and others.
[Sidenote: FOSSIL DIATOMS.]
The abundance of Diatoms in Cretaceous rocks of the Paris basin has
recently been recorded by Cayeux[262]; it would seem that these algae
had already assumed an important rôle as rock-builders in pre-Tertiary
times. Cayeux points out that the silica of these Cretaceous
diatomaceous frustules has often been replaced by carbonate of calcium.
In addition to the occurrence of Diatoms in the various diatomaceous
deposits, their siliceous tests may occasionally be recognised in
argillaceous or other sediments. Shrubsole and Kitton[263] have
described several species of Diatoms from the London Clay of Lower
Eocene age. In many localities in the London basin the clay obtained
from well-sinkings presented the appearance of being dusted with
sulphur-like particles of a dark bronze or golden colour which
glistened in the sunlight. These yellow bodies have been found to
be diatomaceous frustules in which the silica has been replaced by
iron pyrites. The genus _Coscinodiscus_ is one of the commonest forms
recorded from the London Clay[264].
Without further considering individual examples of diatomaceous rocks
we may briefly notice the general facts of the geological history of
the family. As Ehrenberg pointed out several years ago, the Tertiary
and Cretaceous species of diatoms show a very marked resemblance to
living forms. In many cases the species are identical, and the fossil
deposits as a whole seem to differ in no special respect from those now
being built up.
With the exception of two species of Liassic Diatoms, no trustworthy
examples of the Diatomaceae have been found below the Cretaceous
series. The oldest known Diatoms were discovered by Rothpletz[265]
among the fibres of an Upper Lias sponge from Boll in Württemberg.
They occur as small thimble-shaped siliceous tests with coccoliths
and foraminifera in the horny skeleton of _Phymatoderma_, a genus
formerly regarded as an alga. Rothpletz describes two species which he
includes in the genus _Pyxidicula_, _P. bollensis_ and _P. liasica_.
This generic name of Ehrenberg is used by Schütt[266] as a subgenus of
_Stephanopyxis_.
Seeing how great a resemblance there is between the recent and
Cretaceous species, and how many examples there are of Tertiary diatom
deposits, it is not a little surprising that the past history of these
plants has not been traced to earlier periods. In 1876 Castracane[267],
an Italian diatomist, gave an account of certain species of diatoms
said to have been found in a block of coal from Liverpool obtained
from the English Coal-Measures. The species were found to be identical
with recent forms. It is generally agreed that these specimens
cannot have been from the coal itself, but that they must have been
living forms which had come to be associated with the coal. The late
Prof. Williamson spent many years examining thin sections and other
preparations of coal from various parts of the world, but he never
found a trace of any fossil diatom. There is no apparent reason
why diatoms should not be found in Pre-Cretaceous rocks, and the
microscopic investigation of old sediments may well lead to their
discovery. Prof. Bertrand of Lille, who has devoted himself for some
time past to a detailed microscopical examination of coal, informs
me that he has so far failed to discover any trace of Palaeozoic
diatomaceous tests.
[Sidenote: BACTRYLLIUM.]
The genus _Bactryllium_ is often quoted in text-books as a probable
example of a Triassic diatom. It was first described by Heer[268] from
the Trias of Switzerland and North Italy, also from the neighbourhood
of Heidelberg, and regarded as an extinct member of the Diatomaceae.
Heer defined the genus as follows:
“Small bodies, with parallel sides, rounded at either end, the
surface traversed by one or two longitudinal grooves.”
(fig. 32, C.) Several species have been figured by Heer from beds of
Muschelkalk, Keuper and Rhaetic age. He describes the wall as thick and
firm (fig. 32, C. ii.) and probably composed of silica, with a hollow
interior. The specimen shown in fig. 32, C. was found in the Rhaetic
beds, and named by Heer _Bactryllium deplanatum_; it has a length
of 4·5 mm.; the surface is transversely striated and traversed by a
single longitudinal groove. Stefani[269] has given reasons in favour
of removing _Bactryllium_ from the plant to the animal kingdom; he
points out that the specimens are too large for diatoms, and moreover
that they are asymmetrical in form and possessed a calcareous and not
a siliceous shell. He would place the fossil among the Pteropods,
comparing it with such genera as _Cuvierina_ and _Hyalaea_. In view
of Stefani’s opinion we cannot attach any importance to this supposed
diatom, especially as it has generally been regarded as at best but an
unsatisfactory genus.
[Illustration: FIG. 32. _A_, _Lithothamnion mamillosum_ Gümb. (i) In
section, (ii) surface view [after Gümbel. (i) × 320, (ii) nat.
size]. _B_, _Sycidium melo_ Sandb. (i) Surface view, (ii)
transverse section (after Deecke). _C_, _Bactryllium deplanatum_
Heer. (i) Surface view, (ii) section, showing the thick wall and
hollow interior (after Heer). _D_, Calcareous pebble from a lake in
Michigan. Rather less than nat. size (after Murray).]
B. CHLOROPHYCEAE (GREEN ALGAE).
Thallus unseptate, having the form of a vesicle or a variously branched
coenocyte, which may or may not be encrusted with carbonate of lime, or
of filaments composed of cells containing a single nucleus, or of cells
in which more than one nucleus occurs; in other instances consisting
of a plate of cells or a cell-mass. Asexual reproduction by zoospores
and other reproductive cells; sexual reproduction by means of the
conjugation of similar gametes or by the fertilisation of a typical
egg-cell by a motile spermatozoid.
This family of algae is represented at the present day by numerous
and widely distributed marine and fresh-water genera, as well as by
genera growing in moist air or as endophytes in the tissues of higher
plants[270].
Seeing how very few fossil forms have been described which have any
claim to inclusion in this subdivision of the Algae, it is unnecessary
to enumerate or define the various families of the Chlorophyceae. It
is true that many species have been figured as examples of different
genera of green algae, but few of these possess any scientific value.
There is, however, one division of the Chlorophyceae, the Siphoneae,
which must be treated at some length on account of its importance from
a palaeobotanical and geological point of view.
_a._ SIPHONEAE.
Thallus consisting of simple or branched cells very rarely divided
by septa, and containing many nuclei. In certain genera the branches
form a pseudoparenchymatous tissue by their repeated branching, and
as a result of the intimate felting together of the branched cells.
Reproduction is effected either by the conjugation of similar gametes
or by the fertilisation of an egg-cell.
_Vaucheria_ and _Botrydium_ are two well-known British genera of
this order, but most of the recent representatives live in tropical
and subtropical seas. The most striking characteristic feature of
this division of the Chlorophyceae is the fact that the thallus of a
siphoneous alga consists of an unseptate coenocyte; the plant may be
extremely small and simple, or it may reach a length of several inches,
but in all cases the body does not consist of more than one cell or
coenocyte.
From a palaeontological standpoint the Siphoneae are of exceptional
interest. It is impossible to do more than refer to a few of the living
and fossil genera. There are numerous fossil representatives already
known, and there can be little doubt that further research would be
productive of valuable results.
As examples of the order, a few genera may be described belonging to
the three families Caulerpaceae, Codiaceae, and Dasycladaceae.
α. Caulerpaceae.
Thallus unseptate, showing an extraordinary variation in the external
differentiation of the plant-body. Reproduction is effected by means of
detached portions of the parent plant.
The genus _Caulerpa_, represented by a few species in the Mediterranean
and by many tropical forms, has already been alluded to as a striking
example of a plant which appears under a great many different
forms[271]. As a recent writer has said, “Nature seems to have shown in
this genus the utmost possibilities of the siphoneous thallus[272],”
Fragments of coniferous twigs, the tracks and burrows of various
animals and other objects have been described by several authors as
fossil species of _Caulerpa_. As an illustration of the identification
of a very doubtful fossil as a species of _Caulerpites_, reference
may be made to such a form as _C. cactoides_ Göpp.[273] from Silurian
and Cambrian rocks. There are several examples of this fossil in the
Brussels Museum which probably owe their origin to some burrowing
animal, and may be compared with Zeiller’s figures of the tunnels made
by the mole-cricket (fig. 30, 4)[274].
Mr Murray, of the British Museum, has recently described what he
regards as a trustworthy example of a fossil _Caulerpa_ from the
Kimeridge Clay near Weymouth[275]. Specimens of the fossil were first
figured in a book on the geology of the Dorset coast as casts of an
equisetaceous plant[276].
To this fossil Murray has assigned the name _Caulerpa Carruthersi_,
and given to it a scientific diagnosis. The best specimens have the
form of a slender central axis, giving off at fairly regular intervals
whorls of short and somewhat clavate branches; they bear a superficial
resemblance to such a recent species as _Caulerpa cactoides_ Ag. An
examination of several examples of this fossil leads me to express
the opinion that there is not sufficient reason for assigning to them
the name of a recent genus of algae[277]. To use the generic name of
a recent plant without following the common custom of adding on the
termination “ites” (i.e. _Caulerpites_) is as a general rule to be
avoided in dealing with fossil forms; and there are, I believe, no
satisfactory grounds for referring to these fossils as trustworthy
examples of a Mesozoic alga.
In the present case I am disposed to regard the _Caulerpa_-like casts
as of animal rather than plant origin. The clavate branches have
the form of very deep moulds in the hard brown rock which have been
filled in with blue mud. It is hardly conceivable that the branches
of a soft watery plant such as _Caulerpa_ could leave more than a
faint impression on an old sea-floor. The specimens occur in different
positions in the matrix of the rock and they are not confined to
the lines of bedding; in none of the examples is there any trace of
carbonaceous matter in association with the deep moulds. On the whole,
then, this Kimeridge fossil cannot, I believe, be accepted as an
authentic example of a Mesozoic _Caulerpa_.
It is not improbable that some of the supposed fossil algae may be
casts of egg-cases or spawn-clusters of animals. In Ellis’ Natural
History of the Corallines[278] there is a drawing representing a number
of disc-like ovaries attached to a tough ligament, and referred to the
mollusc _Buccinum_, which bears a certain resemblance to the Weymouth
fossil. A similar body is figured by Fuchs[279] in an important memoir
on supposed fossil algae.
It is not suggested that the _Caulerpa Carruthersi_ of Murray should be
regarded as the cast of some molluscan egg-case attached to a slender
axis, but it is important to bear in mind the possibility of matching
such extremely doubtful fossils with other organic bodies than the
thallus of a _Caulerpa_. In an example of an egg-case in the Cambridge
Zoological Museum, referred to a species of _Pyrula_, there is a hard,
long and slender axis, bearing a series of semicircular chambers
divided into radial compartments. The whole is hard and horny and might
well be preserved as a fossil.
β. Codiaceae.
The members of this Order present a considerable diversity of form as
regards the shape of the plant-body; the thallus of some species is
encrusted with carbonate of lime. The order is widely distributed in
tropical and temperate seas.
Among the recent genera _Penicillus_ and _Codium_ may be chosen as
important types from the point of view of fossil representatives.
_Codium._
The thallus of _Codium_ consists of a spongy mass of tubular
cell-branches which are differentiated into two fairly distinct
regions, an outer peripheral layer in which the branches have long
club-shaped terminations, and an inner region consisting of loosely
interwoven filaments.
_Codium Bursa_ L. and _C. tomentosum_ Huds. are two well-known British
species, the former presents the appearance of a spongy ball of
cells, and in the latter the thallus is divided up into dichotomously
forked branches[280]. In this genus the thallus is not encrusted with
carbonate of lime, at least in recent species.
_Sphaerocodium._ Fig. 37, D.
Rothpletz[281] instituted this genus for certain small spherical
or oval bodies varying from 1 mm. to 2 cm. in diameter, which have
been found on crinoid stems or shell fragments of Triassic age. Each
spherical body consists of dichotomously branched single-celled
filaments, between 50 and 100µ in breadth, and from 300–500µ in height.
The tubular cavities occasionally swell out into spherical spaces which
are regarded by Rothpletz as sporangia.
There is not sufficient evidence that _Sphaerocodium Bornemanni_ Roth.
has been correctly referred to the Codiaceae. The sporangia-like
swellings described by the author of the species are not by any means
conclusive as characters of important taxonomic value. Figure 37, D,
illustrates the general structure of the fossil as seen in a transverse
section of one of the calcareous grains.
Like _Girvanella_, which has been referred by some writers to the
Siphoneae, _Sphaerocodium_ occurs in the form of oolitic grains. In
the Triassic Raibler and St Cassian beds of the Tyrol, as well as in
rocks of Rhaetic age in the Eastern Alps, it makes up large masses
of limestone. Rothpletz compares the structure of this genus with
that of the recent alga _Codium adhaerens_ Ag., but it is wiser to
regard such tubular structures as _Girvanella_, _Siphonema_[282] and
_Sphaerocodium_ as closely allied organisms, which are probably algae,
but too imperfectly known to be referred to any particular family.
_Penicillus._
The recent genus _Penicillus_ is one of those algae formerly included
among animals. Fig. 33, O, has been copied from a drawing of a species
of _Penicillus_ given by Lamouroux[283] under the generic name of
_Nesea_ in his treatise on the genera of Polyps published in 1821. He
describes the genus as a brush-like Polyp with a simple stem.
The thallus consists of a stout stem terminating in a brush-like tuft
of fine dichotomously-branched filaments. The apical branches are
divided by regular constrictions into short oval or rod-like segments
which may be encrusted with carbonate of lime. A few of the segments
from the terminal tuft of a recent _Penicillus_ are shown in fig. 35,
E. Each of these calcareous segments has the form of an oval shell
perforated at each end, and the wall is pierced by numerous fine
canals. _Penicillus_ is represented by about 10 recent species, which
with one exception live in tropical seas.
The recognition of _Penicillus_, or a very similar type, in a fossil
condition is due to Munier-Chalmas[284]. This keen observer has
rendered great service to palaeobotany by directing attention to the
calcareous algae in the Paris basin beds, and by proving that many of
the fossils from these Tertiary deposits have been erroneously included
by previous writers among the Foraminifera[285]. It is greatly to be
desired that Prof. Munier-Chalmas may soon publish a monograph on the
fossil Siphoneous forms of which he possesses a unique collection.
_Ovulites._ Figs. 33, K, L, and 35, F.
In his Natural History of Invertebrate Animals, Lamarck described
some small oval bodies from the Calcaire Grossier (Eocene) of the
Paris basin under the name of _Ovulites_. He defined them as
follows:—“Polypier pierreux, libre, ovuliforme ou cylindracé, creux
intérieurement, souvent percé aux deux bouts. Pores très petits,
régulièrement disposés à la surface[286].”
[Illustration: FIG. 33. A and B, _Cymopolia barbata_ (L.); A,
transverse section of the calcareous cylinder. B, verticillate
branches and sporangium after removal of the calcareous matrix (A
and B after Munier-Chalmas). C and D, _Acicularia Andrussowi_ Solms
(C, after Andrussowi; D, after Solms). E, _Acicularia Miocenica_
Reuss; section of a spicula (after Reuss). F and G, _Acicularia
sp._ (after Carpenter), F × 40; G × 20. H, _Acicularia Schencki_
(Möb.) (after Solms). I, _Acetabularia Mediterranea_ Lamx.; section
of the cap (after Falkenberg). K and L, _Ovulites margaritula_
(Lam.) (after Munier-Chalmas); K slightly enlarged; L, a piece of
the thallus more highly magnified. M, _Cymopolia barbata_ (L.)
(after Ellis, nat. size). N, _C. barbata_ (L.); the surface of the
thallus; magnified. O, _Penicillus pyramidalis_ (Lamx.) (after
Lamouroux, nat. size).]
The specimens are referred to two species, _Ovulites margaritula_ and
_O. elongata_.
By some subsequent writers[287] these calcareous fossils, like
miniature birds’ eggs with a hole at either end, were included among
the Zoophytes. Carpenter and others afterwards referred _Ovulites_ to
the Foraminifera, and compared the genus with Lagena[288]. The single
specimens of _Ovulites_ have a length of 2–6 mm. At each end there is
usually a fairly large and somewhat irregular hole (fig. 35, F), and
in some rarer cases there may be two apertures at the broader end of
an Ovulite. A good example of _Ovulites margaritula_ with two pores at
the broader end is figured by Michelin[289]. The surface of the shell
when seen under a low magnifying power appears to be covered over with
regularly arranged circular pores, which are the external openings of
fine canals (fig. 33, L).
In 1878 Munier-Chalmas expressed the opinion, which was supported by
strong evidence, that _Ovulites_ should be referred to the siphoneous
algae[290]. He regarded it as generically identical with _Penicillus_
(_Coralliodendron_, Kützing). It has already been pointed out that
in _Penicillus_ the apical tuft of filaments is partially calcareous
(fig. 33, O)[291]. The individual calcareous segments agree almost
exactly with the fossil _Ovulites_. As a rule the Ovulites occur
as separate egg- or rod-like bodies, but Munier-Chalmas informs me
that occasionally two or three have been found joined end to end in
their natural position. The terminal holes in the fossil specimens
represent the apertures left after the detachment of the calcareous
segments from the uncalcified filaments of the alga. The segments
with two holes at the broader end were no doubt situated at the base
of dichotomising branches as shown in fig. 33, K. The restoration of
_Ovulites_, shown in fig. 33, K, bears a striking resemblance to the
figure of an Australian _Penicillus_ given by Harvey in his _Phycologia
Australica_[292].
It is probable that these Eocene forms agreed closely in habit with
the recent species of _Penicillus_. The portions preserved as fossils
are segments of the filaments which probably formed a terminal brush
of fine branches supported on a stem. The retention of the original
generic name _Ovulites_ is on the whole better than the inclusion of
the fossil species in the recent genus. The Tertiary species lived in
warm seas of the Lower and Middle Eocene of England, Belgium, France
and Italy.
_Halimeda._
An example of an Eocene species of _Halimeda_ has been recorded by
Fuchs from Greifenstein under the name of _Halimeda Saportae_[293]. The
impression has the form of a branched plant consisting of wedge-shaped
or oval segments, and there is a close resemblance to the thallus of a
recent _Halimeda_, e.g. _H. gracilis_ Harv. It is not improbable that
Fuchs’ determination is correct, but without more definite evidence
than is afforded by a mere impression it is a little rash to make use
of the recent generic name.
γ. Dasycladaceae.
In this family of Siphoneae are included a number of genera represented
by species living in tropical and subtropical seas.
The thallus consists of an elongated axial cell fixed to the substratum
by basal rhizoids, and bearing whorls of lateral appendages of limited
growth which may be either simple or branched. Many of the lateral
branches bear sporangia or spores. The thallus is in many species
encrusted with carbonate of lime.
The two genera _Acetabularia_ and _Cymopolia_ may be briefly described
as recent types which are represented by trustworthy fossil forms.
[Illustration: FIG. 34. _Acetabularia mediterranea_ Lamx. From a
specimen in the Cambridge Botanical Museum (nat. size).]
_Acetabularia._ Figs. 33, I, and 34.
With the exception of _A. mediterranea_ Lamx. (fig. 34) the few living
species of this genus are confined to tropical seas.
The habit of _Acetabularia_ is well illustrated by the photograph of a
cluster of plants of _A. mediterranea_ Lamx.[294] reproduced in fig.
34. The thallus consists of a delicate stalk attached to the substratum
by a tuft of basal holdfasts, and expanded distally into a small
circular disc 10–12 mm. in diameter and more or less concave above.
This terminal cap is made up of a number of laterally fused appendages
given off from the upper part of the stalk in the form of a crowded
whorl. The whole thallus resembles a small and long-stalked calcareous
fungus. In each radially elongated compartment of the fertile cap
(fig. 33, I) there are several sporangia (_gametangia_) developed;
these eventually open and produce numerous ciliated gametes which
give rise to zygospores by conjugation. Fig. 33, I, represents the
cap of an _Acetabularia_ in radial section and surface-view; the two
radial compartments seen in section contain the elliptical gametangia;
the circular markings at the base of the figure are scars of sterile
deciduous branches.
The whole plant is unicellular, each chamber in the disc being in open
communication with the stem of the plant.
_Acicularia._ Fig. 33, C–H.
In a recent monograph on the Acetabularieae, Solms-Laubach[295] has
described a new type of these algae which is of special importance
from the point of view of the past history of the family. Möbius
described an example of _Acetabularia_ in 1889 under the name _A.
Schencki_; this species has since been placed in D’Archiac’s genus
_Acicularia_[296]. _Acicularia Schencki_[297] bears a close resemblance
as regards external form to _Acetabularia mediterranea_. In the latter
species the walls of the terminal disc compartments are calcified,
and the cavity of each of the laterally fused members contains
numerous free spores; in _Acicularia_, the cavity of each disc-ray is
occupied by a calcareous substance in the form of a spicule containing
numerous cavities in each of which is a single sporangium. A single
spicule is seen in fig. 33, H, showing the spherical pockets in which
the sporangia were originally situated. This species, _Acicularia
Schencki_, has been recorded from Martinique, Guadeloupe, Brazil, and a
few other places.
The genus _Acicularia_ was founded by D’Archiac for certain minute
calcareous spicules found in the Eocene sands (Calcaire Grossier)
of the Paris basin. D’Archiac describes one species, _Acicularia
pavantina_, which he defines as follows:—“Polypier aciculaire, élargi,
et légèrement comprimé à sa partie supérieure, qui est échancrée au
milieu. Surface couverte de petits pores simples, nombreux, disposés
irrégulièrement[298].” The same species is figured also in Michelin’s
_Iconographie Zoophytologique_, and described as an organism of
which the exact zoological position is uncertain[299]. After these
fossils had been placed in various divisions of the animal kingdom,
Carpenter[300] described several specimens as portions of foraminifera.
Finally, Munier-Chalmas removed _Acicularia_ to the plant kingdom,
and “with rare divination” placed the genus among the Acetabularieae.
The history of our knowledge of the true nature of _Acicularia_ is
of unusual interest. Some of the specimens of this genus figured in
Carpenter’s monograph have the form of imperfect long and narrow bodies
tapering to a point at one end and broad at the other (fig. 33, F and
G); they are joined together laterally and pitted with numerous small
cavities. From the resemblance of such specimens to a fragment of the
terminal fertile disc of the recent Acetabularias, Munier-Chalmas
referred the fossils to this type of algae. In the living species
which were then known the radiating chambers of the disc contained
loose sporangia, without any calcareous matrix filling the cavity of
the chambers. In the fossil Acicularias, on the other hand, the manner
of preservation of the pitted calcareous spicules pointed to the
occurrence of sporangia embedded in cavities in a calcareous matrix.
Subsequent to Munier-Chalmas’ somewhat daring conclusions as to the
relation of _Acicularia_ to _Acetabularia_, Solms-Laubach found that
the species originally described by Möbius as _Acetabularia Schencki_
from Guadeloupe presented exactly those characters in which the fossil
specimens differ from _Acetabularia_. The genus _Acicularia_ formerly
restricted to fossil species is now applied also to this single living
species _Acicularia Schencki_.
The genus is thus defined by Solms-Laubach:—
“Discus fertilis terminalis e radiis inter se conjunctis formatus,
coronis et inferiore et superiore praeditis, sporae massa mucosa
calce incrustata coalitae, pro radio spiculam solidam cuneatam
formantes[301].”
As Solms-Laubach points out in his recent monograph, Munier-Chalmas’
conjecture, “which had little to support it in the fossil material, has
been more recently proved true in the most brilliant fashion by the
discovery of a living species of this genus.”
• • • • •
1. _Acicularia Andrussowi_ Solms[302]. Fig. 33, C and D. This species
was first described by Andrussow[303] as _Acetabularia miocenica_ from
the Crimea. It occurs in Miocene rocks south of Sevastopol, and, with
_Ostrea_ and _Pecten_, forms masses of white limestone.
In each sporangial ray of the disc the cavity contains a calcareous
spicula bearing spore cavities in four rows. “Round each spore-cavity
there is a circular zone which stands out, when viewed in reflected
light, through its white colour against the central mass of the
spicule, though a sharp contour is not visible[304].” Fig. 33, C, is
taken from a somewhat diagrammatic sketch by Andrussow; it shows ten of
the fertile rays of the disc. The thick walls of the chambers are seen
in the two lowest rays, and in the next two rays the spore-cavities
are represented. A more accurate drawing, from Solms-Laubach’s memoir,
is reproduced in fig. 33, D. The calcareous spicule with numerous
spore-cavities shown in fig. 33, H, is from a fertile ray of the recent
species _Acicularia Schencki_. This corresponds to the spore-containing
calcareous matrix in each ray of the disc of _Acicularia Andrussowi_
Solms. The spicule copied in fig. 33, F from one of Carpenter’s
drawings[305] of an Eocene specimen bears the closest resemblance
to the recent spicule of fig. 33, H, and emphasizes the very close
relationship between the fossil forms and the single rare tropical
species.
2. _Acicularia miocenica_ Reuss. Another Tertiary species has been
described under this name by Reuss[306] from the Miocene of the Vienna
district, from the Leithakalk of Moravia and elsewhere. It agrees very
closely with the recent species _A. Schencki_. A section of one of
the spicules of this species is shown in fig. 33, E; the dark patches
represent the pockets in the calcareous spicule which were originally
occupied by sporangia and spores.
_Cymopolia_. Fig. 33, A, B, M and N.
The genus _Cymopolia_ is at present represented by two species, _C.
barbata_ (L.) and _C. mexicana_, Ag., living in the Gulf of Mexico and
off the Canary Islands.
_Cymopolia_ and _Acetabularia_, with several other calcareous algae,
are figured by Ellis and other writers as members of the animal
kingdom. Ellis speaks of the species of _Cymopolia_ which he figures as
the Rosary Bead-Coralline of Jamaica.
Fig. 33, M, has been drawn from a figure published by Ellis in his
_Natural History of the Corallines_ published in 1755[307]. The thallus
has the form of a repeatedly forked body, of which the branches are
divided into cylindrical joints thickly encrusted with carbonate of
lime, but constricted and uncalcified at the limits of each segment. A
tuft of hairs is given off from the terminal segment of each branch.
The axis of each branch of the thallus is occupied by a cylindrical
and unseptate cell which gives off crowded whorls of lateral branches.
In the lower part of fig. 33, M, the calcareous investment has been
removed, and the branches are seen as fine hair-like appendages of the
central cell. The branches given off from the constricted portions of
the axis are unbranched simple appendages, but the others terminate
in bladder-like swellings, each of which bears an apical sporangium.
The sporangia are surrounded and enclosed by the swollen tips of
four to six branches which spring from the summit of the sporangial
branch. Fig. 33, A, represents part of a transverse section through
the calcareous outer portion of a branch of _Cymopolia_; the darker
portions or cavities in the calcareous matrix were originally occupied
by the lateral branches and sporangia[308].
In Fig. 33, B, the sporangial branch with the terminal sporangium and
three of the investing branches are more clearly shown, the surrounding
calcareous investment and the thallus having been removed by the action
of an acid.
In a transverse section of a branch from which the organic matter
had been removed, and only the calcareous matrix left, one would
see a central circular cavity surrounded by a thick calcareous wall
perforated by radially disposed canals and containing globular
cavities; the canals and cavities being occupied in the living plant by
branches and sporangia respectively.
The two circular cavities shown in the figure mark the position of the
sporangia which are borne on branches with somewhat swollen tips. From
the summit the left-hand sporangial branch shown in fig. 33, A, three
of the secondary branches are represented by channels in the calcareous
matrix; the two black dots on the face of the sporangiophore being the
scars of the remaining two secondary branches.
By the lateral contact of the swollen ends of the ultimate branches
enclosing the sporangia the whole surface of the thallus, when examined
with a lens, presents a pitted appearance. Each pit or circular
depression (fig. 33, N) marks the position of the swollen tip of a
branch.
This form of thallus represents a type which is met with in several
members of the Dasycladaceae. It would carry us beyond the limits
of a short account to describe additional recent genera which throw
light on the numerous fossil species. For further information as to
the recent members of the family, the student should refer to Murray’s
_Seaweeds_[309], and for a more detailed memoir on the group to
Wille’s recent contribution to the _Pflanzenfamilien_[310] of Engler
and Prantl. Among the various special contributions to our knowledge
of the Dasycladaceae, those by Munier-Chalmas[311], Cramer[312],
Solms-Laubach[313], and Church[314], may be mentioned.
[Sidenote: PALAEOZOIC SIPHONEAE.]
The publication of a short preliminary note by Prof. Munier-Chalmas in
the _Comptes Rendus_ for 1877 was the means of calling attention to the
exceptional importance of the calcareous Siphoneae as algae possessing
an interesting past history, of which satisfactory records had been
preserved in rocks of various ages. Decaisne had pointed out in 1842
that certain marine organisms previously regarded as animals should
be transferred to the plant kingdom. Such seaweeds as _Halimeda_,
_Udotea_, _Penicillus_ and others were thus assigned to their correct
position. Many fossil algae belonging to this group continued to be
dealt with as Foraminifera until Munier-Chalmas demonstrated their true
affinities. In Gümbel’s monograph on the so-called Nullipores found in
limestone rocks, published in 1871[315], several examples of siphoneous
algae are included among the fossil Protozoa.
In recent years there have been several additions to an already
long list of fossil Siphoneae. In addition to the numerous and
well-preserved specimens, representing a large number of generic and
specific forms, which have been collected from the Eocene of the Paris
basin, there is plenty of evidence of the abundance of the members of
the Dasycladaceae in the Triassic seas. In the Triassic limestones
of the Tyrol, as well as in other regions, the calcareous bodies of
siphoneous algae have played no inconsiderable part as agents of
rock-building[316]. Genera have been recorded from Silurian and other
Palaeozoic horizons, and there is no doubt that the Verticillate
Siphoneae of to-day are the remnants of an extremely ancient family,
which in former periods was represented by a much more widely
distributed and more varied assemblage of species. There is probably
no more promising field of work in the domain of fossil algae than the
further investigation of the numerous forms included in Munier-Chalmas’
class of Siphoneae Verticillatae. A brief description of a few genera
from different geological horizons must suffice to draw attention to
the character of the data for a phylogenetic history of this group.
The fossil examples of the genus _Cymopolia_ (_Polytrypa_) were
originally described by Defrance[317] in the _Dictionnaire des Sciences
Naturelles_ as small polyps under the generic name _Polytrypa_.
In the Eocene sands of the Paris basin there have been found numerous
specimens of short, calcareous tubes which Munier-Chalmas has shewn
are no doubt the isolated segments of an alga practically identical
with the recent _Cymopolia_. A section[318] through one of the
fossil segments presents precisely the same features as those which
are represented in fig. 33, A. The habit of the Eocene alga and its
minute structure were apparently almost identical with those of the
recent species, _Cymopolia barbata_. The two drawings of _Cymopolia_
reproduced in fig. 33, A and B, have been copied from Munier-Chalmas’
note in the _Comptes Rendus_[319]; the corresponding figures given
by this author of the Eocene species (_Cymopolia elongata_ Deb.) are
practically identical with figs. A and B, and show no points of real
difference. The segments of the thallus of the fossil species, as
figured by Defrance[320], appear to be rather longer than those of the
recent species. The calcareous investment of the axial cell of the
thallus was traversed by regular verticils of branches or ‘leaves’; the
central branch of each whorl terminates in an oval sporangial cavity,
exactly as in fig. 33, A and B; and from the top of this branch there
is given off a ring of slender prolongations which terminate on the
surface of the calcareous tube as regularly disposed depressions, which
were no doubt originally occupied by their swollen distal ends as in
the recent species.
_Vermiporella._
This generic name was proposed by Stolley for certain branched and
curved tubes found in Silurian boulders from the North German
drift[321]. The tubes have a diameter of ·5–1 mm., and are perforated
by radial canals which probably mark the position of verticils of
branches given off at right angles to the central axis. The surface of
the tubes is divided into regular hexagonal areas.
The resemblance of these Silurian fossils to _Diplopora_ and other
genera favours their inclusion in the Verticillate Siphoneae.
_Sycidium._ Fig. 32, B.
The fossils included in this genus were first described by Sandberger
from the middle Devonian rocks of the Eifel, and referred by him to the
animal kingdom. More recently Deecke has suggested the removal of the
genus to the calcareous Siphoneae, and such a view appears perfectly
reasonable, although without more data it is not possible to speak with
absolute certainty.
_Sycidium melo._ (Sandb.) Fig. 32, _B._ The specimen represented in
fig. 32, B (i), (ii), drawn from Deecke’s figures[322], has the form of
a small oval calcareous body, 1 mm. in transverse diameter and 1–1·3
mm. in longitudinal diameter. It is pointed at one end and flattened at
the other. At the flatter end there is a circular depression, continued
into a funnel-shaped cavity, and on the walls of this cavity there are
18–20 radially disposed ribs, which extend over the surface of the
whole body. A series of transverse ribs intersects the vertical ribs at
right angles. The calcareous wall is perforated by numerous whorls of
circular pores, and the internal cavity is a simple undivided space.
Each of these oval bodies (fig. 33, B) is probably the segment of a
thallus, and the perforations in the wall may have been originally
occupied by lateral prolongations from the unseptate axial cell of the
thallus. _Sycidium_ bears a fairly close resemblance to the Tertiary
_Ovulites_.
_Diplopora._ Fig. 35, A and B.
This genus of algae is characteristic of Triassic rocks, and is
especially abundant in Muschelkalk and Lower Keuper limestones of the
Alps, Silesia, and elsewhere. The thallus, or rather the calcareous
portion of the thallus, has the form of a thick-walled tube, with a
diameter of about 4 mm., and occasionally reaching a length of 50 mm.
At one end the tube has a rounded and closed termination, and the
wall is pierced throughout its whole length by regular whorls of fine
canals. _Diplopora_ agrees with _Cymopolia_ in its main features.
[Illustration: FIG. 35. A, B, _Diplopora._ × 2. C, D, _Gyroporella_
(after Benecke. × 4). E, Calcareous segments of _Penicillus_, from
a specimen in the British Museum. × 5. F, a single segment of
_Ovulites margaritula_ Lam. × 4. G, _Confervites chantransioides_
Born. (after Bornemann. × 150).]
Fig. 35, A, affords a diagrammatic view of a _Diplopora_ tube, and
shews the arrangement of the numerous whorls of canals. In fig. 35,
B, a piece of limestone is represented containing several Diploporas
cut across transversely and more or less obliquely. In an obliquely
transverse section of a tube perforated by horizontal canals the
cavities of the canals necessarily appear as holes or discontinuous
canals in the substance of the calcareous wall. The manner of
occurrence of the specimens points to the abundance of this genus
in the Triassic seas, and suggests that the calcareous tubes of
_Diplopora_ may have been important factors in the building up of
limestone sediments[323]. In many instances no doubt the carbonate of
lime of the thallus has been dissolved and recrystallised, and the
original form completely obliterated. As in the rocks built up largely
of calcareous Florideae (p. 185) which have lost their structure, it
is a legitimate inference that some of the limestone rocks which shew
no trace of organic structure may have been in part derived from the
calcareous incrustation of various algal genera.
_Gyroporella._ Fig. 35, C and D.
In this genus from the Alpine Trias the structure of the calcareous
tube is very similar to that in _Diplopora_, but in _Gyroporella_ the
canals form less distinct whorls and are closed externally by a small
plate, as seen in figs. 35, C and D.
As Solms-Laubach has pointed out, the branch-systems of _Diplopora_,
_Gyroporella_ and other older genera are much simpler than in the
Tertiary genera _Dactylopora_ and others[324].
A species of _Gyroporella_, _G. bellerophontis_, has recently been
described by Rothpletz[325] from Permian rocks in the Southern Tyrol.
The thallus is tubular in form and has a diameter of ·5–1 mm.
_Dactylopora._
The genus _Dactylopora_ was founded by Lamarck[326] on some fossil
specimens from the Calcaire Grossier and included among the Zoophytes.
D’Orbigny afterwards included it among the Foraminifera, and the
structure of the calcareous body has been described by Carpenter[327]
and other writers on the Foraminifera. In a specimen of _Dactylopora
cylindracea_ Lam. from the Paris basin, for which I am indebted to
Munier-Chalmas, the tubular thallus measures 4 mm. in diameter; at the
complete end it is closed and bluntly rounded. The wall of the tube is
perforated by numerous canals, and contains oval cavities which were no
doubt originally occupied by sporangia. The shape of the specimens is
similar to that of _Diplopora_, but the canals and cavities present a
characteristic and more complex appearance, when seen in a transverse
section of the wall, than in the older genus _Diplopora_. Gümbel has
given a detailed account of this Tertiary genus in his memoir on _Die
sogenannten Nulliporen_[328]; he distinguishes between _Dactyloporella_
and _Gyroporella_ by the existence of cavities in the calcareous wall
of the tube in the former genus, and by their absence in the latter.
The oval cavities in a _Dactyloporella_ were originally occupied by
sporangia; in _Diplopora_ and _Gyroporella_ the sporangia were probably
borne externally and on an uncalcified portion of the thallus.
• • • • •
In addition to the few examples of fossil species described above there
are numerous others of considerable interest, which illustrate the
great wealth of form among the Tertiary and other representatives of
the Verticillate Siphoneae.
Reference has already been made to _Vermiporella_ as an example
of a Silurian genus. Other genera have been described by Stolley
from Silurian boulders in the North-German drift under the names
_Palaeoporella_, _Dasyporella_ and _Rhabdoporella_[329]; the latter
genus is compared with the Triassic _Diplopora_, and the two preceding
with the recent _Bornetella_.
Schlüter has transferred a supposed Devonian Foraminiferal genus,
_Coelotrochium_[330], to the list of Palaeozoic Siphoneae.
Munier-Chalmas regards some of the fossils described by Saporta under
the name of _Goniolina_[331], and classed among the inflorescences of
pro-angiospermous plants, as examples of Jurassic Siphoneae. The shape
and surface-features of some of the examples of _Goniolina_ suggest a
comparison with Echinoid spines, but the resemblance which many of the
forms in the Sorbonne collection present to large calcareous Siphoneae
is still more striking. A comparison of Saporta’s fig. 5, Pl. xxxiii.
and fig. 4, Pl. xxxii. in volume iv. of the _Flore Jurassique_, with
the figures given by Solms-Laubach[332] and Cramer[333] of species of
_Bornetella_ brings out a close similarity between _Goniolina_ and
recent algae; the chief difference being the greater size of the fossil
forms. The possibility of confounding Echinoid spines with calcareous
Siphoneae is illustrated by Rothpletz[334], who has expressed the
opinion that Gümbel’s _Haploporella fasciculata_ is not an alga but the
spine of a sea-urchin.
Among Cretaceous forms, in addition to _Goniolina_, which passes
upwards from Jurassic rocks, _Triploporella_[335] and other genera have
been recorded.
_Uteria_[336] is an interesting type of Tertiary genera; it occurs
in the form of barrel-shaped rings, which are probably the detached
segments of a form in which the central axial cell was encrusted
with carbonate of lime, but the sporangia and the whorls of branches
differed from those of _Cymopolia_ in being without a calcareous
investment.
_b._ CONFERVOIDEAE.
Without attempting to describe at length the fossil forms referred to
this division of the Chlorophyceae, there is one fossil which deserves
a passing notice. Brongniart in 1828[337] instituted the generic term
_Confervites_ for filamentous fossils resembling recent species of
confervoid algae. Numerous fossils have been referred to this genus
by different authors, but they are for the most part valueless and
need not be further considered. In 1887 Bornemann described some new
forms which he referred to this genus from the Cambrian rocks of
Sardinia. He describes the red marble of San Pietra, near Masne,
as being in places full of the delicate remains of algae having the
form of branched filaments, and appearing in sections of the rock as
white lines on a dark crystalline matrix. In fig. 35, G, one of these
Sardinian specimens is represented. This form is named _Confervites
Chantransioides_[338]; the thallus consists of branched cell-filaments,
having a breadth of 6–7µ, and composed of ovate cells. It is possible
that this is a fragment of a Cambrian alga, but the figures and
descriptions do not afford by any means convincing evidence. From
post-Tertiary beds various genera, such as _Vaucheria_ and others, have
been recorded, but they possess but little botanical value.
C. INCERTAE SEDIS.
_Fossils in Boghead ‘Coal’ referred by some authors to
the Chlorophyceae._
During the last few years much has been written by two French authors,
Dr Renault and Prof. Bertrand, on the subject of the so-called Boghead
of France, Scotland, and other countries. They hold the view that the
formation of the extensive beds of this carbonaceous material was due
to the accumulation and preservation of enormous numbers of minute
algae which lived in Permo-Carboniferous lakes.
In an article contributed to _Science-Progress_ in 1895 I ventured
to express doubts as to the correctness of the conclusions of MM.
Renault and Bertrand[339]. Since then Prof. Bertrand has very kindly
demonstrated to me many of his microscopic preparations of various
Bogheads, and I am indebted to Prof. Bayley Balfour of Edinburgh for an
opportunity of examining a series of sections of the Scotch Boghead.
The examination of these specimens has convinced me of the difficulties
of the problems which many investigators have tried to solve, but it
has by no means led me to entirely adopt the views expressed by MM.
Bertrand and Renault.
[Sidenote: BOGHEAD.]
The Boghead or Torbanite of Scotland was rendered famous by a
protracted lawsuit tried in Edinburgh from July 29th to August 4th,
1853. A lease had been granted by Mr and Mrs Gillespie, of Torbanehill,
in Fifeshire, to Messrs James Russell and Son, coal-masters of Falkirk,
of “the whole _coal_, ironstone, iron-ore, limestone, and fire-clay
(but not to comprehend copper, or any other minerals whatsoever, except
those specified) with lands of Torbanehill[340].” After the Boghead had
been worked for two years the Gillespies challenged the right of Messrs
Russell, and argued that the valuable _mineral_ Torbanite was not
included among the substances named in the agreement. The defendants
maintained that it was a _coal_, known as gas-, cannel- or parrot-coal.
A verdict was given for the defendants. Some of the scientific
experts who gave evidence at the trial considered that the Boghead
afforded indications of organic structure, while others regarded it as
essentially mineral in origin.
The Torbanite or Boghead is a close-grained brown rock, of peculiar
toughness and having a subconchoidal fracture. It contains about 65%
carbon, with some hydrogen, oxygen, sulphur, and mineral substances. A
thin section examined under the microscope presents the appearance of
a dark and amorphous matrix, containing numerous oval, spherical and
irregularly shaped bright orange-yellow patches. Fig. 36, 1 shows the
manner of occurrence of the yellow bodies in a piece of Scotch Boghead,
as seen in a slightly magnified horizontal section. Under a higher
power the light patches in the figure reveal traces of a faint radial
striation, which in some cases suggests the occurrence of a number of
oval or polygonal cells.
The Autun Boghead possesses practically the same structure. The yellow
bodies are often sufficiently abundant to impart a bright yellow colour
to a thin section. If the section is vertical the coloured bodies are
seen to be arranged in more or less regular layers parallel to the
plane of bedding.
The Kerosene shale of New South Wales agrees closely with the Scotch
and French Boghead; it is approximately of the same geological age, and
is largely made up of orange or yellow bodies similar to those of the
European Boghead, but much more clearly preserved.
The nature and manner of formation of the various forms of coal should
be dealt with in a later chapter devoted to the subject of plants as
rock-builders, but in view of the recent statements as to the algal
nature of these bituminous deposits it may not be out of place to state
briefly the main conclusions of the French authors.
MM. Renault and Bertrand regard each of the yellow bodies in the
European and Australian Boghead as the thallus of an alga. To the
form which is most abundant in the Kerosene shale they have given
the generic name of _Reinschia_, while that in the Scotch and French
Boghead is named _Pila_.
_Reinschia._ Fig. 36, 3.
A section of a piece of Kerosene shale at right angles to the bedding
appears to be made up of fairly regular layers of flattened elliptical
sacs of an orange or yellow colour. Each sac or thallus is about
300µ in length and 150µ broad (fig. 36, 3). A single row of cells
constitutes the wall surrounding the central globular cavity. The cells
are more or less pyriform in shape, and the cell-cavities are filled
with a dark substance, described by Renault and Bertrand as protoplasm,
and the cell-walls are fairly thick. In some of the larger specimens
there are often found a few smaller sacs enclosed in the cavity of
the partially disorganised mother-thallus. In the larger specimens
the wall is usually invaginated in several places, giving the whole
thallus a lobed or brain-like appearance. The supposed alga, which
makes up ⁹⁄₁₀ths of the contents of a block of Kerosene shale, is named
_Reinschia Australis_; it is regarded by the authors of the species as
nearly related to the Hydrodictyaceae or Volvocineae.
[Illustration: FIG. 36, 1. Section of a piece of Scotch Torbanite.
Slightly enlarged. 2. _Pila bibractensis_ from the Autun Boghead, ×
282 (after Bertrand). 3. _Reinschia Australis_, from the Kerosene
shale of New South Wales, × 592 (after Bertrand).]
In the Kerosene shale from certain localities in New South Wales
Bertrand recognises a second form of thallus, which he refers to the
genus _Pila_, characteristic of the European Bogheads.
_Pila_. Fig. 36, 2.
The “thallus” characteristic of the Scotch Boghead has been named _Pila
scotica_, and that of the Autun Boghead, _Pila bibractensis_.
In the latter form, which has been studied in more detail by MM.
Renault and Bertrand, the thallus consists of about 6–700 cells, and
is irregularly ellipsoidal in form, from ·189–·225mm. in length,
and ·136–·160mm. broad. The surface-cells are radially disposed and
pyramidal in shape, the internal cells are polygonal in outline and
less regularly arranged (fig. 36, 2). The Pila thalli make up ¾ths of
the mass in an average sample of the Autun Boghead. The Autun Boghead
often contains siliceous nodules, and sections of these occasionally
include cells of a _Pila_ in which the protoplasmic contents and
nuclei have been described by the French authors. The evidence for
the existence of these supposed nuclei is, however, not entirely
satisfactory; sections of silicified thalli which were shown to me by
Prof. Bertrand did not satisfy me as to the minute histological details
recognised by Bertrand and Renault.
The species of _Pila_ are compared with the recent genus _Celastrum_,
and regarded as most nearly allied to the Chroococcaceae or
Pleurococcaceae among recent algae. Prof. Bornet[341] has suggested
_Gomphosphaeria_ as a genus which presents a resemblance to the Autun
_Pila_.
In addition to the Bogheads of Autun, Torbanehill, and New South Wales,
there are similar Palaeozoic deposits in Russia, America, and various
other parts of the world. Full details of the structure of Boghead and
the supposed algae referred to _Reinschia_, _Pila_, and other genera
will be found in the writings of Bertrand and Renault[342].
The Kerosene shale of New South Wales affords the most striking and
well-preserved examples of the cellular orange and yellow bodies
referred to as the globular thalli of algae. It is almost impossible to
conceive a purely inorganic material assuming such forms as those which
occur in the Australian Boghead. On the other hand, it is hardly less
easy to understand the possibility of such explanations as have been
suggested of the organic origin of these characteristic bodies.
The ground-mass or matrix of the Boghead is referred to a brown ulmic
precipitate thrown down on the floor of a Permian or Carboniferous
lake, probably under the action of calcareous water. In this material
there accumulated countless thalli of minute gelatinous algae, which
probably at certain seasons completely covered the surface of the
waters, as the _fleurs d’eau_ in many of our fresh-water lakes. In
addition to the thalli of _Reinschia_ and _Pila_ the Bogheads contain
a few remains of various plant fragments, pollen-grains, and pieces
of wood. Fish-scales and the coprolites of reptiles and fishes occur
in some of the beds. On a piece of Kerosene shale in the Woodwardian
Museum, Cambridge, there are two well-preserved graphitic impressions
of the tongue-shaped fronds of _Glossopteris Browniana_, Brongn. There
can be little doubt that the beds of Boghead were deposited under water
as members of a regular sequence of sedimentary strata. The yellow
bodies which form so great a part of the beds are practically all of
the same type. _Reinschia_ and _Pila_ cannot always be distinguished,
and it would seem that there are no adequate grounds for instituting
two distinct genera and referring them to different families of recent
algae.
Stated briefly, my conclusion is that the algae of the French
authors may be definite organic bodies, but it is unwise to attempt
to determine their affinities within such narrow limits as have been
referred to in the above _résumé_. The structure of the bituminous
deposits is worthy of careful study, and it is by no means impossible
that further research might lead us to accept the view of the earlier
investigators, that the brightly coloured organic-like bodies may be
inorganic in origin.
D. RHODOPHYCEAE. (FLORIDEAE. RED ALGAE.)
The thallus of the members of this group assumes various forms,
and consists of branched cell-filaments of a more or less complex
structure. Cells of the thallus contain a red colouring matter in
addition to the green chlorophyll. The reproduction is asexual and
sexual; the formation of asexual reproductive cells (_tetraspores_) in
groups of four in sporangia is a characteristic method of reproduction.
Sexual reproduction is effected by means of distinct male and female
cells.
With the exception of a few fresh-water genera all the red algae are
marine. The Rhodophyceae, like the Cyanophyceae and Chlorophyceae,
include a shell-boring form which has been found in the common
razor-shell[343]. Several genera live as endophytes in the tissues
of other algae. The recent species of this section of algae are
characteristic of temperate and tropical seas. One subdivision of the
red algae, the Corallinaceae, is extremely important from a geological
point of view and must be dealt with in some detail.
CORALLINACEAE.
The thallus is usually encrusted with carbonate of lime; it is of a
branched cylindrical form in the well-known _Corallina officinalis_,
Linn. of the British coasts, of an encrusting and foliaceous type, in
the genus _Lithophyllum_, and of a more coral-like form in the genus
_Lithothamnion_. The reproductive organs occur in conceptacles, having
the form of small depressed cavities in the thallus, or projecting as
warty swellings above the surface of the plant. Asexual reproduction
is by means of tetraspores formed in conceptacles resembling those
containing the sexual cells. The Corallinaceae may be subdivided into
the two families Melobesieae and Corallineae[344].
=Melobesieae.= Thallus encrusting, leaf- or coral-like;
unsegmented.
(_Melobesia_, _Lithophyllum_, _Lithothamnion_.)
=Corallineae.= Cylindrical filamentous and segmented thallus.
(_Amphiroa_ and _Corallina_.)
The genus _Corallina_ is the best known British representative of
the Corallinaceae. With other members of the group it was long
regarded as a coralline animal, and it is only comparatively recently
that the plant-nature of these forms has been generally admitted.
_Lithophyllum_, _Lithothamnion_, _Melobesia_, and other genera of
the Corallinaceae and some of the Siphoneae play a very important
part in the building and cementing of coral-reefs. The pink or
rose-coloured calcareous thallus of some of these calcareous algae
or Nullipores imparts to coral-reefs a characteristic appearance.
In some cases, indeed, the coral-reefs are very largely composed
of algae. Saville Kent[345] describes the Corallines or Nullipores
of the Australian Barrier-reef as furnishing a considerable quota
towards the composition of the coral rock. Mr Stanley Gardiner, who
accompanied the coral-boring expedition to the island of Funafuti, has
kindly allowed me to quote the following extract from his notes, which
affords an interesting example of the importance of calcareous algae as
reef-building organisms. “It is quite a misnomer to speak of the outer
edge of a reef like this (Rotuma Island) as being formed of coral. It
would be far better to call it a Nullipore reef, as it is completely
encrusted by these algae, while outside in the perfectly clear water,
10 to 15 fathoms in depth, the bottom has a most brilliant appearance
from masses of red, white and pink Nullipores, with only a stray coral
here and there.”
Agassiz[346] has given an account of the occurrence of immense masses
of Nullipores (_Udotea_, _Halimeda_ etc.) in the Florida reefs; his
description is illustrated by good figures of these algae.
In the Mediterranean there are true Nullipore reefs, which are
interesting geologically as well as botanically. Walther[347] has
described one of these limestone-banks in the Gulf of Naples which
occurs about 1 kilometre from the coast and 30 metres below the surface
of the water. Every dredging, he says, brings up numberless masses
of _Lithothamnion fasciculatum_ (Lamarck), and _L. crassum_ (Phil.).
Between the branches of the algae, gasteropods and other animals become
completely enclosed by the growing plants, while diatoms, foraminifera,
and other forms of life are abundant. Water percolating through the
mass gradually destroys the structure of the algal thalli, and in
places reduces the whole bank to a compact structureless limestone.
The same author[348] has also called attention to the importance of
_Lithophyllum_ as a constructive element in the coral-reefs off the
Sinai peninsula.
_Lithothamnion_ a typical genus of the Corallinaceae may be briefly
described.
_Lithothamnion._ Fig. 37.
Philippi[349] was the first writer to describe this and other genera as
plants. He gave the following definition of _Lithothamnion_:
“Stirps calcarea rigida, e ramis cylindricis vel compressiusculis
dichotoma ramosis constans.”
The thallus of _Lithothamnion_ grows attached to the face of a rock
or other foundation, and forms a hard, stony mass, assuming various
coralline shapes. The exposed face may have the form of numerous short
branches or of an irregular warty surface.
In section (fig. 37, A.) the lower part of the thallus is seen to
be made up of rows of cells radiating out from a central point, and
the upper portion consists of vertical and horizontal rows of cells.
The whole body is divided up into a large number of small cells by
anticlinal and periclinal walls, and possesses an evident cellular as
distinct from a tubular structure. Conceptacles containing reproductive
organs are either sunk in the thallus or project above the surface. The
two types of structure in a single thallus are shown in fig. 37, A,
also a conceptacle containing tetraspores.
[Illustration: FIG. 37, _A._ Section of a recent _Lithothamnion_ (after
Rosanoff[350], × 200). _B._ Section of _Lithothamnion suganum_,
Roth (after Rothpletz[351], × 100). _C._ A conceptacle with
tetraspores from a Tertiary _Lithothamnion_ (after Früh[352], ×
300). _D._ _Sphaerocodium Bornemanni_ Roth, (after Rothpletz, ×
150).]
In the closely allied _Lithophyllum_ the thallus is encrusting, and
in section it presents the same appearance as the lower part of a
Lithothamnion thallus.
Species of _Lithothamnion_ occur in the Mediterranean Sea, and are
abundant in the arctic regions[353], while on the British coasts the
genus is represented by four species[354]. Some large specimens
of _Lithothamnion_ and _Lithophyllum_ are exhibited in one of the
show-cases in the botanical department of the British Museum. For
the best figures and descriptions of recent species reference should
be made to the works of Hauck, Rosanoff, Rosenvinge, Kjellman and
Solms-Laubach[355].
It is to be expected that such calcareous algae as _Lithothamnion_
should be widely represented by fossil forms. In addition to the
botanical importance of the data furnished by the fossil species as
to the past history of the Corallinaceae, there is much of geological
interest to be learnt from a study of the manner of occurrence of both
the fossil and recent representatives. As agents of rock-building the
coralline algae are especially important. The late Prof. Unger[356] in
1858 gave an account of the so-called Leithakalk of the Tertiary Vienna
basin, and recognised the importance of fossil algae as rock-forming
organisms. The Miocene Leithakalk, which is widely used in Vienna as a
building stone[357], consists in part of limestone rocks consisting to
a large extent of _Lithothamnion_.
Since the publication of Unger’s work several writers have described
numerous fossil species of _Lithothamnion_ from various geological
horizons. A few examples will suffice to illustrate the range and
structure of this and other genera of the Corallinaceae. In dealing
with the fossil species it is often impossible to make use of those
characters which are of primary importance in the recognition of recent
species. The fossil thallus is usually too intimately associated with
the surrounding rock to admit of any use being made of external form
as a diagnostic feature. The size and form of the cells must be taken
as the chief basis on which to determine specific differences. In
the absence of conceptacles or reproductive organs it is not always
easy to distinguish calcareous algae from fossil Hydrozoa or Bryozoa.
In many instances, however, apart from the nature and size of the
elements composing the thallus, the conceptacles afford a valuable
aid to identification. An example of a fossil conceptacle containing
tetraspores is shown in fig. 37, C; it is from a Tertiary species of
_Lithothamnion_, described by Früh from Montévraz in Switzerland.
• • • • •
1. _Lithothamnion mamillosum_ Gümb. Fig. 32, A (i) and (ii). (p.
155.) This species was first recorded by Gümbel[358] from the Upper
Cretaceous (Danian) rocks of Petersbergs, near Maëstricht, on the
Belgian frontier. It was originally described as a Bryozoan. The
thallus has the form of an encrusting calcareous structure bearing
on its upper surface thick nodular branches, as shown in fig. 32, A
(ii); in section, A (i), the thallus consists of a regular series of
rectangular cells.
The specific name _mamillosum_ has also been given to a recent species
by Hauck[359], but probably in ignorance of the existence of Gümbel’s
Cretaceous species.
• • • • •
2. _Lithothamnion suganum_ Roth. Fig. 37, B. The section of this
form given in fig. 37, B shows three oval conceptacles filled
with crystalline material. The two lower conceptacles originally
communicated with the surface of the thallus, but as in recent species
the deeper portions of the algal body became covered over by additions
to the surface, forming merely dead foundations for new and overlying
living tissues.
The cells of the thallus have a breadth of 7–9µ, and a length of 9–12µ.
The specimen was obtained from a Lithothamnion bank, probably of Upper
Oligocene age, in Val Sugana[360], in the Austrian Tyrol.
Numerous other species of Jurassic, Cretaceous and Tertiary age
might be quoted, but the above may suffice to illustrate the general
characters and mode of occurrence of the genus. It is important that
the student should become familiar with the _Lithothamnion_ and
_Lithophyllum_ types of thallus, in view of their frequent occurrence
in crystalline limestone rocks and in such comparatively recent
deposits as those of upraised coral-reefs. The coral-rock of Barbados
and other West-Indian islands affords a good illustration of the
manner of occurrence of fossil coralline algae in association with
corals and other organisms[361].
In the fossil species of _Lithothamnion_ hitherto recorded there do
not appear to be any important features in which they differ from
recent forms; the geological history of the genus so far as it is
known, favours the view that the generic characters are of considerable
antiquity.
_Solenopora._ Fig. 38.
Mr A. Brown[362], of Aberdeen, has recently brought forward good
evidence for including various calcareous fossils, described by several
authors under different names and referred to various genera of fossil
animals, in the genus _Solenopora_, which he places among the coralline
algae.
Species of this genus have been described from England, Scotland,
Esthonia, Russia, and other countries. The geological range of
_Solenopora_ appears to be from Ordovician to Jurassic rocks; in some
cases it is an important constituent of beds of limestone.
• • • • •
_Solenopora compacta_ (Billings). Fig. 38. This species was originally
described by Billings as _Stromatopora compacta_, and afterwards
defined by Nicholson and Etheridge. The thallus forms sub-spheroidal
masses, from the size of a hemp-seed to that of an orange. The external
surface is lobulate; the fractured surface has a porcellaneous and
sometimes a fibrous appearance, and is usually white or light brown
in colour. In vertical section (fig. 38, B) the cells are elongated
and arranged in a radiating and parallel fashion; they often occur in
concentric layers. The cells have a diameter of about ¹⁄₁₇ mm. and
possess distinctly undulating walls, as seen in a tangential section
(fig. 38, A). Brown describes certain larger cells in the thallus (fig.
38, A) as sporangia[363], but it is difficult to recognise any distinct
sporangial cavities in the drawing. The example figured is from the
Trenton limestone of Canada; a variety of the same species has been
recorded from the Ordovician rocks of Girvan in Ayrshire. There appear
to be good reasons for accepting Brown’s conclusion that _Solenopora_
belongs to the Corallinaceae rather than to the Hydrozoa, among which
it was originally included. After comparing _Solenopora_ with recent
genera of Florideae, Brown concludes that “the forms of the cells and
cell-walls, the method of increase, and the arrangement of the tissue
cells in the various species of _Solenopora_ bear strong evidence of
relationship between that genus and the calcareous algae[364].”
[Illustration: FIG. 38. Solenopora compacta (Billings). A. Tangential
section. × 100. B. Vertical section. × 50. (After Brown.)]
The importance of the calcareous Rhodophyceae has been frequently
emphasised by recent researches, and our knowledge of the rock-building
forms is already fairly extensive. We possess evidence of the existence
of species of different genera in Ordovician seas, as well as in those
of the Silurian, Triassic, Jurassic, and more recent periods. It is
reasonable to prophesy that further researches into the structure
of ancient limestones will considerably extend our knowledge of the
geological and botanical history of the Corallinaceae.
Numerous fossils have been described as examples of other genera[365]
of Rhodophyceae than those included in the Corallinaceae, but these
possess little or no scientific value and need not be considered.
E. PHAEOPHYCEAE (BROWN ALGAE).
Olive-brown algae, thallus often leathery in texture, composed of
cell-filaments or parenchymatous tissue, in some cases exhibiting
a considerable degree of internal differentiation. The sexual
reproductive organs may be either in the form of passive egg-cells
and motile antherozoids or of motile cells showing no external sexual
difference.
With one or two exceptions all the genera are marine. They have a wide
distribution at the present day, and are especially characteristic
of far northern and extreme southern latitudes. The gigantic forms
_Lessonia_, _Macrocystis_ and others already alluded to, belong to this
group; also the genus _Sargassum_, of which the numberless floating
plants constitute the characteristic vegetation of the Sargasso Sea.
Palaeobotanical literature is full of descriptions of supposed fossil
representatives of the brown algae, but only a few of the recorded
species possess more than a very doubtful value; most of them are
worthless as trustworthy botanical records. Many of the numerous
impressions referred to as species of _Fucoides_ and other genera
present a superficial resemblance to the thallus of the common
Bladder-wrack and other brown seaweeds. Such similarity of form,
however, in the case of flat and branched algal-like fossils is of no
scientific value. In many instances the impressions are probably those
of an alga, but they are of no botanical interest. The flat and forked
type of thallus of _Fucus_, _Chondrus crispus_ (L.) and other members
of the Phaeophyceae is met with also among the red and green algae, to
say nothing of its occurrence in the group of thalloid Liverworts, or
of the almost identical form of various members of the animal kingdom.
The variety of form of the thallus in one species is well illustrated
by the common _Chondrus crispus_ (L.). This alga was described by
Turner[366] in his classic work on the _Fuci_ under the name of _Fucus
crispus_ as “a marine Proteus.” It affords an interesting example of
the different appearance presented by the same species under different
conditions, and at the same time it furnishes another proof of the
futility of relying on imperfectly preserved external features as
taxonomic characters of primary importance.
An example of a supposed Jurassic _Fucus_ is shown in fig. 49, and
briefly described in the Chapter dealing with fossil Bryophytes.
Several species of Flysch Algae have recently been referred by
Rothpletz[367] to the Phaeophyceae under the provisional generic name
_Phycopsis_, but they are of no special botanical interest.
The extremely interesting genus _Nematophycus_ has lately been assigned
by a Canadian author[368] to a position in the Phaeophyceae. Although
the particular points on which he chiefly relies are not perhaps
thoroughly established, there are certain considerations which lead us
to include _Nematophycus_ as a doubtful member of the present group of
algae.
_Nematophycus._
The stem attains a diameter of between 2 and 3 feet in the largest
specimens; it is made up either of comparatively wide and loosely
arranged tubes pursuing a slightly irregular vertical course
accompanied by a plexus of much narrower tubes, or of tubes varying
in diameter but not divisible into two distinct types. Rings of
growth occur in some forms but not in others. Radially elongated or
isodiametric spaces occur in the stem tissues in which the tubes are
less abundant.
Reproductive organs unknown, with the possible exception of some very
doubtful bodies described as spores.
In 1856 Sir William Dawson proposed the generic name _Prototaxites_
for some large silicified trunks discovered in the Lower and Middle
Devonian rocks of Canada. A few years later the same writer[369]
published a detailed account of the new fossils and arrived at the
conclusion that the Devonian stem showed definite points of affinity
with the recent genus _Taxus_, and the generic name suggests that he
regarded it as the type of Coniferous trees belonging to the sub-family
Taxineae. The reasons for this determination were afterwards shown
by Carruthers to be erroneous. Dawson thought he recognised pits and
spiral thickenings in the walls of the tubular elements, as well as
pointed ends in some of the latter. The spiral markings were in reality
small hyphal tubes passing obliquely across the face of the wider
tubes, and the apparent ends of the supposed tracheids were deceptive
appearances due to the fact that the tubes had in some cases been cut
through in an oblique direction. In 1870 Carruthers[370] expressed the
opinion that Dawson’s _Prototaxites_ was a “colossal fossil seaweed”
and not a coniferous plant. The same author[371] in 1872 published
a full and able account of the genus, and conclusively proved that
_Prototaxites_ could not be accepted as a Phanerogam; he brought
forward almost convincing evidence in favour of including the genus
among the algae. The name _Prototaxites_ was now changed for that of
_Nematophycus_. Carruthers compares the rings of growth in the fossil
stems with those in the large Antarctic _Lessonia_ stems, but he
regards the histological characters as pointing to the Siphoneae as the
most likely group of recent algae in which to include the Palaeozoic
genus.
We may pass over various notes and additional contributions by Dawson,
who did not admit the corrections to his original descriptions which
Carruthers’ work supplied. In 1889 an important memoir appeared by
Penhallow[372] of Montreal in which he confirmed Carruthers’ decision
as to the algal nature of _Prototaxites_; he contributed some new facts
to the previous account by Carruthers, and expressed himself in favour
of regarding the fossil plant as a near ally of the recent Laminariae.
The next addition to our botanical knowledge of this genus was made by
Barber[373] who described a new specific type of _Nematophycus_—_N.
Storriei_—found by Storrie in beds of Wenlock limestone age near
Cardiff. Solms-Laubach[374], in a recent memoir on Devonian plants,
recorded the occurrence of another species of this genus in Middle
Devonian rocks near Gräfrath on the Lower Rhine. Lastly Penhallow[375],
in describing a new species, lays stress on the resemblance of some
of the tubular elements in the stem to the sieve-hyphae of the recent
seaweeds _Macrocystis_ and _Laminaria_. He concludes that the new facts
he records make it clear that _Nematophycus_ “is an alga, and of an
alliance with the Laminarias.” The recent evidence brought forward by
Penhallow is not entirely satisfactory; the drawings and descriptions
of the supposed trumpet-shaped sieve-hyphae are not conclusive. On the
whole it is probably the better course to speak of _Nematophycus_ as a
possible ally of the brown algae rather than as an extinct type of the
Siphoneae, but until our knowledge is more complete it is practically
impossible to decide the exact position of this Siluro-Devonian genus.
Solms-Laubach[376] has suggested that the generic name _Nematophyton_,
used by Penhallow in preference to Carruthers’ term _Nematophycus_,
is the more suitable as being a neutral designation and not one which
assumes a definite botanical position. In view of the nature of the
evidence in favour of the algal affinities of the fossil, the reasons
for discarding Carruthers’ original name are hardly sufficient.
Before discussing more fully the distribution and botanical position of
_Nematophycus_ we may describe at length one of the best known species,
and give a short account of some other forms.
• • • • •
1. _Nematophycus Logani_ (Daws.). Fig. 39, A–E. The stem possesses well
marked concentric rings of growth due to a periodic difference in size
of the large tubular elements. The tissues consist of two distinct
kinds of tubular elements, the larger tubes loosely arranged and
pursuing a fairly regular longitudinal course, and having a diameter of
13–35µ; the smaller tubes, with a diameter of 5–6µ, ramify in different
directions and form a loose plexus among the larger and more regularly
disposed elements. Branching occurs in both kinds of tubes; septa have
been recognised only in the smaller tubes. Irregular and discontinuous
radial spaces traverse the stem tissues, having a superficial
resemblance in their manner of occurrence to the medullary rays of the
higher plants.
The best specimens of this species were obtained by Sir William
Dawson from the Devonian Sandstones of Gaspé in New Brunswick. The
largest stems had a diameter of 3 feet and reached a length of several
feet[377]; in some examples Dawson found lateral appendages attached
to the stem which he described as “spreading roots.” Externally the
specimens were occasionally covered with a layer of friable coal,
and internally the tissues were found to be more or less perfectly
preserved by the infiltration of a siliceous solution. Most of the
examples of _Nematophycus_ from Britain and Germany are much smaller
and less perfectly preserved than those from Canada. The Peter Redpath
Museum, Montreal, contains several very large blocks of _Nematophycus_,
in many of which one sees the concentric rings of growth clearly etched
out by weathering agents in a cross section of a large stem.
In fig. 39, A, a sketch is given of a thin transverse section of a
stem, drawn natural size. The lines of growth are clearly seen, and
as in coniferous stems the breadth of the concentric zones varies
considerably. The short lines traversing the tissues in a radial
direction represent the medullary-ray-like spaces referred to in the
specific diagnosis. A transverse section examined under a low-power
objective presents the appearance of a number of thick-walled and
comparatively wide tubes loosely arranged; they may be in contact or
separated from one another. If the microscope be carefully focussed
through the thickness of the section the transversely-cut tubes appear
to move laterally, producing a curiously dazzling effect if the
objective is raised or lowered rapidly. This lateral movement is due
to the undulating vertical course of the tubes. Under a higher power
the lighter-coloured matrix in which the tubes are embedded shows a
number of very much smaller and thinner-walled hyphal elements; some
of these are cut across transversely, others more or less obliquely
and others again longitudinally. These smaller tubes constitute an
irregular plexus surrounding and ramifying between the larger elements.
The diameter of the larger tubes decreases for a certain distance in
a radial direction as seen in a transverse section, and this change
in size gives rise to the appearance of concentric lines indicating
periodic changes in growth.
[Illustration: FIG. 39. _Nematophycus Logani_ (Daws.). A. Part of a
transverse section from a specimen in the British Museum. (Nat.
size.) B. Transverse section from specimens in Mr Barber’s
possession. C. Longitudinal section. (B and C × 160.) D. Transverse
section showing a radial space. E. Transverse section; a few
‘cells’ more highly magnified. D and E from a specimen in the
British Museum.]
The radial spaces are characterised by the partial absence of the
larger tubes, and as seen in longitudinal sections these spaces
constitute regions in which the smaller tubes branch very freely. Fig.
39, B, represents a small piece of a transverse section seen under a
fairly high power. In fig. 39, C, the tubes are seen in longitudinal
section. The larger elements are unseptate and not very regular in
their vertical course through the stem; the smaller elements are
seen as fine tubes lying between and across the larger tubes. In the
sections I have examined no undoubted transverse septa were detected in
any of the tubular elements.
The question as to the possible connection between the larger and
smaller elements is one which is not as yet satisfactorily disposed
of. Penhallow[378] regards the finer hyphal elements as branches of
the larger tubes, but Barber[379], who has carefully examined good
material of _Nematophycus Logani_, was unable to detect any organic
connection between the two. My own observations are in accord with
those of Barber. Further details and numerous figures of this species
of _Nematophycus_ will be found in the memoirs of Carruthers, Penhallow
and Barber.
• • • • •
Some specimens of silicified _Nematophycus_ stems afford particularly
instructive examples of the state of preservation or method of
mineralisation as a source of error in histological work. The sketches
reproduced in fig. 39, D and E, were made from a section of a large
specimen of _Nematophycus_ in the British Museum. In fig. D we have
one of the radial spaces containing some indistinct small elements,
the tissue surrounding the space appears to consist of polygonal cells
suggesting ordinary parenchymatous tissue. In fig. E a few of these
‘cells’ are seen more clearly, they have black and ragged walls, and
often contain very small and faint circles of which the precise nature
is uncertain. The true interpretation of this form of structure
was first supplied by Penhallow[380]. The black network simulating
parenchymatous tissue consists of the substance of _Nematophycus_
tubes which has been completely redistributed during fossilisation
and collected along fairly regular lines, as seen in figs. D and E.
The original structure has been almost completely destroyed, and
the material composing the walls of the large tubes has finally
been rearranged as a network, interrupted here and there by the
characteristic radial spaces which remain as evidence of the original
_Nematophycus_ characters. It is possible in some cases to trace
every gradation from sections exhibiting the normal structure through
those having the appearance shown in figs. D and E to others in which
the structure is completely lost. Penhallow describes this method
of fossilisation in _N. crassus_ (Daws.); an examination of several
specimens in the National Collection leads me to entirely confirm
his general conclusions, and also to the opinion that _N. Logani_
shows exactly the same manner of mineralisation as _N. crassus_. The
chief point of interest as regards this method of preservation lies
in the fact that a fossil described by Dawson[381] as _Celluloxylon
primaevum_, and referred to as a probable conifer, is undoubtedly a
badly preserved _Nematophycus_. Penhallow examined Dawson’s specimens
and obtained convincing evidence of their identity with certain forms
of highly altered _Nematophycus_ stems.
• • • • •
2. _Nematophycus Storriei_ Barber. Fig. 40. The specimens on which
Barber[382] founded this species were obtained by Mr Storrie from the
Tymawr quarry near Cardiff, in beds of Wenlock age. The fragmentary
nature of the material is largely compensated for by the excellence of
the preservation. We may briefly define the species as follows:
The stem consists of separate interlacing undivided and usually
unbranched tubes of varying diameter. Spaces more or less isodiametric
in dimensions are scattered through the tissue. The spaces constitute
regions in which the tubular elements branch freely.
The main distinguishing features of this British species are (i) the
absence of two distinct and well-defined forms of tubular elements.
The main part of the stem consists of thick walled tubes similar to
those of _N. Logani_, but the spaces between them are occupied by
thinner-walled and smaller tubes varying considerably in diameter;
(ii) the form of the spaces which are not radially elongated as in _N.
Logani_.
[Illustration: FIG. 40. _Nematophycus Storriei_ Barb. Longitudinal
section, from a photograph by Mr C. A. Barber. × 45.]
Fig. 40 shows the undulating course of the tubes as seen in a
longitudinal section; the black colour of some of the elements is
due to the fact that the surface of the wall is seen, while in the
lighter-coloured portions of the tubes the wall has been cut through.
The lighter patch about the middle of the figure shows the form of one
of the spaces in which the tubes are freely branched.
In addition to the two species already described six others have been
recorded, but with these we need not concern ourselves in detail.
One of these species, _N. Hicksi_, was found by Dr Hicks[383] in the
Denbighshire grits quarry of Pen-y-Glog near Corwen in North Wales. The
position of these beds has recently been determined by Mr Lake[384]
as corresponding to that of the Wenlock limestone. This species and
_N. Storriei_ are both Silurian examples of the genus. It is possible,
as Barber has suggested, that the specimens described under these two
names should be referred to one species. The specimens found by Hicks
were small and imperfectly preserved fragments; Etheridge has given
a full description of their structure, and Barber has subsequently
examined the material. The preservation is not such as will admit
of any very precise specific diagnosis; the fragments are correctly
referred to _Nematophycus_, but their specific characters cannot be
clearly determined.
Solms-Laubach[385] has described some fragments of another species
of _Nematophycus_ from the Devonian rocks of the Lower Rhine. His
specimens are chiefly interesting as extending the geographical
range of the genus, and as affording examples of a curious method of
preservation. The specimens obtained were small fragments, flattened
and very dark brown in colour. The tubular elements consisted of an
external membrane of black coal, enclosing a central core of dark red
iron-oxide. On burning the fragment on a piece of platinum foil the
coal composing the wall of the tubes was removed and the deep-red casts
of the tube-cavities remained[386]. The investigation of the structural
characters of this imperfect material was conducted by reflected light.
Under certain conditions, when it is impossible to obtain thin sections
for examination by transmitted light, it is possible to accomplish
much, as shown by Solms-Laubach’s work, by means of observation with
direct light.
The last species to be noticed is _Nematophycus Ortoni_ recently
described by Penhallow. There are no concentric rings of growth,
no radial spaces and no smaller hyphae in the tissues of this
type of stem. In longitudinal section, the tubes show occasional
local expansions of the lumen which Penhallow compares with the
‘trumpet-hyphae’ of some recent brown algae. No actual sieve-plates
or transverse walls have been detected, but the general appearance of
the tubes is considered to afford distinct evidence of the original
existence of such walls. The figures accompanying the description do
not carry conviction as to the correctness of the reference of the
tubes to imperfectly preserved sieve-hyphae.
The following list, taken, with a few alterations, from Penhallow’s
memoir[387], shows the geographical and geological range of the species
of _Nematophycus_ hitherto recorded.
╭ Lower Devonian of Gaspé.
_Nematophycus Logani_ (Daws.) ┤ Silurian [Wenlock] of England.
╰ Silurian of New Brunswick.
_N. Hicksi_ (Eth.) Silurian. (Wenlock) of N. Wales.
_N. crassus_ (Daws.)[388] Middle Devonian of Gaspé and New
York.
_N. laxus_ (Daws.) Lower Devonian of Gaspé.
_N. tenuis_ (Daws.) Lower Devonian of Gaspé.
_N. Storriei_ (Barb.) Silurian (Wenlock) of Wales
(Cardiff).
_N. dechenianus_ (Pied.) Upper Devonian of Germany (Gräfrath).
_N. Ortoni_ (Pen.) Upper Erian of Ohio.
In summing up our information as to the structure of _Nematophycus_ we
find there are certain points not definitely settled, and which are
of considerable importance. The few recorded instances of spore-like
bodies by Penhallow and Barber are not satisfactory; we are still
ignorant of the nature of the reproductive organs. Such instances of
lateral appendages as have been referred to do not throw much light
on the habit of the plant. So far as we know at present the stem of
_Nematophycus_ was not differentiated internally into a cortical and
central region. It may be that the specimens have been only partially
preserved, and the coaly layer which occasionally surrounds a stem may
represent a carbonised cortex which has never been petrified. The large
and loosely arranged tubes constitute the chief characteristic feature
of the genus; in some cases (_N. Logani_) there is an accompanying
plexus of smaller hyphae, in others (_N. Storriei_) there is no
definite division of the tissue into two sets of tubes of uniform size,
and in _N. Ortoni_ the tubular elements are all of the large type.
Penhallow has recognised the branching of large tubes in _N. Logani_
and _N. crassus_ giving rise to the small hyphal elements. In most
specimens, however, no such mode of origin of the smaller tubes can be
detected. The spaces which interrupt the homogeneity of the tissues
in some forms have been described as branching depots, on account of
the frequent occurrence in these areas of much branched hyphae. The
function of these spaces (fig. 39, D, and fig. 40) may be connected
with aeration of the stem-tissues.
As Carruthers first pointed out the unseptate nature of the elements
and the occurrence of large and small tubes forming a comparatively lax
tissue suggested affinities with such recent genera as _Penicillus_,
_Halimeda_, _Udotea_ and other members of the Siphoneae. In those fossil
stems which possess tubes of two distinct sizes, we cannot as a rule
trace any organic connection between the two sets of tubular elements.
Transverse septa have been detected in the tubes of some specimens
of _N. Logani_. These considerations and the large size and habit of
growth of the stem leave one sceptical as to the wisdom of assigning
the fossil genus to the Siphoneae. On the other hand, apart from
the doubtful sieve-hyphae of Penhallow, the manner of growth of the
plant, the concentric rings, marked by a decrease in the diameter of
the tubes, the lax arrangement and irregular course of the elements,
afford points of agreement with some recent Phaeophyceae. The stem
of a _Laminaria_ (fig. 29) or of a _Lessonia_ are the most obvious
structures with which to compare _Nematophycus_. The medullary region
of a _Laminaria_ or _Fucus_ and of other genera presents a certain
resemblance to the tissues of the fossil stems. On the whole we may
be content to leave _Nematophycus_ for the present as probably an
extinct type of alga, more closely allied to the large members of the
Phaeophyceae than to any other recent seaweeds.
_Pachytheca._
(A fossil of uncertain affinity.)
There is another fossil occasionally associated with _Nematophycus_ and
referred by many writers to the Algae, which calls for a brief notice.
_Pachytheca_ is too doubtful a genus to justify a detailed treatment in
the present work. Although, as I have elsewhere suggested[389], we are
hardly in a position to speak with any degree of certainty as to its
affinity, it is not improbable that it may eventually be shown to be an
alga.
Without attempting a full diagnosis of the genus, we may briefly refer
to its most striking characters.
_Pachytheca_ usually occurs in the form of small spherical bodies,
about ·5 cm. in diameter, in Old Red Sandstone or Silurian rocks. In
section a single sphere is found to consist of two well marked regions;
in the centre, of a number of ramifying and irregularly placed narrow
tubes, and in the peripheral or cortical region, of numerous regular
and radially disposed simple or forked septate tubes. The tubular
elements of the two regions are in organic connection.
The name was proposed by Sir Joseph Hooker for some specimens found
by Dr Strickland[390] in the Ludlow bone-bed (Silurian) of Woolhope
and May-Hill. Examples were subsequently recorded from the Wenlock
limestone of Malvern and from Silurian and Old Red Sandstone rocks of
other districts. Hicks[391] found _Pachytheca_ in the Pen-y-Glog grits
of Corwen in association with _Nematophycus_, and the two fossils have
been found together elsewhere. This association led to the suggestion
that _Pachytheca_ might be the sporangium of _Nematophycus_, and
Dawson[392], in conformity with his belief in the coniferous character
of the latter plant, referred to _Pachytheca_ as a true seed.
The best sections of this fossil have been prepared with remarkable
skill by Mr Storrie of Cardiff; they were carefully examined and
described by Barber in two memoirs[393] published in the _Annals
of Botany_, the account being illustrated by several well executed
drawings and microphotographs.
Among other difficulties to contend against in the interpretation of
_Pachytheca_ there is that of mineralisation. The preservation is such
as to render the discrimination of original structure as distinct from
structural features of secondary origin, consequent on a particular
manner of crystallisation of the siliceous material, a matter of
considerable difficulty.
Suggestions as to the nature of _Pachytheca_ have been particularly
numerous; it has been referred to most classes of plants and relegated
by some writers to the animal kingdom. The most recent addition to our
knowledge of this problematic fossil was the discovery of a specimen
by Mr Storrie in which the _Pachytheca_ sphere rested in a small cup,
like an acorn fruit in its cupule. This specimen was figured and
described by Mr George Murray[394] in 1895; he expresses the opinion
that the discovery makes the taxonomic position of the genus still more
obscure. Solms-Laubach briefly refers to _Pachytheca_ in connection
with _Nematophycus_, and regards its precise nature almost as much
an unsolved riddle now as it was when first discovered. For a fuller
account of this fossil reference must be made to the contributions
of Hooker[395], Barber[396] and others. The literature is quoted by
Barber and more recently by Solms-Laubach[397]. There are several
specimens and microscopic sections of _Pachytheca_ in the geological
and botanical departments of the British Museum. The genus has been
recorded from Shropshire, North Wales, Malvern, Herefordshire,
Perthshire and other British localities, as well as from Canada; it
occurs in both Silurian and Old Red Sandstone rocks.
_Algites._
A generic name for those fossils which in all probability belong to
the class Algae, but which, by reason of the absence of reproductive
organs, internal structure, or characters of a trustworthy nature in
the determination of affinity, cannot be referred with any degree of
certainty to a particular recent genus or family.
This term was suggested in 1894[398] as a provisional and comprehensive
designation under which might be included such impressions or casts
as might reasonably be referred to Algae. The practice of applying to
alga-like fossils names suggestive of a definite alliance with recent
genera is as a rule unsound. It would simplify nomenclature, and
avoid the multiplication of generic names, if the term _Algites_ were
applied to such algal fossils from rocks of various ages as afford
no trustworthy data by which their family or generic affinity can be
established.
V. MYXOMYCETES (MYCETOZOA).
This class of organisms affords an interesting example of the
impossibility of maintaining a hard and fast line between the animal
and plant kingdom. Zoologists and Botanists usually include the
Myxomycetes[399] in the text-books of their respective subjects,
and the name Animal-fungi which has been applied to these organisms
expresses their dual relationship. They constitute one of three groups
which we may include in that intermediate zone or ‘buffer-state’
between the two kingdoms. From a palaeobotanical point of view the
Myxomycetes are of little interest, but a very brief reference may be
made to them rather for the sake of avoiding unnecessary incompleteness
in our classification than from their importance as possible fossils.
They are organisms without chlorophyll, consisting of a naked mass
of protoplasm, known as the _plasmodium_, which may attain a size of
several inches. Such plasmodia creep over the surface of decaying
organic substrata, and in forming their asexual reproductive cells
they are converted into somewhat complex fruits containing spores. The
spores produce motile swarm-cells, which eventually coalesce together
to form a new plasmodium.
A few examples of fossil Myxomycetes have been recorded from the
Palaeozoic and more recent formations, but none of them are entirely
beyond suspicion. We may mention three examples of fossils referred to
this group, but only one of these is entitled to serious consideration.
_Myxomycetes Mangini_ Ren.[400] It is not uncommon to find distinct
traces of original or secondary cell-contents in well preserved
petrified plant-tissues. There is often a difficulty, however, in
distinguishing between the true cell-contents and the cells of some
parasitic or saprophytic intruder. In some petrified corky tissue
in a silicified nodule from the Permo-Carboniferous beds of Autun,
Renault has recently discovered what he believes to be traces of a
Myxomycetous plasmodium. The cork-cells would be without protoplasmic
contents of their own, and their cavities contain a number of fine
strands stretching from the cell-walls in different directions and
uniting in places as irregular or more or less spherical masses. The
drawings given by Renault of these irregular reticulated structures
with scattered patches of what may possibly be petrified plasmodial
protoplasm bear a striking resemblance to the plasmodium of a
Myxomycete. A figure of the capillitium of a species of _Leocarpus_
figured by Schröter[401] in his account of the Myxomycetes in Engler
and Prantl’s work is very similar to that of Renault’s ‘plasmodium.’
It is by no means inconceivable that the _Myxomycetes Mangini_ may be
correctly referred to this group, but the wisdom of assigning a name to
such structures may well be questioned.
• • • • •
The other two examples call for little notice. Messrs Cash and
Hick[402] in a paper on fossil fungi from the Coal-Measures refer to
some small spherical bodies as possibly the spores of a Myxomycete.
They might be referred equally well to numerous other organisms.
Göppert and Menge[403] in their monograph on plants in the Baltic
Tertiary Amber, express the opinion that an ill-defined tangle of
threads which they figure may be a Myxomycete.
It would serve no useful purpose to quote other instances of possible
representatives of fossil Mycetozoa; but the consideration of the above
examples may serve to emphasize the desirability of refraining from
converting a possibility into an apparently recognised fact by the
application of definite generic and specific names.
VI. FUNGI.
The most striking difference between the fungi and algae is the
absence of chlorophyll in the former, and the consequent inability of
fungi to manufacture their organic compounds from inorganic material.
Fungi live therefore either as parasites or saprophytes, and as the
same species may pass part of its life in a living host to occur at
another stage of its development as a saprophyte, it is impossible to
distinguish definitely between parasitic and saprophytic forms. The
vegetative body of a fungus, that is the portion which is concerned
with providing nourishment and preparing the plastic food-substance
for the reproductive organs, is known as the _mycelium_. It consists
either of a single and branched tubular cell known as a _hypha_, or
of several hyphae or thread-like elements (filamentous fungi). The
hyphal filaments may be closely packed together and form a felted
mass of compact tissue, which in cross section closely simulates the
parenchyma of the higher plants. This pseudoparenchymatous form of
thallus is particularly well illustrated by the so-called _sclerotia_;
these are sharply defined and often tuberous masses of hyphal tissue
covered by a firm rind and containing supplies of food in the inner
hyphae. They are able to remain in a quiescent state for some time,
and to resist unfavourable conditions until germination and the
formation of a new individual take place. The reproductive structures
assume various forms; in some of the simpler fungi (Phycomycetes)
sexual organs occur, as in the parallel group of Siphoneae among the
algae, but in the higher fungi the reproduction is usually entirely
asexual. An interesting case has recently been recorded among the
more highly differentiated fungi in which distinct sexuality has been
established[404]. In addition to the reproductive organs, such as
oogonia and antheridia, the asexual cells or spores are borne either
in special sporangia, or they occur as exposed _conidia_ on supporting
hyphae or _conidiophores_. Thick-walled and resistant resting-spores of
various forms are also met with.
Without going into further details we may very briefly refer to the
larger subdivisions of this group of Thallophytes.
PHYCOMYCETES. Mycelium usually consisting of a single cell.
ZYGOMYCETES, Reproduction by means of conidia, and in many
OOMYCETES, cases also by the conjugation of two similar
including hyphae or by the fertilisation of an egg-cell
=Chytridiaceae=, contained in an oogonium.
&c.
MESOMYCETES,
including the Intermediate between the Phycomycetes and
Sub-classes the higher fungi. Multicellular hyphae. No
HEMIASCI and sexual organs.
HEMIBASIDII.
MYCOMYCETES. Septate vegetative mycelium. No sexual
including the reproduction—as a general rule. Asexual conidia
Sub-classes and other forms of spores. In the Ascomycetes
ASCOMYCETES and the spores are found in characteristic
BASIDIOMYCETES. club-shaped cases or asci; in the Basidiomycetes
the spores are borne on special branches from
swollen cells known as _basidia_. The sporophore
or spore-bearing body in this group may attain a
considerable size (e.g. _Agaricus_, _Polyporus_,
&c.) and exhibit a distinct internal
differentiation.
Before describing a few examples of fossil fungi, it is important
to consider the general question of their manner of occurrence and
determination. Considering the small size and delicate nature of most
fungi, it is not surprising that we have but few satisfactory records
of well-defined fossil forms. The large leathery sporophores of
_Polyporus_ and other genera of the Basidiomycetes, which are familiar
objects as yellow or brown brackets projecting from the trunks of
diseased forest trees, have been found in a fairly perfect condition
in the Cambridgeshire peat-beds, and examples have been described also
by continental writers[405]. As a general rule, however, we have to
depend on the chance mineralisation or petrifaction of the hyphae of a
fungus-mycelium which has invaded the living or dead tissues of some
higher plant. In the literature on fossil plants there are numerous
recorded species of fungi founded on dark coloured spots and blotches
on the impression of a leaf. Most of such records are worthless; the
external features being usually too imperfect to allow of accurate
identification. The occurrence of recent fungi as discolourations on
leaves is exceedingly common, and the characteristic _perithecia_ or
compact and more or less spherical cases enclosing a group of sporangia
in certain Ascomycetous species, might be readily preserved in a fossil
condition.
[Sidenote: ASCOMYCETES.]
Some examples of possible Ascomycetous fungi have been recently
recorded by Potonié from leaves and other portions of plants of
Permian age. There is a distinct superficial resemblance between the
specimens he figures and the fructifications of recent Ascomycetes,
but in the absence of internal structure, it would be rash to do more
than suggest the probable nature of the markings he describes. For
one of the fungus-like impressions Potonié proposes the generic name
_Rosellinites_; he compares certain irregularly shaped projections on
a piece of Permian wood with the perithecia of _Rosellinia_, a member
of the Sphaeriaceae, and describes them as _Rosellinites Beyschlagii_
Pot.[406] Various other records of similar Ascomycetes-like fossils may
be found in palaeobotanical literature[407], but it is unnecessary to
examine these in detail. Unless we are able to determine the nature of
the supposed fungus by microscopical methods our identifications cannot
in most cases be of any great value.
An example of the perithecia of a fungus (_Rosellinia congregata_
[Beck])[408] has been recorded from the Oligocene of Saxony, which
would appear to rest on a more satisfactory basis than is often the
case. In this particular instance the small projections on a piece of
fossil coniferous stem present a form which naturally suggests a fungus
perithecium. In cases where the black spots on a fossil stem or leaf
possess a definite form and structure, it is perfectly legitimate to
refer them to a group of fungi; but in very many instances the forms
referred to such genera as _Sphaerites_ and others are of little or
no value. Many forms of scale-insects and galls on leaves present an
obvious superficial resemblance to epiphyllous fungi, and might readily
be mistaken for the fructifications of certain Ascomycetous species. As
examples of scale-insects simulating fungi, reference may be made to
such genera of the Coccineae as _Aspidiotus_, _Diaspis_, _Lecanium_,
_Coccus_, and others. The female insects lying on the surface of a
leaf, if preserved as a fossil impression, might easily be mistaken for
perithecia[409].
Another pitfall in fossil mycology may be illustrated by a description
of a supposed fungus, _Sclerotites Salisburiae_[410], Mass. on a
Tertiary _Ginkgo_ leaf. The figure given by Massalongo represents a
_Ginkgo_ leaf with well marked veins, the lamina between the veins
being traversed by short discontinuous and longitudinally-running
lines; the latter are referred to as the fungus. In a recent _Ginkgo_
leaf one may easily detect with the naked eye a number of short lines
between and parallel to the veins, which if examined in section
are found to be secretory canals. There can be little doubt that
_Sclerotites Salisburiae_ owes its existence to the preservation of
these canals.
The list of fossil fungi given by Meschinelli in Saccardo’s _Sylloge
Fungorum_[411] includes certain species which are of no botanical
value, and should have no place in any list which claims to be
authentic.
Among the numerous examples of fossil ‘fungi’ which have no claim to
be classed with plants, there are some which are in all probability
the galleries of wood-eating animals. The radiating grooves frequently
found on the inner face of the bark of a pine tree made by species of
the beetle _Bostrychus_ might be mistaken for the impressions of the
firm strands of mycelial tissue of some Basidiomycetous fungus.
In some notes on fossil fungi by J. F. James[412] contributed to
the American Journal of Mycology in 1893, it is pointed out that a
supposed fungus described by Lesquereux from the Lower Coal-Measures
as _Rhizomorpha Sigillariae_[413], bears a strong likeness to some
insect-burrows, such as those of _Bostrychus_.
“A new fungus from the Coal-Measures” described by Herzer in 1893[414]
may probably be referred to animal agency. In any case there is no
evidence as to the fungoid nature of the object represented in the
figure accompanying Herzer’s description.
[Sidenote: BASIDIOMYCETES.]
More trustworthy evidence of fossil fungi is afforded by the marks
of disease in petrified tissue and by the presence of true mycelia.
In examining closely the calcareous and siliceous plant-tissues from
the Coal-Measures and other geological horizons, one occasionally
sees fine thread-like hyphae ramifying through the cells or tracheal
cavities; in many cases the hyphae bear no reproductive organs and
cannot as a rule be referred to a particular type of fungus. If the
hyphal filaments are unseptate, they most likely belong to some
Phycomycetous species; or if they are obviously septate the Mesomycetes
or the Mycomycetes are the more probable groups. Occasionally there
may be found indications of the characteristic _clamp-connections_ in
the septate filaments; a small semicircular branch, which is given off
from a mycelium immediately above a transverse wall, bends round to
fuse with the filament just below the septum, thus serving as a small
loop-line connecting the cell-cavity above and below a cross wall. Such
clamp-connections are usually confined to the hyphae of Basidiomycetes
and thus serve as a useful aid in identification. A good example of a
clamp-connection in a fossil mycelium is figured by Conwentz[415] in
his monograph on the Baltic amber-trees of Oligocene age. The stout
and thick type of hypha found in some fossil woods agrees closely with
that of _Polyporus_, _Agaricus melleus_ and other well-known recent
Basidiomycetes.
In a section of a piece of lignified coniferous wood recently brought
by Col. Feilden from Kolguev island[416], the brown and stout hyphae
of a fungus are clearly seen as distinct dark lines traversing the
tracheal tissue. The occurrence of septa and the large diameter of the
mycelial branches at once suggest a comparison with such recent forms
as _Agaricus melleus_, _Polyporus_ and other Basidiomycetes. The age of
the Kolguev wood is not known with any certainty.
The vesicular swellings such as those represented in fig. 41, A, B, D
and E, may easily be misinterpreted. Such spherical expansions in a
mycelium, either terminal or intercalary, may be sporangia, oogonia or
large resting-spores, or non-fungal cell-contents, and it is usually
impossible in the absence of the contents to determine their precise
nature. Hartig[417] and others have drawn attention to the occurrence
of such bladder-like swellings in the mycelia of recent fungi, which
have nothing to do with reproductive purposes; under certain conditions
the hyphae of a fungus growing in the cavity of a cell or trachea may
form such vesicles, and these, as in fig. 42, D, _m_ may completely
fill up the cavity of a large tracheid.
Some good examples of bladder-like swellings, such as occur in the
mycelium of _Agaricus melleus_ and other recent fungi, have been
figured by Conwentz[418] in fossil wood of Tertiary age from Karlsdorf.
The swellings in this fossil fungus might easily be mistaken for
oogonia or sporangia; especially as they are few in number and
spherical in form.
A similar appearance is presented by a mass of tyloses in the cavity of
an old vessel or tracheid; and vesicular cell-contents, as in the cells
of fig. 41, A, 2–5, may closely simulate a number of thin-walled fungal
spores or sporangia.
A good example of such a vesicular tissue, in addition to that already
quoted, is afforded by a specimen of an Eocene fern, _Osmundites
Dowkeri_ Carr.[419] described by Carruthers in 1870. The ground-tissue
cells contain traces of distinct fungal hyphae (fig. 41, B), and in
many of the parenchymatous elements the cavity is completely filled
with spherical vesicles; in other cases one finds hyphae in the centre
of the cell while vesicles line the wall, as shewn in fig. 41, B.
Carruthers refers to these bladders as starch grains, and this may
be their true nature; their appearance and abundant occurrence in
the parenchyma certainly suggest vesicular cell-contents rather than
fungal cells. I could detect no proof of any connection between the
hyphae and bladders, and the absence of the latter in the cavities of
the tracheids, fig. 41, C, favoured the view of their being either
starch-grains or other vacuolated contents similar to that in the cells
of the Portland Cycad (fig. 41, A) referred to on p. 88.
[Sidenote: PATHOLOGY OF FOSSIL TISSUES.]
The vacuolated cell-contents partially filling the cells in fig. 41, D,
present a striking resemblance to the contents of the cells 2–5 in fig.
41, A. In fig. D the frothy and contracted substance might be easily
mistaken for a parasitic or saprophytic fungus, but this resemblance
is entirely misleading. It is by no means uncommon to find the cells
of recent plants occupied by such vacuolated contents, especially in
diseased tissues in which a pathological effect produces an appearance
which has more than once misled the most practised observers.
In the important work recently published by Renault on the
Permo-Carboniferous flora of Autun, there is a small spore-like body
described as a teleutospore, and classed with the Puccineae[420].
We have as yet no satisfactory evidence of the existence of this
section of Fungi in Palaeozoic times, and Renault’s description of
_Teleutospora Milloti_ from Autun might be seriously misleading if
accepted without reference to his figure. The fragment he describes
cannot be accepted as sufficient evidence for the existence of a
Palaeozoic _Puccinia_.
The same author refers another Palaeozoic fungus to the Mucorineae
under the name of _Mucor Combrensis_[421]; this identification is based
on a mycelium having a resemblance to the branched thallus of _Mucor_,
but in the absence of reproductive organs such resemblance is hardly
adequate as a means of recognition.
The occurrence of hyphal cells in calcareous shells and corals has
already been alluded to.[422] In addition to the examples referred
to above, there is one which has been described by Etheridge[423]
from a Permo-Carboniferous coral. This observer records the occurrence
of tubular cavities in the calices of _Stenopora crinita_ Lonsd.,
and attributes their origin to a fungus which he names _Palaeoperone
endophytica_; he mentions one case in which a tube contains fine
spherical spore-like bodies which he compares with the spores of a
_Saprolegnia_. As pointed out above (p. 128), it is almost impossible
to decide how far these tubes in shells and corals should be attributed
to fungi, and how far to algae.
[Illustration: FIG. 41. A. Cells of _Cycadeoidea gigantea_ Sew. × 355.
B and C. Parenchymatous cells and scalariform tracheids of
_Osmundites Dowkeri_ Carr. × 230. D. Epidermal cells of _Memecylon_
(_Melastomaceae_) with vacuolated contents. E. _Peronosporites
antiquarius_ Smith, (No. 1923 in the Williamson collection). × 230.
F. _Zygosporites_. × 230. (A, B, C and E drawn from specimens in
the British Museum; D from a drawing by Prof. Marshall Ward; F from
a specimen in the Botanical Laboratory Collection, Cambridge.)]
[Illustration: FIG. 42. A, B, C. Tracheids of coniferous wood attacked
by _Trametes radiciperda_ Hart. (_Polyporus annosus_ Fr.) D and E.
Tracheids attacked by _Agaricus melleus_ Vahl. A, × 650, B–E, ×
360. (After Hartig.)]
Passing from the direct evidence obtained from the presence of fungal
hyphae in petrified tissues, we must draw attention to the indirect
evidence of fungal action afforded by many fossil plants. It is
important to be familiar with at least the more striking effects of
fungal ravages in recent wood in order that we may escape some of the
mistakes to which pathological phenomena may lead us in the case of
fossils[424].
The gradual dissociation of the elements in a piece of fossil wood
owing to the destruction of the middle lamellae, the occurrence of
various forms of slit-like apertures in the walls of tracheids (fig.
42, E) and the production of a system of fine parallel striation on
the walls of a vessel are among the results produced by parasitic and
saprophytic fungi. With the help of a ferment secreted by its hyphae,
a fungus is able to eat away either the thickening cell layers or the
middle lamellae or both, and if, as in fig. 42, A, only the middle
lamellae are left one might easily regard such tissue in a fossil
condition as consisting of delicate thin-walled elements. The oblique
striae on the walls of a tracheid may often be due to the action of a
ferment which has dissolved the membrane in such a manner as to etch
out a system of spiral lines, probably as a consequence of the original
structure of the tracheids. In distinguishing between the woods of
Conifers the presence of spiral thickening layers in the wood element
is an important diagnostic character, and it is necessary to guard
against the confusion of purely secondary structures, due to fungal
action, with original features which may be of value in determining the
generic affinity of a piece of fossil wood.
_Oochytrium Lepidodendri_, Ren. Fig. 43, 1.
Under this name Renault has recently described a filamentous fungus
endophytic in the cavities of the scalariform tracheids of a
_Lepidodendron_[425]. The mycelium has the form of slender branched
hyphae with transverse septa. Numerous ovoid and more or less spherical
sporangia occur as terminal swellings of the mycelial threads. The
long axis of the ovoid forms measures 12–15 µ, and the shorter axis
9–10 µ; the contents may be seen as a slightly contracted mass in the
sporangial cavity. In some of the sporangia one sees a short apical
prolongation in the form of a small elongated papilla, as shown in fig.
43, 1. Renault refers this fungus to the Chytridineae, and compares it
with _Cladochytrium_, _Woronina_, _Olpidium_, and other recent genera.
In the immediate neighbourhood of two of the sporangia shown in the
uppermost tracheid of fig. 43, 1, there are seen a few minute dark dots
which are described as spores petrified in the act of escaping from a
lateral pore. This interpretation strikes one as lacking in scientific
caution.
The sporangia of _Hyphochytrium infestans_[426], as figured by Fischer
in Rabenhorst’s work bear a close resemblance to those of the fossil.
It would seem very probable that Renault’s species may be reasonably
referred to the Chytridineae, as he proposes.
[Illustration: FIG. 43. 1. _Oochytrium Lepidodendri_, Ren. (After
Renault.) 2. _Polyporus vaporarius_ Fr. var. _succinea_. (After
Conwentz.) 3. _Cladosporites bipartitus_ Fel. (After Felix.) 4.
_Haplographites cateniger_ Fel. (After Felix.)]
_Peronosporites antiquarius_ W. Smith. Fig. 41, E.
In an address to the Geologists’ Association delivered by Mr Carruthers
in 1876 a brief reference, accompanied by a small-scale drawing, is
made to the discovery of a fungus in the scalariform tracheids of a
_Lepidodendron_ from the English Coal-Measures[427]. In the following
year Worthington Smith published a fuller account of the fungus, and
proposed for it the above name[428], which he chose on the ground of
a close similarity between the mycelium and reproductive organs of
the fossil form and recent members of the Peronosporeae. In Smith’s
description the mycelium is described as bearing spherical swellings
containing zoospores. These spherical organs are fairly abundant and
not infrequently met with in sections of petrified plant-tissues from
the English Coal-Measures; they may be oogonia or sporangia, or in
some cases mere vesicular expansions of a purely vegetative hypha.
No confirmation has been given to the supposed spores referred to
by Smith. Prof. Williamson and others have carefully examined the
specimens, but they have failed to detect any trace of reproductive
cells enclosed in the spherical sacs[429]. The mycelium does not appear
to show any satisfactory evidence of its being septate as figured by
Smith.
The example shown in fig. 41 E has been drawn from one of the
Williamson specimens: it illustrates the form and manner of occurrence
of the characteristic swellings. It is probable that some at least
of the vesicles are either sporangia or oogonia, but we cannot speak
with absolute confidence as to their precise nature. The general
habit and structure of the fungus favour its inclusion in the class
of _Phycomycetes_. The occurrence of several of the vesicles close
together on short hyphal branches, as shown in Williamson’s figures,
suggests the spherical swellings on vegetative hyphae, but it is
impossible to speak with absolute confidence. There is a close
resemblance between this English form and one recently described by
Renault as _Palaeomyces gracilis_ Ren.[430]; the two fossils should
probably be placed in the same genus.
The examples referred to below and originally recorded by Cash and Hick
no doubt belong to the same type as Smith’s _Peronosporites_.
The sketches reproduced in fig. 44 have been drawn from specimens
originally described by Cash and Hick in 1878[431]. The sections were
cut from a calcareous nodule from the Halifax Coal-Measures containing
fragments of various plants and among others a piece of cortical
tissue, probably of a _Lepidodendron_ or _Stigmaria_. In a transverse
section of this tissue one sees under a moderately high power that
the cells have become partially separated from one another by the
destruction of the middle lamellae (fig. 44 A). The cell-cavities and
the spaces between the isolated cells contain numerous fine fungal
hyphae, which here and there terminate in spherical swellings. One
such swelling is shown under a low power in fig. 44 A, in the middle
uppermost cell, and more highly magnified in fig. 44 B. In fig. C
there are two such swellings (the larger one having a diameter of ·003
mm.) in contact, but the connection does not appear to be organic.
The cell-walls of the infected tissue present a ragged and untidy
appearance, and in places (_e.g._ fig. 44 D) the membrane has been
pierced by some of the mycelial branches.
[Illustration: FIG. 44. Cells with fungal hyphae. A. A piece of
disorganised tissue, showing the separation of the cells. B. Part
of A more highly magnified. C. A single cell containing two swollen
hyphae. D. Partially destroyed cell-membranes pierced by fungal
hyphae. (Drawn from sections in the Edinburgh Botanical Museum,
originally described by Cash and Hick.)]
This fungus bears a close resemblance to _Peronosporites antiquarius_,
but it is impossible to determine its precise botanical position
without further data. In Cash and Hick’s paper in which the above
fungus is briefly dealt with, some small spore-like bodies are figured
which the authors speak of as possibly a Myxomycetous fungus[432].
There is however no sound reason for such a supposition.
As examples of Ascomycetous fungi found in silicified wood of Tertiary
age, two species may be quoted from Felix.
_Cladosporites bipartitus_ Felix[433], fig. 43, 3.
The mycelium and conidia of this form were discovered in some Eocene
silicified wood from Perekeschkul near Baku, on the shores of the
Caspian. The conidia are elliptical or pyriform in shape and divided by
a transverse septum into two cells. No traces were found of any special
conidiophores. The mycelium consists of septate branched hyphae,
rendered conspicuous by a brown colouration. Felix compares the fossil
with the recent genera _Cephalothecium_ and _Cladosporium_.
_Haplographites cateniger_ Felix[434], fig. 43, 4.
The conidia of this form were found to be fairly abundant in the
silicified tissue investigated by Felix; they occur usually in chains
of 2 to 6 conidia having an ovoid or flask-shaped form, with a thick
membrane (fig. 43, 4). The mycelium consists of branched hyphae divided
into long cylindrical cells by transverse septa; occasional instances
were found of an H-shaped fusion between lateral branches of parallel
hyphae.
Felix compares this species with examples of the genera _Haptographium_
and _Dematium_ of the family Sphaeriaceae; it was found in the woody
tissue of a dicotyledonous stem from Perekeschkul.
_Zygosporites_ sp.
The object represented in fig. 41 F consists of a stalked spherical
sac bearing a number of radiating arms which are divided distally
into delicate terminations. We find similar bodies figured by
Williamson[435] in his IXth and Xth Memoirs on the Coal-Measure plants;
he includes some of them under the generic term _Zygosporites_, and
compares them with the zygospores of the freshwater algae Desmideae.
Hitherto these spore-like fossils have only been recorded as isolated
spheres, but in the example shown in fig. 41 F there is a distinct
tubular and thin-walled stalk attached to the _Zygosporites_. The
specimen was found in the partially disorganised cortical tissue of a
_Lyginodendron_ stem from the English Coal-Measures. It is difficult
to decide as to the precise nature of the fossil, but the presence of
the hyphal stalk points to a fungus rather than an alga as the most
probable type of plant with which to connect it. It may possibly be a
sporangium of a fungus comparable with the common mould _Mucor_, or it
may be a zygospore formed by the conjugation of two hyphae of which
only one has been preserved.
[Sidenote: POLYPORUS.]
For an example of a fossil representative of the Basidiomycetes we may
turn to the excellent monograph by Conwentz on the Baltic amber trees,
and quote one of the forms which he has described.
_Polyporus vaporarius_ Fr. _f. succinea_[436], fig. 43, 2.
In several preparations of the wood preserved by petrifaction in amber
Conwentz found distinct indications of the ravages of a fungus, which
suggested the presence of the recent species _Polyporus vaporarius_ Fr.
With the help of the indirect evidence afforded by the pathological
effects as seen in the tissues of the host-plant, and the direct
evidence of the fungal mycelium Conwentz was led to this identification.
The mycelium is brown in colour, in part thick-walled, and in part with
thin walls, transversely septate and not much branched. In the portion
of one of Conwentz’ figures reproduced in fig. 43, 2, the rents and
holes in the tracheid walls are clearly shown; they afford the indirect
evidence of fungal attacks, and are of the same nature as those shown
in fig. 42, B, C and E.
• • • • •
Enough has been said to call attention to the paucity of exact data on
which to generalise as to the geological history of fungi. The types
selected for description or passing allusion have not been chosen in
each case because of their special intrinsic value, but rather as
convenient examples by which to illustrate authentic records or to
serve as warnings against possible sources of error.
It would seem that we have fairly good and conclusive evidence of the
existence in Permo-Carboniferous times of Phycomycetous fungi, but
it is not until we pass to post-Palaeozoic or even Tertiary plants
that we discover satisfactory representatives of the higher fungi or
Mycomycetes. If special attention were paid to the investigation of
fossil fungi, it is quite possible that our knowledge of the past
history of the group might be considerably extended. It is essential
that the greatest caution should be exercised in the identification
of forms and in their reference to definite families; otherwise our
lists of fossil species will serve to mislead, and to emphasize the
untrustworthy character of palaeobotanical data. Unless we feel
satisfied as to the position of a fossil fungus it is unwise to use
a generic term suggestive of a definite family or recent genus. Such
a name as Renault has used in one instance, _Palaeomyces_, might be
employed as a useful and comprehensive designation.
VII. CHAROPHYTA.
CHARACEÆ. NITELLEÆ.
It has been the general custom to include the Characeæ or Stoneworts
among the Chlorophyceae (green algae), of which they form a distinctly
isolated family. On the whole, it would seem better to follow the
course lately adopted by Migula[437] and allow the Characeæ to rank
as a family of a distinct group, Charophyta. While agreeing in many
respects with plants higher in the scale than Thallophytes, the
Stoneworts do not sufficiently resemble the Bryophyta to be included in
that group.
The Charophyta are plants containing chlorophyll, living in fresh and
brackish water; the stem is jointed, and bears at the nodes whorls of
leaves, on which are borne the reproductive organs. The antheridia
are spherical in shape and of complex structure, containing numerous
biciliate antherozoids. The oogonia are oval in form and contain a
single large egg-cell. The Chara-plant is developed from a _protonema_
formed from the germinating oospore. Vegetative reproduction is
effected by means of bulbils, accessory shoots, etc.
The Nitelleæ have not been recognised in a fossil condition. The
absence or feeble development of a calcareous incrustation renders the
genera of this family less likely to be preserved than such a genus as
_Chara_.
Chareae.
Leaves and stems with or without a cortical investment. Fruit with a
five-celled _corona_. The envelope of the ‘fruit’ and other parts of
the plant are frequently encrusted with carbonate of lime.
In the genus _Chara_, the best known member of the family, the plant as
a whole resembles in its general habit and external differentiation of
parts the higher plants. The stem consists of long internodes separated
by short nodes bearing whorls of leaves. Each internode consists of
a long cylindrical cell, which becomes enclosed by a cortical sheath
composed of rows of cells which have grown upwards and downwards from
the peripheral nodal cells. The cortical cells are usually spirally
twisted and impart to the stem a characteristic appearance; they
are divided by transverse walls into numerous cells some of which
occasionally grow out into short processes (fig. 45 _c_). The leaves
repeat on a smaller scale the structural features of the stem, but
possess a limited growth, whereas the stem has an unlimited power
of growth by means of a large hemispherical apical cell. Branches
arise in the axils of the leaves. The plants are either monoecious or
dioecious. The oogonium is elliptical in shape, and is borne on a short
stalk-cell, it contains a single oosphere. The wall of the oogonium is
formed of five spirally twisted cells which have grown over it from the
five peripheral cells of a leaf-node. The tips of the investing cells
project at the apex in the form of a terminal crown or _corona_ (fig.
45, _E_, _c_). The antheridia have a complex structure, and produce a
very large number of motile antherozoids.
[Illustration: FIG. 45. _A_ and _B_. _Chara Knowltoni_ Sew. From a
section in the British Museum. _C._ Stem of _Chara foetida_ A.
Br. in transverse section (after Migula. × 18). _D._ Interior of
oogonium of _C. foetida_. _E._ Oogonium of _C. foetida_ (_D_ and
_E_ after Migula. × 50).]
After fertilisation, the egg-cell becomes surrounded by a membrane, at
first colourless, but afterwards yellow or brown. The inner cell-walls
of the cells surrounding the oospore become thicker and darker in
colour; the outer walls remain thin and eventually fall away. The
lateral walls may or may not become thickened. In most of the Chareae a
calcareous deposit is formed between the hard shell and the outer walls
of the cells enveloping the oospore. This calcareous shell is developed
subsequently to the thickening and hardening of the inner walls of the
fruit-case. The cells of the corona and stalk do not become calcareous.
In the fossil Charas, it is this calcareous shell that is preserved.
In the members of the Chareae the stems are usually encrusted with
carbonate of lime, and thus have a much better chance of preservation
than the slightly calcareous Nitelleæ.
_Chara._
The generic characters have already been described in the brief account
of the family Chareae.
The generic name was proposed by Vaillant in 1719[438], and adopted by
Linnaeus, who classed the Stoneworts with aquatic phanerogams. As long
ago as 1623[439] a figure of _Chara_ was published by Caspar Bauhin
as a form of _Equisetum_. The generic name _Chara_ has usually been
applied to recent and fossil species alike. The existing species have
a wide distribution; _Chara foetida_, A. Br., a common British form,
occurs in practically all parts of the world. Stems and calcareous
‘fruit-cases’ occur fairly commonly in a fossil state, and differ but
little from recent species, at least as regards essential features.
It is difficult to say at what geological horizon the Stoneworts
are first represented. The first certain traces of _Chara_ occur in
Jurassic rocks, but certain spirally marked subspherical bodies have
been recorded from Devonian and Carboniferous strata, which closely
resemble Chara oogonia, and may be Palaeozoic representatives of the
genus.
In 1889 Mr Knowlton[440] of the American Geological Survey described
some ‘problematic organisms’ found in Devonian rocks at the falls of
the Ohio. Examples of these fossils are shown in fig. 46 _b_ and _c_;
the spirally grooved body measures from 1·50 to 1·80 mm. in diameter,
and about 1·70 mm. in length. The Chara-like character of the fossils
had been previously suggested by Meek[441] in 1873. Without going into
the arguments for or against placing these fossils in the Chareae,
they may at least be mentioned as possible but not certain Palaeozoic
forms of _Chara_ or an allied genus.
[Illustration: FIG. 46. _a_. _Chara Bleicheri_ Sap. × 30. _b_ and _c_.
Devonian _Chara_? sp. _circa_ × 12. _d_ and _e_. _Chara Wrighti_
Forbes. _circa_ × 12.]
1. _Chara Bleicheri_, Saporta. Fig. 46, _a_.
In this form the ‘fruits’ are minute and subspherical, ·39-·44 mm.
long, and ·35-·40 mm. broad, showing in side view 5–6 slightly oblique
spiral bands. Each spiral band bears a row of slightly projecting
tubercles.
This species was first described by Saporta[442] from the Oxfordian
(Jurassic) rocks of the Department of Lot in France; it is compared by
the author of the species with _Chara Jaccardi_ Heer, described by Heer
from the Upper Jurassic rocks of Switzerland.
2. _C. Knowltoni_, Seward. Fig. 45, _a_ and _b_, and Fig. 47.
The Oogonia are broadly oval, about ·5mm. in length, and at the
broadest part of about the same breadth. The surface is marked by
eleven or twelve bands in the form of a flattened spiral. The stems
possess investing cortical cells.
This species was founded on specimens from the Wealden beds of
Sussex[443], but numerous examples of Chara ‘fruits’ and stems have
long been known from the uppermost Jurassic rocks of the Dorset coast
and the Isle of Wight, which may probably be included in this species.
These fossil Charas are abundant[444] in the Chert beds of Purbeck age
seen in the cliffs near Swanage. Pieces of corticated stems from this
locality are represented in fig. 45 _A_ and _B_.
The cortical cells surrounding a large internodal cell are very clearly
seen in the section shown in fig. 45 _B_, and in the longitudinal view
in fig. 45 _A_. The resemblance of these specimens to the stems of
recent Stoneworts is very striking.
[Illustration: FIG. 47. _Chara Knowltoni_, Sew. × 30.]
The single oogonium of fig. 47 was found in the Wealden beds near
Hastings.
3. _Chara Wrighti_, Forbes. Fig. 46, _d_ and _e_.
This species is characterised by globular or somewhat elliptical
oogonia, with six or seven spiral bands.
It is very abundant in the Lower Headon beds of Hordwell Cliffs on the
Hampshire coast[445]. Various species of _Chara_ are commonly met with
in the Oligocene beds of the Isle of Wight and Hampshire, as well as
in the Paris basin beds, and elsewhere. Well preserved ‘fruits’ and
stem fragments are met with in a siliceous rock of Upper Oligocene age
imported from Montmorency in the Paris basin, and used as a stone for
grinding phosphates at some chemical works near Upware, a few miles
from Cambridge.
Many other species of fossil Charas are known from various horizons
and localities, but the above examples suffice as illustrative types.
In Post-Tertiary deposits masses of _Chara_ and plant fragments
occasionally occur forming blocks of Travertine. Examples of such Chara
beds have been recorded by Sharpe from Northampton[446], by Lyell[447]
from Forfarshire, and by other writers from several other districts.
Beds of calcareous marl are occasionally seen as whitish streaks in the
peat of the Fenland[448]; these often consist in great part of Charas.
A season’s growth of _Chara_ in a shallow lake or mere in the Fens may
appear as a white line in a section of peaty and other material which
has been formed on the site of old pools or lakes.
The recognition of specific characters in the isolated Chara ‘fruits’
usually met with in a fossil state is exceedingly unsatisfactory; the
features usually relied on in the living species are not preserved, and
great care should be taken in the separation of the various forms.
CHAPTER VIII.
BRYOPHYTA (Muscineae).
I. HEPATICAE (Liverworts). II. MUSCI (Mosses).
The Bryophyta are small plants, varying in size from 1 mm. to about
30 cm., creeping or erect, having a thalloid, or more usually a
foliose body, consisting of a cell-mass exhibiting in most cases a
distinct internal differentiation. They possess no true roots and no
true vascular tissue. The life-history of the members of the group is
characterised by a well-marked and definite alternation of generations.
The Moss or Liverwort plant is the sexual generation (_gametophyte_),
and as a result of the fertilisation of an egg-cell the asexual or
spore-bearing generation (_sporophyte_) is produced. The sporophyte
never exhibits a differentiation into stem and leaves. Asexual and
vegetative reproduction are effected by means of spores, bulbils, or
detached portions of the plant-body. Sexual reproduction is by means of
biciliate antherozoids produced in _antheridia_ and egg-cells formed
singly in _archegonia_.
In the Bryophytes the distinguishing characteristics are more
constant and well-defined than in the Thallophytes. In the former the
plant never consists of a single cell or coenocyte, but is always
multicellular, and exhibits in most cases a definite physiological
division of labour as expressed in the histological differentiation
of distinct tissue-systems. In the Thallophytes there is no true
alternation of generation in the same sense as in the Mosses and
Liverworts and in the higher plants. In the Bryophytes the sexual
reproduction has reached a higher stage of development and a much
greater constancy as regards the nature of the reproductive organs. On
the germination of the spore there is usually formed a fairly distinct
structure known as the _protonema_, from which the Moss or Liverwort
developes as a bud[449].
╭ MARCHANTIALES.
I. HEPATICAE. ┤ ANTHOCEROTALES.
╰ JUNGERMANNIALES.
The vegetative plant-body possesses a different organisation on the
ventral and dorsal side; it has the form of a thalloid creeping plant
(Thalloid Liverworts), or of a delicate stem with thin appendages or
leaves without a midrib (Foliose Liverworts). In most cases the body
of the plant is made up of parenchymatous tissue, showing but little
internal differentiation; in one or two genera a few strengthening or
mechanical fibres occur among the thinner walled ground-tissue. On the
germination of the spore, a feebly developed protonema is produced,
which gives rise to the Liverwort plant. Reproduction as in the group
Bryophyta.
[Sidenote: DETERMINATION OF LIVERWORTS.]
The Liverworts have a very wide geographical distribution, and are
specially abundant in moist shady situations; they grow on stones
or damp soil, and occur as epiphytes on other plants. _Marchantia_,
_Pellia_, and _Jungermannia_ are among the better known British
representatives of the class.
Considering the soft nature of the body of recent Liverworts, it is
not surprising that they are poorly represented in a fossil state. In
the absence of the sexual reproductive organs, or of the sporophytes,
which have scarcely ever been preserved, exact identification is almost
hopeless. The difficulties already referred to in dealing with the
algae, as regards the misleading similarity between the form of the
thallus and the bodies of other plants, have to be faced in the case of
the Liverworts. Many of the thalloid Liverworts, if preserved in the
form of a cast or impression without internal structure or reproductive
organs, could hardly be distinguished from various genera of algae
in which the thallus has the form of a forked plate-like body. Such
genera as _Pellia_, _Marchantia_, _Lunularia_, _Reboulia_, and others
bear a striking resemblance to _Fucus_, _Chondrus_ and many other algae.
Imperfect specimens of certain Lichens, not to mention some of
the Polyzoa, might easily be mistaken for Liverworts. Among the
higher plants, there are some forms of the _Podostemaceae_ which
simulate in habit both thalloid and foliose Liverworts as well as
Mosses[450]. The members of this Dicotyledonous family are described
as water-plants with a Moss- or Liverwort-like form; they occur on
rocks in quickly-flowing water in the tropics. In one instance a
recent Podostemaceous genus has been described as a member of the
Anthocerotales; the genus _Blandowia_[451], referred to by Willdenow as
a Liverwort, has since been recognised as one of the _Podostemaceae_.
The resemblance between some of the foliose _Hepaticae_ and genera
of Mosses is often very close. In certain Mosses, such as _Hookeria
pennata_[452], the large two-ranked leaves suggest the branches of a
_Selaginella_.
[Illustration: FIG. 48. A. _Tristichia hypnoides_ Spreng. From a
specimen in the British Museum. B. _Podocarpus cupressina_ Br.
and Ben. (After Brown and Bennett[453].) C. _Selaginella Oregana_
Eat. From a plant in the Cambridge Botanic Garden. A, B and C very
slightly reduced.]
The plant reproduced in fig. 48 A (_Tristichia_), one of the
Podostemaceae, might easily be mistaken for a foliose Liverwort if
found as a fragmentary fossil. Such species of _Selaginella_ as _S.
Oregana_ Eat. and _S. rupestris_ Spring (fig. 48 C) have a distinctly
moss-like habit and do not present a very obvious resemblance to the
more typical and better known Selaginellas. The twig of a _Podocarpus_
(_P. cupressina_)[454] in fig. 48 B affords an instance of a conifer
which simulates to some extent certain of the larger-leaved Liverworts;
it bears a resemblance also to some fossil fragments referred to
_Selaginellites_ or _Lycopodites_. A small fossil specimen figured by
Nathorst[455] from Japan as possibly a _Lycopodites_ may be compared
with a coniferous twig, and with some of the larger Liverworts, _e.g._
species of _Plagiochila_[456]. _Podocarpus cupressina_ is, however,
chiefly instructive as an example of the striking differences which are
met with among species of the same genus; it differs considerably from
the ordinary species of _Podocarpus_, and might well be identified as a
member of some other group than that of the Coniferae.
We have no records of Palaeozoic Hepaticae. The fossils which Zeiller
has figured in his _Flore de Brive_ as _Schizopteris dichotoma_
Gümb.[457] and _S. trichomanoides_ Göpp. bear a resemblance to some
forms of hepatics, but there is no satisfactory evidence for removing
them from the position assigned to them by the French writer. In
Mesozoic rocks a few specimens are known which bear a close resemblance
as regards the form of the thalloid body to recent Liverworts, but
the identification of such fossils cannot be absolutely trusted. Two
French authors, MM. Fliche and Bleicher[458], have described a plant
from Lower Oolite rocks near Nancy as a species of _Marchantia_,
_M. oolithus_, but they point out the close agreement of such
forked laminar structures to algae and lichens. From Tertiary and
Post-Tertiary beds a certain number of fossil species have been
recorded, but they possess no special botanical interest.
ORDER MARCHANTIALES.
The plant-body is always thalloid, bearing rhizoids on the lower
surface, and having an epidermis with pores limiting the upper or
dorsal surface.
_Marchantites._
This convenient generic name was proposed by Brongniart in 1849[459];
it may be briefly defined as follows:
Vegetative body of laminar form, with apparently dichotomous
branches, and agreeing in habit with the recent thalloid Hepaticae,
as represented by such a genus as _Marchantia_.
The name _Marchantites_ is preferable to _Marchantia_, as the latter
implies identity with the recent genus, whereas the former is used in a
wide sense and refers rather to a definite form of vegetative body than
to a particular generic type.
1. _Marchantites erectus_ (Leckenby). Fig. 49. This species may be
described as follows: The thalloid body is divided into spreading
dichotomously branched segments, obtusely pointed apically. The
slightly wrinkled surface shows a distinct and comparatively broad
darker and shorter median band, with lighter coloured and thinner
margins.
[Illustration: FIG. 49. _Marchantites erectus_ (Leck.). From the
type-specimen in the Woodwardian Museum. Nat. size.]
In 1864 Leckenby described this plant from the Lower Oolite beds
of the Yorkshire coast near Scarborough, as _Fucoides erectus_,
regarding it as a fossil alga. I recently pointed out that the general
appearance and mode of occurrence of the specimens suggest a liverwort
rather than an alga, and proposed the substitution of the genus
_Marchantites_[460]. It would, however, be unwise to speak with any
great confidence as to the real affinities of the fossil.
The example shown in the figure is the type-specimen of Leckenby[461]:
the breadth of the branches is about 3 mm. Under a low magnifying power
the surface shows distinct and somewhat oblique wrinklings, the general
appearance being very similar to that of some recent forms of the genus
_Marchantia_.
A closely allied species has recently been described from the
Wealden beds of Ecclesbourne, near Hastings, on the Sussex coast, as
_Marchantites Zeilleri_ Sew.[462].
In a recent monograph on Jurassic plants from Poland, apparently
containing much that is of the greatest value, but which is
unfortunately written in the Polish language, Raciborski[463] describes
a new species of thalloid Liverwort under the name of _Paleohepatica
Rostafinski_. The specimens are barren plants larger than any Jurassic
species hitherto described; they agree closely in habit with Saporta’s
Tertiary species _Marchantites Sezannensis_.
2. _Marchantites Sezannensis Saporta._ Fig. 50. The body is broadly
linear and dichotomously branched, with a somewhat undulating margin.
Midrib on the dorsal surface depressed, but more prominent on the
ventral surface. The upper surface is divided into hexagonal areas, in
each of which occurs a central pore. There are two rows of scales along
the median line on the lower surface. Stalked male receptacles.
Brongniart[464] first mentioned this fossil hepatic, which was found in
the calcareous travertine of Sézanne of Oligocene age in the Province
of Marne. The specimens figured by Saporta[465] show very clearly
the characters of one of the _Marchantiaceae_, and in this case we
have the additional evidence of the characteristic male receptacles
which are given off from a point towards the apex of the lobes, and
arise from a slight median depression. In one of Saporta’s figures
(reproduced in fig. 50 _A_) there are represented some median scars
which may mark the position of cups similar to those which occur on
recent species of _Marchantia_, and in which gemmæ or bulbils are
produced.
[Illustration: FIG. 50. _Marchantites Sezannensis_ Sap. _A._ Surface
view of the thallus; _g_, ? cups with gemmæ. _B._ A male branch.
_C._ A portion of _A_ magnified to show the surface features.
(After Saporta.)]
The collection of Sézanne fossils in the Sorbonne includes some very
beautiful casts of _Marchantites_ in which the structural details
are preserved much more perfectly than in the examples described by
Saporta. In a few specimens which Prof. Munier-Chalmas recently showed
me the reproductive branches were exceedingly well shown. The fossils
occur as moulds in the travertine, and the museum specimens are in the
form of plaster-casts taken from the natural moulds.
Several species of Liverworts belonging to the Marchantiales and
Jungermanniales have been recorded from the amber of North Germany,
of Oligocene age. These appear to be represented by small fragments,
such as are figured by Göppert and Berendt[466] in their monograph
on the amber plants, published in 1845. The determinations have since
been revised by Gottsche[467], who recognises species of _Frullania_,
_Jungermannia_, and other genera.
╭ SPHAGNALES.
II. MUSCI. ┤ ANDREAEALES.
╰ BRYALES.
The plant-body (gametophyte) in the Musci consists of a stem bearing
thin leaves, usually spirally disposed, rarely in two rows. The
internal differentiation of the stem is generally well marked, and
in some cases is comparable in complexity with the structure of the
higher plants. A protonema arises from the spore, having the form of a
branched filamentous, or more rarely a thalloid structure. Reproduction
as in the group Bryophyta.
Mosses like Liverworts have an extremely wide distribution, and occur
in various habitats. In many districts vast tracts of country are
practically monopolised by peat-forming genera, such as _Sphagnum_ and
other Mosses. Some genera are found on rocks at high altitudes in dry
regions, a few grow as saprophytes, and many occur either as epiphytes
on the leaves and stems of other plants, or carpeting the ground under
the shade of forest trees.
[Sidenote: DETERMINATION OF MOSSES.]
In the simpler Mosses, the stem consists of a parenchymatous
ground-tissue with a few outer layers of thicker-walled and smaller
cells. In others there is a distinct central cylinder which occupies
the axis of the stem, and consists of long and narrow cells; in the
more complex forms the structure of the axial tissues suggests the
central cylinder or _stele_ of higher plants. The genus _Polytrichum_,
so abundant on English moors, illustrates this higher type of stem
differentiation. In a transverse section of the stem the peripheral
tissue is seen to be composed of thick-walled cells, passing internally
into large parenchymatous tissue. The axial part is occupied by a
definite central cylinder consisting in the centre of elongated
elements with dark-coloured and thick walls having thin transverse
septa; surrounding this central tissue there are thinner walled
elements, of which some closely agree in form with the sieve-tubes of
the higher plants. The central tissue may be regarded as a rudimentary
type of xylem, and the surrounding tissue as a rudimentary phloem. Each
leaf is traversed by a median conducting strand which passes into the
stem and eventually becomes connected with the axial cylinder.
The fertilisation of the egg-cell gives rise to the development of a
long slender stalk terminating distally in a large spore-capsule. In
section the stalk or seta closely resembles the leafy axis of the moss
plant. Considering the fairly close approach of some of the mosses to
the higher plants as regards histological characters, it is conceivable
that imperfectly petrified stems of fossil mosses might be mistaken for
twigs of Vascular Cryptogams.
Like Liverworts, Mosses have left very few traces of their existence
in plant-bearing rocks. Without the aid of the characteristic
moss-‘fruit’ or sporogonium it is almost impossible to recognise
fossil moss-plant fragments. In species of the tropical genera
_Spiridens_ and _Dawsonia_, e.g. _S. longifolius_[468] Lind. or _D.
superba_[469] Grev. and _D. polytrichoides_[470] R. Br., the plant
reaches a considerable length, and resembles twigs of plants higher in
the scale than the Bryophytes. The finer branches of species of the
extinct genus _Lepidodendron_ are extremely moss-like in appearance.
Again, _Cyathophyllum bulbosum_ Muell[471], with its two kinds of
leaves arranged in rows, is not at all unlike species of _Selaginella_
or the hepatic genus _Gottschea_. It is by no means improbable that
some of the Palaeozoic specimens described as twigs of _Lycopodites_,
_Selaginites_, or _Lepidodendron_, may be portions of mosses. The
fertile branches of _Lycopodium phlegmaria_ in a fossil condition might
be easily mistaken for fragments of a moss. In some conifers with
small and crowded scale-leaves there is a certain resemblance to the
stouter forms of moss stems. Such possible sources of error should be
prominently kept in view when we are considering the value of negative
evidence as regards the geological history of the Musci.
A recent writer[472] on mosses has expressed the opinion that no doubt
the Musci played an exceedingly important rôle in past time. Although
we have no proof that this was so, yet it is far from improbable, and
the absence of fossil mosses must no doubt be attributed in part to
their failure to be preserved in a fossil state.
In the numerous samples of Coal-Measure vegetation preserved in
extraordinary perfection in the calcareous nodules of England, no
certain trace of a moss has so far been discovered. The most delicate
tissue in the larger Palaeozoic plants has often been preserved, and
in view of such possibilities of petrifaction it might appear strange
that if moss-like plants existed no fragments had been preserved.
Their absence is, however, no proof of the non-existence of Palaeozoic
mosses, but it is a fact which certainly tends towards the assumption
that mosses were probably not very abundant in the Coal Period forests.
Epiphytic mosses frequently occur on the stems and leaves of ferns
and other plants in tropical forests. Such small and comparatively
delicate plants would, however, be easily rubbed off or destroyed in
the process of fossilisation, and it is extremely rare to find among
petrified Palaeozoic plants the external features well preserved. It is
probable that the forests extended over low lying and swampy regions,
and that, in part, the trees were rooted in a submerged surface. Under
such conditions of growth there would not be the same abundance of
Bryophytes as in most of our modern forests.
To whatever cause the absence of mosses may be best attributed, it is
a fact that should not be too strongly emphasised in discussions on
plant-evolution.
_Muscites._
This comprehensive genus may be defined as follows:—
Stem filiform, simple or branched, bearing small sessile leaves, with
a delicate lamina, without veins or with a single median vein, arranged
in a spiral manner on the stem.
_Muscites_[473] is one of those convenient generic designations
which limited knowledge and incomplete data render necessary in
palaeontology. Fossil plants which in their general habit bear a
sufficiently striking resemblance to recent mosses, may be included
under this generic name.
[Illustration: FIG. 51. _Muscites polytrichaceus_ Ren. and Zeill.
(after Renault and Zeiller).]
1. _Muscites polytrichaceus_ Renault and Zeiller. In this species
the stems are about 3–4 cm. long and 1·3 m. broad, usually simple,
but sometimes giving off a few branches, and marked externally by
very delicate longitudinal grooves. The leaves are alternate, closely
arranged, lanceolate, with an acute apex, gradually narrowed towards
the base, 1–2 mm. long, traversed by a single median vein.
One of the French specimens, on which the species was founded[474],
is shown in fig. 51, and the form of the leaves is more clearly seen
in the small enlarged piece of stem. The authors of the species point
out that the tufted habit of the specimens, their small size, and the
membranous character of the leaves, all point to the Musci as the
Class to which the plant should be referred in spite of the absence of
reproductive organs.
Among recent mosses, the genus _Rhizogonium_,—one of the
_Mniaceae_,—and _Polytrichum_ are spoken of as offering a close
resemblance to the fossil form. The type-specimen was found in the
Coal-Measures of Commentry, and is now in the Museum of the École des
Mines in Paris; the figure given by MM. Renault and Zeiller faithfully
represents the appearance of the plant.
• • • • •
It has been suggested[475] that some small twigs figured by
Lesquereux[476] from the Coal-Measures of North America as _Lycopodites
Meeki_ Lesq., may possibly be mosses. The specimens do not appear
to be at all convincing, and cannot well be included as probable
representatives of Palaeozoic Musci. _Lycopodites Meeki_ Lesq. bears a
close resemblance to the recent _Selaginella Oregana_ shown in fig. 48,
C.
From Mesozoic rocks we have no absolutely trustworthy fossil mosses.
The late Prof. Heer[477] has quoted the occurrence of certain fossil
Caterpillars in Liassic beds as indicative of the existence of mosses,
but evidence of this kind cannot be accepted as scientifically sound.
In 1850 Buckman[478] described and figured a few fragments of plants
from a freshwater limestone at the base of the Lias series near
Bristol. Among others he described certain specimens as examples of a
fossil Monocotyledon, under the generic name _Najadita_. Mr Starkie
Gardner[479] subsequently examined the specimens, and suggested that
the Lias fragments referred to _Najadita_ should be compared with the
recent freshwater moss _Fontinalis_. In this opinion he was supported
by Mr Carruthers and Mr Murray of the British Museum. In a footnote
to the memoir in which this suggestion is made, Gardner refers to a
moss-capsule from the same beds, which he had received from Mr Brodie.
Through the kindness of the latter gentleman, I have had an opportunity
of examining the supposed capsule, and have no hesitation in describing
it as absolutely indeterminable. It is in the form of an irregularly
oval brown stain on the surface of the rock, with the suggestion of a
stalk at one end, but there are no grounds for describing the specimen
as a moss-capsule, or indeed anything else. The type-specimens figured
by Brodie and subsequently referred to a moss are now in the British
Museum; they are small and imperfect fragments of slender stems
bearing rather long oval leaves which might well have belonged to a
moss. The material is however too fragmentary to allow of accurate
diagnosis or determination.
2. _Muscites ferrugineus_ (Ludg.). This species possesses a slender
stem bearing crowded ovate-acuminate leaves. The capsules are
cup-shaped, borne on a short stalk, with a circular opening without
marginal teeth. This fossil was first figured and described by
Ludwig[480] from a brown ironstone of Miocene age at Dernbach in
Nassau. The author of the species placed it in the recent genus
_Gymnostomum_, and Schimper[481] afterwards changed the generic name to
_Sphagnum_, at the same time altering the specific name to _Ludwigi_.
The evidence is hardly strong enough to justify a generic designation
which implies identity with a particular recent genus, and it is a much
safer plan to adopt the non-committal term _Muscites_, at the same time
retaining Ludwig’s original specific name. Without having examined
the type-specimen it is impossible to express a definite opinion as
to the accuracy of the description given by Ludwig; if the capsule is
correctly identified it is the oldest example hitherto recorded of a
fossil moss-sporogonium.
CHAPTER IX.
PTERIDOPHYTA (Vascular Cryptogams).
The Pteridophytes include plants which vary in size from a few
millimetres[482] to several metres in height. The spore on germination
gives rise to a small thalloid structure, the _prothallium_, on which
the sexual organs are developed; this is the _gametophyte_ or sexual
generation. The sexual organs have the form of typical archegonia
and antheridia. From the fertilised egg-cell there is developed the
Pteridophyte plant or _sporophyte_, which bears the spores. This
asexual generation shows a well-marked external differentiation into
stem and leaves, and bears true roots. Internally the tissues exhibit
a high degree of differentiation into distinct tissue-systems. True
vascular bundles occur, which may or may not be capable of secondary
thickening by means of a _cambium_, _i.e._ a definitely localised zone
of meristematic tissue. The sporangia are borne either on the ordinary
foliage leaves or on special spore-bearing leaves called _sporophylls_,
which differ in a greater or less degree from the sterile leaves.
The majority of the best known and most important Palaeozoic genera
are either true Vascular Cryptogams, or possess certain of the
pteridophytic characteristics combined with those of higher plants.
It is not merely the commoner and more familiar recent genera with
which the student of extinct types must be acquainted, but it is
extremely important that he should make himself familiar with the
rarer, less known and more isolated recent forms, which often throw
most light on the affinities of the older representatives of the group.
It is often the case, the more isolated living plants are, the more
likely are they to afford valuable assistance in the interpretation
of genera representing a class, which reached its maximum development
in the earlier periods of the earth’s history. The importance of
paying special attention to such recent plants as may be looked upon
as survivals of a class now tending towards extinction, will be more
thoroughly realised after the extinct vascular cryptogams have been
dealt with.
A comparison of the Pteridophyta and Bryophyta brings out certain
points of divergence. In the first place, the sporophyte assumes in
the former class a much more prominent rôle, and the gametophyte has
suffered very considerable reduction. The gametophyte, _i.e._ the
structure which is formed on the germination of the asexually-produced
spore, is usually short-lived, small, and more or less dependent on
the sporophyte for its nutrition. In a few cases only is it capable
of providing itself with the essential elements of food. On the
other hand, the sporophyte, at a very early stage of its development
becomes free from the gametophyte and is entirely self-supporting.
Reproduction is effected as in the Bryophyta by sexual reproductive
organs and by asexual methods. Not only have we in the Pteridophytes a
much more complete external division of the plant-body into definite
members, which subserve distinct functions, and behave as well-defined
physiological organs adapted for taking a certain share in the
life-functions of the individual, but the internal differentiation has
reached a much higher stage. True vascular tissue, consisting of xylem
and phloem, occurs for the first time in this class. The whole plant is
traversed by one or more vascular strands composed of xylem and phloem
elements, which are respectively concerned with the distribution of
inorganic and organic food substances.
The Pteridophyta include the most important fossil plants. It is from a
study of the internal structure of various extinct representatives of
this class, that palaeobotanists have been able to contribute facts
of the greatest interest and importance towards the advancement of
botanical science.
The botanist’s chief aim in the anatomical investigation of Palaeozoic
genera is to discover data which point the way to a solution of the
problems of plant-evolution. In the abundant material afforded by the
petrified remnants of ancient floras we have the means of tracing the
past history of existing groups or individual forms, and it is from the
Palaeozoic Pteridophytes that our most valuable results have been so
far obtained.
In this and the following chapters of Volume I. two divisions of
the Pteridophyta are dealt with in such detail as the nature of the
book allows. In the earlier chapters of Volume II. the remaining
representatives of this class will be described. As in the preceding
chapters such recent plants will be described as are most essential for
the correct interpretation of the fossil forms.
It is impossible to do more than confine our attention to a few
only of the genera of living plants which directly concern us;
some acquaintance with the general facts of plant morphology must
be assumed. Among the most useful text-books or books of reference
on the Pteridophyta the student may consult those mentioned in the
footnote[483].
I. EQUISETALES.
Leaves usually small in proportion to the size of the whole plant,
arranged in whorls at the nodes. Sporangia borne on specially modified
sporophylls or sporangiophores, which are aggregated to form a definite
strobilus or spore-bearing cone.
EQUISETACEAE. (Recent Species.)
The leaves are in whorls, coherent in the form of a sheath, and
traversed by longitudinal veins which do not fork or anastomose.
The stem is divided into comparatively long internodes separated by
the leaf-bearing nodes, and the branches arise in the leaf-axils at
the nodes. The fertile leaves or sporophylls differ from the sterile
leaves, and usually occur in definite aggregations or strobili
containing spores of one kind (_isosporous_). In the single living
genus _Equisetum_, the outer coat of the mature spore forms two
hygroscopically sensitive filamentous structures or _elaters_. On the
germination of the spore the gametophyte is developed in the form of
a small lobed prothallium 1–2 cm. in length. In most cases there are
distinct male and female prothallia.
The genus _Equisetum_ L., the common Horse-tail, is the sole living
representative of this Family. It occurs as a common native plant
in Britain, and has a wide geographical distribution. Species of
_Equisetum_ are abundant in the temperate zones of both hemispheres,
and occur in arctic as well as tropical latitudes. Wallace[484]
speaks of Horse-tails, “very like our own species,” growing at a
height of 5000 feet on the Pangerango mountain in Java. In favourable
situations the large British Horse-tail, _Equisetum maximum_ Lam. (=
_E. Telmateia_ Erhb.), occasionally reaches a height of about six feet,
and growing in thick clusters forms miniature forests of trees with
slender erect stems and regular circles of long and thin branches. A
tropical species, _Equisetum giganteum_ Linn.[2] living in the marshes
of Mexico and Cuba[485], and extending southward to Buenos Ayres and
Chili, reaches a height of twenty to forty feet, but the stem always
remains slender, and does not exceed an inch in diameter. Groves of
such tall slender plants on the eastern slopes of the Andes[486]
suggest to the palaeobotanist an enfeebled forest-growth recalling
the arborescent Calamites of a Palaeozoic vegetation. The twenty-five
existing species of _Equisetum_ are remnants of various generic types
of former epochs, and possess a special interest from the point of view
of the geological history of plants. A brief description of the main
characters of the recent genus will enable the student to appreciate
the points of difference and agreement between the extinct and present
representatives of the Equisetales.
[Illustration: FIG. 52. _Equisetum maximum_ Lam. A. Fertile shoot with
strobilus and sterile leaf-sheaths [after Luerssen (89); slightly
less than nat. size]. B. Sporophyll bearing open sporangia (after
Luerssen; slightly enlarged). C. Part of a transverse section
(diagrammatic); _v_, vallecular canals, _e_, endodermis, _c_,
carinal canals (after Luerssen; × 20). D. _Equisetum arvense_ L.
Part of a transverse section of an internode of a sterile shoot.
_v_, cortex, _e_, endodermis, _x_, xylem tracheids, _a_ remains
of annular tracheids of the protoxylem, _c_, carinal canal (after
Strasburger; × 90).]
_Equisetum._
The plant consists of a perennial underground creeping rhizome,
branching into secondary rhizomes, divided into well-marked nodes and
internodes. From the nodes are given off two sets of buds, which may
develope into ascending aerial shoots or descending roots. At each node
is a leaf-sheath more or less deeply divided along the upper margin
into teeth representing the tips of coherent leaves (fig. 52, A).
In some species one or more internodes of underground branches become
considerably swollen and assume the form of ovate or elliptical
starch-storing tubers, which are capable of giving rise to new plants
by vegetative reproduction. Tubers, either singly or in chains, occur
in _E. arvense_ Linn., _E. sylvaticum_ Linn., _E. maximum_ Lam., among
British species.
[Illustration: FIG. 53. Rhizome (R) of _Equisetum palustre_ L. with a
thin shoot giving off roots and tuberous branches from a node
[after Duval-Jouve (64)].]
In the example shown in fig. 53 (_Equisetum palustre_ L.[487]) the
stout rhizome R gives off from its node, marked by a small and
irregular leaf-sheath, two thin roots and a single shoot. The latter
has a leaf-sheath at its base, and from the second node, with a
larger leaf-sheath, there have been developed branches with tuberous
internodes; the constrictions between the tubers and the tips of the
terminal tubers bear small leaf-sheaths. Branched roots are also given
off from the upper node of the erect shoot.
Near the surface of the ground the buds on the rhizome nodes develope
into green erect shoots. The shoot axis is marked out into long
internodes separated by nodes bearing the leaf-sheaths. The surface
of each internode is traversed by regular and more or less prominent
longitudinal ridges and grooves; each ridge marking the position of
an internal longitudinal vascular strand. In the axil of each leaf,
that is in the axil of each portion of a leaf-sheath corresponding to
a marginal uni-nerved tooth, there is produced a lateral bud which may
either remain dormant or break through the leaf-sheath and emerge as
a lateral branch. At the base of each branch an adventitious root may
be formed from a cell immediately below the first leaf-sheath, but
in aerial shoots the roots usually remain undeveloped. The lateral
branches repeat on a smaller scale the general features of the main
axis. In some species, the shoots are unbranched, and in others the
slender branches arise in crowded whorls from each node. Leaves, roots
and branches are given off in whorls, and the whorls from each node
alternate with those from the node next above and next below.
[Sidenote: ANATOMY OF EQUISETUM.]
In some species of _Equisetum_ the aerial stem terminates in a
conical group of sporophylls, while in others the strobilus is formed
at the apex of a pale-coloured fertile shoot, which never attains
any considerable length and dies down early in the season of growth
(fig. 52, A). Below the terminal cone or strobilus there occur one
or two modified leaf-sheaths. Such a ring of incompletely developed
leaves intervening between the cone of sporangiophores and the normal
leaves, is known as the _annulus_. The annulus is seen in fig. 52, A,
immediately below the lowest whorl of sporophylls; it has the form
of a low sheath with a ragged margin. In the region of the cone the
internodes remain shorter, and the whorls of appendages, known as
sporophylls or sporangiophores, have the form of stalked structures
terminating distally in a hexagonal peltate disc, which bears on its
inner face a ring of five to ten oval sporangia (fig. 52, B). Each
sporangium contains numerous spores which eventually escape by the
longitudinal dehiscence of the sporangial wall. The opening of the
sporangia is probably assisted by the movements of the characteristic
elaters formed from the outer wall of each spore.
The spores, which are capable of living only a short time, grow into
aerial green prothallia, 1–2 cm. in length; these have the form
of irregularly and more or less deeply lobed structures. On the
larger and more deeply lobed prothallia the archegonia or female
reproductive organs are borne, and the smaller or male prothallia bear
the antheridia. On the fertilisation of an egg-cell, the _Equisetum_
plant is gradually developed. For a short time parasitic on the female
prothallium or gametophyte, the young plant soon takes root in the
ground and becomes completely independent.
As seen in transverse section through a young stem near the apex, the
axis consists of a mass of parenchyma, in which may be distinguished a
central larger-celled tissue, surrounded by a ring of smaller-celled
groups marking the position of a circle of embryonic vascular strands.
In each young vascular strand, a few of the cells next the pith may be
seen to have thicker walls and to be provided with a ring-like internal
thickening; these have passed over into the condition of annular
tracheids and represent the _protoxylem_ elements. At a later stage, a
transverse section through the stem shows a central hollow pith, formed
by the tearing apart and subsequent disappearance of the medullary
parenchymatous cells, which were unable to keep pace with the growth in
thickness of the stem. The pith cavity is bridged across at each node
by a multi-layered plate of parenchyma, which forms the so-called nodal
_diaphragm_. The inner edge of each vascular strand is now found to be
occupied by a small irregularly circular canal (fig. 52, C, c, and D,
c) in which may be seen some of the rings of protoxylem tracheids (D,
a) which have been torn apart and almost completely destroyed. These
canals, known as _carinal canals_, have arisen by the tearing and
disruption of the thin-walled cells in the immediate neighbourhood of
the protoxylem. Each carinal canal is bounded by a layer of elongated
parenchymatous cells which form part of the xylem of the vascular
bundle, and is succeeded internally by the general ground-tissue
of the stem. The xylem parenchyma next a carinal canal is succeeded
externally by phloem tissue, consisting of short protoplasmic cells and
longer elements, without nuclei and poor in contents; the latter may be
regarded as sieve-tubes. On either side of the phloem, the xylem occurs
in two separate bands or groups of annular and reticulately thickened
tracheids. In some species, _e.g._ _Equisetum xylochaetum_ Metten.[488]
and _E. giganteum_[489] L. a native of South America, the xylem has
the form of two bands composed of fairly numerous tracheids, but in
most species the xylem tracheids occur in small groups, as shown in
the figure of _E. maximum_ (fig. 52, D). In the shape of the vascular
bundle, and in the formation of the carinal canal, there is a distinct
resemblance between the vascular bundles of _Equisetum_ and those of
a monocotyledonous stem. These collateral stem-bundles of xylem and
phloem traverse each internode as distinct strands, and at the nodes
each strand forks into two branches (fig. 54, A), which anastomose with
the alternating bundles passing into the stem from the leaf-sheath.
Thus the vascular strands of each internode alternate in position with
those of the next internode.
[Illustration: FIG. 54. A. Plan of the vascular bundles in the stem of
an _Equisetum_; _b_, branches passing out to buds (after
Strasburger); _l_, vascular strands passing to the leaf-segments.
B. Longitudinal section through a node of _E. arvense_ L. (after
Duval-Jouve; × 20). Explanation in the text.]
There are certain points connected with the vascular bundles in the
nodal region of a shoot, which have an important bearing on the
structure of fossil equisetaceous stems. Fig. 54 B represents a
diagrammatic longitudinal section through the node of a rhizome of
_Equisetum arvense_ from which a root _h_ is passing off in a downward
direction, and a branch in an upward direction. The black band _c_
in the parent stem shows the position of the vascular strands; in
the region of the node the vascular tissue attains a considerable
thickness, as seen at _d_ in the figure. The bands passing out to
the left from _d_ go to supply the branch and root respectively. The
increased breadth of the xylem strands at the node is due to the
intercalation of a number of short tracheids. Fig. 55, 4 shows a
transverse section through a mature node of _Equisetum maximum_; _px_
marks the position of the protoxylem and _e_ that of the endodermis.
On comparing this section with that of the internodal vascular bundle
in fig. 52, D, the much greater development of wood in the former is
obvious; the carinal canal of the internodal bundle is absent in the
section through a node. The disposition of the xylem tracheids in fig.
55, 4 shows a certain regularity which, though not very well marked,
suggests the development of wood elements as the result of cambial
activity. Longitudinal sections through the nodal region demonstrate
the existence of “cells similar to those of an ordinary cambium, and a
cell-formation resulting from their division which is similar to that
in an ordinary secondary thickening.”[490] The short tracheids which
make up this nodal mass of xylem differ from those in the internodal
bundle in their smaller size, and in being reticulately thickened.
There is, therefore, evidence that in the nodes of some _Equisetum_
stems additional xylem elements are produced by a method of growth
comparable with the cambial activity which brings about the growth in
thickness of a forest-tree[491]. The significance of these statements
will be realised when the structure of the extinct genus _Calamites_ is
described and compared with that of _Equisetum_.
The small drawing in fig. 55, 3 shows part of the ring of thick
nodal wood; the section cuts through two bundles about their point
of bifurcation, the strand _x_ is passing out in a radial direction
to a lateral branch, the strand to the right of _x_ and the separate
fragment of a strand to the left of _x_ are portions of leaf-trace
bundles on their way to the leaf-sheath. Reverting to fig. 54, B, the
other structures seen in the section are the leaf-sheaths (_l_ and
_m_), the vallecular canal (_f_), the epidermis, cortex and pith (_k_,
_e_ and _a_) of the stem. The epidermis which has been ruptured by the
root and branch is indicated at _i_, _i_; the dotted lines traversing
the upper part of the pith of the lateral branch mark the position of a
nodal diaphragm.
[Illustration: FIG. 55. 1. Transverse section of a root of _Equisetum
variegatum_ Schl., _e_ endodermis, or outer layer of the
phloeoterma (after Pfitzer; × 160). 2. Transverse section of
rhizome of _E. maximum_, slightly enlarged. 3. Transverse section
through a node of _E. maximum_, _x_, branch of vascular strand
(slightly enlarged). 4. Transverse section through a node of _E.
maximum_ showing the mass of xylem, _px_ protoxylem (× 175). (Figs.
3 and 4 after Cormack.)]
Immediately external to each vascular strand, as seen in transverse
section, there is a layer of cells containing starch, and this
is followed by a distinct endodermis, of which the cells show the
characteristic black dot in the cuticularised radial walls (fig. 52,
D). Beyond the endodermis there is the large-celled parenchyma of
the rest of the cortex. Tannin cells occur here and there scattered
among the ground tissue. On the same radius on which each vascular
strand occurs, the cortical parenchyma passes into a mass of
sub-epidermal thick-walled mechanical tissue or stereome. Alternating
with the ridges of stereome, the grooves are occupied by thin-walled
chlorophyll-containing tissue which carries on most of the assimilating
functions, and communicates with the external atmosphere by means of
stomata arranged in vertical rows down each internode. The continuity
of the cortical tissue is interrupted by the occurrence of large
longitudinal _vallecular canals_ alternating in position with the stem
ridges and vascular strands (fig. 52, C, _v_). The epidermis consists
of a single layer of cells, containing stomata, and with the outer
cell-walls impregnated with silica.
In certain species of _Equisetum_, _e.g._ _E. palustre_ L., the whole
circle of vascular strands is enclosed by an endodermis, and has the
structure typical of a monostelic stem. In others _e.g._ _E. litorale_
Kühl. each vascular strand is surrounded by a separate endodermis,
and in some forms _e.g._ _E. sylvaticum_ L. there is an inner as well
as an outer endodermal layer[492]. Without discussing the explanation
given to this variation in the occurrence of the endodermis, it may be
stated that in all species of _Equisetum_ the stem may be regarded as
monostelic[493].
In the rhizome the structure agrees in the main with that of the
green shoots, but the vallecular canals attain a larger size, and the
pith is solid. A slightly enlarged transverse section of a rhizome
of _Equisetum maximum_ is shown in fig. 55, 2, the small circles
surrounding the pith mark the position of the vascular bundles and
carinal canals; the much larger spaces between the central cylinder and
the surface of the stem are the vallecular canals.
The central cylinder or stele of the root is of the diarch, triarch or
tetrarch type; _i.e._ there may be 2, 3 or 4 groups of protoxylem in
the xylem of the root stele. The axial portion is occupied by large
tracheids, and the smaller tracheids of the xylem occur as radially
disposed groups, alternating with groups of phloem. External to the
xylem and phloem strands there occur two layers of cells, usually
spoken of as a double endodermis, but it has been suggested that it
is preferable to describe the double layer as the _phloeoterma_[494],
of which the inner layer has the functions of a pericycle, and the
outer that of an endodermis. A transverse section of a root is seen
in fig. 55, 1, the dark cells on the left are part of a thick band of
sclerenchyma in the cortex of the root, the layer e is the outer layer
of the phloeoterma.
Without describing in detail the development[495] of the sporangia,
it should be noted that the sporangial wall is at first 3 to 4 cells
thick, but it eventually consists of a single layer. The cells have
spiral thickening bands on the ventral surface, and rings on the
cells where the longitudinal splitting takes place. Each sporangium
is supplied by a vascular bundle which is given off from that of the
sporangiophore axis. The strobili are isosporous.
╭ A. EQUISETITES.
│ B. PHYLLOTHECA.
II. FOSSIL EQUISETALES. ┤ C. SCHIZONEURA.
│ D. CALAMITES.
╰ E. ARCHAEOCALAMITES.
In dealing with the fossil Equisetales, we will first consider
the genera _Equisetites_, _Phyllotheca_ and _Schizoneura_, and
afterwards describe the older and better known genera _Calamites_
and _Archaeocalamites_. A thoroughly satisfactory classification of
the members of the Equisetales is practically impossible without
more data than we at present possess. It has been the custom to
include _Equisetites_, _Phyllotheca_ and _Schizoneura_ in the family
Equisetaceae, and to refer _Calamites_ and _Archaeocalamites_ to the
Calamarieae; such a division rests in part on assumption, and cannot
be considered final. When we attempt to define the Equisetales and the
two families Equisetaceae and Calamarieae, we find ourselves seriously
hampered by lack of knowledge of certain important characters, which
should be taken into account in framing diagnoses. There is little
harm in retaining provisionally the two families already referred to,
if we do not allow a purely arbitrary classification to prejudice our
opinions as to the affinities of the several members of the Equisetales.
The Equisetaceae might be defined as a family including plants which
were usually herbaceous but in some cases arborescent, bearing
verticils of leaves in the form of sheaths more or less deeply divided
into segments or teeth. The strobili were isosporous and consisted of
a central axis bearing verticils of distally expanded sporophylls with
sporangia, as in _Equisetum_. The genus _Equisetites_ might be included
in this family, but it must be admitted that we know next to nothing
as to its anatomy, and we cannot be sure that the strobili were always
isosporous.
The genus _Schizoneura_ is too imperfectly known to be defined with any
approach to completeness, or to be assigned to a family defined within
certain prescribed limits. _Phyllotheca_ is another genus about which
we possess but little satisfactory knowledge; we are still without
evidence as to its structure, and the descriptions of the few strobili
that are known are not consistent. Recent work points to a probability
of _Phyllotheca_ being closely allied to _Annularia_, a genus included
in the Calamarieae, and standing for a certain type of Calamitean
foliage-shoots.
In comparing the Calamarieae with the Equisetaceae, the alternation of
sterile and fertile whorls in the strobilus, and the free linear leaves
at the nodes instead of leaf-sheaths are two characters made use of as
distinguishing features of the genus _Calamites_ as the type of the
Calamarieae. On the other hand, the strobili of _Phyllotheca_ appear to
agree with those of _Calamites_ rather than with those of _Equisetum_,
and strobili of _Archaeocalamites_ have been found exhibiting the
typical _Equisetum_ characters. The sheath-like form of the leaves
is not necessarily peculiar to the Equisetaceae, and we have evidence
that leaf-sheaths occurred on the nodes of Calamitean plants. In
_Archaeocalamites_ the leaves possess characteristic features, and can
hardly be said to agree more closely with those of Calamites than with
the leaves of _Phyllotheca_ or _Sphenophyllum_, a genus belonging to
another class of Pteridophytes.
On the whole, then, without discussing further the possibilities of a
subdivision of the Equisetales, we may regard the genera _Calamites_,
_Archaeocalamites_, _Equisetites_, _Equisetum_, _Phyllotheca_ and
_Schizoneura_ as so many members of the Equisetales, without insisting
on a classification which cannot be supported by satisfactory evidence.
Our knowledge of _Calamites_ is fairly complete. Abundant and
well-preserved material from the Coal-Measures of England, and from
Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled
palaeobotanists to investigate the anatomical characters of both the
vegetative and reproductive structures of this genus. We are in a
position to give a detailed diagnosis of Calamitean stems, roots and
strobili, and to determine the place of this type of plant in a system
of classification. _Calamites_ not only illustrates the possibilities
of palaeobotanical research, but it demonstrates the importance of
fossil forms as foundations on which to construct the most rational
classification of existing plants. The close alliance between
_Calamites_ and the recent Equisetaceae has been clearly established,
and certain characteristics of the former genus render necessary an
extension and modification of the definition of the class to which
both _Calamites_ and _Equisetites_ belong. The Calamites broaden our
conception of the Equisetaceous alliance, and by their resemblance
to other extinct Palaeozoic types they furnish us with important
links towards a phylogenetic series, which the other members of the
Equisetales do not supply.
From the Upper Devonian to the Permian epoch _Calamites_ and other
closely related types played a prominent part in the vegetation of
the world. We have no good evidence for the existence of _Calamites_
in Triassic times; in its place there were gigantic Equisetums which
resembled modern Horse-tails in a remarkable degree. In the succeeding
Jurassic period tree-like Equisetums were still in existence, and
species of _Equisetites_ are met with in rocks of this age in nearly
all parts of the world. A few widely distributed species are known from
Wealden rocks, but as we ascend the geologic series from the Jurassic
strata, the Equisetums become less numerous and the individual plants
gradually assume proportions practically identical with those of
existing forms.
A. _Equisetites._
The generic name _Equisetites_ was proposed by Sternberg in
1838[496] as a convenient designation for fossil stems bearing a
close resemblance to recent species of _Equisetum_. Some authors
have preferred to apply the name _Equisetum_ to fossil and recent
species alike, but in spite of the apparent identity in the external
characters of the fossil stems with those of existing Horse-tails, and
a close similarity as regards the cones, there are certain reasons
for retaining Sternberg’s generic name. It is important to avoid such
nomenclature as might appear to express more than the facts admit. If
the custom of adding the termination -_ites_ to the root of a recent
generic term is generally followed, it at once serves to show that the
plants so named are fossil and not recent species. Moreover, in the
case of fossil Equisetums we know nothing of their internal structure,
and our comparisons are limited to external characters. Stems, cones,
tubers, and leaves are often very well preserved as sandstone casts
with distinct surface-markings, but we are still in want of petrified
specimens. There is indeed evidence that some of the Triassic and
Jurassic species of _Equisetites_, like the older Calamites, possessed
the power of secondary growth in thickness, but our deductions are
based solely on external characters.
In the following pages a few of the better known species of
_Equisetites_ are briefly described, the examples being chosen partly
with a view to illustrate the geological history of the genus, and
partly to contribute something towards a fuller knowledge of particular
species. One of the most striking facts to be gleaned from a general
survey of the past history of the Equisetaceae is the persistence since
the latter part of the Palaeozoic period of that type of plant which
is represented by existing Equisetums. There is perhaps no genus in
existence which illustrates more vividly than _Equisetum_ the survival
of an extremely ancient group, which is represented to-day by numerous
and widely spread species. The Equisetaceous characteristics mark
an isolated division of existing Vascular Cryptogams, and without
reference to extinct types it is practically impossible to do more than
vaguely guess at the genealogical connections of the family. When we go
back to Palaeozoic plants there are indications of guiding lines which
point the way to connecting branches between the older Equisetales
and other classes of Pteridophytes. The recently discovered genus
_Cheirostrobus_[497] is especially important from this point of view.
[Sidenote: LEAF-SHEATHS OF EQUISETITES.]
The accurate description of species, and the determination of the value
of such differences as are exhibited in the surface characters of
structureless casts, are practically impossible in many of the fossil
forms. In certain living Horse-tails we find striking differences
between fertile and sterile shoots, and between branches of different
orders. The isolated occurrence of fragments of fossil stems often
leads to an artificial separation of ‘species’ largely founded on
differences in diameter, or on slight variations in the form of the
leaf-sheaths. It is wiser to admit that in many cases we are without
the means of accurate diagnosis, and that the specific names applied
to fossil Equisetums do not always possess much value as criteria of
taxonomic differences.
The specimens of fossil Equisetums are usually readily recognised by
the coherent leaf-segments in the form of nodal sheaths resembling
those of recent species. The tissues of the cortex and central
cylinder are occasionally represented by a thin layer of coal pressed
on to the surface of a sandstone cast, or covering a flattened
stem-impression on a piece of shale. It is sometimes possible under
the microscope to recognise on the carbonised epidermal tissues
the remains of a surface-ornamentation similar to that in recent
species, which is due to the occurrence of siliceous patches on the
superficial cells. Longitudinal rows of stomata may also be detected
under favourable conditions of preservation. The nodal diaphragms
of stems have occasionally been preserved apart, but such circular
and radially-striated bodies may be misleading if found as isolated
objects. Casts of the wide hollow pith of _Equisetites_, with
longitudinal ridges and grooves, and fairly deep nodal constrictions,
have often been mistaken for the medullary casts of _Calamites_.
Several species of _Equisetites_ have been recorded from the Upper
Coal-Measures and overlying Permian rocks, but these present special
difficulties. In one instance described below, (_Equisetites
Hemingwayi_ Kidst.), the species was founded on a cast of what appeared
to be a strobilus made up of sporophylls similar to those in an
_Equisetum_ cone. In other Permo-Carboniferous species the choice of
the generic name _Equisetites_ has been determined by the occurrence
of leaf-sheaths either isolated or attached to the node of a stem. The
question to consider is, how far may the Equisetum-like leaf-sheath
be regarded as a characteristic feature of _Equisetites_ as distinct
from _Calamites_? In the genus _Calamites_ the leaves are generally
described as simple linear leaves arranged in a whorl at the nodes,
but not coherent in the form of a sheath (fig. 85). The fusion of
the segments into a continuous sheath or collar is regarded as a
distinguishing characteristic of _Equisetites_ and _Equisetum_. The
typical leaf-sheath of a recent Horse-tail has already been described.
In some species we have fairly large and persistent free teeth on the
upper margin of the leaf-sheath, but in other Equisetums the rim of
the sheath is practically straight and has a truncated appearance,
the distal ends of the segments being separated from one another by
very slight depressions, as in a portion of the sheath of _Equisetum
ramosissimum_ Desf. of fig. 58, _C_. In other leaf-sheaths of this
species there are delicate and pointed teeth adherent to the margin
of the coherent segments; the teeth are deciduous, and after they have
fallen the sheath presents a truncated appearance. This difference
between the sheaths to which the teeth are still attached and those
from which they have fallen is illustrated by fig. 58, _B_ and _C_;
it is one which should be borne in mind in the description of fossil
species, and has probably been responsible for erroneous specific
diagnoses. In some recent Horse-tails the sheath is occasionally
divided in one or two places by a slit reaching to the base of the
coherent segments[498]; this shows a tendency of the segments towards
the free manner of occurrence which is usually considered a Calamitean
character. In certain fossils referred to the genus _Annularia_, the
nodes bear whorls of long and narrow leaves which are fused basally
into a collar (fig. 58, _D_). There are good grounds for believing that
at least some Annularias were the foliage shoots of true Calamites.
Again, in some species of _Calamitina_, a sub-genus of _Calamites_,
the leaves appear to have been united basally into a narrow sheath.
We see, then, that it is a mistake to attach great importance to the
separate or coherent character of leaf-segments in attempting to draw
a line between the true _Calamites_ and _Equisetites_. Potonié[499]
while pointing out that this distinction does not possess much value
as a generic character, retains the genus _Equisetites_ for certain
Palaeozoic Equisetum-like leaf-sheaths.
[Illustration: FIG. 56. Calamitean leaf-sheath. From a specimen in the
Woodwardian Museum. _a_, base of leaf-sheath; (very slightly
reduced).]
Fig. 56 represents a rather faint impression of a leaf-sheath and
nodal diaphragm. The specimen is from the Coal-Measures of Ardwick,
Manchester. The letter _a_ probably points to the attachment of the
sheath to the node of the stem. The flattened sheath is indistinctly
divided into segments, and at the middle of the free margin there
appears to be a single free tooth. The lower part of the specimen, as
seen in the figure, shows the position of the nodal diaphragm. Between
the diaphragm and the sheath there are several slight ridges converging
towards the nodal line; these agree with the characteristic ridges and
grooves of Calamite casts which are described in detail in Chapter X.
There is another specimen in the British Museum which illustrates,
rather more clearly than that shown in fig. 56, the association of a
fused leaf-sheath with a type of cast usually regarded as belonging
to a Calamitean stem. Some leaf-sheaths of Permian age described by
Zeiller[500] as _Equisetites Vaujolyi_ bear a close resemblance to
the sheath in fig. 58 E. The nature of the true Calamite leaves is
considered more fully on a later page.
[Sidenote: PALAEOZOIC EQUISETITES.]
The examples of supposed _Equisetites_ sheaths referred to below may
serve to illustrate the kind of evidence on which this genus has
been recorded from Upper Palaeozoic rocks. I have retained the name
_Equisetites_ in the description of the species, but it would probably
be better to speak of such specimens as ‘Calamitean leaf-sheaths’
rather than to describe them as definite species of _Equisetites_.
We have not as yet any thoroughly satisfactory evidence that the
_Equisetites_ of Triassic and post-Triassic times existed in the
vegetation of earlier periods.
In Grand’Eury’s _Flore du Gard_[501] a fossil strobilus is figured
under the name _Calamostachys tenuissima_ Grand’Eury, which consists of
a slender axis bearing series of sporophylls and sporangia apparently
resembling those of an _Equisetum_. There are no sterile appendages or
bracts alternating with the sporophylls; and the absence of the former
suggests a comparison with _Equisetites_ rather than _Calamites_.
Grand’Eury refers to the fossil as “parfois à peine perceptible,” and a
recent examination of the specimen leads me to thoroughly endorse this
description. It was impossible to recognise the features represented
in Grand’Eury’s drawing. Setting aside this fossil, there are other
strobili recorded by Renault[502] and referred by him to the genus
_Bornia_ (_Archaeocalamites_), which also exhibit the Equisetum-like
character; the axis bears sporophylls only and no sterile bracts. It
would appear then that in the Palaeozoic period the Equisetaceous
strobilus, as we know it in _Equisetum_, was represented in some of the
members of the Equisetales.
[Illustration: FIG. 57. A. _Equisetites Hemingwayi_ Kidst. From a
specimen in the British Museum. ⅔ nat. size. B. Diaphragm and
sheath of an Equisetaceous plant, from the Coal-Measures. ⅔ nat.
size. From a specimen in the British Museum.]
1. _Equisetites Hemingwayi_ Kidst. Fig. 57, _A_.
Mr Kidston[503] founded this species on a few specimens of cones
found in the Middle Coal-Measures of Barnsley in Yorkshire. The best
example of the cone described by Kidston has a length of 2·5 cm.,
and a breadth of 1·5 cm.; the surface is divided up into several
hexagonal areas 4 mm. high and 5 mm. wide. Each of these plates shows
a fairly prominent projecting point in its centre; this is regarded
as the point of attachment of the sporangiophore axis which expanded
distally into a hexagonal plate bearing sporangia. An examination of
Mr Kidston’s specimens enabled me to recognise the close resemblance
which he insists on between the fossils and such a recent Equisetaceous
strobilus as that of _Equisetum limosum_ Sm. Nothing is known of the
structure of the fossils beyond the character of the superficial
pattern of the impressions, and it is impossible to speak with absolute
confidence as to their nature. The author of the species makes
use of the generic name _Equisetum_; but in view of our ignorance
of structural features it is better to adopt the more usual term
_Equisetites_.
Since Kidston’s description was published I noticed a specimen in the
British Museum collection which throws some further light on this
doubtful fossil. Part of this specimen is shown in fig. 57, _A_. The
stem is 21 cm. in length and about 5 mm. broad; it is divided into
distinct nodes and internodes; the former being a little exaggerated in
the drawing. The surface is marked by fine and irregular striations,
and in one or two places there occur broken pieces of narrow linear
leaves in the neighbourhood of a node. Portions of four cones occurring
in contact with the stem, appear to be sessile on the nodes, but
the preservation is not sufficiently good to enable one to speak
with certainty as to the manner of attachment. Each cone consists of
regular hexagonal depressions, which agree exactly with the surface
characters of Kidston’s type-specimen. The manner of occurrence of the
cones points to a lateral and not a terminal attachment. The stem does
not show any traces of Equisetaceous leaf-sheaths at the nodes, and
such fragments of leaves as occur appear to have the form of separate
linear segments; they are not such as are met with on _Equisetites_.
It agrees with some of the slender foliage-shoots of Calamitean plants
often described under the generic name _Asterophyllites_. As regards
the cones; they differ from the known Calamitean strobili in the
absence of sterile bracts, and appear to consist entirely of distally
expanded sporophylls as in _Equisetum_. The general impression afforded
by the fossil is that we have not sufficient evidence for definitely
associating this stem and cones with a true _Equisetites_. We may,
however, adhere to this generic title until more satisfactory data are
available.
2. _Equisetites spatulatus_ Zeill. Fig. 58, _A_.
This species is chosen as an example of a French _Equisetites_ of
Permian age. It was recently founded by Zeiller[504] on some specimens
of imperfect leaf-sheaths, and defined as follows:—
Sheaths spreading, erect, formed of numerous uninerved coherent
leaves, convex on the dorsal surface, spatulate in form, 5–6 cm.
in length and 2–3 mm. broad at the base, and 5–10 mm. broad at the
apex, rounded at the distal end.
The specimen shown in fig. 58, _A_, represents part of a flattened
sheath, the narrower crenulated end being the base of the sheath.
The limits of the coherent segments and the position of the veins
are clearly marked. Zeiller’s description accurately represents the
character of the sheaths. They agree closely with an Equisetaceous
leaf-sheath, but as I have already pointed out, we cannot feel certain
that sheaths of this kind were not originally attached to a Calamite
stem.
The portion of a leaf-sheath and a diaphragm represented in fig. 57,
_B_, agrees closely with Zeiller’s examples. This specimen is from
the English Coal-Measures, but it is not advisable to attempt any
specific diagnosis on such fragmentary material. It is questionable,
indeed, if these detached fossil leaf-sheaths should be designated by
specific names. Another similar form of sheath, hardly distinguishable
from Zeiller’s species, has recently been described by Potonié from the
Permian (Rothliegende) of Thuringia.
[Illustration: FIG. 58.
_A._ _Equisetites spatulatus_, Zeill. Leaf-sheath. ⅘ nat. size.
(After Zeiller.)
_B._ _E. columnaris_, Brongn. From a specimen in the British Museum.
¾ nat. size.
_C._ _Equisetum ramosissimum_, Desf. × 2.
_D._ _Annularia stellata_ (Schloth.). Leaf-sheath. Slightly enlarged.
(After Potonié.)
_E._ _Equisetites zeaeformis_ (Schloth.). Leaf-sheath. ⅘ nat. size.
(After Potonié.)
_F._ _E. lateralis_, Phill. From a specimen in the Scarborough
Museum. Nat. size.]
3. _Equisetites zeaeformis_ (Schloth.)[505] Fig. 58, _E_.
The sheaths consist of linear segments fused laterally as in
_Equisetum_. In some specimens the component parts of the sheath are
more or less separate from one another, and in this form they are
apparently identical with the leaves of _Calamites_ (_Calamitina_)
_varians_, Sternb. The example shown in fig. 58, _E_ is probably a
young leaf-sheath; the segments are fused, and each is traversed by
a single vein represented by a dark line in the figure. The regular
crenulated lower margin is the base of the sheath, and corresponds to
the upper portion of fig. 58, _A_. This species affords, therefore, an
interesting illustration of the difficulty of separating _Equisetites_
leaves from those of true _Calamites_. Potonié has suggested that the
leaf-sheath of a young Calamite might well be split up into distinct
linear segments as the result of the increase in girth of the stem.
• • • • •
Other Palaeozoic species of _Equisetites_ have been recorded, but with
one exception these need not be dealt with, as they do not add anything
to our knowledge of botanical importance. The specimen described
in the _Flore de Commentry_ as _Equisetites Monyi_, by Renault and
Zeiller[506], differs from most of the other Palaeozoic species of
_Equisetites_, in the fact that we have a stem with short internodes
bearing a leaf-sheath at each node divided into comparatively long
and distinct teeth. This species presents a close agreement with
specimens of _Calamitina_, but Renault and Zeiller consider that it is
generically distinct. They suggest that the English species, originally
described and figured by Lindley and Hutton[507] as _Hippurites
gigantea_, and now usually spoken of as _Calamitina_, should be
named _Equisetites_. It would probably be better to adopt the name
_Calamitina_ for the French species. The type-specimen of this species
is in the Natural History Museum, Paris.
[Sidenote: EQUISETITES PLATYODON.]
[Illustration: FIG. 59. _Equisetites platyodon_ Brongn. (After
Schoenlein, slightly reduced.)]
When we pass from the Permian to the Triassic period, we find large
casts of very modern-looking Equisetaceous stems which must clearly be
referred to the genus _Equisetites_. The portion of a stem represented
in fig. 59 known as _Equisetites platyodon_ Brongn.[508] affords an
example of a Triassic Equisetaceous stem with a clearly preserved
leaf-sheath. The stem measures about 6 cm. in diameter. One of the
oldest known Triassic species is _Equisetites Mougeoti_[509] (Brongn.)
from the Bunter series of the Vosges.
The Keuper species _E. arenaceus_ is, however, more completely known.
The specimens referred to this species are very striking fossils; they
agree in all external characters with recent Horse-tails but greatly
exceed them in dimensions.
4. _Equisetites arenaceus_ Bronn.
This plant has been found in the Triassic rocks of various parts of
Germany and France; it occurs in the Lettenkohl group (Lower Keuper),
as well as in the Middle Keuper of Stuttgart and elsewhere. The species
may be defined as follows:—
Rhizome from 8–14 cm. in diameter, with short internodes,
bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in
diameter, bearing whorls of branches, and leaf-sheaths made up
of 110–120 coherent uni-nerved linear segments terminating in an
apical lanceolate tooth. Strobili oval, consisting of crowded
sporangiophores with pentagonal and hexagonal peltate terminations.
The casts of branches, rhizomes, tubers, buds and cones enable us to
form a fairly exact estimate of the size and general appearance of
this largest fossil Horse-tail. The Strassburg Museum contains many
good examples of this species, and a few specimens may be seen in
the British Museum. In the École des Mines, Paris, there are some
exceptionally clear impressions of cones of this species from a lignite
mine in the Vosges.
It is estimated that the plant reached a height of 8 to 10 meters,
about equal to that of the tallest recent species of _Equisetum_,
but in the diameter of the stems the Triassic plant far exceeded any
existing species.
It is interesting to determine as far as possible, in the absence of
petrified specimens, if this Keuper species increased in girth by
means of a cambium. There are occasionally found sandstone casts of
the pith-cavity which present an appearance very similar to that of
Calamitean medullary casts[510]. The nodes are marked by comparatively
deep constrictions, which probably represent the projecting nodal wood.
The surface of the casts is traversed by regular ridges and grooves
as in an ordinary Calamite, and it is probable that in _Equisetites
arenaceus_, as in _Calamites_, these surface-features are the
impression of the inner face of a cylinder of secondary wood (_cf._
p. 310). Excellent figures of this species of _Equisetites_ are given
by Schimper in his Atlas of fossil plants[511], also by Schimper and
Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].
5. _Equisetites columnaris_ Brongn. Figs. 11 and 58, _B_.
This species, which is by far the best known British _Equisetites_, was
founded by Brongniart[514] on some specimens from the Lower Oolite beds
of the Yorkshire coast. Casts of stems are familiar to those who have
collected fossils on the coast between Whitby and Scarborough; they
are often found in an erect position in the sandstone, and are usually
described as occurring in the actual place of growth. As previously
pointed out (p. 72), such stems have generally been deposited by water,
and have assumed a vertical position (fig. 11). Young and Bird[515]
figured a specimen of this species in 1822, and in view of its striking
resemblance to the sugar-cane, they regarded the fossil as being of the
same family as _Saccharum officinarum_, if not specifically identical.
A specimen was described by König[516] in 1829, from the Lower
Oolite rocks of Brora in the north of Scotland under the name of
_Oncylogonatum carbonarium_, but Brongniart[517] pointed out its
identity with the English species _Equisetites columnaris_.
Our acquaintance with this species is practically limited to the
casts of stems. A typical stem of _E. columnaris_ measures 3 to 6
cm. in diameter and has fairly long internodes. The largest stem in
the British Museum collection has internodes about 14 cm. long and a
diameter of about 5 cm. In some cases the stem casts show irregular
lateral projections in the neighbourhood of a node, but there is no
evidence that the aerial shoots of this species gave off verticils of
branches. In habit _E. columnaris_ probably closely resembled such
recent species as _Equisetum hiemale_ L., _E. trachyodon_ A. Br. and
others.
The stems often show a distinct swelling at the nodes; this may be due,
at least in part, to the existence of transverse nodal diaphragms which
enabled the dead shoots to resist contraction in the region of the
nodes. The leaf-sheaths consist of numerous long and narrow segments
often truncated distally, as in fig. 58, _B_, and as in the sheath of
such a recent Horse-tail as _E. ramosissimum_ shown in fig. 58, _C_. In
some specimens one occasionally finds indications of delicate acuminate
teeth extending above the limits of a truncated sheath. Brongniart
speaks of the existence of caducous acuminate teeth in his diagnosis of
the species, and the example represented in fig. 58, _B_, demonstrates
the existence of such deciduous appendages. There is a very close
resemblance between the fossil sheath of fig. 58, _B_, with and without
the teeth, and the leaf-sheath of the recent _Equisetum_ in fig. 58,
_C_. In some specimens of _E. columnaris_ in which the cast is covered
with a carbonaceous film, each segment in a leaf-sheath is seen to be
slightly depressed in the median portion, which is often distinctly
marked by numerous small dots, the edges of the segment being flat and
smooth. The median region is that in which the stomata are found and on
which deposits of silica occur.
6. _Equisetites Beani_ (Bunb.). Figs. 60–62.
Bunbury[518] proposed the name _Calamites Beani_ for some fossil stems
from the Lower Oolite beds of the Yorkshire coast, which Bean had
previously referred to in unpublished notes as _C. giganteus_. The
latter name was not adopted by Bunbury on account of the possible
confusion between this species and the Palaeozoic species _Calamites
gigas_ Brongn. The generic name _Calamites_ must be replaced by
_Equisetites_ now that we are familiar with more perfect specimens
which demonstrate the Equisetean characters of the plant.
[Illustration: FIG. 60. _Equisetites Beani_ (Bunb.). ⅔ nat. size.
[After Starkie Gardner (86) Pl. IX. fig. 2.]]
Schimper[519] speaks of this species as possibly the pith-cast of
_Equisetites columnaris_, but his opinion cannot be maintained; the
species first described by Bunbury has considerably larger stems than
those of _E. columnaris_. It is not impossible, however, that _E.
columnaris_ and _E. Beani_ may be portions of the same species. The
chief difference between these forms is that of size; but we have not
sufficient data to justify the inclusion of both forms under one name.
Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem
cast of _E. Beani_ under the name of _Calamites Beani_, but the species
has received little attention at the hands of recent writers. In
1886 Starkie Gardner[521] figured a specimen which was identified by
Williamson as an example of Bunbury’s species; but the latter pointed
out the greater resemblance, as regards the external appearance of the
Jurassic stem, to some of the recent arborescent Gramineae[522] than to
the Equisetaceae. Williamson, with his usual caution, adds that such
appearances have very little taxonomic value. Fig. 60 is reproduced
from the block used by Gardner in his memoir on Mesozoic Angiosperms;
he quotes the specimen as possibly a Monocotyledonous stem. The
fossil is an imperfect cast of a stem showing two clearly marked
nodal regions, but no trace of leaf-sheaths. A recent examination
of specimens in the museums of Whitby, Scarborough, York and London
has convinced me that the plant named by Bunbury _Calamites Beani_
is a large _Equisetites_. As a rule the specimens do not show any
indications of the leaf-sheaths, but in a few cases the sheaths have
left fairly distinct impressions.
[Illustration: FIG. 61. _Equisetites Beani_ (Bunb.). From a specimen in
the British Museum, ⅔ nat. size. (No. V. 2725.)]
In the portion of stem shown in fig. 61 the impressions of the leaf
segments are clearly marked. This specimen affords much better evidence
of the Equisetaceous character of the plant than those which are simply
internal casts. The narrow projecting lines extending upwards from the
nodes in the figured specimen probably represent the divisions between
the several segments of each leaf-sheath.
In the museums of Whitby and Scarborough there are some long specimens,
in one case 44 cm. in length, and 33 cm. in circumference, which
are probably casts of the broad pith-cavity. These casts are often
transversely broken across at the nodes, so that they consist of three
or four separate pieces which fit together by clean-cut faces. This
manner of occurrence is most probably due to the existence of large and
resistant nodal diaphragms which separated the sand-casts of adjacent
internodes. In the York museum there are some large diaphragms, 10
cm. in diameter, preserved separately in a piece of rock containing
a cast of _Equisetites Beani_. The nodal diaphragms of some of the
Carboniferous Calamites were the seat of cork development[523], and
it may be that the frequent preservation of Equisetaceous diaphragms
in Triassic and Jurassic rocks is due to the protection afforded by a
corky investment.
The stem shown in fig. 62 appears to be a portion of a shoot of _E.
Beani_ not far from its apical region. From the lower nodes there
extend clearly marked and regular lines or slight grooves tapering
gradually towards the next higher node; these are no doubt the
impressions of segments of leaf-sheaths. The sheaths themselves have
been detached and only their impressions remain. The flattened bands at
the node of the stem in fig. 60, and shown also in fig. 61, mark the
place of attachment of the leaf-sheaths. On some of these nodal bands
one is able to recognise small scars which are most likely the casts of
outgoing leaf-trace bundles.
Some of the internal casts of this species are marked by numerous
closely arranged longitudinal lines, which are probably the impressions
of the inner face of a central woody cylinder. In the smaller specimen
shown in fig. 62 we have the apical portion of a shoot in which the
uppermost internodes are in an unexpanded condition.
[Illustration: FIG. 62. _Equisetites Beani_ (Bunb.). From a specimen in
the Scarborough Museum. Very slightly reduced.]
It is impossible to give a satisfactory diagnosis of this species
without better material. The plant is characterised chiefly by the
great breadth of the stem, and by the possession of leaf-sheaths
consisting of numerous long and narrow segments. _Equisetites Beani_
must have almost equalled in size the Triassic species, _E. arenaceus_,
described above.
7. _Equisetites lateralis_ Phill. Figs. 58, _F_, 63, and 64.
This species is described at some length as affording a useful
illustration of the misleading character of certain features which
are entirely due to methods of preservation. The specific name was
proposed by Phillips in his first edition of the _Geology of the
Yorkshire Coast_ for some very imperfect stems from the Lower Oolite
rocks near Whitby[524]. The choice of the term _lateralis_ illustrates
a misconception; it was given to the plant in the belief that certain
characteristic wheel-like marks on the stems were the scars of
branches. Lindley and Hutton[525] figured a specimen of this species
in their _Fossil Flora_, and quoted a remark by “Mr Williamson junior”
(afterwards Prof. Williamson) that the so-called scars often occur
as isolated discs in the neighbourhood of the stems. Bunbury[526]
described an example of the same species with narrow spreading leaves
like those of a Palaeozoic _Asterophyllites_, and proposed this generic
name as more appropriate than _Equisetites_. In all probability
the example shown in fig. 63 is that which Bunbury described. It
is certainly the same as one figured by Zigno[527] as _Calamites
lateralis_ in his _Flora fossilis formationis Oolithicae_.
[Illustration: FIG. 63. _Equisetites lateralis_ Phill. From a specimen
in the British Museum. Slightly reduced.]
This specimen illustrates a further misconception in the diagnosis of
the species. The long linear appendages spreading from the nodes are,
I believe, slender branches and not leaves; they have not the form
of delicate filmy markings on the rock face, but are comparatively
thick and almost woody in appearance. The true leaves are distinctly
indicated at the nodes, and exhibit the ordinary features of toothed
sheaths.
Heer[528] proposed to transfer Phillips’ species to the genus
_Phyllotheca_, and Schimper[529] preferred the generic term
_Schizoneura_. The suggestion for the use of these two names would
probably not have been made had the presence of the _Equisetum_ sheaths
been recognised.
The circular depressions a short distance above each node are the
‘branch scars’ of various writers. Schimper suggested that these
radially marked circles might be displaced nodal diaphragms.
Andrae[530] figured the same objects in 1853 but regarded them as
branch scars, although in the specimen he describes, there are several
of them lying apart from the stems, and to one of them is attached
a portion of a leaf-sheath. Solms-Laubach[531] points out that the
internodal position of these supposed scars is an obvious difficulty;
we should not expect to find branches arising from an internode. After
referring to some specimens in the Oxford museum, he adds—“In presence
of these facts the usual explanation of these structures appears to
me, as to Heer, very doubtful.... We are driven to the very arbitrary
assumption that they represent the lowest nodes of the lateral
branches which were inserted above the line of the nodes of the stem.”
Circular discs similar to those of _E. lateralis_ have been found in
the Jurassic rocks of Siberia[532] and elsewhere. There are one or
two examples of such discs from Siberia in the British Museum. If the
nodal diaphragms were fairly hard and stout, it is easy to conceive
that they might have been pressed out of their original position when
the stems were flattened in the process of fossilisation. It is not
quite clear what the radial spoke-like lines of the discs are due to;
possibly they mark the position of bands of more resistant tissue or
of outgoing strands of vascular bundles. A detached diaphragm is seen
in fig. 64 C; in the centre it consists of a flat plate of tissue, and
the peripheral region is traversed by the radiating lines. In the stem
of fig. 64, A the deeply divided leaf-sheaths are clearly seen, and an
imperfect impression of a diaphragm is preserved on the face of the
middle internode. In fig. 64 B a flattened leaf-sheath is shown with
the free acuminate teeth fused basally into a continuous collar[533].
The short piece of stem of _Equisetites lateralis_ shown in fig. 58,
_F_, shows how the free teeth may be outspread in a manner which bears
some resemblance to the leaves of _Phyllotheca_, but a comparison with
the specimens already described, and a careful examination of this
specimen itself, demonstrate the generic identity of the species with
_Equisetites_. The carbonaceous film on the surface of such stems as
those of fig. 58, F, and 64, A, shows a characteristic shagreen texture
which may possibly be due to the presence of silica in the epidermis as
in recent Horse-tails.
There is another species of _Equisetites_, _E. Münsteri_, Schk., from
a lower geological horizon which has been compared with _E. lateralis_,
and lends support to the view that the so-called branch-scars are
nodal diaphragms[534]. This species also affords additional evidence
in favour of retaining the generic name _Equisetites_ for Phillips’
species. _Equisetites Münsteri_ is a typical Rhaetic plant; it has been
found at Beyreuth and Kuhnbach, as well as in Switzerland, Hungary
and elsewhere. A specimen of _Equisetites_ originally described by
Buckman as _E. Brodii_[535], from the Lower Lias of Worcestershire,
may possibly be identical with _E. Münsteri_. The leaf-sheaths of this
Rhaetic species consist of broad segments prolonged into acuminate
teeth; some of the examples figured by Schenk[536] show clearly marked
impressions of displaced nodal diaphragms exactly as in _E. lateralis_.
Another form, _Equisetum rotiferum_ described by Tenison-Woods[537]
from Australia, is closely allied to, or possibly identical with _E.
lateralis_.
[Illustration: FIG. 64. _Equisetites lateralis_ Phill. A. Part of a
stem showing leaf-sheaths and an imperfect diaphragm. B. A single
flattened leaf-sheath. C. A detached nodal diaphragm. From a
specimen in the York Museum. Slightly reduced.]
8. _Equisetites Burchardti_ Dunker[538]. Fig. 65.
This species of _Equisetites_ is fairly common in the Wealden beds of
the Sussex coast near Hastings, and also in Westphalia.
[Illustration: FIG. 65. _Equisetites Burchardti_ Dunk. Showing a node
with two tubers and a root. From a specimen in the British Museum.
Nat. size.]
It is characterised by having long and slender internodes, bearing at
the nodes leaf-sheaths with five or six pointed segments, and by the
frequent formation of branch-tubers. These tuberous branches closely
resemble those which are formed on the underground shoots of _Equisetum
arvense_ L., _E. sylvaticum_ L. and others; they occur either singly
or in chains[539]. In the specimen shown in the figure the left-hand
tuber is remarkably well preserved, its surface is somewhat sunk and
shrivelled, and the apex is surrounded by a nodal leaf-sheath. A thin
branched root is given off just below the point of insertion of the
oval tuber.
No other species of _Equisetites_ affords such numerous examples
of tubers as this Wealden plant. By some of the earlier writers the
detached tubers of _E. Burchardti_ were described as fossil seeds under
the name _Carpolithus_.
[Illustration: FIG. 66. _Equisetites Yokoyamae_ Sew. From specimens in
the British Museum. Nat. size.]
The specimens shown in fig. 66 have been referred to another species,
_E. Yokoyamae_ Sew.[540]; they were obtained from the Wealden beds
of Sussex, but according to Mr Rufford, who discovered them, the
smaller tubers of this species are not found in association with
those of _E. Burchardti_. The stems are very narrow and the tubers
have a characteristic elliptical form; the species is of little value
botanically, but it affords another instance of the common occurrence
of these tuberous branches in the Wealden Equisetums.
Similar fossil tubers, on a much larger scale, have been found in
association with the Triassic _Equisetites arenaceus_; with _E.
Parlatori_ Heer[541], a Tertiary species from Switzerland, and with
other Mesozoic and Tertiary stems. _E. Burejensis_[542], described by
Heer from the Jurassic rocks of Siberia, bears a close resemblance to
the Wealden species.
• • • • •
The description of the above species by no means exhausts the material
which is available towards a history of fossil Equisetums. The
examples which have been selected may serve to illustrate the kind of
specimens that are usually met with, as well as some of the possible
sources of error which have to be borne in mind in the description of
species.
Such Tertiary species as have been recorded need not be considered;
they furnish us with no facts of particular interest from a
morphological point of view. The wide distribution of _Equisetites_,
especially during the Jurassic period, is one of the most interesting
lessons to be learnt from a review of the fossil forms. No doubt a
detailed comparison of the several species from different parts of the
world would lead us to reduce the number of specific names; and at the
same time it would emphasize the apparent identity of fossils which
have been described from widely separated latitudes under different
names.
Specimens of _Equisetites_ are occasionally found in plant-bearing
beds apart from the other members of a Flora; this isolated manner of
occurrence suggests that the plant grew in a different station from
that occupied by Cycads and other elements of the vegetation[543].
A selection of Triassic and Jurassic species arranged in a tabular form
demonstrates the world-wide distribution of this persistent type of
plant[544].
B. _Phyllotheca._
The generic name _Phyllotheca_ was proposed by Brongniart[545] in
1828 for some small fossil stems from the Hawkesbury river, near
Port Jackson, Australia. The stems of this genus are divided into
nodes and internodes and possess leaf-sheaths as in _Equisetum_, but
_Phyllotheca_ differs from other Equisetaceous plants in the form of
the leaves and in the character of its sporophylls. We may define the
genus as follows:—
Plants resembling in habit the recent Equisetums. Stems simple or
branched, divided into distinct nodes and internodes, the latter
marked by longitudinal ridges and grooves; from the nodes are given
off leaf-sheaths consisting of linear-lanceolate uninerved segments
coherent basally, but having the form of free narrow teeth for the
greater part of their length. The long free teeth are usually spread
out in the form of a cup and not adpressed to the stem, the tips of the
teeth are often incurved.
The sporangia are borne on peltate sporangiophores attached to the stem
between whorls of sterile leaves.
Our knowledge of _Phyllotheca_ is unfortunately far from complete. The
chief characteristic of the vegetative shoots consists in the cup-like
leaf-sheaths; these are divided up into several linear segments, which
differ from the teeth of an _Equisetum_ leaf-sheath in their greater
length and in their more open and spreading habit of growth. The large
loose sheaths of the fertile shoots of some recent Horse-tails bear a
certain resemblance to the sheaths of _Phyllotheca_. The diagnosis of
the fertile shoots is founded principally on some Permian specimens of
the genus described by Schmalhausen from Russia[546] and redescribed
more recently by Solms-Laubach[547]. Prof. Zeiller[548] has, however,
lately received some examples of _Phyllotheca_ from the Coal-Measures
of Asia Minor which bear strobili like those of the genus _Annularia_,
a type which is dealt with in the succeeding chapter. A description of
a few species will serve to illustrate the features usually associated
with this generic type, as well as to emphasize the unsatisfactory
state of our knowledge as to the real significance of such supposed
generic characteristics.
There are a few fossil stems from Permian rocks of Siberia,
from Jurassic strata in Italy, and from Lower Mesozoic and
Permo-Carboniferous beds in South America, South Africa, India and
Australia which do not conform in all points to the usually accepted
definition of _Equisetites_, and so justify their inclusion in an
allied genus. On the other hand there are numerous instances of stems
or branches which have been referred to _Phyllotheca_ on insufficient
grounds. Our knowledge of this Equisetaceous plant has recently been
extended by Zeiller[549], who has recorded its occurrence in the
Coal-Measures of Asia Minor associated with typical Upper Carboniferous
plants. The same author[550] has also brought forward good evidence for
the Permian age of the beds in Siberia and Altai, where _Phyllotheca_
has long been known. It is true that Zigno’s species of the genus
occurs in Italian Jurassic rocks, but on the whole it would seem that
this genus is rather a Permian than a Jurassic type. The species
which Zeiller describes under the name _Phyllotheca Rallii_ from
the Coal-Measures of Herakleion (Asia Minor) shows some points of
contact with _Annularia_. It is much to be desired, however, that
we might learn more as to the reproductive organs of this member of
the Equisetales; until we possess a closer acquaintance with the
fructification we cannot hope to arrive at any satisfactory conclusion
as to the exact position of the genus among the Calamarian and
Equisetaceous forms. M. Zeiller[551] informs me that his specimens of
_P. Rallii_, which are to be fully described in a forthcoming work,
include fossil strobili resembling those of _Annularia radiata_.
The verticils of linear leaves fused basally into a sheath agree
in appearance with the star-like leaves of _Annularia_, but in
_Phyllotheca Rallii_ the segments appear to spread in all directions
and are not extended in one plane as in the typical _Annularia_[552].
1. _Phyllotheca deliquescens_ (Göpp.).
In an account of some fossil plants collected by Tchikatcheff in
Altai, Göppert[553] describes and figures two imperfect stems of an
Equisetum-like plant. Owing to the apparent absence of nodal lines on
the surface of the stem the generic name _Anarthrocanna_ is proposed
for the fossils; and the manner in which the main axis appears to break
up into slender branches suggested the specific name _deliquescens_.
Schmalhausen[554] afterwards recognised the generic identity of
Göppert’s fragments with the Indian and Australian stems referred to
the genus _Phyllotheca_ by McCoy[555] and Bunbury[556].
We may define the species as follows:—
Stem reaching a diameter of 2–3 cm. with internodes as much as 4
cm. long, the surface of which is traversed by longitudinal ridges
and grooves which are continuous and not alternate at the nodes.
Branches arise in verticils from the nodes. The leaves have the form
of funnel-shaped sheaths split up into narrow and spreading linear
segments, each of which is traversed by a median vein. The fertile
shoot terminates in a loose strobilus bearing alternating whorls of
sterile bracts and sporangiophores.
The specimens on which this diagnosis is founded are for the most part
fragments of sterile branches. Some of these present the appearance of
Calamitean stems in which the ridges and grooves continue in straight
lines from one internode to the next. Similar stem-casts have been
referred by some writers to the allied genus _Schizoneura_, and it
would appear to be a hopeless task to decide with certainty under
which generic designation such specimens should be described. The
portion of stem shown in fig. 67 affords an example of an Equisetaceous
plant, probably in the form of a cast of a hollow pith, which might
be referred to either _Phyllotheca_ or _Schizoneura_. The specimen
was found in certain South African rocks which are probably of
Permo-Carboniferous age[557]. It agrees closely with some stems from
India described by Feistmantel[558] as _Schizoneura gondwanensis_, and
it also resembles equally closely the Australian specimens referred
by Feistmantel[559] to _Phyllotheca australis_ and some stems of
_Phyllotheca indica_ figured by Bunbury[560].
The longitudinal ridges and grooves shown in fig. 67 probably represent
the broad medullary rays and the projecting wedges of secondary wood
surrounding a large hollow pith, as in _Calamites_. In the Calamitean
casts the ridges and grooves of each internode usually alternate in
position with those of the next, as in _Equisetum_ (fig. 54, A), but in
_Phyllotheca_, _Schizoneura_ and _Archaeocalamites_ there is no such
regular alternation at the nodes of the internodal vascular strands.
[Illustration: FIG. 67. _Phyllotheca?_ ¾ nat. size. From a South
African specimen of Permo-Carboniferous age in the British Museum.]
In _Phyllotheca_ and _Schizoneura_ there are no casts of ‘infranodal
canals’ below each nodal line, but these are by no means always found
in true Calamites. It is therefore practically impossible to determine
the generic position of such fossils as that shown in fig. 67 without
further evidence than is afforded by leafless casts.
A few examples of _Phyllotheca deliquescens_ have been described by
Schmalhausen in which a branch bears clusters of sporangiophores,
alternating with verticils of sterile bracts. The sporangiophores
appear to have the form of stalked peltate appendages bearing
sporangia, very similar to the sporangiophores of _Equisetum_.
Solms-Laubach[561] has examined the best of Schmalhausen’s specimens,
and a carefully drawn figure of one of the fertile branches is given in
his _Fossil Botany_.
The significance of this manner of occurrence of sporangiophores
and whorls of sterile bracts on the fertile branch will be better
understood after a description of the strobilus of _Calamites_. In
_Phyllotheca_ the sporangiophores appear to have been given off in
whorls, which were separated from one another by whorls of sterile
bracts, whereas in _Equisetum_ there are no sterile appendages
associated with the sporangiophores of the strobilus, with the
exception of the annulus at the base of the cone. Heer[562] first drew
attention to the fact that in _Phyllotheca_ we have a form of strobilus
or fertile shoot to a certain extent intermediate in character between
_Equisetum_ and _Calamites_.
In abnormal fertile shoots of _Equisetum_, sporophylls occasionally
occur above and below a sterile leaf-sheath. Potonié[563] has figured
such an example in which an apical strobilus is succeeded at a lower
level by a sterile leaf-sheath, and this again by a second cluster
of sporophylls. As Potonié points out, this alternation of fertile
and sterile members affords an interesting resemblance between
_Phyllotheca_ and _Equisetum_. It suggests a partial reversion towards
the Calamitean type of strobilus.
2. _Phyllotheca Brongniarti_ Zigno. Fig. 68, A.
This species of _Phyllotheca_ from the Lower Oolite rocks of Italy
is known only in the form of sterile branches. The leaves are fused
basally into an open cup-like sheath which is dissected into several
spreading and incurved linear segments. The internodes are striated
longitudinally; they are about 2 mm. in diameter and 10 mm. in length.
The specimen represented in fig. 68, A, was originally described by
the Italian palaeobotanist Zigno[564]; it serves to illustrate the
points of difference between this genus and the ordinary _Equisetum_.
The open and spreading sheaths clasping the nodes and the erect
solitary branches give the plant a distinctive appearance.
[Illustration: FIG. 68.
A. _Phyllotheca Brongniarti_, Zigno. Nat. size. (After Zigno.)
B. _Calamocladus frondosus_, Grand’Eury. (After Grand’Eury.)
Slightly enlarged.
C. _Phyllotheca indica_, Bunb. Part of a leaf-sheath. From a specimen
in the Museum of the Geological Society. Slightly enlarged.]
3. _Phyllotheca indica_ Bunb. and _P. australis_ Brongn. Fig. 68, C.
Sir Charles Bunbury[565] described several imperfect specimens from the
Nagpur district of India under this name, but he expressed the opinion
that it was not clear to him if the plant was specifically distinct
from the _Phyllotheca australis_ Brongn. previously recorded from New
South Wales. Feistmantel[566] subsequently described a few other Indian
specimens, but did not materially add to our knowledge of the genus.
Bunbury’s specimens were obtained from Bharatwádá in Nagpur, in beds
belonging to the Damuda series of the Lower Gondwana rocks, usually
regarded as of about the same age as the Permian rocks of Europe.
_Phyllotheca indica_ is represented by broken and imperfect fragments
of leaf-bearing stems. The species is thus diagnosed by Bunbury:—“Stem
branched, furrowed; sheaths lax, somewhat bell-shaped, distinctly
striated; leaves narrow linear, with a strong and distinct midrib,
widely spreading and often recurved, nearly twice as long as the
sheaths.” An examination of the specimens in the Museum of the
Geological Society of London, on which this account was based, has led
me to the opinion that it is practically impossible to distinguish
the Indian examples from _P. australis_ described by Brongniart[567]
from New South Wales. The few specimens of the latter species which
I have had an opportunity of examining bear out this view. In the
smaller branches the axis of _P. indica_ is divided into rather short
internodes on which the ridges and grooves are faintly marked. In
the larger stems the ridges and grooves are much more prominent, and
continuous in direction from one internode to the next; a few branches
are given off from the nodes of some of the specimens. The leaves are
not very well preserved; they consist of a narrow collar-like basal
sheath divided up into numerous, long and narrow segments, which are
several times as long as the breadth of the sheath, and not merely
twice as long as Bunbury described them. Each leaf-sheath has the form
of a very shallow cup-like rim clasping the stem at a node, with long
free spreading segments which are often bent back in their distal
region. The general habit of the leafy branches appears to be identical
with that of _P. australis_ as figured by McCoy.
Prof. Zeiller informs me that in the type-specimen on which Brongniart
founded the species, _P. australis_, the sheath appears to be closely
applied to the stem with a verticil of narrow spreading segments
radiating from its margin. It may be, therefore, that in the Australian
form there was not such an open and cup-like sheath as in _P. indica_;
but it would be difficult, without better material before us, to feel
confidence in any well marked specific distinctions between the Indian
and Australian Phyllothecas.
On the broader stems, such as that of fig. 67, we have clearly marked
narrow grooves and broader and slightly convex ridges, which present
an appearance identical with that of some Calamitean stems. In the
specimen figured by Bunbury[568] in his Pl. X, fig. 6, there is a
circular depression on the line of the node which represents the
impression of the basal end of a branch; on the edges of the node
there are indications of two other lateral branches. The nature of
this stem-cast points umnistakeably to a woody stem like that of
_Calamites_. The precise meaning of the ridges and grooves on the cast
is described in the Chapter dealing with Calamitean plants.
[Sidenote: CALAMOCLADUS.]
Grand’Eury[569] in his monograph on the coal-basin of Gard, has
recently described under the name of _Calamocladus frondosus_ what he
believes to be the leaf-bearing axes of a Calamitean plant. The thicker
branches are almost exactly identical in appearance with the broader
specimens of _Phyllotheca_. The finer branches of _Calamocladus_
bear cup-like leaf-sheaths which are divided into long and narrow
recurved segments (fig. 67, B), precisely as in _Phyllotheca_. These
comparisons lead one to the opinion that the _Phyllotheca_ of Australia
and India may be a close ally of the Permo-Carboniferous Calamitean
plants. The form of the leaf-whorls of _Annularia_ (Calamarian
leaf-bearing branches) and of _Calamocladus_ is of the same type as
in _Phyllotheca_; the character of the medullary casts is also the
same. The nature of the fertile shoot of _Phyllotheca_ described by
Schmalhausen from Siberia, with its alternating whorls of sterile and
fertile leaves, is another point of agreement between this genus and
Calamitean plants. An Equisetaceous species has been described from
the Newcastle Coal-Measures of Australia by Etheridge[570] in which
there are two forms of leaves, some of which closely resemble those of
_Phyllotheca indica_, while others are compared with the sterile bracts
of _Cingularia_, a Calamitean genus instituted by Weiss[571].
When we turn to other recorded forms of _Phyllotheca_ many of
them appear on examination to have been placed in this genus on
unsatisfactory grounds. Heer figures several stem fragments from the
Jurassic rocks of Siberia as _P. Sibirica_ Heer[572], and it was the
resemblance between this form and the English _Equisetites lateralis_
which led to the substitution of _Phyllotheca_ for _Equisetites_ in
the latter species. Without examining Heer’s material it is impossible
to criticise his conclusions with any completeness, but several of
his specimens, appear to possess leaf-sheaths more like those of
_Equisetum_ than of _Phyllotheca_.
The frequent occurrence of isolated diaphragms and the comparatively
long acuminate teeth of the leaf-sheath afford obvious points of
resemblance to _Equisetites lateralis_. Some of the examples figured
by Heer appear to be stem fragments, with numerous long and narrow
filiform leaves different in appearance from those of other specimens
which he figures. It may be that some of the less distinct pieces
of stems are badly torn specimens in which the internodes have been
divided into filiform threads. Heer also figures a fertile axis
associated with the sterile stems, and this does not, as Heer admits,
show the alternating sterile bracts such as Schmalhausen has described.
So far as it is possible to judge from an examination of Heer’s figures
and a few specimens from Siberia in the British Museum—and this is by
no means a safe basis on which to found definite opinions—there appears
to be little evidence in favour of separating the fossils described
as _Phyllotheca Sibirica_ from _Equisetites_. This Siberian form may
indeed be specifically identical with _Equisetites lateralis_ Phill.
Various species of _Phyllotheca_ have been described from Jurassic and
Upper Palaeozoic rocks in Australia. Some of these possess cup-like
leaf-sheaths, and in the case of the thicker specimens they show
continuous ridges and grooves on the internodes, as well as a habit
of branching similar to that in some of the Italian Phyllothecas. In
some of the stems it is however difficult to recognise any characters
which justify the use of the term _Phyllotheca_. A fragment figured by
Tenison-Woods[573] as a new species of _Phyllotheca_, _P. carnosa_,
from Ipswich, Queensland, affords an example of the worthless material
on which species have not infrequently been founded. The author of
the species describes his single specimen as a “faint impression”;
the figure accompanying his description suggests a fragment of some
coniferous branch, as Feistmantel has pointed out in his monograph on
Australian plants.
It is important that a thorough comparative examination should be
made of the various fossil Phyllothecas with a view to determine
their scientific value, and to discover how far the separation of
_Phyllotheca_ and _Equisetites_ is legitimate in each case. There is
too often a tendency to allow geographical distribution to decide the
adoption of a particular generic name, and this seems to have been
especially the case as regards several Mesozoic and Palaeozoic Southern
Hemisphere plants.
The geological and geographical range of _Phyllotheca_ is a question of
considerable interest, but as already pointed out it is desirable to
carefully examine the various records of the genus before attempting
to generalise as to the range of the species. _Phyllotheca_ is often
spoken of as a characteristic member of the _Glossopteris_ Flora of the
Southern Hemisphere, and its geological age is usually considered to be
Mesozoic rather than Palaeozoic.
C. _Schizoneura._
The plants included under this genus were originally designated
by Brongniart[574] _Convallarites_ and classed as Monocotyledons.
Some years later Schimper and Mougeot[575] had the opportunity of
examining more perfect material from the Bunter beds of the Vosges,
and proposed the new name _Schizoneura_ in place of Brongniart’s term,
on the grounds that the specimens were in all probability portions of
Equisetaceous stems, and not Monocotyledons. Our knowledge of this
genus is very limited, but the characteristics are on the whole better
defined than in the case of _Phyllotheca_. The following diagnosis
illustrates the chief features of _Schizoneura_.
Hollow stems with nodes and internodes as in _Equisetum_; the surface
of the internodes is traversed by regular ridges and grooves, which
are continuous and not alternate in their course from one internode to
the next. The leaf-sheaths are large and consist of several coherent
segments; the sheaths are usually split into two or more elongate ovate
lobes, and each lobe contains more than one vein. Fertile shoots are
unknown.
Two of the best known and most satisfactory species are _Schizoneura
gondwanensis_ Feist. and _S. paradoxa_ Schimp. and Moug.
_Schizoneura gondwanensis_ Feist. Fig. 69, A and B.
This species is represented by numerous specimens from the Lower
Gondwana rocks of India[576]; it is characterised by narrow articulated
stems which bear large leaf-sheaths at the nodes. The sheaths may have
the form of two large and spreading elongate-oval lobes, each of which
is traversed by several veins (fig. 69, B), or the lobes may be further
dissected into long linear single-veined segments, as in fig. 69, A. It
is supposed that in the young condition each node bears a leaf-sheath
consisting of laterally coherent segments which, as development
proceeds, split into two or more lobes. Feistmantel records this
species from the Talchir, Damuda and Panchet divisions of the Lower
Gondwana series of India; these divisions are regarded as equivalent to
the Permo-Carboniferous and Triassic rocks of Europe. The two specimens
shown in fig. 69 are from the Lower Gondwana rocks of the Raniganj
Coal-field, India.
As already pointed out[577], some of the specimens of flat and
broader stems referred by Feistmantel to _Schizoneura_ are identical
in appearance with stems which have been described from India and
elsewhere as species of _Phyllotheca_.
[Illustration: FIG. 69. _Schizoneura gondwanensis_ Feist. (After
Feistmantel; slightly reduced.)]
There are a few specimens of _S. gondwanensis_ in the British Museum,
but the genus is poorly represented in European collections.
A similar plant was described in 1844 by Schimper and Mougeot[578]
from the Bunter rocks of the Vosges as _Schizoneura paradoxa_. This
species bears a very close resemblance to the Indian forms, and indeed
it is difficult to point to any distinction of taxonomic importance.
Feistmantel considers that the European plant has rather fewer
segments in the leaf-sheaths, and that the Indian plant had somewhat
stronger stems. Both of these differences are such as might easily be
found on branches of the same species. It is, however, interesting to
notice the very close resemblance between the Lower Trias European
plant and the somewhat older member of the _Glossopteris_ flora
recorded from India and other regions, which probably once formed
part of that Southern Hemisphere Continent which is known as Gondwana
Land[579].
CHAPTER X.
I. EQUISETALES (_continued_).
(CALAMARIEAE.)
In order to minimise repetition and digression the following account of
the Calamarieae is divided into sections, under each of which a certain
part of the subject is more particularly dealt with. After a brief
sketch of the history of our knowledge of _Calamites_, and a short
description of the characteristics of the genus, the morphological
features are more fully considered. A description of the most striking
features of the better known Calamitean types is followed by a short
discussion on the question of nomenclature and classification, and
reference is made to the manner of occurrence of _Calamites_ and to
some of the possible sources of error in identification.
D. CALAMITES.
I. Historical Sketch.
In the following account of the Calamarieae the generic name
_Calamites_ is used in a somewhat comprehensive sense. As previous
writers have pointed out, it is probable that under this generic name
there may be included more than one type of plant worthy of generic
designation. Owing to the various opinions which have been held by
different authors, as to the relationship and botanical position of
plants now generally included in the Calamarieae, there has been no
little confusion in nomenclature. Facts as to the nature of the genus
_Calamites_ have occasionally to be selected from writings containing
many speculative and erroneous views, but the data at our disposal
enable us to give a fairly complete account of the morphology of this
Palaeozoic plant.
In the earliest works on fossil plants we find several figures of
_Calamites_, which are in most cases described as those of fossil
reeds or grasses. The _Herbarium diluvianum_ of Scheuchzer[580]
contains a figure of a Calamitean cast which is described as probably
a reed. Another specimen is figured by Volkmann[581] in his _Silesia
subterranea_ and compared with a piece of sugar-cane. A similar
flattened cast in the old Woodwardian collection at Cambridge is
described by Woodward[582] as “part of a broad long flat leaf,
appearing to be of some _Iris_, or rather an Aloe, but ’tis striated
without.” Schulze[583], one of the earlier German writers, figured a
Calamitean branch bearing verticils of leaves, and described the fossil
as probably the impression of an Equisetaceous plant. It has been
pointed out by another German writer that the Equisetaceous character
of _Calamites_ was recognised by laymen many years before specialists
shared this view.
One of the most interesting and important of all the older records of
_Calamites_ is that published by Suckow[584] in 1784. Suckow is usually
quoted as the author of the generic name _Calamites_; he does not
attempt any diagnosis of the plant, but merely speaks of the specimens
he is describing as “Calamiten.” The examples figured in this classic
paper are characteristic casts from the Coal-Measures of Western
Germany. Suckow describes them as ribbed stems, which were found in
an oblique position in the strata and termed by the workmen Jupiter’s
nails (“Nägel”). Previous writers had regarded the fossils as casts
of reeds, but Suckow correctly points out that the ribbed character
is hardly consistent with the view that the casts are those of reeds
or grasses. He goes on to say that the material filling up the hollow
pith of a reed would not have impressed upon it a number of ribs and
grooves such as occur on the Calamites. He considers it more probable
that the casts are those of some well-developed tree, probably a
foreign plant. _Equisetum giganteum_ L. is mentioned as a species with
which _Calamites_ may be compared, although the stem of the Palaeozoic
genus was much larger than that of the recent Horse-tail. The tree
of which the Calamites are the casts must, he adds, have possessed
a ribbed stem, and the bark must also have been marked by vertical
ribs and grooves on its _inner face_. It is clear, therefore, that
Suckow inclined to the view that _Calamites_ should be regarded as an
_internal cast_ of a woody plant. Such an interpretation of the fossils
was generally accepted by palaeobotanists only a comparatively few
years ago, and the first suggestion of this view is usually attributed
to Germar, Dawes, and other authors who wrote more than fifty years
later than Suckow.
One of the earliest notices of _Calamites_ in the present century is
by Steinhauer[585], who published a memoir in the Transactions of the
American Philosophical Society in 1818 on _Fossil reliquia of unknown
vegetables in the Carboniferous rocks_. He gives some good figures of
Calamitean casts under the generic name of _Phytolithus_, one of those
general terms often used by the older writers on fossils. Among English
authors, Martin[586] may be mentioned as figuring casts of _Calamites_,
which he describes as probably grass stems. By far the best of
the earlier figures are those by Artis[587] in his _Antediluvian
Phytology_. This writer does not discuss the botanical nature of the
specimens beyond a brief reference to the views of earlier authors.
Adolphe Brongniart[588], writing in 1822, expresses the opinion that
the Calamites are related to the genus _Equisetum_, and refers to
M. de Candolle as having first suggested this view. In a later work
Brongniart[589] includes species of _Calamites_ as figured by Suckow,
Schlotheim, Sternberg and Artis in the family _Equisetaceae_. Lindley
and Hutton[590] give several figures of Calamites in their _Fossil
flora_, but do not commit themselves to an Equisetaceous affinity.
An important advance was made in 1835 by Cotta[591], a German writer,
who gave a short account of the internal structure of some Calamite
stems, which he referred to a new genus _Calamitea_. The British Museum
collection includes some silicified fragments of the stems figured
and described by Cotta in his _Dendrolithen_. Some of the specimens
described by this author as examples of _Calamitea_ have since been
recognised as members of another family.
[Sidenote: PETZHOLDT AND UNGER.]
In 1840 Unger[592] published a note on the structure and affinities of
_Calamites_, and expressed his belief in the close relationship of the
Palaeozoic plant and recent Horse-tails.
An important contribution to our knowledge of _Calamites_ was supplied
by Petzholdt[593] in 1841. His main contention was the Equisetaceous
character of this Palaeozoic genus. The external resemblance between
Calamite casts and _Equisetum_ stems had long been recognised, but
after Cotta’s account of the internal structure it was believed that
the apparent relation between _Equisetum_ and _Calamites_ was not
confirmed by the facts of anatomy. Petzholdt based his conclusions
on certain partially preserved Permian stems from Plauenscher Grund,
near Dresden. Although his account of the fossils is not accurate his
general conclusions are correct. The specimens described by Petzholdt
differ from the common Calamite casts in having some carbonised
remnants of cortical and woody tissue. A transverse section of one of
the Plauenscher Grund fossils is shown in fig. 70. The irregular black
patches were described by Petzholdt as portions of cortical tissue,
while he regarded the spaces as marking the position of canals like the
vallecular canals in an _Equisetum_. Our more complete knowledge of the
structure of a Calamite stem enables us to correlate the patches in
which no tissue has been preserved with the broad medullary rays, which
separated the wedge-shaped groups of xylem elements; the latter being
more resistant were converted into a black coaly substance, while the
cells of the medullary rays left little or no trace in the sandstone
matrix. The thin black line, which forms the limit of the drawing in
fig. 70, external to the carbonised wood, no doubt marks the limit
of the cortex, and the appendage indicated in the lower part of the
figure may possibly be an adventitious root. It is interesting to note
that Unger[594] in 1844 expressed the opinion, which we now know to be
correct, that the coaly mass in the specimens described by Petzholdt
represented the wood, and that there was no proof of the existence of
canals in the cortex as Petzholdt believed.
[Illustration: FIG. 70. Transverse section of a Calamite stem, showing
carbonised remnants of secondary wood. From a specimen (no. 40934),
presented to the British Museum by Dr Petzholdt from Plauenscher
Grund, Dresden. ½ nat. size.]
Turning to Brongniart’s later work[595] we find an important
proposal which led to no little controversy. While retaining the
genus _Calamites_ for such specimens as possess a thin bark and a
ribbed external surface, showing occasional branch-scars at the
nodes, and having such characters as warrant their inclusion in the
Equisetaceae, he proposes a second generic name for other specimens
which had hitherto been included in _Calamites_. The fossils assigned
to his new genus _Calamodendron_ are described as having a thick woody
stem, and as differing from _Equisetum_ in their arborescent nature.
Brongniart’s genus _Calamodendron_ is made to include the plants for
which Cotta instituted the name _Calamitea_, and it is placed among
the Gymnosperms. This distinction between the Vascular Cryptogam
_Calamites_ and the supposed Gymnosperm _Calamodendron_ is based on the
presence of secondary wood in the latter type of stem. The prominence
formerly assigned to the power of secondary thickening possessed by a
plant as a taxonomic feature, is now known to have been the result of
imperfect knowledge. The occurrence of a cambium layer and the ability
of a plant to increase in girth by the activity of a definite meristem,
is a feature which some recent Vascular Cryptogams[596] share with the
higher plants; and in former ages many of the Pteridophytes possessed
this method of growth in a striking degree.
Although Brongniart’s distinction between _Calamites_ and
_Calamodendron_ has not been borne out by subsequent researches, the
latter term is still used as a convenient designation for a special
type of Calamitean structure. One of the earliest accounts of the
anatomy of _Calamodendron_ stems is by Mougeot[597], who published
figures and descriptions of two species, _Calamodendron striatum_ and
_C. bistriatum_.
Some years later Göppert[598], who was one of the greatest of the
older palaeobotanists, instituted another genus, _Arthropitys_[599],
for certain specimens of silicified stems from the Permian rocks of
Chemnitz in Saxony, which Cotta had previously placed in his genus
_Calamitea_ under the name of _Calamitea bistriata_[600]. Göppert
rightly decided that the plants so named by Cotta differed in important
histological characters from other species of _Calamitea_. The generic
name _Arthropitys_ has been widely adopted for a type of Calamitean
stem characterised by definite structural features. The great majority
of the petrified Calamite stems found in the English Coal-Measures
belong to Göppert’s _Arthropitys_.
[Sidenote: WILLIAMSON.]
The next proposal to be noticed is one by Williamson[601] in 1868;
he instituted the generic name _Calamopitys_ for a few examples
of English stems, which differed in the structure of the wood and
primary medullary rays from previously recorded types. We have thus
four names which all stand for generic types of Calamitean stems. Of
these _Calamodendron_ and _Arthropitys_ are still used as convenient
designations for stems with well-defined anatomical characters.
The genus _Calamitea_ is no longer in use, and Williamson’s name
_Calamopitys_ had previously been made use of by Unger[602] for plants
which do not belong to the Calamarieae. As it is convenient to have
some term to apply to such stems as those which Williamson made the
type of _Calamopitys_, the name _Arthrodendron_ is suggested by my
friend Dr Scott[603] as a substitute for Williamson’s genus.
The twofold division of the Calamites instituted by Brongniart has
already been alluded to, and for many years it was generally agreed
that both Pteridophytes and Gymnosperms were represented among the
Palaeozoic fossils known as Calamites. The work of Prof. Williamson
was largely instrumental in proving the unsound basis for this
artificial separation; he insisted on the inclusion of all Calamites
in the Vascular Cryptogams, irrespective of the presence or absence
of secondary wood. By degrees the adherents of Brongniart’s views
acknowledged the force of the English botanist’s contention. It is
one of the many signs of the value of Williamson’s work that there is
now almost complete accord among palaeobotanical writers as to the
affinities of Calamitean plants.
In the following account of the Calamites, the generic name
_Calamites_ is used in a wide sense as including stems possessing
different types of internal structure; when it is possible to recognise
any of these structural types the terms _Calamodendron_, _Arthropitys_
or _Arthrodendron_ are used as subgenera. The reasons for this
nomenclature are discussed in a later part of the Chapter.
╭ This term was originally applied
Genus _Calamites_, Suckow, 1714 ┤ to the common pith-casts of
│ Calamitean stems, _without reference
╰ to_ _internal structure_.
Subgenera _Calamodendron_, Brongniart, 1849 ╮ These names have
_Arthropitys_ Göppert, 1864 ├ primarily reference
_Arthrodendron_ Scott 1897 │ to _internal_
(= _Calamopitys_ Williamson, 1871) ╯ _structure_.
II. Description of the anatomy of Calamites.
_a. Stems. b. Leaves. c. Roots. d. Cones._
No fossils are better known to collectors of Coal-Measure plants than
the casts and impressions of the numerous species of _Calamites_. In
sandstone quarries of Upper Carboniferous rocks there are frequently
found cylindrical or somewhat flattened fossils, varying from one to
several inches in diameter, marked on the surface by longitudinal
ridges and grooves, and at more or less regular intervals by regular
transverse constrictions. Similar specimens are still more abundant
as flattened casts in the blocks of shale found on the rubbish heaps
of collieries. The sandstone casts are often separated from the
surrounding rock by a loose brown or black crumbling material, and the
specimens in the shale are frequently covered by a thin layer of coal.
Most of the earlier writers regarded such specimens as the impressions
of the ribbed stems of plants similar to or identical with reeds or
grasses. Suckow, and afterwards Dawes and others, expressed the opinion
that the ordinary Calamite cast represented a hardened mass of sand
or marl, which had filled up the pith of a stem either originally
fistular or rendered hollow by decay. The investigation of the internal
structure confirmed this view, and proved that the surface-features of
a Calamite stem do not represent the external markings of the original
plant, but the form of the inner face of the cylinder of wood. The ribs
represent the medullary rays of the original stem or branch, and the
intervening grooves mark the position of the strands of xylem which are
arranged in a ring round a large hollow pith[604].
With this brief preliminary account we may pass to a detailed
description of the anatomical characters of _Calamites_.
The genus _Calamites_ may be briefly defined as follows:—
Arborescent plants reaching a height of several meters, and having a
diameter of proportional size. In habit of growth the Calamites bore
a close resemblance to _Equisetum_; an underground rhizome giving off
lateral branches and erect aerial shoots bearing branches, either in
whorls from regularly recurring branch-bearing nodes, or two or three
from each node; and in some cases the stems bore occasional branches
from widely separated nodes. The leaves were disposed in whorls either
as star-shaped verticils on slender foliage shoots, or in the form
of a circle of long narrow leaves on the node of a thicker branch.
Adventitious roots were developed from the nodal regions of underground
and aerial stems. The cones had the form of long and narrow strobili
consisting of a central axis bearing whorls of sterile and fertile
appendages; the latter in the form of sporangiophores bearing groups of
sporangia. The strobili were heterosporous in some cases, isosporous
in others. The stems had a large hollow pith bridged across by a
transverse diaphragm at the nodes in the centre of the single stele;
the latter consisted of a ring of collateral bundles separated from one
another by primary medullary rays. Each group of xylem was composed of
spiral, annular, scalariform and occasionally reticulate tracheids,
the position of the protoxylem being marked by a longitudinal
carinal canal. The shoots and roots grew in thickness by means of a
regular cambium layer. The cortex consisted of parenchymatous and
sclerenchymatous cells, with scattered secretory sacs. The increase in
girth of the central cylinder was often accompanied by a considerable
development of cortical periderm. The roots differed from the shoots
in having no carinal canals, and in the possession of a solid pith and
centripetally developed primary xylem groups alternating with strands
of phloem.
The above incomplete diagnosis includes only some of the more important
structural features of the genus. Thanks to the researches begun by the
late Mr Binney of Manchester and considerably extended by Carruthers,
Williamson and later investigators, we are now in a position to give
a fairly complete account of _Calamites_. The type of stem most
frequently met with in a petrified condition in the English rocks is
that to which Göppert applied the name _Arthropitys_, and it is this
subgenus that forms the subject of the following description. Our
knowledge of Calamitean anatomy is based on the examination of numerous
fragments of petrified twigs and other portions of different specific
types of the genus. It is seldom possible to differentiate specifically
between the isolated fragments of stems and branches which are met
with in calcareous or siliceous nodules. As so frequently happens in
fossil-plant material, large specimens showing good surface features
and broken fragments with well-preserved internal structure have to be
dealt with separately.
[Sidenote: YOUNG STEM.]
_a. Stems._
A transverse section of a young twig, such as is represented in fig.
71, illustrates the chief characteristics of the _primary structure_
of a young branch of _Calamites_. The figure has been drawn from
a section originally described by Hick[605] in 1894. A very young
Calamite twig bears an exceedingly close resemblance to the stem of
a recent _Equisetum_. The axial region of the stem may be occupied
by parenchymatous cells, or the absence of cells in the centre may
indicate the beginning of the gradual formation of the hollow pith,
which is one of the characteristics of _Calamites_. The student of
petrified Palaeozoic plants must constantly be on his guard against
the possible misinterpretation of Stigmarian ‘rootlets,’ which are
frequently found in intimate association with fossil tissues. The
intrusion of these rootlets is admirably illustrated by a section
of a Calamite stem in the Williamson Collection (No. 1558) in which
the hollow pith, 2 cm. broad, contains more than a dozen Stigmarian
appendages.
[Illustration: FIG. 71. Transverse section of a young Calamite stem.
_c_, carinal canals; _mr_, primary medullary rays; _a, b, and d_,
cortex; _e_, epidermis. From a section in the Manchester Museum,
Owens College, × 60.]
In the figured specimen of a Calamite twig (fig. 71) there is a clearly
marked differentiation into a cortical region and a large stele or
central cylinder. The pith-cells are already partially disorganised,
but there still remain a few fairly large parenchymatous cells internal
to the ring of vascular bundles. The few irregular projections into the
cavity of the large pith consist of small fragments of cells, which may
be the result of fungal action. Mycelia of fungi are occasionally met
with in the tissues of older Calamite stems.
The position of the primary xylem groups is shown by the conspicuous
and regularly placed canals, _c_; these have been formed in precisely
the same manner as the corresponding spaces in an _Equisetum_ stem,
and they are spoken of in both genera as the carinal canals. Each
canal owes its origin to the disorganization and tearing apart of
the protoxylem elements and the surrounding cells. This may be
occasionally seen in examples of very young Calamites; the canals of
a young twig often contain apparently isolated rings which are coils
of elongated spiral threads. Fig. 72, _B_ represents the canal of a
twig, cut in an oblique direction, in which the remains of spiral
tracheids are distinctly seen. In the stem of fig. 71 the development
has not advanced far enough to enable us to clearly define the exact
limits of each xylem strand. The smaller elements bordering the canals
constitute the primary xylem, they are fairly distinct on the outer
margin of some of the canals seen in the section. Between the small
patches of primary xylem the outward extensions of the parenchyma of
the pith constitute the primary medullary rays, _mr_. The distinct line
encircling the canals and primary xylem has been described by Hick as
marking the position of the endodermis, but it may possibly owe its
existence to the tearing of the tissues along the line where cambial
activity is just beginning. This layer of delicate dividing cells
would constitute a natural line of weakness. External to this line
we have a zone of tissue _a_, _d_, containing here and there larger
cells with black contents, which are no doubt secretory sacs. It is
impossible to distinguish with certainty any definite phloem groups,
but in other specimens these have been recognised immediately external
to each primary xylem group; the bundles were typically collateral in
structure. Towards the periphery of the twig the preservation is much
less perfect; the outer portion of the inner cortex, _d_, consists
of rather smaller and thicker-walled cells, but this is succeeded
by an ill-defined zone containing a few scattered cells, _b_, which
have been more perfectly preserved. The twig is too young to show any
secondary tissue in the cortex; but the tangential walls in some of
the cortical cells afford evidence of meristematic activity, which
probably represents the beginning of cork-formation. The limiting line,
_e_, possibly represents the cuticularised outer walls of an epidermal
layer. The irregularly wavy character of the surface of the specimen
is probably the result of shrinking, and does not indicate original
surface features.
[Sidenote: VASCULAR SYSTEM.]
In examining sections of calcareous nodules from the coal seams one
meets with numerous fragments of small Calamitean twigs with little
or no secondary wood; in some of these there is a small number of
carinal canals, in others the canals are much more abundant. The
former probably represent the smaller ramifications of a plant, and
the latter may be regarded as the young stages of branches capable of
developing into stout woody shoots[606]. Longitudinal sections of small
branches teach us that the xylem elements next the carinal canals are
either spiral or reticulate in character, the older tracheids being
for the most part of the scalariform type, with bordered pits on the
radial walls. This and other histological characters are admirably
shown in the illustrations accompanying Williamson and Scott’s memoir
on _Calamites_. The student should treat the account of the anatomy
of _Calamites_ given in these pages as introductory to the much more
complete description by these authors. They thus describe the course of
the vascular bundles in a Calamitean branch:—
“The bundle-system of _Calamites_ bears a general resemblance to that
of _Equisetum_. A single leaf-trace enters the stem from each leaf,
and passes vertically downwards to the next node. In the simplest
cases the bundle here forks, its two branches attaching themselves to
the alternating bundles which enter the stem at this node. In other
cases both the forks attach themselves to the same bundle, so that,
in this case, there is no regular alternation. In other cases, again,
the bundle runs past one node without forking, and ultimately forms a
junction with the traces of the second node below its starting-point.
These variations may all occur in the same specimen. The xylem at the
node usually forms a continuous ring, for where the regular dichotomous
forks of the bundles are absent their place is usually taken by
anastomoses[607].”
As in _Equisetum_, the xylem at the nodes possesses certain
characteristic features which distinguish it from the internodal
strands. It has already been pointed out that the xylem of _Equisetum_
increases in breadth at the nodes (p. 251, fig. 55, 4); the same is
true of _Calamites_. In fig. 72, _C_, we have part of a radial section
of a Calamite twig in which the broad mass of short nodal tracheids is
clearly shown; this nodal wood forms a prominent projection towards
the pith. In the lower part of the section the remains of some spiral
protoxylem tracheids are seen in a carinal canal.
[Illustration: FIG. 72.
_A._ External xylem elements and cambium, _c_, with imperfect phloem,
× 100.
_B._ Carinal canal containing protoxylem, _px._ × 65.
_C._ Radial longitudinal section through nodal xylem, _px._ × 35.
_D._ Phloem elements; _s_, sieve-tubes; _p_, _p_, parenchymatous
cells.
(_A–C._ After Williamson and Scott. _D._ After Renault.)]
The tracheids of the nodal wood are often reticularly pitted, and so
differ in appearance from the ordinary scalariform elements.
It is rare to find the phloem clearly preserved, but in specimens where
it has been possible to examine this portion of the vascular bundles,
it is found to consist of elongated cambiform cells and sieve-tubes.
An unusually perfect specimen has been described by Renault[608] in
which the phloem elements are preserved in silica. Fig. 72, _D_, is
copied from one of Renault’s drawings, the sieve-tubes, _s_, _s_,
show several distinct sieve-plates on the lateral walls of the tubes,
reminding one to some extent of the sieve-tubes in a Bracken Fern.
The cells, _p_, _p_, associated with the sieve-tubes are square-ended
elongated parenchymatous elements. Another characteristic feature
illustrated by longitudinal sections is the nodal diaphragm; except in
the smallest branches the interior of each internode is hollow, and the
ring of vascular bundles is separated from the pith-cavity by a band
of parenchymatous tissue. At each node this parenchyma extends across
the central cavity in the form of a nodal diaphragm, as in the stem of
_Equisetum_.
By far the greater number of the petrified fragments of _Calamites_
afford proof of cambial activity, and possess obvious secondary
tissues. In exceptionally perfect specimens the xylem tracheids are
found to be succeeded externally by a few flattened thin-walled cells
which are in a meristematic condition (fig. 72, _A_, _c_); these
constitute the cambium zone, and it is the _secondary structure_ that
results from the activity of the meristematic cells that we have now to
consider.
[Sidenote: SECONDARY THICKENING.]
In petrified examples of branches in which the secondary thickening
has reached a fairly advanced stage, the wood is usually the outermost
tissue preserved, the more external tissues having been detached along
the line of cambium cells. It is only in a few cases that we are able
to examine all the tissues of older examples.
The specimen represented in fig. 73 illustrates very clearly the
extension of the hollow pith up to the inner surface of the vascular
ring; the disorganisation of the pith-cells which had already begun in
the twig of fig. 71 has here advanced much further. The bluntly rounded
projections represent the prominent primary xylem strands, each of
which is traversed by the characteristic carinal canal. Alternating
with the wedge-shaped groups of secondary xylem, _x_, we have the broad
principal medullary rays, _mr_, which become slightly narrower towards
the outside. The inner face of each of these wide rays has a concave
form, due to the less resistent nature of the medullary-ray cells as
compared with the stronger xylem. The regularly sinuous form of the
inner face of the vascular cylinder enables one to realise how the
Calamite-casts (figs. 82, 99, and 101) have come to have the regular
ridges and grooves on their surface. The broad ridges on the cast mark
the position of the wide medullary rays, while the grooves correspond
to the more prominent ends of the vascular strands. The tissues
external to the wood have not been preserved in the example shown in
fig. 73. Some silicified specimens described by Stur[609] from Bohemia
and now in the Museum of the Austrian Geological Survey, Vienna,
admirably illustrate the connection between the surface features of a
Calamite cast and the anatomy of the stem.
[Illustration: FIG. 73.
Transverse section of a Calamite stem.
_mr_, medullary ray. After Williamson.
_x_, _x_, xylem. (No. 1933 A.A. in the Williamson Collection.)]
[Sidenote: ARTHROPITYS.]
In the large section of a calcareous nodule diagrammatically shown in
fig. 17 II. (p. 85) the secondary wood of a slightly flattened Calamite
is the most prominent plant fragment. The pith-cavity has been almost
obliterated by the lateral compression of the woody cylinder, but the
presence of the carinal canals along the inner edge of the wood may
still be readily recognised. The appearance presented by a transverse
section of the secondary wood of a Calamite is that of regular
radial series of rather small rectangular tracheids, with occasional
secondary medullary rays consisting of narrow and radially elongated
parenchymatous cells. The principal rays[610] in the _Arthropitys_ type
of a Calamite stem are often found to gradually decrease in breadth
as they pass into the secondary wood, until in the outer portion of
the wood the primary medullary rays are practically obliterated by the
formation of interfascicular xylem.
In fig. 74, _A_, we have a portion of a single xylem group of a
thick woody stem. The stem from which the figure has been drawn was
originally described by Binney[611] as _Calamodendron commune_; we
now recognise it as a typical example of the subgenus _Arthropitys_.
The specific term _communis_ was used by Ettingshausen[612] in 1855
in a comprehensive sense to include more than twenty species of the
genus _Calamites_, but since Binney’s use of the term it has come to
be associated with a definite type of _Arthropitys_ stem, in which
the primary medullary rays decrease rapidly in breadth towards the
periphery of the wood. The wood of Binney’s stem[613] measures 2·5
cm. across, but the pith-cavity has been crushed to the limits of a
narrow band represented in the figure by the shaded portion. The strand
of cells, _s_, in the pith is a portion of a Stigmarian appendage
(“rootlet”), which penetrated into the hollow stem of the Calamite
and became petrified by the same agency to which the preservation of
the stem is due. These intruded Stigmarian appendages are of constant
occurrence in the calcareous nodules; their intimate association with
the tissues of other plants is often a serious source of error in the
identification of petrified tissues. The inner portion of one of the
xylem groups is shown in fig. 74, _A_. External to the carinal canal,
the xylem tracheids are disposed in regular series and associated with
numerous narrow secondary medullary rays. The width of the xylem wedge
increases gradually as we pass outwards, this is due to the formation
of interfascicular xylem, which in the more peripheral portion of
the stem extends across the primary medullary rays. The few primary
medullary-ray cells shown in the drawing illustrate the characteristic
tangentially elongated form and large size of the parenchymatous
elements. Williamson and Scott have pointed out that the tangentially
elongated form of the medullary-ray cells is the result of active
growth, and not merely the expression of the tangential stretching of
the stem consequent on secondary thickening.
[Illustration: FIG. 74.
_A._ Transverse section of part of a Calamite stem. [_Calamites
(Arthropitys) communis_ (Binney).]
_s_, Stigmarian appendage. _x_, xylem. From a specimen in the
Binney Collection, Cambridge, × 50.
_B._ Transverse section of a stem.
_h_, hypodermal tissue; _c_, inner cortex. From a specimen in the
Williamson Collection (no. 62). × 35.]
A glance at the complete transverse section of the stem,—of which a
small portion is shown in fig. 74 _A_,—suggests the existence of annual
rings in the wood, but this appearance of rings is merely the result
of compression. The secondary wood of a Calamite does not exhibit any
regular zones of growth comparable with the annual rings of our forest
trees.
[Illustration: FIG. 75. Longitudinal tangential section near the inner
edge of the wood of the Calamite of fig. 74.
_x_, _x_, secondary xylem and medullary rays; _m_, principal
medullary ray. From a section in the Binney Collection, × 50.]
Before passing to other examples of Calamitean stems, reference may be
made to the sections shown in figs. 75 and 76, which illustrate some
further points in the structure of Binney’s stems. In fig. 75 the xylem
tracheids are shown at _x_, and between them the secondary medullary
rays present the appearance of long and narrow parenchymatous cells;
as the section is tangential the characteristic scalariform character
of the tracheids is not shown, the ladder-like bordered pits being
confined to the radial walls of the tracheal elements. The much greater
length than breadth of the cells which form the rays associated with
the xylem tracheids, is a characteristic feature in Calamitean stems.
The breadth of the principal ray, _m_, shows that the section has
passed through the wood a short distance from the pith; in a tangential
section cut further into the wood the breadth of the principal rays
would be considerably reduced. The large medullary-ray tissue consists
of square-walled parenchymatous cells. The more highly magnified
section, in fig. 76, shows a central group of parenchyma containing
a few transversely cut tracheids, but the two kinds of elements are
not clearly differentiated in the figure; this group of cells is an
outgoing leaf-trace which is enclosed by the strongly curved tracheids
of the stem. The section is taken from the node of a stem where several
leaf-trace bundles are passing out to a whorl of leaves; the few cells
intercalated between the tracheids belong to the parenchyma of the
secondary medullary rays.
[Illustration: FIG. 76. Longitudinal tangential section of the same
Calamite as that of figs. 74 and 75, showing a leaf-trace
and curved tracheids at a node. From a section in the Binney
Collection, × 100.]
[Sidenote: ARTHROPITYS. SURFACE FEATURES.]
In the small portion of a stem represented in fig. 74 _B_, the cortical
tissues have been partially preserved; at the inner edge, next the
hollow pith, there are two xylem groups, each with a carinal canal, and
between them is part of a broad “principal” medullary ray[614]. The
cambium has not been preserved, but beyond this region we have some
of the large cells, _c_, of the inner cortex; these are followed by a
few remnants of a smaller-celled tissue, and external to this part of
the cortex there is a series of triangular groups, _h_, consisting of
small thick-walled cells alternating with spaces which were originally
occupied by more delicate parenchyma. The darker groups constitute
hypodermal strands of mechanical tissue or stereome which lent support
to the stem. The surface of a stem possessing such supporting strands
would probably assume a longitudinally wrinkled or grooved appearance
on drying; the intervening parenchyma, contracting and yielding more
readily, would tend to produce shallow grooves alternating with the
ridges above the stereome strands.
The complete section of the stem of which a small portion is shown
in fig. 74 _B_, is figured by Williamson[615] in his 12th memoir on
Coal-Measure plants. The section was obtained from Ashton-under-Lyne
in Lancashire; it illustrates very clearly a method of preservation
which is occasionally met with among petrified plants. The walls of the
various tissue elements are black in colour and somewhat ragged, and
the general appearance of the section is similar to that of a section
of a charred piece of stem. It is possible that the Calamite twig was
reduced to charcoal before petrifaction by a lightning flash or some
other cause.
It is often said that the surface of a Calamite stem was probably
marked by regular ridges and grooves similar to those of the
pith-cast, and that such external features are connected with the
arrangement of the tissues in the vascular cylinder. The indication
of grooves and ridges on the bark of fossil Calamites is no doubt the
result of the existence in the hypoderm of firm strands alternating
with strands of less resistant cells. It is very common to find
Calamite pith-casts covered with a layer of coal presenting a ribbed
surface, but this is simply due to the moulding of the coaly film on an
internal pith-cast. The broad grooves on such a specimen as that of
fig. 77 are, on the other hand, probably an indication of the existence
of hypoderm bands similar to those in fig. 74 _B_, _h_. The specimen
from which fig. 77 is drawn shows many interesting features. The figure
given by Grand’Eury, of which fig. 77 is a copy, is somewhat idealised,
but the various surfaces can be made out in the fossil. The surface
of the coaly envelope surrounding the pith-cast, _a_, is distinctly
grooved, but the depressions have nothing to do with the surface
features of the wood or the pith-cast; they are no doubt due to the
occurrence of alternating bands of thick- and thin-walled tissue in the
hypodermal region of the cortex; the peripheral strands of bast cells
would stand out as prominent ribs as the stem tissue contracted during
fossilisation. At _b_ (fig. 77) we have a view of the wood in which the
position of the principal rays is indicated by fine longitudinal lines
at regular intervals; the oval projections just below the nodal line
are probably the casts of infranodal canals (_cf._ p. 324). At _a_ the
characteristic pith-cast is seen with a small branch-scar on the node.
The scar on the middle node, _N_ 2, is probably that of a root, and a
root _R_ is still attached to the node, _N_ 3.
[Illustration: FIG. 77. Portion of a Calamite stem, showing the surface
of the bark, _c_; the wood, _b_; the surface of the pith-cast,
_a_. _N._1-_N._3. Nodes. _R._ Root. (After Grand’Eury. Partially
restored from a specimen in the École des Mines, Paris.) ¾ nat.
size.]
[Sidenote: PERIDERM IN STEMS.]
An interesting feature observed in some specimens of older Calamite
branches is the development of periderm or cork. This is illustrated on
a large scale by a unique specimen originally described by Williamson
in 1878[616]. Figs. 78 and 79 represent transverse and longitudinal
sections of this stem. This unusually large petrified stem was found
in the Coal-Measures of Oldham, in Lancashire. In the slightly reduced
drawing, fig. 78, the large and somewhat flattened pith, _p_, 4·2 cm.
in diameter, is shown towards the bottom of the figure. Surrounding
this we have 58 or 59 wedge-shaped projecting xylem groups and broad
medullary rays; the latter soon become indistinguishable as they
are traced radially through the thick mass of secondary wood, 5 cm.
wide, composed of scalariform tracheids and secondary medullary rays
(fig. 78, 3). The secondary wood presents the features characteristic
of _Calamites_ (_Arthropitys_) _communis_ (Binney). External to
the wood there is a broken-up mass, about 5·5 cm. wide composed of
regularly arranged (fig. 78, 2) and rather thick-walled cells; this
consists of periderm, a secondary tissue, which has been developed
by a cork-cambium during the increase in girth of the plant. The
more delicate cortical tissues have not been preserved, and the more
resistant portion of the bark has been broken up into small pieces of
corky tissue, among which are seen numerous Stigmarian appendages,
pieces of sporangia and other plant fragments. These associated
structures cannot of course be shown in the small-scale drawing of the
figure.
[Illustration: FIG. 78.
1. Transverse section of a thick Calamite stem.
_p_, pith; _x_, secondary wood; _c_, bark. (⅔ nat. size.)
2. Periderm cells of bark.
3. Xylem and medullary rays. (2 and 3, × 80.)
From a specimen in the Williamson Collection (no. 79).]
In the radial longitudinal section (fig. 79) we see the pith with
the projecting wood and the remains of a diaphragm at the node. The
mottled or watered appearance of the wood is due to numerous medullary
rays which sweep across the tracheids. The periderm elements, as seen
in longitudinal section, are fibrous in form.
[Illustration: FIG. 79. Longitudinal section of the specimen stem in
fig. 78. From a specimen in the Williamson Collection, British
Museum (no. 80). ⅔ nat. size.]
[Sidenote: CALLUS WOOD.]
The development of cork in a younger Calamite stem is clearly shown in
a specimen described by Williamson and Scott in their Memoir of 1894.
In a transverse section of the stem several large cells of the inner
cortex are seen to be in process of division by tangential walls, and
giving rise to radially arranged periderm tissue[617].
The section diagrammatically sketched in fig. 80 is that of a Calamite
twig in which the wood appears to have been injured, and the wound has
been almost covered over by the formation of callus wood. The young
trees in a Palaeozoic forest might easily be injured by some of the
large amphibians, which were the highest representatives of animal
life during the Carboniferous period, just as our forest trees are
often barked by deer, rabbits, and other animals. Fissures might also
be formed by the expansion of the bark under the heating influence of
the sun’s rays[618]. Such a specimen as that of fig. 80 gives an air
of living reality to the petrified fragments of the Coal period trees.
It is well known how a wound on the branch of a forest tree becomes
gradually overgrown by the activity of the cambium giving rise to a
thick callus, which gradually closes over the wounded surface in the
form of two lips of wood which finally meet over the middle of the
scar. The two lips of callus are clearly shown in the fossil branch
arching over the tear in the wood just beyond the ring of carinal
canals. The tissue external to the wood represents the imperfectly
preserved cortex. A section which was cut parallel to that of fig. 80
shows a continuous band of wood beyond the wound, and the latter has
the form of a small triangular gap; this section appears to have passed
across the wound where it was narrower and has already been closed over
by the callus. The formation of a rather different kind of callus wood
has been described by Renault[619] and by Williamson and Scott[620],
in stems where aborted or deciduous branches have been overgrown and
sealed up by cambial activity.
[Illustration: FIG. 80. Diagrammatic sketch of a transverse section of
a Calamite twig, showing callus wood. From a specimen in the
Cambridge Botanical Laboratory Collection. × _ca._ 10.]
[Illustration: FIG. 81. _Calamites._ Longitudinal section (_R_, radial;
_T_, tangential) of a small branch. _b_, position of a lateral
branch. From a specimen (no. 1937) in the Williamson Collection.
Slightly enlarged.]
Some of the features to be noticed in longitudinal sections of Calamite
stems have already been described, at least as regards younger
branches. The specimen shown in fig. 81 illustrates the general
appearance of a stem as seen in tangential and radial section. In the
lower portion, _T_, the course of the vascular bundles is shown by
the black lines which represent the xylem tracheids, bifurcating and
usually alternating at each node. Between the xylem strands are the
broad principal medullary rays. At _b_ a branch has been cut through
on its passage out from the parent stem, just above the nodal line. In
tangential sections of Calamite stems one frequently sees both branches
and leaf-trace bundles (fig. 83, _A_), passing horizontally through
the wood and enclosed by strongly curved and twisted tracheids. In the
upper part of the figure (81, _R_), the section has passed through the
centre of the stem, and the wood is seen in radial view; each node is
bridged across by a diaphragm of parenchymatous cells capable of giving
rise to a surface layer of periderm[621].
An outgoing branch, as seen in a tangential section of a stem, consists
of a parenchymatous pith surrounded by a ring of vascular bundles, in
which the characteristic carinal canals have not yet been formed, but
if the section has cut the branch further from its base, there may be
seen a circle of irregular gaps marking the position of the carinal
canals. Such gaps are often occupied by thin parenchyma, and contain
protoxylem elements. The outgoing branches, as seen in a tangential
section of a Calamite stem, are seen to be connected with the wood of
the parent stem by curved and sinuous tracheids, which give to the
stem-wood a curiously characteristic appearance[622], as if the xylem
elements had been pushed aside and contorted by the pressure of the
outgoing member. A tangential section through a Pine stem[623] in the
region of a lateral branch presents precisely the same features as in
_Calamites_. The branches are given off from the stem immediately above
a node and usually between two outgoing leaf-trace bundles.
[Sidenote: RHIZOME OF CALAMITES.]
Specimens of pith-casts occasionally present the appearance of a
curved and rapidly tapered ram’s horn, and the narrow end of such a
cast is sometimes found in contact with the node of another cast. This
juxtaposition of casts is shown unusually well in fig. 82. In some of
the published restorations of _Calamites_ the plant is represented as
having thick branches attached to the main stem by little more than
a point. Williamson[624] clearly explained this apparently unusual
and indeed physically impossible method of branching, by means of
sections of petrified stems. The branches seen in fig. 82 are of
course pith-casts, and in the living plant the pith of each branch was
surrounded by a mass of secondary wood developed from as many primary
groups of xylem as there are grooves on the surface of the cast, each
of the grooves on an internode corresponding to the projecting edge
of a xylem group. At the junction of one branch with another the pith
was much narrower and the enclosing wood thicker, so that the tapered
ends of the cast merely show the continuity by a narrow union between
the pith-cavities of different branches. Most probably the casts of
fig. 82 are those of a branched rhizome which grew underground, giving
off aerial shoots and adventitious roots. There is a fairly close
resemblance between the Calamite casts of fig. 82 and a stout branching
rhizome of a Bamboo, _e.g._ _Bambusa arundinacea_ Willd.; it is not
surprising that the earlier writers looked upon the Calamite as a
reed-like plant.
Before leaving the consideration of stem structures there is another
feature to which attention must be drawn. On the casts shown in fig. 82
there is a circle of small oval scars situated just below the nodes,
these are clearly shown at _c_, _c_, _c_. Each of the scars is in
reality a slight projection from the upper end of an internodal ridge.
As the ridges correspond to the broad inner faces of medullary rays,
the small projection at the upper end of each ridge is a cast of a
depression or canal which existed in the medullary tissue of the living
plant. There have been various suggestions as to the meaning of these
oval projections; several writers have referred to them as the points
of attachments of roots or other appendages, but Williamson proved them
to be the casts of canal-like gaps which traversed the upper ends of
principal medullary rays in a horizontal direction. In a tangential
section of a Calamite stem the summit of each primary medullary ray
often contains a group of smaller elements which are in process of
disorganisation, and in some cases these cells give place to an oval
and somewhat irregular canal. In the diagrammatic tangential section
represented in fig. 83, _A_ the upper end of each ray is perforated
by a large oval space, which has been formed as the result of the
breaking down of a horizontal band of cells. Williamson designated
these spaces _infranodal canals_. While proving that they had nothing
to do with the attachment of lateral members, he suggested that they
might be concerned with secretion; but their physiological significance
is still a matter of speculation. The casts of infranodal canals are
especially large and conspicuous in the subgenus _Arthrodendron_,
a form of Calamite characterised by certain histological features
to be referred to later. Williamson[625] originally regarded the
presence of infranodal canals as one of the distinguishing features of
_Arthrodendron_, but they occur also in the casts of the commoner type
_Arthropitys_. As a rule we have only the cast of the inner ends of the
infranodal canals preserved as slight projections like those in fig.
83, _A_; but in one exceptionally interesting pith-cast described by
Williamson, these casts of the infranodal canals have been preserved as
slender spoke-like columns radiating from the upper ends of the ridges
of the infranodal region of a pith-cast.
[Illustration: FIG. 82. Branched rhizome of _Calamites_. ½ nat. size.
_C_, _C_, nodes showing casts of infranodal canals.
From a specimen in the Manchester Museum, Owens College.]
This specimen, which was figured by Williamson[626] in two of his
papers, and by Lyell[627] in the fifth edition of his _Elementary
Geology_, is historically interesting as being one of the first
important plants obtained by Williamson early in the fifties, when he
began his researches into the structure of Carboniferous plants. A
joiner, who was employed by Williamson to make a piece of machinery
for grinding fossils, brought a number of sandstone fragments as an
offering to his employer, whom he found to be interested in stones.
The specimens “were in the main the merest rubbish, but amongst them,”
writes Williamson, “I detected a fragment which was equally elegant
and remarkable.... In later days, when the specimen so oddly and
accidentally obtained, came to be intelligently studied, its history
became clear enough, and the priceless fragment is now one of the most
precious gems in my cabinet[628].”
_Comparison of three types of structure met with in Calamitean
stems_,—Arthropitys, Arthrodendron, _and_ Calamodendron.
[Sidenote: ARTHROPITYS.]
The anatomical features which have so far been described as
characteristic of _Calamites_ represent the common type met with in the
English Coal-Measures. The same type occurs also in France, Germany and
elsewhere. It is that form of stem known as _Arthropitys_, a sub-genus
of _Calamites_.
_Arthropitys_ may be briefly diagnosed as follows,—confining our
attention to the structure of the stem: A ring of collateral bundles
surrounds a large hollow pith, each primary xylem strand terminates
internally in a more or less bluntly rounded apex traversed by a
longitudinally carinal canal. The principal medullary rays consist of
large-celled parenchyma, of which the individual elements are usually
tangentially elongated as seen in transverse section, and four or five
times longer than broad as seen in a tangential longitudinal section.
The secondary xylem consists of scalariform and reticulately pitted
tracheids; the interfascicular xylem may be formed completely across
each primary ray at an early stage in the growth of the stem[629], or
it may be developed more gradually so as to leave a tapering principal
ray of parenchyma between each primary xylem bundle. In the latter
case the principal rays present the characteristic appearance shown in
figs. 71, 74, _A_, 75 and 78, a type of stem which we may refer to as
_Calamites_ (_Arthropitys_) _communis_. In the former case the stem
presents the appearance shown in fig. 83, _D_[630]. A third variety
of _Arthropitys_ stem is one which was originally named by Göppert
_Arthropitys bistriata_; in this form the principal rays retain their
individuality as bands of parenchyma throughout the whole thickness
of the wood[631]. Such stems as those of figs. 73 and 74, _B_, may be
young examples of _Arthropitys communis_ or possibly of _A. bistriata_.
The narrow secondary medullary rays of _Arthropitys_ usually consist of
a single row of cells which are three to five times higher than broad,
as seen in tangential longitudinal section. Infranodal canals occur in
some examples of _Arthropitys_.
[Sidenote: ARTHRODENDRON.]
In the subgenus _Arthrodendron_, a type of stem first recognised
by Williamson and named by him _Calamopitys_[632], the principal
medullary rays consist of _prosenchymatous cells_ (_i.e._ elongated
pointed elements) and not parenchyma. These elongated elements are
not pitted like tracheids, and they are shorter and broader than the
xylem elements. In some examples of this subgenus the primary rays
are bridged across at an early stage by the formation of secondary
interfascicular xylem, and in others they persist as bands of ray
tissue, as in _Arthropitys_. Other characteristics of _Arthrodendron_
are the abundance of reticulated instead of scalariform tracheids in
the secondary wood, and the large size of the infranodal canals.
Fig. 83, _D_ represents part of a transverse section of
_Arthrodendron_; in this stem the rays have been occupied by
interfascicular xylem at a very early stage of the secondary growth.
The section from which fig. 83, _D_ is drawn was described by
Williamson in 1871; the complete section shows about 80 carinal canals
and primary xylem groups. The prosenchymatous form of the principal
medullary rays is seen in fig 83, _C_, and the reticulate pitting on
the radial wall of a tracheid is shown in fig. 83, _B_. Fig. 83, _A_
illustrates the large infranodal canals as seen in a tangential section
of a stem. The same section shows also the course of the vascular
bundles characteristic of _Calamites_ as of _Equisetum_, and the
position of outgoing leaf-traces is represented by unshaded areas in
the black vascular strands.
[Illustration: FIG. 83. _Calamites_ (_Arthrodendron_).
_A._ Tangential section (diagrammatic) showing the course of the
vascular strands, also leaf-traces and infranodal canals.
_B._ Radial face of a tracheid.
_C._ Prosenchymatous elements of a principal medullary ray.
_D._ Transverse section of the wood. (After Williamson.) No. 36 in
the Williamson Collection.]
The subgenus _Arthrodendron_ is very rarely met with, and our
information as to this type is far from complete[633].
The third subgenus _Calamodendron_ has not been discovered in English
rocks, and our knowledge of this type is derived from French and
German silicified specimens[634]. There is the same large hollow
pith surrounded by a ring of collateral bundles with carinal canals,
as in the two preceding subgenera. The tracheids are scalariform
and reticulate, and the secondary medullary rays consist of rows of
parenchymatous cells which are longer than broad, as in _Arthropitys_
and _Arthrodendron_.
[Illustration: FIG. 84. _Calamites_ (_Calamodendron_) _intermedium_, Ren.
Transverse section through two vascular bundles.
_a_, _a_, xylem tracheids, _b_, _b_, bands of prosenchyma, _c_,
medullary ray. (After Renault.)]
The most characteristic feature of _Calamodendron_ is the occurrence of
several rows of radially disposed thick-walled prosenchymatous elements
(fig. 84, _b_) on either flank of each wedge-shaped group of xylem.
Each principal ray is thus nearly filled up by bands of fibrous cells
on the sides of adjacent xylem groups, but the centre of each principal
ray is occupied by a narrow band of parenchyma (fig. 84, _c_). The
relative breadth of the xylem and prosenchymatous bands has been made
use of by Renault as a specific character in _Calamodendron_ stems.
Fig. 84 is copied from a drawing recently published by this French
author of a new species of _Calamodendron_, _C. intermedium_[635]. In
this case the bands of fibrous cells, _b_, are slightly broader, as
seen in a transverse section of the stem, than the bands of xylem
tracheids, _a_. The narrow band, _c_, consists of four rows of the
parenchymatous tissue of a medullary ray. At the inner end of each
group of tracheids there is a large carinal canal.
The question of the recognition of the pith-casts of stems possessing
the structure of any of the three subgenera of _Calamites_ is referred
to in a later section of this chapter.
_b._ _Leaves_
_Leaves of Calamites and Calamitean foliage-shoots, including an
account of_ (α) Calamocladus (Asterophyllites) _and_ (β)
Annularia.
Our knowledge of the structure and manner of occurrence of Calamite
leaves is very incomplete. There are numerous foliage-shoots among the
fossils of the Coal-Measures which are no doubt Calamitean, but as
they are nearly always found apart from the main branches and stems,
it is generally impossible to do more than speak of them as probably
the leaf-bearing branches of a Calamite. The familiar fossils known
as _Asterophyllites_, and in recent years often referred to the genus
_Calamocladus_, are no doubt Calamitean shoots; but they are usually
found as isolated fragments, and it is seldom that we are able to refer
them to definite forms of _Calamites_. Another common Coal-Measure
genus, _Annularia_, is also Calamitean, and at least some of the
species are no doubt leafy shoots of _Calamites_. Although it is
generally accepted that the fossils referred to as _Asterophyllites_
or _Calamocladus_ are portions of _Calamites_, and not distinct
plants, it is convenient, and indeed necessary, to retain such a term
as _Calamocladus_ as a means of recording foliage-shoots, which may
possess both a botanical and a geological value.
Some of the Calamite casts, especially those referred to the subgenus
_Calamitina_, are occasionally found with leaves attached to the nodes.
In some stems the leaves are arranged in a close verticil, and each
leaf has a narrow linear form and is traversed by a single median vein.
Figures of Calamite stems with verticils of long and narrow leaves
may be found in Lindley and Hutton[636], and in the writings of many
other authors[637]. In the specimen shown in fig. 85 the leaves are
preserved apart from the stem, but from their close association with
a Calamite cast, and from the proofs afforded by other specimens, it
is quite certain they formed part of a whorl of leaves attached to the
node of a true Calamite, and a stem having that particular type known
as _Calamitina_[638] (figs. 99, 100). It is probable that in some
Calamites, and especially in younger shoots, the leaves had the form of
narrow sheaths split up into linear segments. This question has already
been referred to in dealing with certain Palaeozoic fossils referred to
_Equisetites_[639].
[Illustration: FIG. 85. Linear leaves of a Calamite (_Calamitina_).
After Weiss, slightly reduced.]
A few years ago the late Thomas Hick[640], of Manchester, described
the structure of some leaves which he believed to be those of a
Calamite. He found them attached to a slender axis which possessed the
characteristics of a young Calamite branch. There can be little doubt
that his specimens are true Calamite leaves. The sketches of fig. 86
have been made from the sections originally described by Hick. Fig.
86, 1 shows a leaf in transverse section; on the outside there is a
well-defined epidermal layer with a limiting cuticle. Internal to
this we have radially elongated parenchymatous cells forming a loose
or spongy tissue, the cells being often separated by fairly large
spaces (fig. 86, 5), especially in the region of the blunt lateral
wings of the leaf. Some of these cells contain a single dark dot, which
in all probability is the mineralised nucleus. These pallisade-like
cells probably contained chlorophyll and constituted the assimilating
tissue of the leaf. In the centre there is a circular strand of cells
limited by a layer of larger cells with black contents, enclosing an
inner group of small-celled parenchyma and traversed by a few spiral
or scalariform tracheids constituting the single median vein. It is
hardly possible to recognise any phloem elements in the small vascular
bundle; there appear to be a few narrow tracheids surrounded by larger
parenchymatous elements (fig. 86, 2). At one point in the epidermis
of fig. 86, 1, there appears to be a stoma, but the details are not
very clearly shown (fig. 86, 4); the two cells, _s_, _s_, bordering the
small aperture are probably guard-cells.
[Illustration: FIG. 86. A leaf of _Calamites_.
1. Transverse section; _t_, vascular bundle; _x_, sheath of cells.
× 35.
2. Vascular bundle consisting of a few small tracheids, _t_.
3. A tracheid and a few parenchymatous cells, the latter with nuclei.
4. A stoma; _s_, _s_, guard-cells.
5. Pallisade cells and intercellular spaces.
From a section in the Manchester Museum, Owens College.]
The nature of the assimilating tissue, the comparatively thick band of
thin-walled cells with intercellular spaces, and the exposed position
of the stomata suggest that the plant lived in a fairly damp climate;
at least there is nothing to indicate any adaptation to a dry climate.
In the Binney collection of plants in the Woodwardian Museum,
Cambridge, there is a species of a very small shoot bearing three or
four verticils of leaves which possess the same structure as those of
fig. 86. We may probably regard such twigs as the slender terminal
branches of Calamitean shoots.
α. _Calamocladus_ (_Asterophyllites_).
The generic name _Asterophyllites_ was proposed by Brongniart[641] in
1822 for a fossil previously named by Schlotheim[642] _Casuarinites_,
and afterwards transferred to Sternberg’s genus _Annularia_. In 1828
Brongniart[643] gave the following diagnosis of the fossils which he
included under the genus _Asterophyllites_:—“Stems rarely simple,
usually branched, with opposite branches, which are always disposed in
the same plane; leaves flat, more or less linear, pointed, traversed by
a simple median vein, free to the base.” Lindley and Hutton described
examples of Brongniart’s genus as species of _Hippurites_[644], and
other authors adopted different names for specimens afterwards referred
to _Asterophyllites_.
At a later date Ettingshausen[645] and other writers expressed the
view that the fossils which Brongniart regarded as a distinct genus
were the foliage-shoots of _Calamites_, and Ettingshausen went so far
as to include them in that genus. In view of the generally expressed
opinion as to the Calamitean nature of _Asterophyllites_, Schimper[646]
proposed the convenient generic name _Calamocladus_ for “rami et
ramuli foliosi” of _Calamites_. Some recent authors have adopted this
genus, but others prefer to retain _Asterophyllites_. In a recent
important monograph by Grand’Eury[647] Calamitean foliage-shoots are
included under the two names, _Asterophyllites_ and _Calamocladus_;
the latter type of foliage-shoots he associates with the stems of
the subgenus _Calamodendron_, and the former he connects with those
Calamitean stems which belong to the subgenus _Arthropitys_.
It is an almost hopeless task to attempt to connect the various forms
of foliage-shoots with their respective stems, and to determine what
particular anatomical features characterised the plants bearing
these various forms of shoots. We may adopt Schimper’s generic name
_Calamocladus_ in the same sense as _Asterophyllites_, but as including
such other foliage-shoots as we have reason to believe belonged to
_Calamites_. Those leaf-bearing branches which conform to the type
known as _Annularia_ are however not included in _Calamocladus_, as
we cannot definitely assert that these foliage-shoots belong in all
cases to Calamitean stems. Grand’Eury’s use of _Calamocladus_ in a
more restricted sense is inadvisable as leading to confusion, seeing
that this name was originally defined in a more comprehensive manner
as including Calamitean leaf-bearing branches generally. We may define
_Calamocladus_ as follows:—
Branched or simple articulated branches bearing whorls of uni-nerved
linear leaves at the nodes; the leaves may be either free to the
base or fused basally into a cup-like sheath (_e.g._ Grand’Eury’s
_Calamocladus_). The several acicular linear leaves or segments which
are given off from the nodes spread out radially in an open manner in
all directions; they may be either almost at right angles to the axis
or inclined at different angles. Each segment is traversed by a single
vein and terminates in an acuminate apex.
As a typical example of a Calamitean foliage-shoot the species
_Calamocladus equisetiformis_ (Schloth.) may be briefly described. The
synonymy of the commoner species of fossil plants is a constant source
of confusion and difficulty; in order to illustrate the necessity of
careful comparison of specimens and published illustrations, it may be
helpful to quote a few synonyms of the species more particularly dealt
with. The exhaustive lists drawn up by Kidston in his _Catalogue of
Palaeozoic plants in the British Museum_ will be found extremely useful
by those concerned with a systematic study of the older plants.
[Illustration: FIG. 87. _Calamocladus equisetiformis_ (Schloth.).
From a specimen in the British Museum (McMurtrie Collection, no. V.
2963). _ca._ ⅓ nat. size.]
_Calamocladus equisetiformis_ (Schloth.). Fig. 87.
1809. _Phytolithus_, Martin[648].
1820. _Casuarinites equisetiformis_, Schlotheim[649].
1825. _Bornia equisetiformis_, Sternberg[650].
1828. _Asterophyllites equisetiformis_, Brongniart[651].
1836. _Hippurites longifolia_, Lindley and Hutton[652].
1855. _Calamites equisetiformis_, Ettingshausen[653].
1869. _Calamocladus equisetiformis_, Schimper[654].
1869. _Annularia calamitoides_, Schimper[654].
The above synonyms do not exhaust the list[655], but they suffice to
illustrate the necessity of a careful comparison in drawing up tables
of species, in connection with geographical distribution or for other
purposes.
_Calamocladus equisetiformis_ may be briefly defined as follows:—A
central axis possessing a hollow pith of Calamitean character, divided
externally into well-marked slightly constricted nodes and internodes;
from the nodes long narrow and free leaves are borne in whorls; from
the axils of some of the leaves lateral branches are given off inclined
at a fairly wide angle to the main axis, and bearing crowded verticils
of spreading acicular leaves.
The unusually good specimen, 38·5 cm. long, shown on a much reduced
scale in fig. 87, illustrates the characteristic habit of this form of
_Calamocladus_. It is from the Radstock coal-field of Somersetshire,
one of the best English localities for Coal-Measure plants. An
exceedingly good collection of Radstock plants has recently been
presented to the British Museum by Mr J. McMurtrie; it includes many
fine specimens of _Calamites_. A small example—probably of this
species—from Coalbrook Dale, near Dudley, in Shropshire, and now in the
British Museum, illustrates very well the appearance of a young and
partially expanded Calamitean foliage-shoot. The central axis, 6·5
cm. in length, includes about 15 internodes, and terminates in a bud
covered by several small leaves. Lateral branches are given off at a
wide angle, and small unexpanded buds occur in the axils of several of
the leaves.
As an example of the leaf-bearing branches which Grand’Eury has
recently described as _Calamocladus_, using the genus in a more
restricted sense than is adopted in the present chapter, reference may
be made to the fragment shown in fig. 68, _A_. The foliage-shoots of
this type bore verticils of linear leaves, coherent basally in the form
of a cup, at the ends of branches and not in a succession of whorls on
each branch. The association of reproductive organs, in the form of
long and narrow strobili, with _Calamocladus_ is referred to in the
sequel.
The specimens described by Grand’Eury are in the École des Mines
Museum, Paris; some of the shoots which are well preserved bear a
resemblance in habit of growth to the genus _Archaeocalamites_.
β. _Annularia._
In 1820 this generic name was applied by Sternberg[656] to some
specimens of branches bearing verticils of linear leaves. In 1828
Brongniart[657] thus defined the genus _Annularia_:—“Slender stem,
articulated, with opposite branches arising above the leaves. Leaves
verticillate, flat, frequently obtuse, traversed by a single vein,
fused basally and of unequal length.”
In the works of earlier writers we find frequent illustrations of
specimens of _Annularia_, which are compared with Asters and other
recent flowering plants. Lehmann[658] contributed a paper to the Royal
Academy of Berlin in 1756, in which he referred to certain fossil
plants as probable examples of flowers, among them being a specimen
of _Annularia_. He refers to the occurrence of fossil ferns and other
plants, and asks why we do not find flowers of the rose or tulip;
his object being “not to acquire vain glory, but to give occasion for
others to look into the matter more clearly.”
The general habit of the fossils which are now included under
_Annularia_ agrees closely with that of _Calamocladus_. There is the
same spreading form and a similar foliage in the two genera, but in
_Annularia_ the members of a whorl are always fused into a basal
sheath, and the segments are not of equal length. We may thus summarise
the characteristic features of the genus:—
Opposite branches are given off in one plane from the nodes of a
main axis; the leaves are in the form of narrow sheaths divided into
numerous and unequal linear or narrow lanceolate segments, each with
a median vein. The segments in each whorl appear to be spread out in
one plane very oblique to the axis of a branch, instead of spreading
radially in all directions; the lateral segments are usually longer
than the upper and lower members of a whorl. The vegetative branches
possess the same type of structure as _Calamites_.
A comparison of _Annularia_ and _Phyllotheca_ has already been
made in Chapter IX. (p. 282). Potonié[659] has recently given a
detailed account of Annularian leaves; he compares them with those
of _Equisetum_, and describes the occurrence on the lamina of each
leaf-segment of a broad central band or midrib, with a groove, probably
containing stomata, on either side. He shows that in well-preserved
specimens of _Annularia_, it is possible to recognise certain minute
surface-features, such as the presence of hairs and stomata, which
enable one to detect a close resemblance between the leaves of Calamite
stems and those of Annularian shoots.
It is not always easy to distinguish between _Annularia_ and
_Calamocladus_; the collar-like basal sheath in the leaves of the
former is a characteristic feature, but that cannot always be
recognised. On the other hand, the leaves of _Calamocladus_ may
sometimes be flattened out on the surface of the rock and simulate the
deeply cut sheaths of _Annularia_. It is difficult to decide how far
the manner of occurrence of Annularian leaves in one plane, which is
commonly insisted on as a generic character, is an original feature, or
how far it is the result of compression in fossilisation. Probably the
leaves of a living _Annularia_ were spread out at right angles to the
axis, as in the ‘verticils’ of such a plant as _Galium_.
Dawson[660] has described some fossils from the Devonian rocks of
Canada as species of _Asterophyllites_; the figures bear a closer
resemblance to the genus _Annularia_. The same author figures some
irregularly whorled impressions as _Protannularia_, which appear to be
identical with a fossil described by Nicholson[661] from the Skiddaw
slates (Ordovician) of Cumberland as _Buthotrephis radiata_, but the
specimens are too imperfect to admit of accurate determination.
_Annularia stellata_ (Schloth.). Fig. 88.
1820. _Casuarinites stellatus_, Schlotheim[662].
1826. _Bornia stellata_, Sternberg[663].
1828. _Annularia longifolia_, Brongniart[664].
1834. _Asterophyllites equisetiformis_, Lindley and Hutton[665].
1868. _Asterophyllites longifolius_, Binney[666].
1887. _Annularia Geinitzi_, Stur[667].
1887. _Annularia westphalica_, Stur.
This species was figured by Scheuchzer[668] in his _Herbarium
Diluvianum_, and compared by him with a species of _Galium_ (Bedstraw).
Brongniart first made use of the generic name _Annularia_ for this
common Coal-Measure species, which may be defined as follows:—
Stem reaching a diameter of about 6–8 cm., with internodes 6–12 cm.
in length, the surface either smooth or faintly ribbed. Primary
branches given off in opposite pairs from the nodes, the lateral
branches giving off smaller branches disposed in the same manner. The
smaller branches bear verticils of leaves at each node; both leaves
and ultimate branches being in one plane. The leaves are narrow,
lanceolate-spathulate in form, broadest about the middle, 1–5 cm. in
length and 1–3 mm. broad, hairy on the upper surface[669]; each leaf is
traversed by a single vein.
[Illustration: FIG. 88. Branch of _Annularia stellata_ (Schloth.). ⅘
nat. size. From a specimen in the Collection of Mr R. Kidston.
Upper Coal-Measures, Radstock.]
Each whorl contains 16–32 segments, which are connected basally into
a collar or narrow sheath; the lateral segments are usually longer
than the upper and lower. The branches are about 6–20 mm. broad, with
finely ribbed internodes 3–7 cm. long, bearing verticils of leaves; the
ultimate branches arise in pairs in the axils of the lateral segments
of the verticils.
The strobili are of the _Calamostachys_[670] type and are borne on the
main branches or possibly on the stem; they have a long and narrow form
and are attached in verticils at the nodes. Each strobilus consists of
a central axis bearing alternate whorls of linear lanceolate sterile
bracts and sporangiophores, about half as numerous as the sterile
bracts; each sporangiophore bears four ovoid sporangia.
The anatomical structure of a specimen referred to _Annularia stellata_
has been described by Renault[671]. The cortex consists of parenchyma
traversed by lacunae and limited peripherally by a denser hypodermal
tissue. In the stele Renault describes 14 xylem strands, each with a
large carinal canal. The pith was apparently large and hollow. The
same author describes an _Annularia_ strobilus in which the lower
sporangiophores bear macrosporangia, and the upper microsporangia.
[Illustration: FIG. 89. _Annularia sphenophylloides_ (Zenk.).
_A._ Strobilus (_Stachannularia calathifera_, Weiss). ⅔ nat. size.
_B._ Vegetative shoot. ⅘ nat. size.
From specimens in the Collection of Mr R. Kidston. Upper
Coal-Measures, Radstock.]
The references in the footnote should be consulted for figures of this
species of _Annularia_; it is from the examination of such specimens
as are referred to in the note that the above diagnosis has been
compiled[672].
_Annularia sphenophylloides_ (Zenk.). Fig. 89.
1833. _Galium sphenophylloides_, Zenker[673].
1865. _Annularia brevifolia_, Heer[674], Strobilus.
1876. _Calamostachys_ (_Stachannularia_) _calathifera_, Weiss[675].
Principal branches 8–12 mm. wide, with internodes 8–10 cm. in length,
giving off two opposite branches at the nodes; from the secondary
branches arise smaller branches in opposite pairs. The leaf-verticils
and branches are all in one plane. Each verticil consists of 12–18
spathulate segments, 3–10 mm. long, cuneiform at the base and broader
above, with an acuminate tip; the lateral segments are slightly longer
than the upper and lower members of a whorl.
The small and crowded leaf-whorls give to this species a characteristic
appearance, which readily distinguishes it from the larger-leaved
forms such as _Annularia stellata_. A fossil figured by Lhwyd[676]
in 1699 as _Rubeola mineralis_ is no doubt an example of _Annularia
sphenophylloides_.
Annularian branches are occasionally found with cones given off
from the axils of some of the leaf-whorls. An interesting specimen,
which is now in the Leipzig Museum, was described by Sterzel in
1882[677], showing cones attached to a vegetative shoot of _Annularia
sphenophylloides_. The long and narrow strobili—2·5 cm. long and
about 6 mm. broad—appear very large in proportion to the size of the
vegetative branches. A fertile shoot consists of a central axis bearing
whorls of bracts alternating with sporangiophores, to each of which
are attached four sporangia. The specimen in fig. 89, _A_, does not
show the details clearly; each transverse constriction represents the
attachment of a whorl of linear bracts; the whole cone appears to
consist of a series of short broad segments. The divisions in the lower
half of each segment mark the position of the sterile bracts, while
those of the upper half represent the outlines of the upper sporangia
of each whorl of sporangiophores, the lower sporangia being hidden by
the ring of linear bracts[678]. On some portions of the specimen of
fig. 89, _A_, it is possible to recognise the outlines of cells on
the coaly surface-film; these probably belong to the sporangium wall.
This type of cone is included under the genus _Calamostachys_, a name
applied to Calamitean strobili with certain morphological characters,
as described on p. 351.
_c. Roots._
In 1871 Williamson[679] described some sections of what he considered
to be a distinct variety of a Calamite stem. The chief peculiarity
which he noticed lay in the absence of carinal canals, and in the solid
pith. Some years later the same observer[680] came to the conclusion
that the specimens were probably those of a plant generically distinct
from _Calamites_; he accordingly proposed a new name _Astromyelon_.
Subsequently Cash and Hick[681] gave an account of some examples
of apparently another form of plant, to which they gave the name
_Myriophylloides Williamsonis_; and Williamson[682] suggested the term
_Helophyton_ as a more suitable generic designation. It was, however,
demonstrated by Spencer[683] that the plant described by Cash and Hick
was identical with Williamson’s _Astromyelon_. Williamson[684] then
gave an account of several specimens of this type illustrating various
stages in the growth and development of the _Astromyelon_ ‘stems,’
which he compared with the rhizome of the recent genus _Marsilia_.
In 1885 Renault[685] published an account of _Astromyelon_ in which he
brought forward good evidence in favour of regarding it as a Calamitean
root. The same author has recently given some excellent figures and
a detailed description of certain specific types of these Calamite
roots, and Williamson and Scott’s memoir on the roots of _Calamites_
has rendered our knowledge of _Astromyelon_ almost complete. Some of
the finest specimens, in which the organic connection between typical
Calamite stems and _Astromyelon_ roots is clearly demonstrated, are in
the Natural History Museum, Paris. There are several sections also from
English material which show the connection between root and stem very
clearly.
[Illustration: FIG. 90. Pith-cast of a Calamite stem, with roots;
embedded in sandstone and shale. (After Grand’Eury.) Much reduced.]
Casts of the hollow pith of Calamite rhizomes or aerial branches are
occasionally found in which slender appendages are given off either
singly or in tufts from the nodal regions. Many examples of such
casts have been figured by Lindley and Hutton[686], Binney[687],
Grand’Eury[688], Weiss[689], Stur, and other writers[690]. The large
stem-cast of fig. 90 illustrates the manner of occurrence of long
branched roots on the nodes of a Calamite growing in sandy or clay
soil. The lower and more darkly shaded portion of the specimen is
covered by a layer of coal representing the carbonised wood and cortex,
which has been moulded on to the sandstone pith-cast. In fig. 77 (p.
316) a fairly thick root is seen, in organic connection with one of the
nodes, _N_ 3, and on _N_ 2 there is a scar of another root.
There are certain external characters by which one may often recognise
a Calamitean root. There is no division into nodes and internodes as
in stems, and as the pith of the root was usually solid the parallel
ribs and grooves of stem-casts are not present. In smaller flattened
roots there may sometimes be seen a central or excentric black line
representing the stele, and the surface of the root presents a curious
wrinkled or shagreen texture, probably due to the shrinkage of the
loose lacunar cortex. The occasional excentric position of the stele is
no doubt due to the displacement of the vascular cylinder as a result
of the rapid decay of the cortical tissues. In the Bergakademie of
Berlin there are some unusually good examples of Calamite casts bearing
well-preserved root-impressions; these include the original specimens
figured by Weiss[691].
No doubt some of the roots figured by various writers under the names
_Pinnularia_[692] and _Hydatica_[693] belong to _Calamites_, but it is
often impossible to identify detached specimens with any certainty.
The section figured diagrammatically in fig. 91 _A_ shows the
characteristic single series of large lacunae, _l_, in the middle
cortical region. In the centre there is a wide solid pith surrounded by
a ring of vascular tissue, _x_. The appearance of the middle cortex is
very like that of the stem of a water-plant such as _Myriophyllum_, the
Water Milfoil; it shows that the Calamite roots grew either in water or
swampy ground. In fig. 91 _B_, the root characters are clearly seen;
the centre of the stele is occupied by large parenchymatous cells which
are rather longer than broad in longitudinal view; at the periphery
there are four protoxylem groups _px_, alternating with four groups
of phloem, _ph_, the latter being situated a little further from the
centre of the stele. The structure is therefore that of a typical
tetrarch root. In the example represented in the figure secondary
thickening has begun, and the cambial cells internal to each phloem
group have given rise to a few radially disposed tracheids, _x²_.
Beyond the phloem there are two layers of parenchyma representing,
as regards position, a pericycle and an endodermis. In the ordinary
pericycle and endodermis of the roots of most plants the cells of
the two layers are on alternate radii, but in the Calamite root, as
in _Equisetum_ roots, the cells of these layers are placed on the
same radii, as seen in the neighbourhood of _x²_ in the figure. This
correspondence of the radial walls of the endodermal and pericyclic
cells points to the development of both layers from one mother-layer,
and suggests the ‘double endodermis’ or phloeoterma of _Equisetum_
(p. 254). The cells in the outer of these two layers have slight
thickenings on the radial walls recalling the usual character of
endodermal cells. The phloeoterma is succeeded by a few layers of
parenchyma, constituting the inner cortex, and beyond this we have
the large lacunae separated from one another by slender trabeculae
of cells. The outer cortex is limited by a well-defined layer of
thick-walled cells, which may be spoken of as the _epidermoidal[694]
layer_. Roots possessing this superficial layer of thicker cells have
no doubt lost the original surface-layer which produced the absorptive
root-hairs.
[Illustration: FIG. 91. _A._ Diagrammatic sketch of a transverse
section of a young root of _Calamites_. _x_, xylem; _l_, lacuna.
After Hick.
_B._ Central cylinder (stele) of root, _px_, protoxylem; _ph_,
phloem; _x²_, secondary xylem; _l_, phloeoterma. × 75. After
Williamson and Scott.]
The xylem elements have the form of spiral, reticulate and scalariform
tracheids.
In roots or rootlets smaller than that shown in fig. 91 _B_, the
primary xylem may extend to the centre of the stele, and form a
continuous axial strand; in such examples the structure may be
diarch, triarch or tetrarch. The origin of the cambium agrees
with that in recent roots, the cells immediately external to the
protoxylem tracheids become meristematic, as also those internal to
the phloem. Another root-character is seen in the endogenous origin
of lateral members. Good examples of branching roots are figured by
Williamson[695] and by Williamson and Scott[696].
Older roots[697] are usually found in a decorticated condition. A
transverse section of root in which secondary thickening has been
active for some time presents on a superficial view a close resemblance
to a stem of _Calamites_, but a careful comparison at once reveals
important points of difference. The specimen diagrammatically sketched
in fig. 92 illustrates very clearly the origin of a root from the
node of a Calamite stem. The section has passed through a stem in a
tangential direction, showing the characteristic arrangement of the
vascular bundles _x_, and principal medullary rays _m_. The small
leaf-traces, _t_, _t_, afford another feature characteristic of a
Calamite stem. The portion of stem to the right of the figure has been
slightly displaced, and between this piece and the root _R_, one of
the ubiquitous Stigmarian appendages, _s_, has inserted itself. At _R_
a fairly thick and decorticated root is seen in oblique transverse
section; at the upper end the root tracheids are seen in direct
continuity with the xylem of the stem. In the centre of the root
is the large solid pith surrounded by twelve bluntly pointed xylem
groups, composed in the main of radially disposed scalariform elements
with narrow secondary medullary rays like those in a stem. Between
each xylem group there is a broad medullary ray, which tapers rapidly
towards the outside, and is soon obliterated by the formation of
interfascicular secondary xylem. At _R′_ a portion of another root is
seen in transverse section, and _R″_ the inner part of a single xylem
group is shown more clearly. The solid pith and the absence of carinal
canals are the two most obvious distinguishing features of the roots.
[Illustration: FIG. 92. Tangential section through a node of
_Calamites_, showing a root in organic connection with the stem.
_R_, _R′_. Root (_Astromyelon_) in transverse and oblique section,
_x_, xylem; _m_, primary medullary ray; _t_, leaf-trace; _s_,
Stigmarian appendage.
_R″_, the inner portion of one of the xylem wedges of _R′_ more
highly magnified. Sketched from a section in the Cambridge
Botanical Laboratory Collection.]
As Renault points out, roots of _Calamites_ have been figured by some
writers[698] as examples of stems, but it is usually comparatively
easy to distinguish between roots and stems. On examining the xylem
groups more closely, one notices that the apex of each is occupied by
a triangular group of centripetally-developed primary tracheids, the
narrow spiral protoxylem elements occupying the outwardly directed
apex. The protoxylem apex is usually followed externally by a ray of
one or two radially disposed series of parenchymatous cells. This ray
is not distinguished in fig. 92 _R″_ from the rows of xylem tracheids.
Each xylem group is thus formed partly of centripetal xylem and in part
of secondary centrifugal xylem; the latter is associated with secondary
medullary rays, as in stems, and contains a broader ray (_fascicular
ray_ of Williamson and Scott[699]) immediately opposite each
protoxylem strand. In the roots of recent plants (_e.g._ _Cucurbita_,
_Phaseolus_, &c.) a broad medullary ray is often found opposite the
protoxylem, and such an arrangement is a perfectly normal structure in
roots[700].
Renault has recently described several species of Calamite roots which
he designates by specific names, some of them belonging to stems with
the _Arthropitys_ structure, and others to _Calamodendron_. Some of
the roots figured by the French author have an axial strand of xylem
with 7–15 projecting angles of protoxylem[701]. These he considers
true roots, but the larger specimens with a wide pith he prefers to
regard as stolons. In the latter he mentions the union of the primary
centripetal with the secondary centrifugal wood as a distinguishing
feature. It has been shown, however, that each group of secondary xylem
includes a median ray of parenchyma, and that the whole structure is
essentially that of a root, and not that of a modified stem or stolon.
The organs described by Renault as true roots are probably rootlets,
and as Williamson and Scott have demonstrated, there is every gradation
between the smaller specimens with a solid xylem axis and those with a
large central pith.
It is interesting to note that Renault’s figures of _Calamodendron_
roots show the closest resemblance to those of the subgenus
_Arthropitys_.
_d. Cones._
The occurrence of fossil plants in the form of isolated fragments is a
constant source of difficulty, and is well illustrated by the numerous
examples of strobili which cannot be connected with their parent
stems. We are, however, usually able to recognise Calamitean cones
if the impressions or petrified specimens are fairly well preserved,
but it is seldom possible to correlate particular types of cones with
the corresponding species of foliage-shoots or stems. Palaeobotanical
literature contains numerous illustrations and descriptions of
long and narrow strobili designated by different generic terms such
as _Volkmannia_, _Brukmannia_, _Calamostachys_, _Macrostachya_ and
others; many of these have since been recognised as the cones of
_Calamites_, while some species of _Volkmannia_ have been identified
with _Sphenophyllum_ stems. Before further considering the general
question of Calamite cones, a few examples may be described in detail
as types of fructification which are known to have been borne by
_Calamites_. The examples selected are species of the two provisional
genera _Calamostachys_ and _Palaeostachya_.
The usual form of a Calamite cone is illustrated in fig. 93, which
represents a fertile shoot bearing a few narrow linear leaves of the
_Calamocladus_ type; in the axils of some of these are borne the long
strobili.
[Illustration: FIG. 93. _Calamostachys_ sp. A fertile Calamitean shoot.
From a specimen in the Geological Survey Museum, Jermyn Street,
London. From the Upper Coal-Measures of Monmouthshire (No. 5539).]
_Calamostachys Binneyana_ (Carr.). Figs. 94 and 95.
In 1867 Carruthers[702] gave an account of the structural features
of the species of cones named by him _Volkmannia Ludwigi_ and
_V. Binneyi_, the generic term having been originally used by
Sternberg[703] for some impressions of Carboniferous strobili.
Brongniart[704] in 1849 referred to the various forms of _Volkmannia_
as cones of Asterophyllitean branches, and the latter he regarded
as the foliage-shoots of a Calamite stem. In 1868 Binney[705]
published a description, with several illustrations, of the cones
named by Carruthers _Volkmannia Binneyi_, and referred to them
as the fructification of that type of Calamite stem spoken of
in a previous section of this chapter (p. 311) as _Calamites_
(_Arthropitys_) _communis_ (Binney). This cone is now usually spoken
of as _Calamostachys Binneyana_; the specific name _Binneyana_
being suggested by Schimper[706] in 1869 as more euphonious than
that proposed by Carruthers. In recent years our knowledge of both
_C. Binneyana_ and _C. Ludwigi_ has been considerably extended. We
shall confine our attention in the following account to the former
species[707]. Some excellent figures of the latter species may be found
in Weiss’ Memoir[708] on Calamarieae.
One of the largest examples of _Calamostachys Binneyana_ so far
recorded has a length of 3–4 cm. and a maximum diameter of about 7·5
mm. The axis of the cone bears whorls of sterile leaves or bracts at
equal distances; the linear bracts of each whorl are coherent basally
as a disc or plate of tissue attached at right angles to the central
axis of the cone. The periphery of each of these discs divides up into
twelve linear segments, which curve upwards in a direction more or less
parallel to the strobilus axis, and at right angles to the coherent
portion of each whorl. The manner of occurrence of the whorls is
shown in fig. 94, which has been sketched from a large section in the
Williamson collection. The segments of the successive sterile verticils
alternate with one another, so that in the surface-view of a cone the
long and narrow free bracts appear spirally disposed. Midway between
these alternating sterile verticils there is a series of fertile
appendages, also given off in regular whorls. Each fertile whorl
consists of about half as many members as the segments of a sterile
whorl, and the members of the several fertile whorls are superposed and
not alternate. Each member has the form of a stalk or sporangiophore
given off at right angles from the cone axis; this is expanded distally
into a peltate disc bearing four sporangia attached to its inner face.
In fig. 94 we can only see the basal portions of the sporangiophores,
which are shown in the upper part of the sketch as pointed projections,
_Sp_, from the cone axis. Each sporangiophore is traversed by a
vascular strand which sends off a branch to the base of a sporangium
(fig. 95, A, _t_).
[Illustration: FIG. 94. _Calamostachys Binneyana_ (Carr.) in
longitudinal (radial and tangential) section.
_Sp_, sporangiophores; _S_, sporangia.
(From specimen no. 1022 in the Williamson Collection, British
Museum.)]
The axis of the cone is occupied by a single stele, usually triangular
in section; the stele consists of a solid pith of elongated cells
surrounded by six vascular bundles, two at each corner. A somewhat
irregular gap marks the position of the protoxylem of each strand,
and portions of spiral or annular tracheids may occasionally be
seen in the cavity. These cavities, which may be spoken of as the
carinal canals, disappear at the nodes, where there is a mass of
short reticulately pitted tracheids, as in a Calamite stem. Vascular
bundles pass upwards in an oblique direction from the central stele
to supply the bracts, each of which is traversed by a single strand
of tracheids. The coherent portion, or disc, of each sterile whorl
consists of sclerenchymatous elements towards the upper surface, and
of parenchyma below. The pedicel of the sporangiophores consists of
fairly thick-walled cells traversed by a single vascular strand,
and the peltate distal portions are made up of parenchymatous cells
arranged in a palisade-like form at right angles to the free surface
of the sporangiophores. The vascular strand of the pedicel forks into
two halves just below the peltate head, and these branches again
bifurcate to send a branch to each sporangium. The four sporangia of
each sporangiophore are attached by a narrow band of tissue to the
shield-shaped distal expansion (fig. 95, A).
In a tangential section of a cone, such as the lower portion of fig. 94
and in fig. 95, B, the sporangiophores present the appearance of narrow
stalks (fig. 95, B, _a_) in the middle of a cluster of sporangia,
and the latter appear more or less square in outline. The wall of a
sporangium is made of a single layer of cells (fig. 95, B) which
present a characteristic appearance in surface-view (fig. 95, C), the
thin walls being crossed at right angles by small vertical plates. In
the tangential section of the coherent sterile whorls (fig. 95, B,
_b_ and _b_) the vascular strands are occasionally seen in transverse
section (fig. 95, B, _t_), as they pass outwards to the several free
bracts.
[Illustration: FIG. 95. _Calamostachys Binneyana_ (Carr.).
_A._ A sporangiophore and one sporangium. _t_, vascular bundle. × 45.
_B._ Tangential section showing portions of two sterile discs, _b_,
_b_; a sporangiophore, _a_, with its four sporangia, in two of
which are seen the spores; _t_, vascular bundle. × 35.
_C._ Surface-view of cells of a sporangium wall. × 130.
_D._ Spores and remains of mother-cells. × 130.
(After Williamson and Scott.)]
The spores in _Calamostachys Binneyana_ are all of the same size,
and no macrospores have ever been seen. In well preserved specimens
tetrads of spores may be seen, still enclosed by the wall of the
spore-mother-cell (fig. 95, A and D); and the torn remnants of
the mother-cell sometimes simulate in appearance the elaters of
an _Equisetum_ spore. In surface-view a spore often shows clearly
the three-rayed marking, which is a characteristic feature of
daughter-cells formed in a tetrad from a mother-cell. The spores of
a tetrad are in some cases of unequal size, some having developed
more vigorously than others. This unequal growth and nourishment of
spores is clearly shown in fig. 96, which represents a sporangium
of a heterosporous Calamitean strobilus, _C. Casheana_. Williamson
and Scott[709] have described striking examples of spores in
different stages of abortion, and these authors draw attention to
the importance of the phenomenon from the point of view of the
origin of a heterosporous form of cone. The abortion of some of the
members of a spore-tetrad and the consequent increased nutrition of
the more favoured daughter-cells, might well be the starting-point
of a process, which would ultimately lead to the production of well
defined macrospores and microspores. The young microsporangia and
macrosporangia of recent Vascular Cryptogams such as _Selaginella_,
_Salvinia_ and other heterosporous genera are identical in
appearance[710]; it is not until the spore-producing tissue begins to
differentiate into groups of spores, that the sporangia assume the
form of macrosporangia and microsporangia. During the evolution of
the various known types of pteridophytic plants heterospory gradually
succeeded isospory, and this no doubt occurred several times and in
different phyla of the plant kingdom. In the mature sporangia of some
of the Calamitean strobili we have in the inequality of the spores in
one sporangium an indication of the steps by which heterospory arose;
and in the immature sporangia of some recent genera we are carried back
to a stage still nearer the starting-point of the substitution of the
heterosporous for the isosporous condition.
_Calamostachys Casheana_ Will. Fig. 96.
To Williamson[711] again is largely due the information we possess as
to the structure of this type of Calamitean strobilus. Its special
interest lies in the occurrence of macrospores and microspores in the
same cone.
The strobilus axis agrees in structure with that of _C. Binneyana_,
but in _C. Casheana_ a band of secondary xylem forms the peripheral
portion of the triangular stele. Were any further proof needed of the
now well-established fact that secondary growth in thickness is by no
means unknown as an attribute of Vascular Cryptogams, the co-existence
in the same cone of a cambium layer producing secondary wood and
bark, and cryptogamic macrospores and microspores, affords conclusive
evidence[712]. The dogma accepted by many writers for a considerable
number of years that the power of secondary thickening is evidence
against a cryptogamic affinity, has been responsible for no little
confusion in palaeobotanical nomenclature.
On the axis of _Calamostachys Casheana_ there are borne alternate
whorls of fertile and sterile appendages similar to those in the
homosporous _C. Binneyana_, but they are inclined more obliquely to
the axis of the cone. Macrospores and microspores have been found in
sporangia borne on the same sporangiophore.
[Illustration: FIG. 96. _Calamostachys Casheana_ Will.
A sporangium with macrospores and abortive spores. × 65.
(After Williamson and Scott.)]
The spore-tetrads in the macrosporangia occasionally include aborted
sister-cells like those noticed in _C. Binneyana_; this phenomenon
is well illustrated by the unequally nourished spores in the
sporangium of fig. 96, but no such starved spores have been found
in the microsporangia. In this cone, then, heterospory has become
firmly established, but the occurrence of undersized spores in a
macrospore-tetrad leads us back to the probable lines of development of
heterospory, which are seen in _C. Binneyana_ at their starting-point.
In the two species of strobili which have been described,
_Calamostachys Binneyana_ and _C. Casheana_, the sporangiophores or
sporophylls are given off at right angles to the axis, and midway
between the sterile whorls. These are two of the most important
distinguishing features of the Calamitean cones included under the
generic term _Calamostachys_. In another form of cone, which also
belongs to Calamitean stems, the sporangiophores arise in the axil
of the sterile leaves, and are inclined obliquely to the axis of the
cone. To this type the generic name _Palaeostachya_ has been applied
by the late Prof. Weiss[713] of Berlin. The portion of a cone shown in
fig. 97 shows the arrangement of the sterile and fertile appendages
characteristic of _Palaeostachya_.
[Illustration: FIG. 97. _Palaeostachya pedunculata_ Will. Part of a
cone, × 3. (After Weiss.)]
It is practically impossible to distinguish between cones of the
_Calamostachys_ and _Palaeostachya_ type in the case of imperfectly
preserved impressions; indeed we cannot assume that all long and narrow
cones with spirally disposed verticillate bracts are Calamitean. We
must have the additional evidence of internal structure or of the
direct association of the cones with Calamitean foliage.
_Palaeostachya vera_ sp. nov. Fig. 98.
In 1869 Williamson[714] described a fragment of a strobilus which
showed certain anatomical features indicative of a close relationship
or even identity with _Calamites_. Some years later[715] a much more
perfect example was obtained from the Coal-Measures of Lancashire,
and the additional evidence which it afforded definitely confirmed
the earlier views of Williamson. The cone was more fully described by
Williamson in 1888, as “the true fruit of _Calamites_.” It is clearly a
form of Weiss’ genus _Palaeostachya_; Williamson and Scott[716] refer
to it in their Memoir as _Calamites pedunculatus_. It is preferable,
however, to retain the generic designation _Palaeostachya_ for cones
of this type. As the name _P. pedunculata_ has previously been adopted
by Weiss[717] for a cone figured by Williamson[718] in 1874, and
afterwards referred to by that author in writing as _P. pedunculata_,
it is proposed to substitute the specific name _vera_; this specific
name being chosen with a view to put on record the fact that it was
this type of cone that Williamson first proved to be the _true_
fructification of the Calamite.
The axis of _P. vera_ is practically identical in structure with a
Calamitean twig. There is a hollow pith in the centre of the stele
surrounded by a ring of 16–20 collateral bundles, each of which is
accompanied by a carinal canal as in a vegetative shoot. As the
pedicel of the strobilus passes into the cone proper it undergoes some
modification in structure, but retains the characteristic features of
a Calamite. The diagrammatic longitudinal section of fig. 98, which is
copied from a drawing by Williamson[719], shows the broadening of the
vascular strands at the nodes, and here and there a carinal canal is
seen internal to the wood.
The axis of the cone bears whorls of bracts at right angles to the
central column. Each whorl consists of about 30–40 segments coherent
basally into a disc of prosenchymatous and parenchymatous tissue.
The free linear bracts curve sharply upwards from the periphery of
the disc, approximately parallel to the axis of the cone. From each
of these sterile whorls there are given off 16–20 long and slender
obliquely-inclined sporangiophores, _sp_, which arise from the upper
surface of the disc close to the axis. Each sporangiophore no doubt
bore four sporangia, _S_, containing spores of one size,—about ·075
mm. in diameter. The specimens of _Palaeostachya vera_ so far obtained
do not show the actual manner of attachment of the sporangia, but more
complete examples of other species of _Palaeostachya_[720] enable us to
assume with certainty that the sporangiophores terminated in a distal
peltate expansion bearing four sporangia on its inner face.
[Illustration: FIG. 98. Diagrammatic longitudinal section of
_Palaeostachya vera_, sp. nov. _S_, _S_, _S_, sporangia; _x_,
xylem; _sp_, sporangiophore. (After Williamson.)]
A transverse section of the axis of the cone in the region of the
sterile and fertile appendages shows the vascular bundles arranged
in pairs. In a section through the peduncle of the cone, below the
lowest whorl of bracts, the bundles of the stele are situated at equal
distances apart. The cortical tissue of the peduncle is traversed by
a ring of large canals[721] similar to the vallecular canals of an
Equisetum stem.
Isospory is not a constant characteristic of _Palaeostachya_; some
forms have been found with macrospores and microspores[722].
_Other Calamitean cones, and examples illustrating the
connection between Cones and Vegetative Shoots._
It would be out of place in an introduction to Palaeobotany to attempt
an exhaustive account of the various cones which were probably borne
by Calamitean plants, but there are a few general points to which the
attention of the student should be directed. The examples dealt with
in the foregoing description illustrate the fact, that plants included
under the comprehensive genus _Calamites_ bore cones possessing
distinct morphological features. There are, however, other types of
strobili which have been found in organic connection with _Calamites_;
and some of these must be taken into account in dealing with
Calamarian plants. The genera _Volkmannia_, _Brukmannia_, _Huttonia_,
_Macrostachya_, in addition to _Calamostachys_ and _Palaeostachya_ and
others, have been applied by different writers to Calamitean cones. As
Solms-Laubach[723] has suggested, it is wiser to discard _Volkmannia_
and _Brukmannia_, as they have been made to do duty for cones of widely
different forms. It is better to adhere to the provisional generic
names used by Weiss, as they enable us to conveniently systematise the
various Calamarian strobili.
The following classification may be given of the better known cones,
some of which we are able to describe in considerable detail, while
others are still very imperfectly known. We have good evidence that
all these strobili were borne by vegetative shoots of the type of
_Calamites_, _Calamocladus_ or _Annularia_.
1. _Calamostachys_[724] (including _Paracalamostachys_ and
_Stachannularia_).
Cones long and narrow, consisting of a central axis bearing alternate
whorls of sterile and fertile appendages, the latter having the form of
sporangiophores attached at right angles to the axis midway between the
sterile verticils, and bearing four sporangia on the inner face of a
peltate distal expansion.
_Calamostachys Binneyana_ Schimp., _C. Ludwigi_ Carr., _C. Casheana_
Will., may be referred to as examples of this type of cone; also
some of the strobili described by different authors as species of
_Volkmannia_[725], _Brukmannia_[726], &c.
Although one cannot make out the detailed structure of a Calamite cone
in the absence of internal structure, it is often possible to recognise
the essential features in specimens preserved in ironstone nodules,
such as those from Coalbrook Dale in Shropshire, or by carefully
examining the carbonised impressions on shale under a simple microscope.
Weiss applies the term _Paracalamostachys_[727] to cones of the
_Calamostachys_ form, but in which the manner of attachment cannot be
made out. Such a cone as that of fig. 93 should probably be referred to
this sub-type of _Calamostachys_ in the absence of definite evidence as
to the position of the sporangia.
Another term _Stachannularia_, originally used by Weiss as a
genus[728], was afterwards[729] applied to cones of the same general
type as _Calamostachys_, in which the sporangiophores have the form of
thorn-like structures bearing on their upper side a lamellar expansion.
There is however some doubt as to the correct interpretation of the
features associated with cones included in _Stachannularia_; for an
account of such forms reference must be made to the writings of Weiss,
Renault[730], Solms-Laubach[731] and others[732].
_Calamostachys_ cones have been found in organic union with branches
bearing leaves of the _Annularia_ type, also with _Calamocladus_
foliage, and the branches bearing such cones have been found in
actual connection with Calamitean stems. The association of cones and
vegetative stems and branches is shown in tabular form on p. 363.
2. _Palaeostachya[733]._
In this genus the general habit agrees with that of _Calamostachys_,
and in imperfectly preserved specimens it may be impossible to
discriminate between _Calamostachys_ and _Palaeostachya_. The latter
form is characterised by the attachment of the sporangiophores in the
axil of the sterile bracts, or immediately above them, as shown in
figs. 97 and 98.
EXAMPLES. _Palaeostachya vera_ sp. nov., _P. pedunculata_ Will. afford
examples of this form of strobilus. The genus _Palaeostachya_ includes
several species previously described under the genus _Volkmannia_[734].
Strobili of this generic type are known in organic association with
Annularian branches, as well as with _Calamocladus_ and _Calamites_.
3. _Macrostachya._
This generic name was originally applied by Schimper[735] to certain
forms of Calamitean stems, of the type afterwards referred to the
sub-genus _Calamitina_ by Weiss, bearing long and thick cones. The name
is, however, more appropriately restricted to strobili, which differ
from the two preceding genera in their greater length (14–16 cm.) and
in the more crowded and imbricating whorls of bracts. The internodes of
the cones are very short, and each whorl of bracts consists of about
20 coherent members separated at the periphery of the disc into short
pointed teeth. The internal structure of _Macrostachya_ has not been
satisfactorily determined. An account by Renault[736] of a petrified
specimen does not present a very clear idea as to the structural
features of this form of Calamitean strobilus.
THE ASSOCIATION OF CALAMITEAN VEGETATIVE SHOOTS AND CONES.
Strobilus | Foliage-shoot | Stem
------------------------+----------------------+-------------------------
_Calamostachys_ | _Annularia ramosa_ | _Calamites ramosus_
(_Stachannularia_) | Weiss | Artis
_ramosa_ Weiss[737] | |
| |
_C._ (_Stachannularia_) | _A. sphenophylloides_| Stem bearing verticils of
_calathifera_ | Zenk. | long and narrow
Weiss[738] | | leaves[739]. Probably a
| | young _Calamites_
| |
_C._ (_Stachannularia_) | _A. stellata_ |
_tuberculata_ (Stern.)| (Schloth.)[740] | _Calamites_ sp.[741]
|(_A. longifolia_ |
| Brongn.) |
| |
_C. Solmsi_[742] Weiss | _Calamocladus_ sp. | _Calamites_ (_Calamitina_)
| | sp.
| |
_C. longifolia_ | _Calamocladus_ sp. |
(Stern.)[743] | |
| |
_Palaeostachya_ | _Calamocladus_ |
_pedunculata_ | |
Will.[744] | |
| |
_P. arborescens_ | | _Calamites_
(Stern.)[745] | | (_Stylocalamites_)
| | _arborescens_ (Stern.)
| |
_Macrostachya_[746] | _Calamocladus_ | _Calamites_ (_Calamitina_)
| _equisetiformis_ | sp.
| (Schloth.) |
[Sidenote: HUTTONIA.]
The generic name _Huttonia_, suggested by Sternberg[747] in 1837,
is applied to cones which closely resemble _Macrostachya_ in habit,
but differ—so far as our scanty knowledge enables us to judge—in the
arrangement of the members. The student must refer to Weiss[748],
Solms-Laubach[749] and other writers[750] for a further account of
these types, and of another rare and little-known form of cone, called
by Weiss Cingularia[751].
Macrostachyan cones have been found attached to stems of _Calamites_
which are included in the sub-genus _Calamitina_ (p. 367). The larger
size of _Macrostachya_ as a distinguishing feature is not always a
safe test; some cones which belong to _Palaeostachya_ [_e.g._ _P.
arborescens_ Sternb.] and _Calamostachys_ (_e.g._ _C. Solmsi_) are much
thicker and larger than the majority of species of these two genera.
It would appear from the examples selected to illustrate the
connection between strobili and vegetative shoots, that the
_Annularia_ type of branch usually bears cones which conform to the
genus _Calamostachys_ (_Stachannularia_); while the Asterophyllitean
branches—_Calamocladus_—are associated with _Palaeostachya_ and
_Macrostachya_. But this rule is not constant, and we are not in
a position to speak of cones of a particular type as necessarily
characteristic of definite types of Calamitean shoots.
• • • • •
Although it is admitted by the great majority of Palaeobotanists that
the Calamites were all true Vascular Cryptogams, the older view that
some members of the Calamarieae are gymnospermous has not been given up
by Renault[752]. This observer has recently described some seeds which
he believes were borne by Calamitean stems; he admits, however, that
no undoubted female cones of _Calamodendron_ have so far been found.
In view of the unsatisfactory evidence on which Renault’s opinion is
based, we need not further discuss the questions which he raises.
[The following specimens in the Williamson Cabinet in the British
Museum, may be found useful in illustration of the structure of
_Calamites_.
_Stems._ (i. _Arthropitys._) _Young twigs and small branches_ 1, 2,
6, 10, 14, 19, 116*, 1002, 1007, 1020.
_Older stems_ (_transverse sections_) 15–17, 62, 77–87, 115 _a_,
117*, 118*, 120, 122*–124*, 1933 A, 1934, 1941.
(_Tangential sections_) 20, 24, 26, 37, 38, 49, 90, 91, 130, 138,
1937, 1943.
(_Radial sections_) 20, 20 A, 21, 22, 48, 65–68, 83–91, 137*,
138*, 1937.
(ii. _Arthrodendron)_ 36, 37, 38, 52, 54.
_Roots._ 1335, 1347, 1350, 1356.
_Strobili._ i. _Calamostachys Binneyana._ 991, 996, 997, 1000, 1003,
1005, 1007, 1008, 1011, 1013, 1016, 1017, 1022, 1023, 1037 A,
1043.
ii. _C. Casheana._ 1024, 1025, 1587, 1588.
iii. _Palaeostachya vera._ 110, 1564, 1567, 1569, 1579, 1583.]
III. Pith-casts of Calamites.
A. _Calamitina._ B. _Stylocalamites._ C. _Eucalamites._
Palaeobotanical literature contains a large number of species of
_Calamites_ founded on pith-casts alone. Many of these so-called
species are of little or no value botanically, but while we may admit
the futility of attempting to recognise specific types in the same
sense as in the determination of recent plants, it is necessary to
pay attention to such characters as are likely to prove of value for
descriptive and comparative purposes. From the nature of the specimens
it is clear that many of the differences may be such as are likely to
be met with in different branches of the same species, while in others
the pith-casts of distinct species or genera may be almost identical.
The most striking differences observable in Calamite casts are in the
character of the internodes, the infranodal canals, the number and
disposition of branch-scars, and other surface features. Occasionally
it is possible to recognise certain anatomical characters in the coaly
layer which often surrounds a shale- or sandstone-cast, and the surface
of a well preserved cast may give a clue to the nature of the wood in
the faint outlines of cells which can sometimes be detected on the cast
itself[753]. The breadth of the carbonaceous envelope on a cast has
been frequently relied on by some writers as an important character. It
has been suggested[754] that we may arrive at the original thickness
of the wood of a stem by measuring the coaly layer and multiplying
the breadth by 27; the explanation being that a zone of wood 27 mm. in
thickness is reduced in the process of carbonisation to a layer 1 mm.
thick.
The breadth of the coal on the same form of cast may vary considerably;
on this account, and for various other reasons, such a character can
have but little value. Our knowledge of anatomy may often help us
to interpret certain features of internal casts and to appreciate
apparently unimportant details. One occasionally notices that a
Calamite pith-cast has large infranodal canals, and in some specimens
each internodal ridge may be traversed by a narrow median line or small
groove; large infranodal canal casts suggest the type of stem referred
to the subgenus _Arthrodendron_, and the median line on the ridges may
be due to bands of hard tissue in each principal medullary ray.
In attempting to identify pith-casts the student must keep in view
the probable differences presented by the branching rhizome, the main
aerial branches and the finer shoots of the same individual. The long
internodal ridges of some casts may be mistaken for the parallel veins
of such a leaf as _Cordaites_, a Palaeozoic Gymnosperm, if there are
no nodes visible on the specimen. The fossil figured by Lindley and
Hutton[755] as _Poacites_, and regarded by them as a Monocotyledon,
is no doubt a portion of a Calamite with very long internodes. An
interesting example of incorrect determination has recently been
pointed out by Nathorst[756] in the case of certain casts from Bear
Island, originally described by Heer as examples of _Calamites_; the
vertical rows of leaf-trace casts on a _Knorria_ were mistaken for the
ribs of a Calamite stem. The specimens in the Stockholm Museum fully
bear out Nathorst’s interpretation. The undulating course of internodal
ridges and grooves is not in itself a character of specific value. If
a Calamite stem were bent slightly, the wood and medullary-ray tissues
on the concave side might adapt themselves to the shortening of the
stem by becoming more or less folded, and a cast of such a stem would
show undulating ridges and grooves on one side and straight ones on the
other[757].
A convenient classification of Calamite casts was proposed by Weiss
in 1884, founded chiefly on the number and manner of occurrence of
branch-scars—or rather branch-depressions—on the surface of pith-casts.
Weiss[758] recognised the imperfection of his proposed grouping,
and Zeiller[759] has also expressed reasonable doubts as to the
scientific value of such group-characters. Weiss instituted three
subgenera—_Calamitina_, _Eucalamites_ and _Stylocalamites_, which are
made use of as convenient terms in descriptive treatment of Calamite
casts. The following account of a few of the more typical casts may
serve to illustrate the methods employed in the description of such
specimens; the synonomy given for the different species is not intended
to be complete, but it is added with a view to drawing attention to the
necessity for careful comparison in systematic work.
A. _Calamitina._
[Illustration: FIG. 99. _Calamites_ (_Calamitina_) _Göpperti_ (Ett.).
_b_, branch scars.
From a specimen in the Manchester Museum, Owens College. ¼ nat.
size.]
This sub-genus of _Calamites_, as instituted by Weiss[760], includes
Calamitean stems or branches, which are characterised by the periodic
occurrence of branch-whorls usually represented by fairly large oval
or circular scars just above a nodal line (figs. 99, 100 and 101).
The branch-scars may form a row of contiguous discs, or a whorl may
consist of a smaller number of branches which are not in contact
basally. A form described by Weiss as _C. pauciramis_, Weiss[761],
has only one branch in each whorl, as represented by a single large
oval scar on some of the nodes of the cast. A stem of this form is by
no means a typical _Calamitina_, but it serves to show the existence
of forms connecting Weiss’ sub-genera _Calamitina_ and _Eucalamites_.
The number of internodes and nodes between the branch whorls varies
in different specimens, and is indeed not constant in the same
plant. Each nodal line bears numerous elliptical scars which mark
the points of attachment of leaves; each branch-whorl is situated
immediately above a node, and in some forms this nodal line pursues
a somewhat irregular course across the stem, following the outlines
of the several branch-scars[762]. The surface of the internodes is
either perfectly smooth or it is more frequently traversed by short
longitudinal ridges or grooves probably representing fissures in the
bark of the living stem; these are indicated by lines in fig. 99 and
by elongated elliptical ridges in fig. 101. On young stems the leaves
are occasionally found in place, as for example in an example figured
by Weiss[763] (_C. Göpperti_), or we may have leaf-verticils still in
place in much older and thicker branches[764] (cf. fig. 85, p. 330).
It occasionally happens that the bark of _Calamitina_ stems has been
preserved as a detached shell[765] reminding one of the sheets of Birch
bark often met with in forests, the separation being no doubt due in
the fossil as in the recent trees to the manner of occurrence of the
cork-cambium.
In a few cases branches have been preserved still attached to a stem
or branch of higher order; examples of such specimens are figured
by Lindley and Hutton[766], Stur[767], and others. Grand’Eury[768]
has given an idealised drawing of a typical _Calamitina_ bearing a
whorl of branches with the foliage and habit of _Asterophyllites
equisetiformis_. The specimen on which this drawing is based is in the
Natural History Museum, Paris; it shows Asterophyllitean branches in
organic connection with a Calamitean stem, but it is not quite clear if
the stem is a true _Calamitina_. A large drawing of this interesting
specimen is given by Stur[769] in his monograph on _Calamites_, also
a smaller sketch by Renault[770] in his _Cours de botanique fossile_.
Similar branches of the _Asterophyllites_ type attached to an undoubted
_Calamitina_ are figured also by Lindley and Hutton. There is, in
short, good evidence that stems of this sub-genus bore branches with
Asterophyllitean shoots.
The wood of stems of the Calamitina group of _Calamites_, in some
instances at least, was of the _Arthropitys_ type; this has been
shown to be the case in some French specimens from the Commentry
coal-field[771] and in others described by Stur[772]. The pith-casts
of _Calamitina_ are characterised by comparatively short internodes
separated by deep nodal constrictions, as shown in fig. 100. From
Permian specimens from Neu Paka in Bohemia, described by Stur[773], we
learn that there were the usual Calamite diaphragms bridging across
the wide pith-cavity at each node. Such a cast as that shown in fig.
100 is often referred to as _Calamites approximatus_ Brongn.; the
length of the internodes and the periodic occurrence of branch-scars
in the form of circular or oval depressions along a nodal line
enable us to recognise the _Calamitina_ casts. Weiss[774] points out
that in pith-casts of this form the branch-scars occur on the nodal
constriction, and not immediately above the node as is the case on
the surface of a typical _Calamitina_. This distinction is however
of little or no value; the point of attachment of a branch may be
above the nodal line, while on the pith-cast of the same stem the
point of origin of the vascular bundles of the branch is on the nodal
constriction[775].
The specimen shown in fig. 100 illustrates the appearance of a
_Calamitina_ cast. There is a verticil of branch-scars on the lowest
nodal constriction; on the right of the pith-cast the broad band of
wood is faintly indicated by the smooth surface of the rock (_x_).
Other examples demonstrating the existence of a broad woody cylinder in
_Calamitina_ stems have been figured by Weiss[776] and other writers,
and some good examples may be seen in the British Museum.
[Illustration: FIG. 100. _Calamites_ (_Calamitina_) _approximatus_
Brongn. Lower Coal-Measures of Ayrshire.
_x_, impression of the wood.
(From a specimen in the collection of Mr R. Kidston.)]
We have so far noticed the connection of certain forms of pith-casts
(_e.g._ _Calamites approximatus_), and Asterophyllitean shoots with
stems of the sub-genus _Calamitina_.
As regards the strobili our knowledge is far from satisfactory.
Stur[777] figures some fertile branches bearing long and narrow
strobili, either _Palaeostachya_ or _Calamostachys_, in close
association with _Calamitina_ stems, and Renault and Zeiller[778] give
illustrations of the association of _Calamitina_ stems with large
strobili of the _Macrostachya_ form.
Before Weiss proposed the term _Calamitina_, various authors had
figured this form of Calamite under a distinct generic name (_e.g._
_Hippurites_ of Lindley and Hutton[779], _Cyclocladia_[780],
_Macrostachya_[781], &c.). Stems of this type have also been described
by more recent writers under different names, and considerable
confusion has been caused by the use of numerous generic designations
for forms of _Calamitina_. Some small fragments of _Calamitina_ stems
were described by Salter[782] in 1863 as portions of a new species
of the Crustacean _Eurypterus_ (_E. mammatus_). In 1869 Grand’Eury
proposed the generic name _Calamophyllites_[783] for stems bearing
verticils of _Asterophyllites_ shoots; his description of such stems
agrees with Weiss’ _Calamitina_, but as Grand’Eury’s name is used in
a narrower sense as implying a connection with _Asterophyllites_,
it is more convenient to adopt Weiss’ term in spite of the priority
of _Calamophyllites_. In the _Fossil flora of Commentry_ we find
some flattened stems of the _Calamitina_ type described under
different generic names, as _Arthropitys approximatus_[784] and as
_Macrostachya_[785].
The determination of distinct species of the sub-genus _Calamitina_
is rendered almost hopeless by the variation in the different
branches of the same individual, and by the difficulty of connecting
surface-impressions with casts of the pith-cavity.
A typical example of the _Calamitina_ type of _Calamites_ was figured
by Sternberg[786] in 1821 as _Calamites varians_. This has been
adopted by Weiss[787] as a comprehensive species including several
different ‘forms’ of stems, which differ from Sternberg’s fossil in
such points as the number of nodes between the branch-whorls and
the number of branches in each whorl. The result of this system of
nomenclature is the separation of portions of one specific type under
different form-names. It must be clearly recognised that accurate
specific diagnoses are practically impossible when we have to deal with
fragments of plants, some of which are mere pith-casts, while others
show the surface features. The specimen represented in fig. 99 agrees
with a stem described by Ettingshausen[788] in 1855 as _Calamites
Göpperti_, and as a matter of convenience a member of the _Calamitina_
group showing such characters may be referred to as _Calamites_
(_Calamitina_) _Göpperti_ (Ett.). The following list, which includes a
few synonyms of this form, may suffice to illustrate the difficulties
connected with accurate systematic determinations.
_Calamites_ (_Calamitina_) _Göpperti_ (Ett.). Fig. 99.
1855. _Calamites Göpperti_, Ettingshausen[789].
1869. _Calamites_ (_Calamophyllites_) _Göpperti_, Grand’Eury[790].
1874. _Cyclocladia major_, Feistmantel[791].
1874. _Calamites verticillatus_, Williamson[792].
1876. _Calamitina Göpperti_, Weiss[793].
1884. _Calamites_ (_Calamitina_) _varians abbreviatus_, Weiss[794].
1884. _Calamites_ (_Calamitina_) _varians inconstans_, Weiss[795].
1887. _Calamites Sachsei_, Stur[796].
1888. _Calamophyllites Göpperti_, Zeiller[797].
This species is characterised by the smooth bark, which may be
traversed by a few irregular longitudinal fissures; most of the nodes
bear a series of small leaf-scars, and at fairly regular intervals a
node is immediately succeeded by a circle of contiguous branch-scars,
8–12 in a whorl. The pith-cast of this type of stem has short ribbed
internodes separated by rather deep nodal constrictions; the
branch-whorls being represented by a series of pits on the nodal
constrictions recurring at corresponding intervals to the whorls of
branch-scars on the surface of the stem. Leaves narrow and linear
in form, like those on Asterophyllitean branches, are occasionally
associated with this type of stem.
[Illustration: FIG. 101. _Calamites_ (_Calamitina_) sp. From a specimen
in the British Museum. (After Carruthers.) Slightly reduced.]
The fragment of a _Calamitina_ stem shown in fig. 101 is the
counterpart of a specimen originally figured by Steinhauer[798] in 1818
as a species of _Phytolithus_. This may be specifically identical with
_C. Göpperti_; but it is better to speak of so small a specimen as
merely one of the _Calamitina_ stems, to be compared with _Calamites_
(_Calamitina_) _Göpperti_. The specimen measures 14·5 cm. in length and
7 cm. in breadth.
The form of pith-cast represented in fig. 100 is no doubt that of
one of the _Calamitina_ species, but as it is seldom possible to
determine the connection between such casts and the particular
species of stems to which they belong, they are often referred to as
_Calamites_ (_Calamitina_) _approximatus_ (Brongn.). The specimen of
which fig. 100 is a photograph was originally described and figured
by Mr Kidston[799] from the lower Coal-Measures of Ayrshire. Both
_Calamites_ (_Calamitina_) _Göpperti_ (Ett.) and _C._ (_Calamitina_)
_approximatus_ (Brongn.) are recorded from the Transition, Middle and
Lower Coal-Measures[800].
B. _Stylocalamites._
In the members of this sub-genus the branch-scars are either irregular
in their occurrence or absent. In some Calamites the branch-scars are
very few and far between, and other species appear to have been almost
without branches; pith-casts of such stems may be referred to the
sub-genus _Stylocalamites_[801].
An exceedingly common Calamitean cast, _C. Suckowi_ Brongn. (fig. 82)
affords a good illustration of this type of stem. In the specimen shown
in fig. 82 we have a cast of a rhizome, which is rather exceptional in
showing three branches in connection with one another. The appearance
of the fossil suggests a rhizome, rather than an aerial shoot, bearing
lateral branches; the narrowing of the branches and the rapid decrease
in the length of the internodes towards the point of attachment being
features associated with rhizomes rather than with aerial branches.
_Calamites_ (_Stylocalamites_) _Suckowi_, Brongn. Fig. 82.
1818. _Phytolithus sulcatus_, Steinhauer[802].
1825. _Calamites decoratus_, Artis[803].
1828. _Calamites Suckowi_, Brongniart[804].
1833. _Calamites cannaeformis_, Lindley and Hutton[805].
For more complete lists of synonyms of this species reference should
be made to Kidston[806], Zeiller[807], and other authors.
Casts of _Calamites Suckowi_ are characterised by flat or slightly
convex internodal ridges separated by shallow depressions, the ridges
are rounded at the upper end of each internode, and usually bear
circular casts of infranodal canals. There are some unusually large
examples of casts of this species in the British Museum from the
Radstock Coal-Measures; one of these has a length of 81 cm., and a
diameter of 27 cm. Specimens are not infrequently found with verticils
of slender roots in close proximity to the nodes of the cast; figures
of such root-bearing casts have been given by Lindley and Hutton[808],
Weiss[809], and other authors.
Renault[810] has drawn attention to the thinness of the layer of
wood which is often associated with large casts of _C. Suckowi_; he
concludes that the stems must have possessed little or no secondary
wood. In a more recent work by Grand’Eury[811] _Calamites Suckowi_
is spoken of as having had wood of the _Calamodendron_ type, but as
wood of this kind has not been found in England, it is suggested that
the plant may not have assumed an arborescent habit until late in
the Coal-Measure period. During the Lower and Middle Coal-Measures,
at which horizon it commonly occurs in England, it may have been
herbaceous. This suggestion has little to commend it; the close
agreement between _C. Suckowi_ from English and French localities
points to a plant of the same form, and we have no satisfactory
evidence as to any difference in stem-structure in the two cases.
Stur has figured a specimen of a Calamite cast, which he compares with
_C. Suckowi_, surrounded by a band of silicified wood apparently of the
_Arthropitys_ type. From this and other facts it would appear probable
that some of the English stems with the _Arthropitys_ structure
possessed casts referable to _Calamites_ (_Stylocalamites_) _Suckowi_.
We are not in a position to speak with confidence as to the strobili
of _C. Suckowi_, but Stur adduces evidence in support of a connection
between this species of Calamite and certain Asterophyllitean branches
(_Calamocladus equisetiformis_) bearing Calamostachyan cones. He does
not appear to have found the foliage-shoots and stems in organic
contact, but draws this conclusion from the association of the fertile
branches and stems in the same rocks[812]. This species is abundant in
the Lower, Middle and Upper Coal-Measures; it has also been recorded
from the Millstone Grit[813].
C. _Eucalamites._
In this sub-genus branch-scars occur on every node; the scars never
form a contiguous whorl as in _Calamitina_, but there may be from 3
to 10 on each node. The scars of successive nodes often alternate in
position, and thus form more or less regular vertical series as shown
in fig. 102. The most obvious feature as regards the arrangement of
the branch-scars is their spiral disposition on the surface of the
pith-cast. The internodes are fairly uniform in length, and there is
no periodic recurrence of narrower internodes as in _Calamitina_. From
an examination of specimens of _Eucalamites_ in which the pith-cast
is covered with a coaly layer representing the carbonised remains of
the wood and cortex, it would appear that the surface of the stems
was practically smooth. The coaly investment on _Eucalamites_ casts
varies considerably in thickness[814]; it is very unsafe to make use
of the thickness of this layer as a test of the breadth of the wood in
Calamitean stems. The branch-scars as seen in a surface-view of a stem
are situated a little above the nodal lines, while depressions on the
pith-casts occur in the slight nodal constriction or immediately above
it. Small leaf-scars have been described as occurring on the nodes
between the branch-scars in specimens showing the surface features[815].
The species long known as _Calamites cruciatus_ Sternb. is usually
taken as the type of the sub-genus _Eucalamites_. Weiss[816] has
subdivided this species into several ‘forms,’ which he bases on the
number of branch-scars on each node and on other characters; a more
extended subdivision of _C. cruciatus_ has recently been made by
Sterzel[817], who admits the impossibility of separating the specific
forms by means of the data at our disposal, but for purposes of
geological correlation he prefers to express slight differences by
means of definite ‘forms’ or varieties. The more comprehensive use
of the specific name _cruciatus_ as adopted by Zeiller in his _Flore
de Valenciennes_[818] is, I believe, the better method to adopt.
The specimen shown in fig. 102 affords a good example of a typical
_Calamites cruciatus_, it was found in the Middle Coal-Measures near
Barnsley, Yorkshire.
[Illustration: FIG. 102. _Calamites_ (_Eucalamites_) _cruciatus_,
Sternb. From a specimen in the Barnsley Museum, Yorkshire. ½ nat.
size.]
_Calamites_ (_Eucalamites_) _cruciatus_ (Sternb.). Fig. 102.
1826. _Calamites cruciatus_, Sternberg[819].
1828. _Calamites cruciatus_, Brongniart[820].
1831. _Calamites alternans_, Germar and Kaulfuss[821].
1837. _Calamites approximatus_, Lindley and Hutton[822].
1877. _Calamodendrofloyos cruciatus_, Grand’Eury[823].
1878. _Calamodendron cruciatum_, Zeiller[824].
1884. _Calamites_ (_Eucalamites_) _cruciatus ternarius_, Weiss[825].
1884. „ „ „ _quaternarius_, Weiss[825].
1884. „ „ „ _genarius_, Weiss[825].
1884. „ „ _multiramis_, Weiss[825].
1888. _Calamites_ (_Calamodendron_) _cruciatus_, Zeiller[826].
This species occurs in the Upper, Middle and Lower Coal-Measures[827].
The casts of the _cruciatus_ type have been found associated
with wood possessing the structural features of the sub-genus
_Calamodendron_[828], but our knowledge of the structure of the stem,
and of the fertile branches of _C. cruciatus_ is very imperfect. A
restoration of _Calamites_ (_Eucalamites_) _cruciatus_ is given by
Stur[829] in his classic work on the Calamites, but he does not make
quite clear the supposed connection with the stems and the fertile
shoots of the Asterophyllites type[830] which he describes. Another
member of the _Eucalamites_ group, which is better known as regards its
foliage-shoots, is _Calamites ramosus_, a species first described by
Artis[831] in 1825. Stems of this species have been found in connection
with the branches and leaves of the _Annularia_[6] type, bearing
_Calamostachys_[832] cones. In all probability pith-casts included in
the sub-genus _Eucalamites_ belonged to stems with foliage-shoots and
probably also with cones of more than one form.
[Sidenote: NOMENCLATURE.]
In the above account of a few common pith-casts it has been pointed
out that there is occasionally satisfactory evidence for connecting
certain casts with wood of a particular structure, and with sterile and
fertile foliage-shoots of a definite type. It is, however, impossible
in many cases to recognise with any certainty the leaf-bearing
branches and strobili of the different casts of _Calamites_; it is
equally impossible to determine what type of pith-cast or what type
of foliage-shoots belongs to petrified stem-fragments in which it is
possible to investigate the microscopical features. The scattered
and piece-meal nature of the material on which our general knowledge
of Calamitean plants is based, necessitates a system of nomenclature
which is artificial and clumsy; but the apparent absurdity of attaching
different names to fragments, which we believe to be portions of the
same genus, is of convenience from the point of view of the geologist
and the systematist. As our material increases it will be possible to
further simplify the nomenclature for Calamarian plants, but it is
unwise to allow our desire for a simpler terminology to lead us into
proposals which are based rather on suppositions than on established
fact. If it were possible to discriminate between pith-casts of stems
having the different anatomical characters designated by the three
sub-genera, _Arthropitys_, _Arthrodendron_ and _Calamodendron_, the
genus _Calamites_ might be used in a much narrower and probably more
natural sense than that which we have adopted. The tests made use
of by some authors for separating pith-casts of _Calamodendron_
and _Arthropitys_ stems do not appear to be satisfactory; we want
some term to apply to all Calamitean casts irrespective of the
anatomical features of the stems, or of the precise nature of the
foliage-branches. As used in the present chapter, _Calamites_ stands
for plants differing in certain features but possessing common
structural characters, which must be defined in a broad sense so as
to include types which may be worthy of generic rank, but which for
convenience sake are included in a comprehensive generic name. The
attempts to associate certain forms of foliage with _Arthropitys_ on
the one hand and with _Calamodendron_ on the other, cannot be said
to be entirely satisfactory; we still lack data for a trustworthy
diagnosis of distinct Calamarian genera which shall include external
characters as well as histological features. If we restricted the
genus _Calamites_ to stems with an _Arthropitys_ structure and an
Asterophyllitean foliage, we should be driven into unavoidable error.
Within certain limits it is possible to distinguish generically or
even specifically between petrified branches, and we already possess
material enough for fairly complete diagnoses founded on internal
structure; but it is not possible to make a parallel classification
for pith-casts and foliage-shoots. For this reason, and especially
bearing in mind the importance of naming isolated foliage-shoots and
stem-casts for geological purposes, I believe it is better to admit
the artificially wide application of the name _Calamites_, and to
express more accurate knowledge, where possible, by the addition of a
subgeneric term. In dealing with distinctions exhibited by Calamitean
stems it may be advisable to make use of specific names, but we
must keep before us the probability of the pith-cast and petrified
stem-fragment of the same plant receiving different specific names.
If the structural type is designated by a special sub-genus, this
will tend to minimise the anomaly of using more than one binominal
designation for what may be the same individual.
[Sidenote: CALAMITES AND EQUISETUM.]
The following summary may serve to bring together the different generic
and subgeneric terms which have been used in the foregoing account of
_Calamites_.
CALAMITES.
Subgenera having reference to the method of branching as seen in casts
or impressions of the stem-surface or in pith-casts.
_Calamitina_,
_Eucalamites_,
_Stylocalamites_.
Subgenera founded on anatomical characters in stems and branches.
_Arthropitys_,
_Calamodendron_,
_Arthrodendron_ (new sub-genus substituted for _Calamopitys_).
Genus proposed for roots of _Calamites_ before their real nature was
recognised. The name refers to anatomical characters.
_Astromyelon._
Genera of which some species, if not all, are the leaf-bearing branches
of _Calamites_.
_Calamocladus_ (including _Asterophyllites_),
_Annularia_.
Generic names applied to strobili belonging to _Calamites_.
_Calamostachys_,
_Palaeostachya_,
_Macrostachya_,
_etc._
Genus including impressions of Calamite roots.
_Pinnularia._
IV. Conclusion.
A brief sketch of the main features of _Calamites_ suffices to bring
out the many points of agreement between the arborescent Calamite
plants and the recent Equisetums. The slight variation in morphological
character among the present-day Horse-tails, contrasts with the
greater range as regards structural features among the types included
in _Calamites_. The Horse-tails probably represent one of several
lines of development which tend to converge in the Palaeozoic period;
the Calamite itself would appear to mark the culminating point of a
certain phylum of which we have one degenerate but closely allied
descendant in the genus _Equisetum_. We shall, however, be in a better
position to consider the general question of plant-evolution after we
have made ourselves familiar with other types of Palaeozoic plants.
Grand’Eury’s[833] striking descriptions of forests of Calamites in the
Coal-Measures of central France, enable us to form some idea of the
habit of growth of these plants with their stout branching rhizomes and
erect aerial shoots.
By piecing together the evidence derived from different sources we
may form some idea of the appearance of a living Calamite. A stout
branching rhizome ascended obliquely or spread horizontally through
sand or clay, with numerous whorls or tufts of roots penetrating into
swampy soil. From the underground rhizome strong erect branches grew
up as columnar stems to a height of fifty feet or more; in the lower
and thicker portions the bark was fissured and somewhat rugged, but
smoother nearer the summit. Looking up the stem we should see old and
partially obliterated scars marking the position of a ring of lateral
branches, and at a higher level tiers of branches given off at regular
or gradually decreasing intervals, bearing on their upper portions
graceful green branchlets with whorls of narrow linear leaves. On the
younger parts of the main shoot rings of long and narrow leaves were
borne at short intervals, several leaf-circles succeeding one another
in the intervals between each radiating series of branches. On some
of the leaf-bearing branchlets long and slender cones would be found
here and there taking the place of the ordinary leafy twigs. Passing to
the apical region of the stem the lateral branches given off at a less
and less angle would appear more crowded, and at the actual tips there
would be a crowded succession of leaf-segments forming a series of
overlapping circles of narrow sheaths with thin slender teeth bending
over the apex of the tree.
Thus we may feebly attempt to picture to ourselves one of the many
types of Calamite trees in a Palaeozoic forest, growing in a swampy
marsh or on gently sloping ground on the shores of an inland sea, into
which running water carried its burden of sand and mud, and broken
twigs of Calamites and other trees which contributed to the Coal Period
sediments. The large proportions of a Calamite tree are strikingly
illustrated by some of the broad and long pith-casts occasionally
seen in Museums; in the Breslau Collection there is a cast of a stem
belonging to the sub-genus _Calamitina_, which measures about 2 m. in
length and 23 cm. in breadth, with 36 nodes. In the Natural History
Museum, Paris, there is a cast nearly 2 metres long and more than 20
cm. wide, which is referred to the sub-genus _Calamodendron_.
E. ARCHAEOCALAMITES.
In the Upper Devonian and Culm rocks casts of a well-defined Calamitean
plant are characteristic fossils; stems, leaf-bearing branches, roots
and cones have been described by several authors, and the genus
_Archaeocalamites_ has been instituted for their reception. Although
this genus agrees in certain respects with _Calamites_, and as recent
work has shown this agreement extends to internal structure, it has
been the custom to regard the Lower Carboniferous and Devonian plants
as genetically distinct. The surface features of the stem-casts,
the form of the leaves, and apparently the cones, possess certain
distinctive characters which would seem to justify the retention of a
separate generic designation.
We may briefly summarise the characteristics of the genus as follows:—
Pith-casts articulated, with very slightly constricted nodes; the
internodes traversed by longitudinal ribs slightly elevated or
almost flat, separated by shallow grooves. The ribs and grooves are
continuous from one internode to another, and do not usually show
the characteristic alternation of _Calamites_[834]. Along the nodal
line there are occasionally found short longitudinal depressions,
probably marking the points of origin of outgoing bundles. Branches
were given off from the nodes without any regular order; a pith-cast
may have branch-scars on many of the nodes, or there may be no trace
of branches on casts consisting of several nodes. The leaves[835]
are in whorls; in some cases they occur as free, linear, lanceolate
leaves, or on younger branches they are long, filiform and repeatedly
forked. The structure of the wood agrees with that of some forms of
_Arthropitys_. The strobili consist of an articulated axis bearing
whorls of sporangiophores, and each sporangiophore has four sporangia.
Our knowledge of the fertile shoots is, however, very imperfect.
Renault[836] has recently described the structure of the wood in some
small silicified stems of _Archaeocalamites_ from Autun. A large hollow
pith is surrounded by a cylinder of wood consisting of wedge-shaped
groups of xylem tracheids associated with secondary medullary rays;
at the apex of each primary xylem group there is a carinal canal. The
primary medullary rays appear to have been bridged across by bands of
xylem at an early stage of secondary thickening, as in the Calamite of
fig. 83, _D._
[Illustration: FIG. 103. _Archaeocalamites scrobiculatus_ (Schloth.).
From a specimen in the Woodwardian Museum, Cambridge. From the
Carboniferous limestone of Northumberland. ½ nat. size.]
Our knowledge of the cones of _Archaeocalamites_ is far from
satisfactory. Renault[837] has recently described a small fertile
branch bearing a succession of verticils of sporangiophores; each
sporangiophore stands at right angles to the axis of the cone and
bears four sporangia, as in _Calamostachys_. It is not clear how far
there is better evidence than that afforded by the association of
the specimen with pith-casts of stems, for referring this cone to
_Archaeocalamites_, but the association of vegetative and fertile
shoots certainly suggests an organic connection. The cone described by
the French author agrees with _Equisetum_ in the absence of sterile
bracts between the whorls of sporangiophores. It is an interesting fact
that such a distinctly Equisetaceous strobilus is known to have existed
in Lower Carboniferous rocks.
Stur[838] has also described _Archaeocalamites_ at considerable
length; he gives several good figures of stem-casts and foliage-shoots
bearing long and often forked narrow leaves. The same writer describes
specimens of imperfectly preserved cones in which portions of whorls
of forked filiform leaves are given off from the base of the
strobilus[839]. Kidston[840] published an important memoir on the cones
of _Archaeocalamites_ in 1883, in which he advanced good evidence
in support of the view that certain strobili, which were originally
described as Monocotyledonous inflorescences, under the generic
name _Pothocites_[841], are the fertile shoots of this Calamarian
genus. Kidston’s conclusions are based on the occurrence on the
_Pothocites_ cones, of leaves like those of _Archaeocalamites_, on the
non-alternation of the sporangiophores of successive whorls, and on the
close resemblance between his specimens and those described by Stur.
Good specimens of the cones, formerly known as _Pothocites_, may be
seen in the Botanical Museum in the Royal Gardens, Edinburgh; as they
are in the form of casts without internal structure it is difficult to
form a clear conception as to their morphological features.
The fossils included under _Archaeocalamites_ have been referred
by different authors to various genera, and considerable confusion
has arisen in both generic and specific nomenclature. The following
synonomy of the best known species, _A. scrobiculatus_ (Schloth.)
illustrates the unfortunate use of several terms for the same plant.
_Archaeocalamites scrobiculatus_ (Schloth.). Fig. 103.
1720. _Lithoxylon_, Volkmann[842].
1820. _Calamites scrobiculatus_, Schlotheim[843].
1825. _Bornia scrobiculata_, Sternberg[844].
1828. _Calamites radiatus_, Brongniart[845].
1841. _Pothocites Grantoni_, Paterson[846].
1852. _Calamites transitionis_, Göppert[847].
—— _Stigmatocanna Volkmanniana_, _ibid._
—— _Anarthrocanna tuberculata_, _ibid._
—— _Calamites variolatus_, _ibid._
—— _C. obliquus_, _ibid._
—— _C. tenuissimus_, _ibid._
—— _Asterophyllites elegans_, _ibid._
1866. _Calamites laticulatus_, Ettingshausen[848].
—— _Equisetites Göpperti_, _ibid._
—— _Sphenophyllum furcatum_, _ibid._
1873. _Asterophyllites spaniophyllus_, Feistmantel[849].
1880. _Asterocalamites scrobiculatus_, Zeiller[850].
For other lists of synonyms reference may be made to Binney[851],
Stur[852], Kidston[853] and other authors.
Some of the best specimens of this species are to be seen in the
Museums of Breslau and Vienna, which contain the original examples
described by Göppert and Stur. An examination of the original
specimens, figured by Göppert under various names, enables one to
refer them with confidence to the single species, _Archaeocalamites
scrobiculatus_. The generic name _Archaeocalamites_, which has been
employed by some authors, was suggested by Schimper[854] in 1862, as
a subgenus of _Calamites_, on account of the occurrence of a deeply
divided leaf-sheath, attached to the node of a pith-cast, which seemed
to differ from the usual type of Calamitean leaf. The specimens
described by Schimper are in the Strassburg Museum; the leaf-sheath
which he figures is not very accurately represented.
The example given in fig. 103 shews very clearly the continuous course
of the ribs and grooves of the pith-cast. Each rib is traversed by a
narrow median groove which would seem to represent the projecting edge
of some hard tissue in the middle of each principal medullary ray of
the stem. The specimen was found in a Carboniferous limestone quarry,
Northumberland; there is a similar cast from the same locality in the
Museum of the Geological Survey.
_Affinities of_ Archaeocalamites.
This genus agrees very closely with _Calamites_ both in the anatomical
structure of the stem and in the verticillate disposition of the
leaves. The strobili appear to be Equisetaceous in character, and there
is no satisfactory evidence of the existence of whorls of sterile
bracts in the cone, such as occur in _Calamostachys_ and in other
Calamitean strobili. The continuous course of the vascular bundles of
the stem from one internode to the next is the most striking feature
in the ordinary specimens of the genus; but it sometimes happens that
the grooves on a pith-cast shew the same alternation at the node as in
_Calamites_. This is the case in a specimen in the Göppert collection
in the Breslau Museum, and Feistmantel[855] has called attention to
such an alternation in specimens from Rothwaltersdorf. In the true
_Calamites_, on the other hand, the usual nodal alternation of the
vascular strands is by no means a constant character[856]. Stur[857],
Rothpletz[858], and other authors have pointed out the resemblance of
_Archaeocalamites_ to _Sphenophyllum_. The deeply divided leaves of
some Sphenophyllums and those of _Archaeocalamites_ are very similar
in form; and the course of the vascular strands in _Sphenophyllum_
may be compared with that in _Archaeocalamites_. But the striking
difference in the structure of the stele forms a wide gap between the
two genera. We have evidence that the Calamites and Sphenophyllums were
probably descended from a common ancestral stock, and it may be that in
_Archaeocalamites_, some of the _Sphenophyllum_ characters have been
retained; but there is no close affinity between the two plants.
On the whole, considering the age of _Archaeocalamites_ and the few
characters with which we are acquainted, it is probable that this
genus is very closely related to the typical _Calamites_, and may be
regarded as a type which is in the direct line of development of the
more modern Calamite and the living _Equisetum_. Weiss[859] includes
_Archaeocalamites_ as one of his subgenera with _Calamitina_ and
others, and it is quite possible that the genus has not more claim to
stand alone than other forms at present included in the comprehensive
genus _Calamites_.
The student will find detailed descriptions of this genus in the works
which have been referred to in the preceding pages.
CHAPTER XI.
II. SPHENOPHYLLALES.
_Sphenophyllum._
The genus _Sphenophyllum_ is placed in a special class, as representing
a type which cannot be legitimately included in any of the existing
groups of Vascular Cryptogams. Although this Palaeozoic genus possesses
points of contact with various living plants, it is generally admitted
by palaeobotanists that it constitutes a somewhat isolated type among
the Pteridophytes of the Coal-Measures. Our knowledge of the anatomy
of both vegetative shoots and strobili is now fairly complete, and the
facts that we possess are in favour of excluding the genus from any of
the three main divisions of the Pteridophyta.
In Scheuchzer’s _Herbarium Diluvianum_ there is a careful drawing of
some fragments of slender twigs, from an English locality, bearing
verticils of cuneiform leaves, which the author compares with the
common _Galium_[860]. As regards superficial external resemblance, the
_Galium_ of our hedgerows agrees very closely with what must have been
the appearance of fresh green shoots of _Sphenophyllum_.
A twig of the same species of _Sphenophyllum_ is figured by
Schlotheim[861] in the first part of his work on fossil plants;
he regards it as probably a fragment of some species of Palm.
Sternberg[862] was the first to institute a generic name for this
genus of plants, and specimens were described by him in 1825 as a
species of the genus _Rotularia_. The name _Sphenophyllites_ was
proposed by Brongniart[863] in 1822 as a substitute for Schlotheim’s
genus, and in a later work[864] the French author instituted the genus
_Sphenophyllum_. Dawson[865] was the first to make any reference to
the anatomy of this genus; but it is from the examination of the much
more perfect material from St. Étienne, Autun, and other continental
localities, the North of England and Pettycur in Scotland, by Renault,
Williamson, Zeiller and Scott, that our more complete knowledge has
been acquired.
The affinity of _Sphenophyllum_ has always been a matter of
speculation; it has been compared with Dicotyledons, Palms, Conifers
(_Ginkgo_ and _Phyllocladus_), and various Pteridophytes, such as
_Ophioglossum_, _Tmesipteris_, _Marsilia_, _Salvinia_, _Equisetum_ and
the Lycopodiaceae[866].
[Sidenote: DEFINITION.]
We may define the genus _Sphenophyllum_ as follows:—
Stem comparatively slender (1·5–15 mm.?), articulated, usually somewhat
tumid at the nodes; the surface of the internodes is marked by more or
less distinct ribs and grooves which do not alternate at the nodes,
but follow a straight course from one internode to the next. A single
branch is occasionally given off from a node. Adventitious roots are
very rarely seen, their surface does not show the ridges and grooves of
the foliage-shoots.
The leaves are borne in verticils at the nodes, those in the same whorl
being usually of the same size, but in some forms two of the leaves are
distinctly smaller than the others. Each verticil contains normally
6, 9, 12, 18 or more leaves, which are separate to the base and not
fused into a sheath; the number of leaves in a verticil is not always
a multiple of six. They vary in form from cuneiform with a narrow
tapered base, and a lamina traversed by several forked veins, to narrow
uninerved leaves and leaves with a lamina dissected into dichotomously
branched linear segments. The leaves of successive whorls are
superposed.
The strobili are long and narrow in form, having a length in some
cases of 12 cm., and a diameter of 12 mm.; they occur as shortly
stalked lateral branches, or terminate long leaf-bearing shoots. The
axis of the cone bears whorls of numerous linear lanceolate bracts
fused basally into a coherent funnel-shaped disc, bearing on its upper
surface sporangiophores and sporangia.
The strobili are usually isosporous, but possibly heterosporous in some
forms.
The stem is monostelic, with a triarch or hexarch triangular strand
of centripetally developed primary xylem, consisting of reticulate,
scalariform and spiral tracheae; the protoxylem elements being
situated at the blunt corners of the xylem-strand. Foliar bundles are
given off, either singly or in pairs, from each angle of the central
primary strand. The secondary xylem consists of radially disposed
reticulate or scalariform tracheae, developed from a cambium-layer.
The phloem is made up of thin-walled elements, including sieve-tubes
and parenchyma. Both xylem and phloem include secondary medullary
rays of parenchymatous cells. The cortex consists in part of fairly
thick-walled elements; in older stems the greater part of the cortical
region is cut off by the development of deep-seated layers of periderm.
The roots are apparently diarch in structure, with a lacunar and smooth
cortex.
• • • • •
The branch of _Sphenophyllum emarginatum_ Brongn. given in fig. 109
shows the characteristic appearance of the genus as represented by this
well-known species which Brongniart figured in 1822. The Indian species
shown in fig. 111 illustrates the occurrence of unequal leaves in the
same whorl, and in fig. 110, _B_, we have a form of verticil in which
the leaves are deeply divided into filiform segments. A larger-leaved
form is represented by _S. Thoni_, Mahr. (fig. 110, _A_), a species
occasionally met with in Permian rocks.
No specimens of _Sphenophyllum_ have so far been found attached to
a thick stem; they always occur as slender shoots, which sometimes
reach a considerable length. One of the longest examples known is in
the collection of the Austrian Geological Survey; the axis is 4 mm. in
breadth and 85 cm. long, bearing a slender branch 61 cm. in length. The
manner of occurrence of the specimen as a curved slender stem on the
surface of the rock suggests a weak plant, which must have depended
for support on some external aid, either water or another plant. The
anatomical structure and other features do not favour the suggestion
of some writers that _Sphenophyllum_ was a water-plant[867], but there
would seem to be no serious obstacle in the way of regarding it as
possibly a slender plant which flung itself on the branches and stems
of stronger forest trees for support.
A. The anatomy of Sphenophyllum.
The following account of the structural features of the stem and root
is based on the work of Renault[868], Williamson[869] and Williamson
and Scott[870]. We may first consider such characters as have been
recognised in different examples of the genus, and then notice briefly
the distinguishing peculiarities of two well-marked specific types.
_a._ _Stems._
i. Primary structure.
In a transverse section of a young _Sphenophyllum_ stem such as that
diagrammatically sketched in fig. 105, _A_, we find in the centre
the xylem portion of a single stele with a characteristic triangular
form. The primary xylem consists mainly of fairly large tracheae with
numerous pits on their walls; towards the end of each arm the tracheids
become scalariform, and at the apex there is a group of narrower
spiral protoxylem elements. In the British species there is a single
protoxylem group at the apex of each arm, but Renault has described
some French stems in which the stele appears to be hexarch, having two
protoxylem groups at the end of each of the three rays of the stele.
The primary xylem strand of _Sphenophyllum_ has therefore a root-like
structure, the tracheids having been developed centripetally from the
three initial protoxylem groups. This type of structure is typical
of roots, but it also occurs in the stems of some recent Vascular
Cryptogams.
[Illustration: FIG. 104. Diagrammatic longitudinal section of
_Sphenophyllum_.
_c_, outer cortex; _b_, space next the stele, originally occupied
by phloem _etc._; _a_, xylem strand. (After Renault[871].) × 7.]
As a rule the tissue next the xylem has not been petrified, but in
exceptionally well-preserved examples it is seen to consist of a band
of thin-walled elements, of which those in contact with the xylem may
be spoken of as phloem, and those beyond as the pericycle. Succeeding
this band of delicate tissue there is a broader band of thicker-walled
and somewhat elongated elements, constituting the cortex. The specimen
drawn in fig. 105, _A_, shows very prominent grooves in the cortex
opposite the middle of each bay of the primary wood. It is these
grooves that give to the ordinary casts of _Sphenophyllum_ branches
the appearance of longitudinal lines traversing each internode. In a
longitudinal section of a stem, the cortical tissue (fig. 104, _c_)
is found to be broader in the nodal regions, thus giving rise to the
tumid nodes referred to in the diagnosis. The increased breadth at the
nodes does not mean that the xylem is broader in these regions, as it
is in Calamite stems. Small strands of vascular tissue are given off
from the three edges of the triangular stele (fig. 105 _A_) at each
node; these branch in passing through the cortex on their way to the
verticils of leaves. The space _b_ in the diagrammatic section of fig.
104 was originally occupied by the phloem and inner cortex. In some
species of _Sphenophyllum_ the apex of each arm of the xylem strand,
as seen in transverse section, is occupied by a longitudinal canal
surrounded by spiral tracheids, as in the primary xylem of the old stem
shown in fig. 105, _C_.
[Illustration: FIG. 105. _Sphenophyllum._
_A._ Transverse section of young stem.
_B._ Transverse section of the wood of a young stem; _px_,
protoxylem; _x_, secondary xylem. (_A_ and _B_. _Sphenophyllum
plurifoliatum._) × 20.
_C._ Transverse section of an old stem; (_S. insigne_); _a_,
phloem; _b_, periderm; _c_, fascicular secondary xylem;
_d_, interfascicular secondary xylem. × 9. (No. 914 in the
Williamson Collection.)
_D._ Longitudinal section of the reticulate tracheae and medullary
rays; _r_, _r_, _r_, of _S. plurifoliatum_. × 36.
_E._ Similar section of _S. insigne_. × 75. (_D_ and _E_ after
Williamson and Scott.)]
ii. Secondary structure.
With the exception of very young twigs the petrified _Sphenophyllum_
stems usually show a greater or less development of secondary wood.
In the xylem-strand of fig. 105, _B_, the broad concave bays of the
primary wood have been filled in by the development of two rows of
large secondary tracheids, _x_, but opposite the protoxylem groups,
_px_, there are no signs of cambial activity. In the unusually large
stem represented by a rough sketch in fig. 105, _C_, the triangular
primary xylem lies in the centre of a thick mass of secondary vascular
tissue. The secondary and primary wood together have a diameter of
about 5 mm.
After the bays between each protoxylem corner have been filled
in, the formation of secondary wood proceeds uniformly along the
stem radii, but the rows of tracheids and medullary rays which are
developed opposite the corners of the primary strand, _c_, differ in
certain characters from the broader masses of wood opposite the bays.
For convenience, the secondary wood, _c_, opposite the protoxylem
groups has been spoken of as _fascicular wood_, and the rest, _d_, as
_interfascicular wood_.
The secondary xylem consists either of tracheae with numerous bordered
pits on their radial walls (fig. 105, _D_), or of tracheae with broad
and bordered scalariform pits (fig. 105, _E_). The suggestion of
concentric rings of growth in the wood in fig. 105, _C_, is rather
deceptive; there are no well-marked regular rings in _Sphenophyllum_
stems, but irregular bands of smaller elements occasionally interrupt
the uniformity of the secondary xylem. In some stems the medullary rays
have the form of rows of parenchymatous cells, which in tangential
longitudinal section are found to consist frequently of a single row
of radially disposed elements; this type of medullary rays occurs
in the species _Sphenophyllum insigne_, in which the tracheae are
scalariform. Three medullary rays, _r_, are seen on the radial face of
the scalariform tracheids in fig. 105, _E_, which represents a radial
section of this species. In other species, _e.g._ _S. plurifoliatum_,
the medullary rays have a peculiar and characteristic structure;
in a transverse section of the stem they appear as groups of a few
parenchymatous cells in the spaces between the truncated angles of the
large tracheae (fig. 106). In longitudinal section these medullary-ray
elements resemble thick bars stretching radially across the face of the
tracheae (fig. 105, _D_, _r_); the apparent septa or bars are however
thin-walled cells connecting the different groups of medullary-ray
cells, as seen in a transverse section. These radial connecting cells
are occasionally seen as short rays in transverse sections of stems.
The cambium and phloem elements are occasionally preserved in good
specimens of older stems; the former consist of tabular flatted
thin-walled cells, and the latter in some cases include large
sieve-tubes and narrower parenchymatous elements.
The sections shown in fig. 107, _E_ and _F_, illustrate the
preservation of cambial and phloem tissue. In the transverse section
of fig. 107, _F_, the secondary xylem with the medullary rays, _r_, is
succeeded by a few tabular cambium cells, and external to these there
are thin-walled elements of unequal size representing the phloem.
In fig. 107, _E_, the scalariform tracheids are succeeded by narrow
thin-walled cells, and the larger elements with transverse and oblique
septa are no doubt sieve-tubes.
In the large stem of fig. 105, _C_, the xylem is succeeded by
a band of tissue, a, which is no doubt phloem, and external to
this there is a considerable development of periderm (_b_). The
periderm in _Sphenophyllum_ stems had a deep-seated origin, the
phellogen or cork-cambium occasionally being formed in the secondary
phloem-parenchyma, and in other cases in the pericycle, as in the stems
of some living dicotyledons. Williamson and Scott[872] describe stems
in which a succession of phellogens were formed at different levels,
thus producing a scaly type of bark, such as we find in the Pine or the
Plane tree.
[Sidenote: SPHENOPHYLLUM PLURIFOLIATUM.]
Before describing the structure of the strobili of _Sphenophyllum_, we
may briefly point out the distinguishing features of two specific types
of the genus recently described by Williamson and Scott. One of these
species, _S. insigne_, was originally described by Williamson as an
_Asterophyllites_; the numerous narrow linear leaves in each verticil
led to the inclusion of the specimens in the latter genus. The material
on which this species is founded is from the volcanic beds of Pettycur,
Burntisland, on the coast of the Firth of Forth.
1. _Sphenophyllum insigne_ (Will.). Figs. 105, _C_ and _E_,
and 107, _E_ and _F_.
1891. _Asterophyllites insignis_. Williamson[873].
An _intercellular space_ occurs at each angle of the three-rayed
primary xylem strand, and spiral tracheae are abundant. The tracheae
of the secondary wood have _scalariform markings_ on the radial walls.
_Regular medullary rays_ extend through the secondary wood. The phloem
contains large sieve-tubes.
This species occurs in the Calciferous sandstone rocks of Burntisland,
and has lately been recorded from Germany. It characterises a lower
horizon than _S. plurifoliatum_ (Will. and Scott).
2. _Sphenophyllum plurifoliatum_ (Williamson and Scott)[874].
Figs. 105, _A_, _B_, and _D_, and 106.
1891. _Asterophyllites sphenophylloides._ Will.[875]
The specific name _plurifoliatum_ was proposed by Williamson and
Scott for a type of stem originally described by Williamson[876] as
an _Asterophyllites_, from the Coal-Measures of Oldham, Lancashire.
This form of stem has not so far been connected with any of the
older species founded on external characters, but it evidently bore
foliage in which the leaves were deeply divided, as in _Sphenophyllum
trichomatosum_ (fig. 110, _B_).
[Illustration: FIG. 106. Sphenophyllum plurifoliatum, Will. and Scott.
From a photograph by Mr Highly from a section in the Williamson
Collection (no. 899). × 27.]
In this species there are _no canals_ at the angles of the primary
xylem, and there are fewer spiral tracheae than in _S. insigne_. The
tracheae of the secondary wood have _numerous small pits_ on the radial
walls, and the medullary rays are chiefly composed of parenchymatous
cells, which appear in transverse section as _groups of cells_ between
the truncated angles of the tracheae. The characters are fairly well
seen in the xylem portion of a stele shown in fig. 106. The fascicular
wood includes some rows of parenchymatous medullary-ray cells in
addition to the characteristic groups, as seen in the figure. A
slightly oblique transverse section of a stem is often convenient in
the interpretation of histological features; one of the sections of _S.
plurifoliatum_ in the Williamson collection (no. 893), which has been
cut somewhat obliquely, shows very clearly the differences in pitting
exhibited by the different xylem elements.
_b. Roots._
Our knowledge of the anatomy of _Sphenophyllum_ roots is very limited.
Renault has described a somewhat imperfect example of a silicified
root from St. Étienne and Autun. The drawing in fig. 107, _B_, which
is copied from one of Renault’s figures, shows a cylindrical mass of
xylem with a small band of narrower elements occupying the centre, and
surrounded by rows of larger secondary tracheae. The central bipolar
band is described as the diarch primary xylem, around which the
secondary pitted elements have been developed.
It is probable that the specimen described by Renault is a root of
_Sphenophyllum_, but my impression gained from an examination of the
section was that the diarch primary strand is not quite so clear as
in the published figures. Until we possess better material we cannot
attempt any very satisfactory description of the anatomical features of
the roots of this genus.
A section of a _Sphenophyllum_ stem has been figured by Felix[877], in
which a lateral member is being given off; this may possibly represent
the origin of an adventitious root, but the preservation is not
sufficiently distinct to render this certain.
_c. Leaves._
Renault[878] has described some silicified leaves of _Sphenophyllum_
from Autun in which the laminae consist of thin-walled loose
parenchyma, traversed by small groups of tracheids constituting the
simple or forked veins. The epidermis is made up of a single layer of
cells, with here and there indistinct indications of stomata. A more
perfect stoma has, however, been described by Solms-Laubach from the
epidermis of a bract in a strobilus (fig. 107, _A_).
[Illustration: FIG. 107. A. Stoma in a bract of _Sphenophyllostachys_.
B. Root of _Sphenophyllum_. C. _Sphenophyllostachys Römeri_, Solms.
_s_, sporangiophore, _b_, bract. D. Sporangium. E and F. Sections
through the cambium, phloem and secondary xylem of _Sphenophyllum
insigne_ (Will.). _s_, sieve-tube. G. Sporangium and pedicel. A, C,
D. After Solms-Laubach. B. After Renault. E–G. After Williamson and
Scott. E. F. × 100. G. × 115.]
_d. Cones._
The history of the recognition of the cones of _Sphenophyllum_ has
already been briefly alluded to in chapter V., p. 100. The main
points in the structure of the cones of this genus were known for
several years, before the fact was established that they belonged to
_Sphenophyllum_ stems. In 1871 Williamson[879] published an account
of an imperfect fossil strobilus from the Lower Coal-Measures of
Oldham, Lancashire, under the name of _Volkmannia Dawsoni_. The
generic term _Volkmannia_ has been used by different writers for
cones varying considerably in structural features; in the case of
Williamson’s fossil, Weiss[880] substituted the name _Bowmanites_,
a genus instituted by Binney[881] for a strobilus apparently of the
same type as _Volkmannia Dawsoni_. In 1891 Williamson[882] described
some additional specimens of _Bowmanites Dawsoni_, and, as in his
earlier paper, he compared the strobilus with _Asterophyllites_ and
_Sphenophyllum_, but it was still a matter of speculation as to what
was the form of the vegetative branches. Soon after the more complete
account of the English cones was published, Zeiller[883] recognised
a close agreement between some French and Belgian specimens of
_Sphenophyllum_ strobili and the strobilus described by Williamson.
A closer comparison thoroughly established the connection between
_Bowmanites Dawsoni_ and _Sphenophyllum_; and there is little doubt
that this strobilus belongs to the stem known as _Sphenophyllum
cuneifolium_ (Sternb.)—a well-known species of the genus.
[Sidenote: STROBILUS.]
The most important morphological features of the strobilus of
_Sphenophyllum_ may best be illustrated by a detailed account of one
specific type, and by a brief reference to other forms which are
characterised by certain differences in the number and attachment of
the sporangia. When we know that a given strobilus must have grown on
a _Sphenophyllum_ stem, the obvious name to assign to it would seem
to be that of the plant which bore it; but there are advantages in
making use of special generic terms for detached cones, which cannot
be referred with certainty to a particular species of stem. The genus
_Calamostachys_ affords an example of a name which is intended to
denote that a cone so called belongs to a Calamarian plant; similarly
such a name as _Sphenophyllostachys_ may be used for Sphenophylloid
cones which cannot be connected with certainty to particular species
of _Sphenophyllum_. It has been suggested that the genus _Bowmanites_,
first used for a cone which was afterwards recognised as belonging to a
_Sphenophyllum_, should be employed instead of the sesquipedalian term
_Sphenophyllostachys_. The latter is used here as being in accordance
with a generally accepted and convenient system of nomenclature, and
as a name which at once denotes the fact that the fossil is not only a
cone but that it belongs to a _Sphenophyllum_.
_Sphenophyllostachys Dawsoni_ (Will.). Figs. 107, _A_ and _G_, 108.
Probably the strobilus of _Sphenophyllum cuneifolium_ (Sternb.).
[Illustration: FIG. 108. Diagrammatic longitudinal section of a
_Sphenophyllum_ strobilus. The upper figure represents a portion of
a whorl of bracts. (The smaller figure, after Zeiller.)]
The cone consists of a central axis bearing a number of verticils
of bracts coherent in their lower portions in the form of a widely
open funnel-shaped disc, which splits up peripherally into 14–20
linear-lanceolate segments. The free segments of each verticil have an
obliquely ascending or almost vertical position, and extend upwards
for a distance of about six internodes. The smaller drawing in fig.
108 shows the appearance in side view of the narrow bracts of a single
whorl. A transverse section of a strobilus would include, therefore,
sections of several concentric series of ascending bracts. The
verticils of _Sphenophyllostachys Dawsoni_ are probably superposed,
but this point has not been definitely settled. From the upper surface
of the coherent basal portion of each verticil, there are given off
twice as many sporangiophores as there are free segments, and these
are attached close to the line of junction of the axis of the cone
and the funnel-shaped disc. Each sporangiophore has the form of a
slender stalk which bends inwards at its distal end and bears a single
sporangium (_cf._ fig. 107, _D_). The sporangiophores given off from
the same verticil of bracts vary in length. All the sporangiophores
are attached to the coherent bracts at the same distance from the axis
of the cone; but as the sporangia between each verticil of bracts
are arranged in two or three concentric series, it follows that the
length of the sporangiophores varies considerably. The diagrammatic
longitudinal section of a strobilus in fig. 108 shows three concentric
series of sporangia between successive bract-verticils. A similar
diagram was published by Williamson in 1892[884], and afterwards copied
by Potonié[885], but in Williamson’s restoration the sporangiophores
of the three series of sporangia are erroneously represented as
arising from different points on the surface of the bracts. There is
little doubt, as regards the strobilus of _S. cuneifolium_, that the
sporangiophores were given off in a single series close to the axils of
the bracts, as is partially shown in fig. 108.
The central part of the axis of the cone is occupied by a single
triangular stele like that of the stem, except that each ray of the
xylem strand has a comparatively broad blunt termination, and is
not tapered to a narrow arm as in fig. 105, _A_ and _B_. The wood
consists of pitted tracheae, with two groups of protoxylem elements at
each of the truncated angles of the solid strand of xylem. From the
angles of the stele branches of vascular tissue pass out through the
cortex to supply the sterile and fertile segments of each verticil.
One of the transverse sections of the _Sphenophyllum_ cone in the
British Museum Collection (no. 1898 _E_) affords a good example of the
misleading appearance occasionally presented by an intruded ‘rootlet’
of _Stigmaria_; the vascular tissue of the cone has disappeared, and a
Stigmarian appendage with its vascular bundle occupies the position of
the stelar tissues.
The bracts consist of parenchymatous tissue limited externally by an
epidermis containing stomata. A single stoma with subsidiary cells
is represented in fig. 107, _A_. The sporangiophores are composed
internally of thin-walled cells with stronger cells towards the
surface. The longer sporangiophores in a series may be more or less
coherent for part of their length to the upper surface of the verticil
of bracts. In fig. 108 the slender sporangiophores do not appear to
come off always from the same portion of the bracts, but this is due to
some of them lying on the surface of the latter during part of their
course to support the external circle of sporangia. The hook-like
distal end of a sporangiophore, towards the point of attachment of the
sporangium, is characterised by the larger size and greater prominence
of the surface cells; these larger cells, which pass over the upper
surface of a sporangium base, probably constitute a kind of _annulus_
which determines the dehiscence of the sporangial wall[886].
Fig. 107, _G_, represents a sporangiophore and its sporangium cut
through transversely just below the point of attachment of the latter
to the end of the hook-like termination of the former. The spores are
characterised by an irregularly reticulate thickening of the outer coat
or exospore, as seen in the figure.
One of the chief points of interest suggested by a _Sphenophyllum_
cone is the exact morphological nature of the sporangiophores. Are
they branches borne in the axils of bracts, or may we regard each
sporangiophore as a modified leaf, which has become coherent with
the whorls of sterile leaves? Or is a sporangiophore merely a stalk
of a sporangium; or a ventral lobe of a leaf, of which the sterile
bracts represent the dorsal lobes? Although it is impossible without
the evidence of development to decide with certainty between these
alternatives, it would seem most probable that a sporangiophore may
be looked upon as a ventral lobe of a leaf, the sterile lobes forming
the bracts or members of the sterile whorls of the cone. This question
is discussed by Zeiller[887] and Williamson and Scott[888], also more
recently by Scott[889] in his memoir on _Cheirostrobus_.
_Sphenophyllostachys Römeri_ (Solms-Laubach)[890].
Fig. 107, _C_ and _D_.
In another type of _Sphenophyllum_ strobilus, recently described
by Solms-Laubach, the incurved end of each sporangiophore bore two
sporangia. In most respects this species, which has not been found in
connection with a vegetative shoot, agrees with _Sphenophyllostachys
Dawsoni_.
In fig. 107, _C_, which is copied from one of Solms-Laubach’s
drawings[891], we have an oblique transverse section of part of a
strobilus, including portions of two series of sporangia borne on one
verticil of bracts, and at the right-hand edge the section has passed
through the sporangia belonging to another whorl of bracts. There
were probably three concentric series of sporangia attached to each
verticil of bracts, as in the case of fig. 108. The unshaded area,
_b_ (fig. 107, _C_), represents the bracts of two successive sterile
whorls in transverse section. The shaded areas are the sporangia, with
their sporangiophores, _s_. The relative position of the sporangia and
sporangiophores suggests that each pedicel bore two sporangia at its
tip, instead of one, as in the strobilus of _Sphenophyllum cuneifolium_
(Sternb.).
A further variation in the structure of the strobili is illustrated by
some specimens of _S. trichomatosum_ Stur, described by Kidston[892],
from the Coal-Measures of Barnsley. Each whorl of bracts bears a single
series of oval sporangia which appear to be sessile on the basal
portion of the whorl. It is possible that delicate sporangiophores
may have been present, but in the imperfect examples in Kidston’s
collection[893] the sporangia present the appearance of being seated
directly on the surface of the bracts. As the specimens do not show any
internal structure, it would be unwise to lay too much stress on the
apparent absence of the characteristic sporangiophores. In any case,
Kidston’s cones afford an illustration of the occurrence of a single
series of sporangia in each whorl, instead of the pluriseriate manner
of occurrence in some other species.
The statement is occasionally met with that some _Sphenophyllum_ cones
possessed two kinds of spores, but we are still in want of satisfactory
evidence that this was really the case. Renault has described an
imperfect specimen, which he considers points to the heterosporous
nature of a _Sphenophyllum_ cone, but Zeiller and Williamson and Scott
have expressed doubts as to the correctness of Renault’s conclusions.
While admitting the possibility of undoubted heterosporous strobili
being discovered, we are not in a position to refer to _Sphenophyllum_
as having borne strobili containing two kinds of spores[894].
[Sidenote: SPHENOPHYLLUM EMARGINATUM.]
[The following are some of the specimens in the Williamson Cabinet
which illustrate the structure of _Sphenophyllum_:—
_S. plurifoliatum._ 874, 882, 884, 893, 894, 897, 899, 901, 903,
908, 1893.
_S. insigne._ 910, 914, 919, 921, 922, 924, 926, 1420, 1898.
_Sphenophyllostachys._ 1049A–1049C, 1898.]
B. Types of vegetative branches of Sphenophyllum.
1. _Sphenophyllum emarginatum_ (Brongniart). Fig. 109.
1822. _Sphenophyllites emarginatus_, Brongniart[895].
1828. _Sphenophyllum emarginatum_, Brongniart[896].
1828. _Sphenophyllum truncatum_, Brongniart[896].
1828. _Rotularia marsileaefolia_, Bischoff[897].
1862. _Sphenophyllum osnabrugense_, Römer[898].
[Illustration: FIG. 109. _Sphenophyllum emarginatum_ (Brongniart).
From a specimen in the Collection of Mr R. Kidston, Upper
Coal-Measures, Radstock. ⅚ nat. size.]
This species of _Sphenophyllum_ bears verticils of six or eight
wedge-shaped leaves varying in breadth and in the extent of dissection
of the laminae; they are truncated distally, and terminate in a margin
characterised by blunt or obtusely-rounded teeth, each of which
receives a single vein. The larger leaves are usually more or less
deeply divided by a median slit. The narrow base of each leaf receives
a single vein which branches repeatedly in a dichotomous manner in
the substance of the lamina. Several drawings have been given by
Sterzel[899] in a memoir on Permian plants, showing the variation in
leaf-form in _Sphenophyllum emarginatum_, but as Kidston[900] and
Zeiller[901] have pointed out Sterzel’s specimens probably belong to
_S. cuneifolium_ (Sternb.).
Branches are given off singly from the nodes, and the cones are borne
at the tips of branches or branchlets. The cone of _S. emarginatum_
agrees very closely with that of _S. cuneifolium_, and is of the same
type as that shown in fig. 108. The small branch of _S. emarginatum_
represented in fig. 109 does not show clearly the detailed characters
of the species, as the leaf-margins are not well preserved.
In one of the largest specimens of this species which I have seen, in
the Leipzig Museum, the main stem has internodes of about 3·9 cm. in
length, from which a lateral branch with much shorter internodes is
given off from a node.
It is important to notice the close resemblance, as pointed out by
Zeiller, between some of the narrower-leaved forms of _S. emarginatum_
and _S. cuneifolium_ (Sternb.)[902]; but in the latter species the
margins of the leaves have sharp, and not blunt teeth.
The cone described and figured by Weiss[903] as _Bowmanites
germanicus_, since investigated by Solms-Laubach[904], must be referred
to this species. Geinitz[905] figured a cone in 1855 as that of
_S. emarginatum_, but his determination of the species is a little
doubtful. Good figures of the true cone of _S. emarginatum_ have been
given by Zeiller[906] in his _Flore de Valenciennes_, as well as in his
important memoir on the fructification of _Sphenophyllum_.
[Sidenote: LEAVES.]
2. _Sphenophyllum trichomatosum_ Stur. Fig. 110, _B_.
The finely-divided leaves of the single whorl shown in fig. 110, _B_
(from the Middle Coal-Measures of Barnsley, Yorkshire), afford an
example of a form of _Sphenophyllum_ which is represented by such
species as _S. tenerrimum_ Ett.[907], _S. trichomatosum_ Stur[908],
and _S. myriophyllum_[909] Crép. Probably the specimen should be
referred to _S. trichomatosum_, but it is almost impossible to speak
with certainty as to the specific value of an isolated leaf-whorl of
this form. It has long been known that the leaves of _Sphenophyllum_
may vary considerably, as regards the size of the segments, on the
same plant; and the occurrence of such finely-divided leaves has
lent support to an opinion which was formerly held by some writers,
that _Asterophyllites_ and _Sphenophyllum_ could not be regarded as
well-defined separate genera. This heterophylly of _Sphenophyllum_
has thus been responsible for certain mistaken opinions both as to
the relation of the genus to _Calamocladus_[910] (_Asterophyllites_),
and as regards the view that the finely-divided laminae belonged
to submerged leaf-whorls, while the broader segments were those of
floating or subaerial whorls.
There is a very close resemblance between some of the deeply-cut and
linear segments of a _Sphenophyllum_ and the leaves of _Calamocladus_,
but in the former genus the linear segments are found to be connected
basally into a narrow common sheath. The assertion[911] that the
deeply-cut leaves occur on the lower portions of stems is not supported
by the facts. Kidston[912] has pointed out that the cones are often
borne on branches with such leaves, and the same author refers to a
figure by Germar, in which entire and much-divided leaves occur mixed
together in the same individual specimen. M. Zeiller recently pointed
out to me a similar irregular association of broader and narrower
leaf-segments on the same shoots in some large specimens in the École
des Mines, Paris. Cones of _Sphenophyllum tenerrimum_ have been figured
by Stur[913] and others; they are characterised by their small size
and by the dissection of the slender free portions of the narrow
bracts[914].
3. _Sphenophyllum Thoni_ Mahr. Fig. 110, _A_.
Another type of _Sphenophyllum_ is illustrated by _S. Thoni_ Mahr as
shown in fig. 110, _A_. This species was first described by Mahr[915]
from the Coal-Measures of Ilmenau, and has since been figured by
Zeiller and other authors. Each whorl consists of six large obcuneiform
leaves with the broad margin somewhat irregularly fringed. The
unusually good specimen of which fig. 110, _A_, represents a single
verticil was originally described and figured by Zeiller in 1880[916];
it is now in the École des Mines Museum, Paris.
[Illustration: FIG. 110.
_A._ _Sphenophyllum Thoni_, Mahr. (After Zeiller.)
_B._ _Sphenophyllum trichomatosum_, Stur. From a specimen in the
Woodwardian Museum; from the Coal-Measures of Barnsley, Yorks.
_A_ and _B_ ¾ nat. size.]
The leaf-forms illustrated by figs. 109 and 110 are some of the more
extreme types of _Sphenophyllum_ leaves; but these are more or less
connected by a series of intermediate forms. For a more complete
systematic account of the different species the student should consult
such works as those by Coemans and Kickx[917], Zeiller, Schimper, and
others.
4. _Sphenophyllum speciosum_ (Royle). Fig. 111.
1834. _Trizygia speciosa_, Royle[918].
The species shown in fig. 111 has been usually described as a separate
genus _Trizygia_, a name instituted by Royle in 1834 for some Indian
fossils from the Lower Gondwana rocks of India[919]. Zeiller[920] has
lately pointed out the advisability of including this Asiatic type in
the genus _Sphenophyllum_. The slender stem bears verticils of cuneate
leaves in three pairs at each node, the anterior pair being smaller
than the two lateral pairs. The characteristic _Sphenophyllum_ venation
is clearly seen in the enlarged leaf, fig. 111, _B_.
[Illustration: FIG. 111. _Sphenophyllum speciosum_ (Royle).
_A._ Nat. size. _B._ enlarged leaf.
From the Raniganj Coal-field, India. (After Feistmantel.)]
The inequality of the members of a single whorl, which characterises
this Indian plant, is sometimes met with in European species. A
specimen of _Sphenophyllum oblongifolium_, which Prof. Zeiller showed
me in illustration of this point, was practically indistinguishable
from _Trizygia_[921].
In some of the earlier descriptions of the Indian species the generic
name _Sphenophyllum_[922] was used by McClelland and others, but the
supposed difference in the leaf-whorls was made the ground of reverting
to the distinct generic term _Trizygia_. Now that a similar type of
leaf-whorl is known to occur in _Sphenophyllum_, it is better to adopt
that genus rather than to allow the question of locality to unduly
influence the choice of a separate generic name for an Indian plant.
[Sidenote: GEOLOGICAL RANGE.]
C. Affinities, range and habit of Sphenophyllum.
It has been pointed out in the description of _Sphenophyllum_, that
the most widely separated families of recent plants have been selected
by different authors as the nearest living allies of this Palaeozoic
genus. It is now generally admitted that _Sphenophyllum_ is a generic
type apart; it cannot be classed in any family or sub-class of recent
or fossil plants, without considerably extending or modifying the
recognised characteristics of existing divisions of the plant-kingdom.
The anatomical characters of the _Sphenophyllum_ stem are such as one
finds in some recent genera of the Lycopodineae, especially _Psilotum_.
If the stele of _Psilotum_ were composed internally of a solid
strand of xylem, we should have a close correspondence between the
centripetally-developed wood of this genus and that of _Sphenophyllum_.
Similar comparisons might be drawn with other existing genera, but the
more detailed consideration of the affinities of the Palaeozoic plant
will be more easily dealt with after other members of the Pteridophytes
have been described. The recent discovery of an entirely new type of
Carboniferous strobilus in rocks of Calciferous sandstone age on the
shores of the Firth of Forth has thrown new light on the position of
_Sphenophyllum_. _Cheirostrobus Pettycurensis_, the new cone which
Scott has described in an able memoir, affords certain points of
contact with _Sphenophyllum_ on the one hand and with _Calamites_ on
the other. This important question will be dealt with after we have
given an account of _Cheirostrobus_[923]. To put the matter shortly,
_Sphenophyllum_ agrees with some Lycopodinous plants in its anatomical
features; with the Equisetales it is connected by the verticillate
disposition of the leaves, and some of the forms of _Sphenophyllum_
strobili present features which also point to Equisetinous affinities.
In his Presidential address to the Botanical Section at the
British Association Meeting of 1896 Scott[924] thus refers to the
Sphenophyllums:—“We may hazard the guess that this interesting group
may have been derived from some unknown form lying at the root of both
Calamites and Lycopods. The existence of the Sphenophyllae certainly
suggests the probability of a common origin for these two series.” The
result of the subsequent investigation of the new cone _Cheirostrobus_
amply justifies this opinion as to the position of _Sphenophyllum_.
It is probable that _Sphenophyllum_ lived during the Devonian period,
but the unsatisfactory specimens on which Dawson has founded a species
of this age, _S. antiquum_[925], can hardly be said to afford positive
evidence of the Pre-Carboniferous existence of the genus. From the
Culm rocks and other strata older than the Coal-Measures, we have
such species as _S. insigne_ (Will.), _Sphenophyllostachys Römeri_
(Solms-Laubach), and _Sphenophyllum tenerrimum_, Ett.[926] while _S.
emarginatum_[5], Brongn. occurs in the Upper Coal-Measures and in the
Transition rocks. _S. cuneifolium_[927] (Sternb.) has been recorded
from the Transition, Middle and Lower Coal-Measures. _Sphenophyllum
oblongifolium_, Germ.[928], is recorded from Lower Permian rocks, as is
also _S. Thoni_[929], Mahr.
The comparison which has naturally been drawn between _Sphenophyllum_
with its slender stems bearing occasionally dimorphic leaves, and
water-plants is not, I believe, supported by the facts of anatomy
or external characters. The entire and finely-dissected leaves
do not exhibit that regularity of relative disposition which is
characteristic of aquatic plants; the two forms of leaves may occur
indiscriminately on the same branch. The well-developed and thick xylem
is not in accordance with the anatomical features usually associated
with water-plants. It is true that in some living dicotyledons of the
family Leguminosae, which inhabit swampy places, the secondary xylem is
represented by a thick mass of unlignified and thin-walled parenchyma,
as in the genus _Aeschynomene_[930], from which the material of
‘pith’-helmets is obtained; but the wood of _Sphenophyllum_ was
obviously thick-walled and thoroughly lignified.
It is not improbable that the long and slender stems of this plant may
have grown like small lianas in the Coal-Measure forests, supporting
themselves to a large extent on the stouter branches of Calamites
and other trees. The anatomical structure of a _Sphenophyllum_ stem
would seem to be in accord with the requirements of a climbing plant.
It has been shewn[931] that in recent climbing plants the tracheae
and sieve-tubes are characterised by their large diameter, a fact
which may be correlated with the small diameter of climbing stems and
the need for rapid transport of food material. In _Sphenophyllum_
the tracheae of the xylem have a wide bore, and in _S. insigne_ the
phloem contains unusually wide sieve-tubes. The central position of
the stele is another feature which is not inconsistent with a climbing
habit. Schwendener and others[932] have demonstrated that in climbing
organs, as in underground stems and roots, there is a tendency towards
a centripetal concentration of mechanical or strengthening tissue.
The axial xylem strand of _Sphenophyllum_ would afford an efficient
resistance to the tension or pulling force which climbing stems
encounter.
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=Credner, H.= (87) Elemente der Geologie. _Leipzig_, 1887.
=Crépin, F.= (81) L’emploi de la Photographie pour la reproduction des
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=Dawson, J. W.= (59) On fossil plants from the Devonian rocks of
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—— (71) The fossil plants of the Devonian and Upper Silurian
formations of Canada. _Geol. Surv. Canada_, 1871.
—— (81) Notes on new Erian (Devonian) plants. _Quart. Journ. Geol.
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—— (82) Notes on _Prototaxites_ and _Pachytheca_ discovered by Dr
Hicks in the Denbighshire Grits of Corwen, North Wales. _Ibid._
vol. XXXVIII. p. 103, 1882.
—— (88) The geological history of plants. _London_, 1888.
—— (90) On burrows and tracks of invertebrate animals in Palaeozoic
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=Deecke, W.= (83) Über einige neue Siphoneen. _Neues Jahrb._ Jahrg. I.
p. 1, 1883.
=Defrance.= (26) Dictionnaire des Sciences naturelles. Article
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=Delgado, J. F. N.= (86) Étude sur les Bilobites. _Secc. Trav. Géol.
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=Dixon, H. H.= and =Joly, J.= (97) Coccoliths in our coastal waters.
_Nature_, vol. LVI. p. 468, 1897.
=Dixon, H. N.= and =Jameson, H. G.= (96) The Student’s Handbook of
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=Duncan, P. M.= (76) On some unicellular algae parasitic within
Silurian and Tertiary Corals, with a notice of their presence in
_Calceolina sandalina_ and other fossils. _Quart. Journ. Geol.
Soc._ vol. XXXII. p. 205, 1876.
—— (76²) On some Thallophytes parasitic within recent Madreporaria.
_Proc. R. Soc._ vol. XXV. p. 238, 1876.
=Dunker, W.= (46) Monographie der norddeutschen Wealdenbildung.
_Braunschweig_, 1846.
=Duval-Jouve, J.= (64) Histoire naturelle des Equisetum de France.
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=Ehrenberg, C. G.= (36) Über das Massenverhältniss der jetzt lebenden
Kieselfusorien und über ein neues Infusiorien-Conglomerat als
Polirschiefer von Jastrabo in Ungarn. _Abh. k. Akad. Wiss.
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—— (54) Mikrogeologie. _Leipzig_, 1854.
=Ellis, J.= (1755) An essay towards a Natural History of the
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=Engelhardt, H.= (87) Über _Rosselinia conjugata_ (Beck.) eine neue
Pilzart aus der Braunkohlen-formation Sachsens. _Ges. Isis.
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=Etheridge, R.= (81) _Vide_ Hicks, H.
=Etheridge, R. junr.= (92) On the occurrence of microscopic
fungi, allied to the genus _Palaeachyla_, Duncan, in the
Permo-Carboniferous rocks of New South Wales and Queensland.
_Rec. Geol. Surv. N.S.W._ Pt. III. p. 95, 1892.
—— (95) On the occurrence of a plant in the Newcastle or Upper
Coal-Measures possessing characters both of the genera
_Phyllotheca_ Brongn. and _Cingularia_ Weiss. _Ibid._ vol. IV.
Pt. IV. p. 148, 1895.
=Ettingshausen, C. von.= (55) Die Steinkohlenflora von Radnitz in
Böhmen. _Abhand. k. k. geol. Reichsanst. Wien_, vol. II. p. 1,
1855.
—— (66) Die fossile Flora des Mährisch-Schlesischen Dachschiefers.
_Ibid._ vol. XXV. p. 77, 1866.
=Ettingshausen, C. von= and =Gardner, J. S.= (79) A monograph of the
British Eocene Flora. _Palaeontographical Society._ _London_,
1879–1882.
=Feilden, H. W.= (96) Note on the glacial Geology of Arctic Europe and
its islands. Pt. I. Kolguev Island. _Quart. Journ. Geol. Soc._
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=Feistmantel, O.= (73) Das Kohlenvorkommen bei Rothwaltersdorf in
der Grafschaft Glatz und dessen organische Einschlüsse. _Zeit.
Deutsch. Geol. Ges._ vol. XXV. p. 463, 1873.
—— (75) Die Versteinerungen der böhmischen Kohlenablagerungen.
_Palaeontograph._ vol. XXIII. p. 1, 1875–76.
—— (81) The Fossil Flora of the Gondwana System. _Mem. Geol.
Surv. India_, (Ser. XII.) vol. III. 1881. (The Flora of the
Damuda-Panchet Divisions, pp. 1–77. Pls. I. A–XVI. A _bis_, 1880.)
—— (90) Geological and palaeontological relations of the Coal and
plant-bearing beds of Palaeozoic and Mesozoic age in Eastern
Australia and Tasmania. _Mem. Geol. Surv. N. S. Wales.
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=Felix, J.= (86) Untersuchungen über den inneren Bau westfälischer
Carbon-Pflanzen. _Abh. Geol. Landesanst._ 1886, p. 153.
—— (94) Studien über fossile Pilze. _Zeit. deutsch. geol. Ges._ Heft
I. p. 269, 1894.
—— (96) Untersuchungen über den inneren Bau westfälischer
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=Fischer, A.= (92) Die Pilze. _Rabenhorst’s Kryptogamen Flora_, vol. I.
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=Fliche, P.= and =Bleicher=, —. Étude sur la flore de l’oolithe
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=Früh, J.= (90) Zur Kenntniss der gesteinbildenden Algen. _Abh.
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=Fuchs, T.= (94) Über eine fossile _Halimeda_ aus dem Eocänen Sandstein
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—— (95) Studien über Fucoiden und Hieroglyphen. _Denksch. Akad.
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=Gardner, J. S.= (79) _Vide_ Ettingshausen.
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—— (86) On Mesozoic Angiosperms. _Geol. Mag._ vol. III. p. 193, 1886.
—— (87) On the leaf-beds and gravels of Ardtun, Carsaig, &c., in
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=Germar, E. F.= and =Kaulfuss, F.= (31) Über einige merkwürdige
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—— (36²) Die fossilen Farnkräuter. _Nova Acta Leop. Car._ vol. XVII.
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—— (45) Tchihatcheff’s Voyage Scientifique dans l’Altaï oriental,
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—— (52) Über die Flora des Übergangsgebirges. _Nova Acta Ac. Caes.
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—— (57) Über den versteinten Wald von Radowenz bei Adersbach in
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—— (60) Über die fossile Flora der Silurischen, der Devonischen und
unteren Kohlenformation. _Nova Acta Ac. Caes. Leop. Car._ vol.
XXVII. p. 427, 1860.
—— (64) Die fossile Flora der Permischen Formation.
_Palaeontograph._ vol. XII. 1864–65.
=Göppert, H. R.= and =Berendt, G. C.= (45) Der Bernstein und die in ihm
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=Göppert, H. R.= and =Menge, A.= (83) Die Flora des Bernsteins. Vol. I.
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=Grand’Eury, C.= (69) Observations sur les Calamites et les
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—— (82) Mémoire sur la formation de la houille. _Ann. Mines_, vol.
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—— (87) Formation des couches de houille et du terrain houiller.
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=Greville, R. K.= (47) Notice of a new species of _Dawsonia_. _Ann.
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=Harrison, J. B.= (91) _Vide_ Jukes-Browne.
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=Harvey, W. H.= (58) Phycologia Australica. _London_, 1858.
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=Heinricher, E.= (82) Die näheren Vorgänge bei der Sporenbildung der
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=Hick, T.= (78) and (81) _Vide_ Cash, W.
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=Hooker, W. J.= (20) Musci Exotici. Vol. II. _London_, 1820.
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_Quart. Journ. Geol. Soc._ vol. XL. p. 178, 1884.
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=Huxley, T. H.= (93) Science and Hebrew tradition. _Collected Essays_,
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=James, J. F.= (93) Studies in the problematic organisms. No. 2. The
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—— (93³) Fossil fungi. Translated from the French of R. Ferry, with
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=Judeich, J. F.= and =Nitsche, F.= (95) Lehrbuch der mitteleuropäischen
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=Jukes-Browne, A. J.= and =Harrison, J. B.= (91) The Geology of
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=Kent, S.= (93) The Great Barrier Reef of Australia. _London_, 1893.
=Kickx, J. J.= (64) _Vide_ Coemans, E.
=Kidston, R.= (83) Report on fossil plants collected by the Geological
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—— (83²) On the affinities of the genus _Pothocites_, Paterson; with
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—— (86) Catalogue of the Palaeozoic plants in the department of
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—— (88) _Vide_ Young.
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—— (93) On the fossil plants of the Kilmarnock, Galston, and
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=Knowlton, F. H.= (89) Fossil wood and lignite of the Potomac
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—— (89²) Description of a problematic organism from the Devonian at
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—— (94) Fossil plants as aids to geology. _Journ. Geol._ vol. II.
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—— (96) The nomenclature question. _Bot. Gazette_, vol. XXI. p. 82,
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=Koechlin-Schlumberger, J.= _Vide_ Schimper, W. P.
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=König, C.= (29) _Vide_ Murchison (29) p. 298.
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=Lesquereux, L.= (66) Geological Survey of Illinois. Vol. II. _New
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—— (70) _Ibid._ vol. IV. 1870.
—— (79) Atlas to the Coal Flora of Pennsylvania and of the
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—— (80) Description of the Coal Flora of the Carboniferous formation
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—— (84) _Ibid._ vol. III. 1884.
—— (87) A species of Fungus recently discovered in the shale of the
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=Lhwyd (Luidius), E.= (1760) (1699) Lithophylacii Britannici
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=Luerssen, C.= (89) Die Farnpflanzen oder Gefässbündelkryptogamen.
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=Lydekker, R.= (89) _Vide_ Nicholson.
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—— (92) Der äussere Bau der Blätter von _Annularia stellata_
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=Quekett, J.= (54) Lectures on Histology. Vols. I. and II. _London_,
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=Raciborski, M.= (94) Flora Kopalna ogniotrwalych Glinek Krakowskich.
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_Ann. Sci. Nat._ [6] vol. IV. p. 277, 1876.
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—— (96²) Notice sur les travaux scientifiques. _Autun_, 1896.
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=Renault, R.= and =Bertrand= (94) Sur une bactérie coprophile de
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=Rodway, J.= (95) In the Guiana Forest. _London_, 1895.
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—— (94) Ein geologischer Querschnitt durch die Ost-Alpen nebst
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—— (81) _Vide_ Saporta and Marion.
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=Saporta, de G.= and =Marion, A. F.= (81) L’Évolution du règne végétal.
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=Schenk, A.= (65) _Vide_ Schoenlein.
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=Schiffner, V.= and =Müller, C.= (95) Engler und Prantl; Die
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=Schimper, W. P.= (65) Euptychium Muscorum Neocaledonicorum genus novum
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—— (74) _Ibid._ Atlas.
=Schimper, W. P.= and =Koechlin-Schlumberger, J.= (62) Mémoire sur
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=Schimper, W. P.= and =Mougeot= (44) Monographie des plantes fossiles
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=Schlotheim, E. F. von.= (04) Beschreibung merkwürdiger
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=Schoenlein, J. L.= and =Schenk, A.= (65) Abbildungen von fossilen
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=Schröter, J.= (89) Engler und Prantl; Die natürlichen
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=Schulze, C. F.= (1755) Kurtze Betrachtung derer Kräuterabdrücke im
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=Schulze, F.= (55) Über das Vorkommen wohlerhaltener Cellulose in
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=Schütt, F.= (93) Das Pflanzenleben der Hochsee. _Kiel_ and _Leipzig_,
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—— (96) Bacillariales. Engler und Prantl; Die natürlichen
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=Schwendener, S.= (74) Das mechanische Princip im anatomischen Bau der
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=Scott, D. H.= (94) (95) (96) _Vide_ Williamson, W. C.
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—— (96²) Presidential address. (Liverpool Meeting.) _British Assoc.
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=Seeman, B.= (65) A gigantic _Equisetum_. _Journ. Bot._ vol. III. p.
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=Seward, A. C.= (88) On _Calamites undulatus_. _Geol. Mag._ vol. V. p.
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—— (89) _Sphenophyllum_ as a branch of _Asterophyllites_. _Mem. and
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—— (94) Algae as rock-building organisms. _Science Progress_, vol.
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—— (94²) Catalogue of the Mesozoic plants in the Department of
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—— (95) _Ibid._ Pt. II. _London_, 1895.
—— (95²) Coal: its structure and formation. _Science Progress_, vol.
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—— (95³) Notes on _Pachytheca_. _Proc. Phil. Soc. Cambridge_, vol.
VIII. pt. 5, p. 278, 1895.
—— (96) Notes on the geological history of Monocotyledons. _Annals
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—— (96²) _Vide_ Feilden, H. W.
—— (97) On _Cycadeoidea gigantea_, a new Cycadean stem from the
Purbeck beds of Portland. _Quart. Journ. Geol. Soc._ vol. LIII.
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—— (97²) On the association of _Sigillaria_ and _Glossopteris_ in
South Africa. _Ibid._ p. 315, 1897.
=Sharpe, S.= (68) On a remarkable incrustation in Northamptonshire.
_Geol. Mag._ vol. V. p. 563, 1868.
=Sherborn, C. D.= (93) An index to the genera and species of the
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=Shrubsole, W. H.= and =Kitton, F.= (81) The Diatoms of the London
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=Skertchly, J. B. J.= (77) The geology of the Fenland. _Mem. Geol.
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=Smith, W. G.= (77) A fossil _Peronospora_. _Gard. Chron._ Oct. 20,
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=Sollas, W. J.= (86) On a specimen of slate from Bray-Head traversed by
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=Solms-Laubach, Graf zu.= (81) Die corallinen Algen des Golfes von
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—— (91) Fossil Botany. _Oxford_, 1891.
—— (93) Über die Algen-genera _Cymopolia_, _Neomeris_ und
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—— (95) William Crawford Williamson. (Obit. notice.) _Nature_, vol.
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—— (95²) Über Devonische Pflanzenreste aus den Lenneschiefern der
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—— (95³) Monograph of the Acetabularieae. _Trans. Linn. Soc._ [2]
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—— (95⁴) _Bowmanites Römeri_, eine neue
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_Wien_, vol. XLV. Heft 2, p. 225, 1895.
—— (96) Über die seinerzeit von Unger beschriebenen Struktur
bietenden Pflanzenreste des Unterculm von Saalfeld in Thüringen.
_Abhand. k. Preus. Geol. Landesanst._ (N.F.) Heft XXIII.
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—— (97) Über die in den Kalksteinen des Culm von Glätzisch
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INDEX.
_Acetabularia_, 165, 166
_A. mediterranea_, 162, 165, 166
_Achyla_, 127
_Acicularia_, 162, 166, 167
_A. Andrussowi_, 162, 168
_A. miocenica_, 162, 168
_A. pavantina_, 167
_A. Schencki_, 162, 166, 168
_Aegoceras planicosta_, 61
Aeolian rocks, 25
_Aeschynomene_, 414
Africa, South, 80, 284
_Agaricus melleus_, 211, 212, 215
Algae, 138–202
_Algites_, 148, 149, 204, 205
Amazons, 66
Amber, 70, 71, 206, 211, 221, 255
America, 182
_Amphiroa_, 184
_Anarthrocanna_, 283
_A. tuberculata_, 386
Andersson, G., 60
Andrae, K. J., 276
Andreaeales, 236
Andrussow, N., 168
_Annularia_, 255, 260, 282, 283, 289, 329, 332, 333, 336–339, 340,
361, 362, 379, 381
_A. calamitoides_, 335
_A. Geinitzi_, 338
_A. longifolia_, 338, 341
_A. radiata_, 283
_A. ramosa_, 363
_A. sphenophylloides_, 340, 341, 363
_A. stellata_, 338–341, 363
_A. westphalica_, 338
Antigua, 79
_Aphlebia_, 142
_Araucaria imbricata_, 101, 104
_Araucarioxylon Withami_, 81, 82
Archaean system, 34–36
_Archaeocalamites_, 254, 255, 256, 263, 285, 336, 383–388
_A. scrobiculatus_, 385–387
d’Archiac, A., 166, 167
Arctic plants, 16, 17
Arenig series, 37
Arran, 88, 89
_Arthrodendron_, 301, 302, 324, 325, 326, 327, 366, 379, 381
_Arthropitys_, 300, 301, 302, 304, 311, 324, 325, 326, 328, 333, 349,
369, 375, 379, 380, 381, 384
_A. approximata_, 371
_A. bistriata_, 326
Arthropod eggs, 108
Artis, F. T., 4, 104, 297
_Artisia_, 104
Ascomycetes, 208, 209, 220
Asia Minor, Coal-Measures of, 282, 283
_Asterocalamites scrobiculatus_, 386
_Asterophyllites_, 264, 276, 329, 332, 333, 338, 369, 371, 379, 397,
409
_A. elegans_, 386
_A. equisetiformis_, 335, 338
_A. insignis_, 397
_A. longifolius_, 338
_A. spaniophyllus_, 386
_A. sphenophylloides_, 397
_Astromyelon_, 342, 343, 381
Australia, 179, 290, 291
Autun, 6, 46, 83, 179, 181, 182, 206, 390, 399
Azoic system, 36
Bacillariaceae, 150–156
_Bacillus_, 135, 136
_B. amylobacter_, 136
_B. permicus_, 135
_B. Tieghemi_, 135, 136
Bacteria, 75, 132, 138
_Bactryllium_, 154–156
_B. deplanatum_, 155
Bala series, 37
_Balanoglossus_, 145
Balfour, Bayley, 178
Balfour, J. H., 101
_Bambusa arundinacea_, 322
Barbados, 188
Barber, C. A., 193, 197, 198, 203, 204
Barrois, C., 146
Barton, R., 80
de Bary, A., 132
Basidiomycetes, 208, 211, 212
Bates, H. W., 64, 66
Bauhin, C., 225
Bear Island, 366
Bentham, G., 98, 99
_Bergeria_, 101
Bernician series, 43
Bertrand, C. E., 87, 154, 178, 181
Beyrich, E., 52
Bilin, 152
_Bilobites_, 148
Binney, E. W., 9, 10, 304, 311, 343, 351, 386, 401
Bituminous deposits, 178–183
Blackman, V. H., 119, 120
_Blandowia_, 231
Boghead, 178–183
Boring algae, 127, 128
Boring fungi, 127, 128
Bornemann, J. G., 129, 130, 148, 177
Bornet, E., 181
Bornet, E. and Flahault, 128, 129
_Bornetella_, 177
_Bornia_, 263
_B. equisetiformis_, 335
_B. scrobiculata_, 386
_B. stellata_, 338
_Bostrychus_, 210, 211
_Bothrodendron_, 133, 134
_Botrydium_, 157
_Bowmanites_, 401, 402
_B. Dawsoni_, 401
_B. Germanicus_, 408
Brazil, 79
Brodie, P. B., 240
Brongniart, A., 5, 6, 9, 11, 17, 18, 111, 112, 145, 177, 233, 269,
281, 288, 289, 291, 297, 299, 300, 301, 332, 336, 351, 390, 391
Bronn, H. G., 118
Brora, 269
Brown, A., 189, 190
_Brukmannia_, 350, 360, 361
Bryales, 236
Bryophyta, 229–241, 243
Bryozoa, 187
_Buccinum_, 159
Buckman, J., 240, 278
Bunbury, Sir C. J. F., 270, 271, 273, 284, 287, 288, 289
Bunter series, 47, 267, 292, 293
Burntisland, 88, 90, 397
_Buthotrephis radiata_, 338
Büzistock, 28
Calamarieae, 255, 295–388
_Calamitea_, 298, 300, 301, 303
_C. bistriata_, 301
_Calamites_, 18, 19, 77, 86, 94, 104, 114, 252, 254–257, 259, 260,
262, 264, 269, 285, 286, 289, 295–383, 386–388, 413
_Calamites_, sub-genera of, 381
_C. alternans_, 378
_C. approximatus_, 369, 370, 374, 378
_C. arborescens_, 363
_C. Beani_, 270, 273
_C. cannaeformis_, 374
_C. communis_, 326, 351
_C. cruciatus_, 376–378
_C. decoratus_, 374
_C. equisetiformis_, 335
_C. giganteus_, 271
_C. gigas_, 271
_C. Göpperti_, 368, 372–374
_C. lateralis_, 276
_C. laticulatus_, 386
_C. obliquus_, 386
_C. pedunculatus_, 358
_C. ramosus_, 363
_C. Sachsei_, 372
_C. scrobiculatus_, 386
_C. Suckowi_, 372, 374, 375
_C. transitionis_, 386
_C. varians_, 371
_C. variolatus_, 386
_C. verticillatus_, 372
_Calamitina_, 260, 266, 267, 329, 330, 362, 364, 365, 367–376, 381,
383, 388
_C. pauciramis_, 367
_Calamocladus_, 227, 289, 328, 329, 332, 333, 336, 337, 350, 361–364,
381, 409
_C. equisetiformis_, 333–335, 363
_C. frondosus_, 289
_Calamodendron_, 300–302, 328, 349, 364, 375, 378, 379, 380, 382
_C. bistriatum_, 300
_C. commune_, 311, 317
_C. cruciatum_, 378
_C. intermedium_, 328
_C. striatum_, 300
_Calamophyllites_, 371
_C. Göpperti_, 372
_Calamopitys_, 301, 302, 381
_Calamostachys_, 339, 342, 350, 351–357, 362, 371, 379, 381, 384,
387, 402
_C. Binneyana_, 351–355, 356, 357, 361
_C. calathifera_, 341, 363
_C. Casheana_, 355–357, 361
_C. longifolia_, 363
_C. ramosa_, 363
_C. Solmsi_, 363, 364
_C. tenuissima_, 261
_C. tuberculata_, 363
Calciferous sandstone, 43, 148, 397, 412
Callus wood, in _Calamites_, 319, 320
_Calothrix_, 126
Cambrian, 36, 37, 149, 177
de Candolle, A., 297
Carboniferous limestone, 40, 42, 43, 387
Carboniferous system, 39–45
Carinal canals, 249, 306, 307, 308
Carlsbad, 123, 126
Carpenter, W. B., 167
_Carpolithus_, 280
Carruthers, W., 9, 10, 87, 193, 197, 202, 212, 217, 240, 304, 351
Cash, W. and Hick, T., 206, 218, 219, 342
Castracane, F., 154
_Casuarina stricta_, 95, 96
_Casuarinites_, 332
_C. equisetiformis_, 335
_C. stellata_, 338
Caterpillars, Liassic, 240
_Caulerpa_, 157–159
_C. abies-marina_, 142
_C. cactoides_, 142, 158
_C. Carruthersi_, 159
_C. ericifolia_, 142
_C. plumaris_, 142
_C. pusilla_, 142
_C. scalpelliformis_, 142
_C. taxifolia_, 142
Caulerpaceae, 157–159
_Caulerpites_, 142, 158
_C. cactoides_, 158
_Celastrum_, 181
_Celluloxylon primaevum_, 198
_Ceratium_, 117
Chalk, 26, 50, 120
Challenger, H. M. S., 65, 66, 101, 117, 118, 119, 122, 151
_Chara_, 69, 223, 228
_C. Bleicheri_, 226
_C. foetida_, 224, 225
_C. Jaccardi_, 226
_C. Knowltoni_, 224, 226, 227
_C. Wrighti_, 226, 227
Characeæ, 222–228
Chareae, 223–228
Charophyta, 222–228
_Cheirostrobus_, 258, 413
_C. Pettycurensis_, 412
Chert, 227
Chlorophyceae, 127–130, 156–190
_Chondrites_, 142, 144
_C. plumosa_, 148
_C. verisimilis_, 146, 147
_Chondrus_, 231
_C. crispus_, 191
Chroococcaceae, 122, 123, 181
Church, A. H., 171
Chytridineae, 216
_Cingularia_, 290, 364
Cladochytrium, 216
_Cladosporites bipartitus_, 217, 220
Clayton, fossil tree, 71
Coal, 44, 45, 68, 76, 92, 133, 134, 178, 179
Coal-balls, 85, 86
Coalbrook Dale, 361
Coal-Measures, 40–45, 55, 84, 85, 217, 238, 256, 259, 261, 268,
282, 283, 296, 301, 302, 325, 329, 358, 375, 376, 378, 382, 397,
401, 406, 413
Coccoliths, 120
Coccospheres, 118–121
Codiaceae, 159–164
_Codium_, 159, 160
_C. Bursa_, 160
_C. tomentosum_, 160
_Coelotrochium_, 176
Coemans, E. and Kickx, J. J., 411
Coenocyte, 116, 117
Cohn, F., 123, 126
Cole, G., 83
Cone-in-cone structure, 83
_Confervites_, 177
_C. chantransioides_, 178
Confervoideae, 177–178
Conglomerate, 24
Coniferae, 232
_Convallarites_, 291
Conwentz, H., 80, 211, 212, 221
Cook, Capt., 122
Coprolites, 108, 135, 137, 182
Coral reefs, 26, 40, 48
_Corallina officinalis_, 183, 184
Corallinaceae, 183–190
Corallineae, 184
Corallines, 159, 169, 184
_Coralliodendron_, 163
Corals, fossil, 25
Corda, A. J., 5, 8
_Cordaites_, 75, 80, 99, 104, 366
Cormack, B. G., 251, 252
_Coscinodiscus_, 153
Costa, Mendes da, 3
Cotta, C. B., 9, 298
‘Crag’, 53, 62
Craigleith Quarry, Edinburgh, 81
Cramer, C., 171, 177
Credner, H., 149
Crépin, F., 109
Cretaceous system, 50, 51, 188
Cromer Forest bed, 53
_Crossochorda_, 148
_Cruziana_, 144, 145
Culm rocks, 42, 68, 383, 413
Cuticles, fossil, 68, 133
Cyanophyceae, 121, 122–132
_Cyathophyllum bulbosum_, 237
Cycadaceæ, 98
_Cycadeoidea gigantea_, 88, 214
_Cycadorachis_, 114, 115
Cycads, 49, 56, 99, 281
_Cyclocladia_, 371
_C. major_, 372
_Cymopolia_, 165, 169, 170, 172, 177
_C. barbata_, 162, 169, 171
_C. elongata_, 172
_C. Mexicana_, 169
_Dactylopora_, 175, 176
_D. cylindracea_, 176
_Dactyloporella_, 176
Darwin, C., 16, 17, 65, 70, 79, 100
Dasycladaceae, 164–176
_Dasyporella_, 176
Dawes, J. S., 297, 302
Dawson, Sir W., 147, 192, 193, 195, 338, 390, 413
_Dawsonia_, 237
_D. polytrichoides_, 237
_D. superba_, 237
Decorticated stems, 105
Deecke, W., 173
Defrance, J. L. M., 172
_Dendrophycus triassicus_, 146
Denudation, 23, 24, 35
Desmideae, 221
Devonian system, 39, 133, 173, 193, 195, 225, 256, 338, 383, 413
Diatomaceae, 150–156
Diatomite, 151
Diatoms, 117, 118, 123, 141, 185
_Dichothrix_, 123
Dicotyledons, 98, 99
Dictyogens, 99
Dinoflagellata, 118
_Diplopora_, 173–176
Dirt-beds, of Portland, 56, 57
Dismal swamp, 74
Dixon, H. N. and Joly, J., 120
Dorset, coast, 56, 226
Dover coal, 45
Drifting of trees &c., 64
Duncan, M., 127, 129
Duval-jouve, J., 247
Dyas system, 46, 209
Echinoid spines, 177
Ehrenberg, C. G., 117, 118, 151, 153
Elaters, 245
Ellis, J., 159, 169
Endophytic algae, 132
Eocene rocks, 29, 52, 164, 171
_Eophyton_, 144, 148
_Eopteris Morierei_, 106
_Ephedra distachya_, 95, 97
Equisetaceae, 19, 244–254, 255, 256
Equisetales, fossil, 254–388
Equisetales, recent, 244–254
_Equisetites_, 254, 257–273, 282, 291, 330, 413
_E. arenaceus_, 268, 269, 275, 280
_E. Beani_, 270–275
_E. Brodii_, 278
_E. Burchardti_, 279, 280
_E. Burejensis_, 280
_E. columnaris_, 72, 265, 269–271
_E. Göpperti_, 386
_E. Hemingwayi_, 259, 262–264
_E. lateralis_, 265, 275–279
_E. Monyi_, 266
_E. Mougeoti_, 267
_E. Münsteri_, 278, 279
_E. Parlatori_, 280
_E. platyodon_, 267
_E. rotiferum_, 279
_E. spatulatus_, 264, 265
_E. Vaujolyi_, 261
_E. Yokoyamae_, 280
_E. zeaeformis_, 265–266
_Equisetum_, 19, 245–254, 258, 262, 263, 267, 268, 281, 282, 285,
286, 287, 290, 292, 297, 298, 304, 307, 308, 309, 327, 337, 346,
360, 381, 384, 388, 390
_E. arvense_, 246, 247, 251, 279
_E. debile_, 95, 96, 97
_E. giganteum_, 245, 297
_E. hiemale_, 270
_E. limosum_, 263
_E. litorale_, 253
_E. maximum_, 245, 246, 247, 250–253
_E. palustre_, 247, 253
_E. ramosissimum_, 259, 265, 270
_E. sylvaticum_, 247, 279
_E. Telmateia_, 245
_E. trachyodon_, 270
_E. variegatum_, 95, 252
_E. xylochaetum_, 250
_Eryngium Lassauxi_, 99
_E. montanum_, 99
Etheridge, R., 200, 215, 290
Ettingshausen, C. von, 6, 7, 311, 332, 372
_Eucalamites_, 367, 376, 379
_Eurypterus_, 371
_E. mammatus_, 371
Feilden, Col. H. W., 211
Feistmantel, O., 284, 291, 293, 387
Felix, J., 399
Fischer, A., 217
Fish-scales, 127, 182
_Fleurs d’eau_, 132, 182
Fliche, P. and Bleicher, 232
Flint, 62
Florideae, 175, 183–190
Flowers, fossil, 70
Flysch, 147, 148, 192
_Fontinalis_, 240
Foraminifera, 26, 161, 163, 175, 185
Forbes, E., 16
Forbes, H. O., 63
Forest-bed, 53
Forests, fossil, 56
Fossil, meaning of term, 67
Fossil plants, determination of, 93–109
Fossil trees, 74, 79, 80
Fossils in half-relief, 77
Fracastaro, 2
Freeman, E. J., 23
_Frullania_, 236
Frustules of Diatoms, 150
Fuchs, T., 159, 164
_Fucoides_, 142, 191
_F. erectus_, 233
Fucoids, 87, 147, 148
_Fucus_, 191, 192, 202, 231
_F. crispus_, 191
Funafuti Island, 184
Fungi, 207–222, 305
_Galium_, 338, 389
_G. sphenophylloides_, 341
_Gallionella_, 153
Gardiner, Stanley, 184
Gardner, Starkie, 58, 240, 273
Geikie, Sir A., 39
Geinitz, H. B., 408
Geological evolution, 53
Geological history, 22–53
Germar, E. F., 297, 409
Geysers, 92, 126
_Ginkgo_, 58, 210, 390
_G. biloba_, 15
_Girvanella_, 124–126, 160
Globigerine ooze, 118
_Glœocapsa_, 122, 130
_Glœotheca_, 122
_Gloioconis_, 130
_Glossopteris Browniana_, 182
_Glossopteris_ Flora, 291, 294
_Gomphonema_, 153
_Gomphosphaeria_, 181
Gondwana system, 46–47, 84, 288, 292, 411
Gondwana Land, 294
_Goniada maculata_, 143, 144
_Goniolina_, 176, 177
Göppert, H. R., 1, 9, 146, 283, 284, 300, 301, 304, 386, 387
Göppert and Berendt, 235
Göppert and Menge, 206
Gottsche, 236
_Gottschea_, 237
Gramineae, 273
Grand’Croix, 80, 137
Grand’Eury, C., 101, 261, 262, 289, 316, 332, 336, 343, 371, 375, 381
Gray, Asa, 16
Greensand rocks, 50, 145
_Grevillea_, 99
_Gryllotalpa vulgaris_, 146
Guillemard, F. H. H., 90
Gümbel, C. W., 171, 176, 188
_Gymnostomum_, 241
_Gyroporella_, 174, 175, 176
_G. bellerophontis_, 175
_Hakea_, 99
Half-relief fossils, 77
_Halimeda_, 164, 185, 202
_H. gracilis_, 164
_H. Saportae_, 164
_Haplographites cateniger_, 217, 220
_Haploporella fasciculata_, 177
Hartig, R., 212
Harvey, W. H., 164
Hauck, F., 187, 188
Heer, O., 6, 40, 51, 68, 132, 155, 276, 280, 286, 290, 366
Heim, A., 28
_Helophyton_, 342
Henson, 118
Hepaticae, 230–236
Herzer, H., 211
Heterospory, 355, 356, 357, 406
Hick, T., 304, 306, 330
Hicks, H., 199, 200, 203
_Hippurites_, 332, 371
_H. gigantea_, 267
_H. longifolia_, 335
Holmes, W. H., 79
Homotaxis, 31
Hooker, Sir J., 10, 16, 63, 90, 139, 151, 203, 204
_Hookeria pennata_, 231
_Hostinella_, 200
Hughes, T. McKenny, 31
Humboldt, A. von, 14
Hutton, W., 87
_Huttonia_, 360, 363
Huxley, T. H., 31
_Hydatica_, 344
Hydrozoa, 187, 190
_Hyella_, 127
_Hyphochytrium infestans_, 217
Ice-Age, 17, 53
Igneous rocks, 26, 27, 35
Incrustation, 79
Incrusting springs, 68
India, 84, 287–289, 292–294, 411
Infranodal canals, 285, 324, 327, 375
Inversion of strata, 29, 30
Italy, 286, 287, 291
James, J. F., 210
_Jungermannia_, 230, 236
Jurassic system, 48–50, 55, 226, 257, 269, 282, 283, 290, 291
_Kaulfussia æsculifolia_, 97, 98
Keeling Island, 65
Keeping, W., 147
Kellaways rock, 49
Kent, S., 184
Kerguelen Land, 16
Kerosene shale, 179–182
Keuper series, 47, 48, 130
Kickx, J. J., 411
Kidston, R., 44, 82, 148, 263, 264, 334, 374, 375, 385, 386, 406,
408, 409
Kieselguhr, 150
Kiltorkan beds, 39
Kimeridge clay, 49, 78, 158, 159
Kjellman, F. R., 187
Koechlin-Schlumberger, J., 269
Kolguev Island, 211
Kölliker, A., 127
König, C., 269
_Knorria_, 101, 102, 366
Knowlton, F. H., 8, 225
Krakatoa, 58, 131
Kuntze, O., 92
Laggan Bay, Arran, 88
Lake, P., 200
Lamarck, de, 161, 175
_Laminaria_, 140, 141, 194, 202
Lamouroux, J., 161
Lapworth, C., 37, 147
Laurentian rocks, 36
Lavas, Tertiary, 58
Leasowe, 58, 59
Leckenby, J., 233, 234
Lehmann, 336
Leithakalk, 169, 187
_Leocarpus_, 206
_Lepidodendron_, 10, 57, 72, 75, 81, 82, 89, 101, 107, 216, 217, 218,
237
_L. Veltheimianum_, 101
_Lepidostrobus_, 3
_Leptothrix_, 126
Lesquereux, L., 6, 210, 240
_Lessonia_, 140, 141, 191, 193, 202
Lettenkohle, 48, 268
Lhwyd, E., 4, 112, 341
Lias, 48, 49, 61, 120, 154, 229, 240, 278
Limestone, 25
_Limulus_, 145
Lindley, T., 99, 105
Lindley, T. and Hutton, W., 10, 267, 276, 298, 330, 332, 343, 366,
369, 371, 375
Linnaeus, 113, 225
_Lithophyllum_, 183–187
_Lithothamnion_, 183–189
_L. crassum_, 185
_L. fasciculatum_, 185
_L. mamillosum_, 155, 188
_L. suganum_, 186, 188
_Lithoxylon_, 112, 386
Llandeilo flags, 37
London clay, 153
Lough Neagh, 80
Ludlow rocks, 38, 203
Ludwig, R., 212, 241
Lulworth Cove, 56
_Lunularia_, 231
Lycopodiaceae, 390, 412, 413
_Lycopodites_, 232, 237
_L. Meeki_, 240
_Lycopodium phlegmaria_, 237
Lyell, Sir C., 52, 60, 64, 65, 74, 227, 324
_Lyginodendron_, 19, 20, 88, 103, 221
Lyme Regis, 61
McClelland, 412
McCoy, F., 284, 288
McMurtrie, J., 335
_Macrocystis_, 191, 194
_Macrostachya_, 350, 360, 362, 363, 364, 371, 381
Magnesian limestone, 46
Mahr, 410
Mantell, G., 62, 145
Marattiaceae, 98
_Marchantia_, 230–235
_M. oolithus_, 232
Marchantiales, 233–236
_Marchantites_, 233–236
_M. erectus_, 233
_M. Sezannensis_, 235
_M. Zeilleri_, 234
Marcou, J., 46
Marsh, O. C., 79
_Marsilia_, 342, 390
Martin, W., 297
Massalongo, E. G., 6
_Medusa_, 144
Meek, F. B., 235
_Melobesia_, 184
Melobesieae, 184
_Memecylon_, 214
Meschinelli, 210
Mesomycetes, 208, 211
Metamorphism, 30
Michelin, H., 163, 167
_Micrococci_, 138
_Micrococcus_, 135, 136
_M. Guignardi_, 136
_M. Zeilleri_, 134
Migula, W., 222
Millstone grit, 40, 42, 43, 376
Miocene rocks, 52, 187, 241
Mississippi, rafts, 64, 65
Mniaceae, 239
Möbius, K. A., 166, 167
Monocotyledons, 99, 240, 273, 291, 366
Mougeot, A., 300
Mountain limestone, 40, 43
_Mucor_, 213, 221
_M. combrensis_, 213
Mull, leaf-beds in, 58
Munier-Chalmas, E., 161, 163, 167, 168, 171, 172, 176
Murchison, Sir R., 37, 38, 42
Murray, G., 119–121, 125, 158, 159, 170, 204, 240
Murray, J., 118, 120
Murray, Dr, 87
Muschelkalk series, 47
Musci, 236–241
_Muscites_, 238, 239, 241
_M. ferrugineus_, 241
_M. polytrichaceus_, 239
Mycelium, 207
Mycomycetes, 208, 211
_Myeloxylon_, 19, 86
_Myriophylloides Williamsonis_, 342
_Myriophyllum_, 344
Myxomycetes, 205, 206, 220
_M. Mangini_, 205, 206
_Najadita_, 240
Nansen, 151
Nathorst, A. G., 60, 77, 78, 144, 145, 148, 232, 366
_Navicula_, 153
_Nematophycus_, 192–204
_N. crassus_, 198, 201, 202
_N. Dechianus_, 201
_N. Hicksi_, 199, 201
_N. laxus_, 201
_N. Logani_, 194–197, 199, 201, 202
_N. Ortoni_, 200, 201
_N. Storriei_, 193, 198–201
_N. tenuis_, 201
_Nesea_, 161
Neumayr, M., 49
Neu Paka, 369
_Neuropteris Scheuchzeri_, 45
New South Wales, 179, 181, 182, 288
Newberry, J. S., 146
Nicholson, H. A., 338
Nicholson, H. A. and Etheridge, J., 123, 124, 190
Nicol, W., 8
_Nipa fruticans_, 63
Nitelleæ, 222, 223, 225
Nodules, calcareous, 85, 86, 108
Nomenclature, 110–115
_Nostoc_, 123, 132
Nostocaceae, 122, 123
Nuclei, fossil, 87, 88, 331
Nullipores, 171, 184, 185
_Odontocaulis Keepingi_, 147
Old Red Sandstone, 39, 204
_Oldhamia_, 146
_O. antiqua_, 145
_O. radiata_, 145, 146
_Olenellus_, 37
Oligocene, 52, 188, 209, 211, 234
_Olpidium_, 216
_Oncylogonatum carbonarium_, 269
_Onychiopsis Mantelli_, 112
_Onychium_, 112
_Oochytrium Lepidodendri_, 216, 217
Oolite, 49, 72, 269, 271, 276, 286
Oolitic structure, 123–126
_Ophioglossum_, 390
d’Orbigny, A. D., 175
Ordovician, 37, 38, 189, 338
Oscillariaceae, 122
_Osmunda_, 87
_Osmundites Dowkeri_, 212
_Ostracoblabe_, 129
_O. implexa_, 129
_Ovulites_, 161–164, 173
_O. elongata_, 163
_O. margaritula_, 162, 163, 174
Oxford clay, 49
_Pachytheca_, 202–204
_Palaeachyla_, 127, 129
_Palaeomyces_, 221
_P. gracilis_, 218
_Palaeoperone endophytica_, 215
_Palaeostachya_, 350, 357–360, 362, 364, 371, 381
_P. arborescens_, 363, 364
_P. pedunculata_, 357, 358, 362, 363
_P. vera_, 358, 359, 362
_Paleohepatica Rostafinski_, 234
_Pandanus_, 99
_Paracalamostachys_, 361
Paris basin, 161, 167, 171, 176
Parkinson, J., 1, 78
Parsons, J., 4
Peat, 26, 75, 228, 236
_Pellia_, 230, 231
Penarth, 48
Penhallow, D. P., 193, 194, 197, 201, 202, 210
_Penicillus_, 159, 161, 163, 164, 202
_P. pyramidalis_, 162
Peridiniaceae, 117
Peridiniales, 117, 118
_Peridinium_, 117, 118
_P. divergens_, 118
_P. pyrophorum_, 118
Permian system, 45, 47, 130, 182, 209, 256, 282, 283, 298, 300, 391,
408, 413
Permo-Carboniferous, 46, 213, 222, 256, 284, 289, 292, 293
_Peronosporites_, 218
_P. antiquarius_, 214, 217, 219
Petrifaction, 79–90, 92
Petrified wood, 79–90
Petrifying springs, 68
Pettycur, 88, 390, 397
Petzholdt, A., 298, 299
Phaeophyceae, 150, 191–202
Philippi, R. A., 185
Phillips, J., 275, 276
Phloeoterma, 254
_Phycodes_, 149
Phycomycetes, 218
_Phycopsis_, 192
_Phyllotheca_, 254–256, 276, 277, 281–289, 292, 293, 337
_P. australis_, 284, 287–289
_P. Brongniarti_, 286, 287
_P. carnosa_, 291
_P. deliquescens_, 283
_P. indica_, 284, 287–289
_P. Rallii_, 283
_P. sibirica_, 290
_Phymatoderma_, 120, 146, 154
_Phytolithus_, 297, 335, 373
_P. sulcatus_, 374
_Pila_, 180–182
_P. bibractensis_, 181
_P. scotica_, 181
_Pinnularia_, 344, 381
Pith-casts, of _Calamites_, 365–380
_Plagiochila_, 232
Plankton, 117, 118, 152
Plauenscher Grund, 298
Pleurococcaceae, 181
Pliocene rocks, 52, 53
Plot, R., 4
_Poacites_, 366
_Podocarpus cupressina_, 231, 232
Podostemaceae, 231
Pollen-grains, 182
_Polydonia frondosa_, 144
Polygonaceæ, 96
_Polygonum equisetiforme_, 96, 97
_Polyporus_, 208, 211, 212
_P. vaporarius_, 217, 221
_Polytrichum_, 236, 239
_Polytrypa_, 172
_Polyzoa_, 142, 231
Portland, Dirt-beds, 49, 56, 57
_Pothocites_, 385
_P. Grantoni_, 386
Potonié, R., 209, 259, 286, 337, 403
Precambrian rocks, 36
Presl, R., 5
Priority, rule of, 113
_Protannularia_, 338
Proteaceæ, 98
Protonema, 230
_Prototaxites_, 192, 193
_Psilotum_, 412
Pteridophyta, 242–414
Pteropods, 156
_Puccinia_, 213
Purbeck, 49, 56, 57, 227
_Pyrula_, 158
_Pyxidicula_, 154
Quekett, J., 127
_Rachiopteris_, 86
Radiolaria, 42, 118
Radstock, 335, 375
_Reboulia_, 231
Reefs, 184, 185
Reid, Clement, 60
Reinsch, 106
_Reinschia_, 180–182
Renault, B., 108, 130, 134, 136, 138, 178, 206, 213, 216, 217, 218,
221, 222, 309, 320, 328, 339, 340, 342, 348, 349, 362–364, 369,
375, 384, 390, 392, 399, 406
Renault, B. and Bertrand, C. E., 135, 137, 178, 180, 181
Renault, B. and Zeiller, R., 240, 266, 371
_Restio tetraphylla_, 95
Reuss, A. E., 169
Rhabdoliths, 120
Rhabdospheres, 118, 121
_Rhacophyllum_, 142
Rhaetic series, 48, 155, 278, 279
_Rhizodendron oppoliense_, 84
_Rhizogonium_, 239
Rhodophyceae, 127, 183, 190
_Richea dracophylla_, 99
Richmond, Virginia, 152
Rill-marks, 144
_Rivularia_, 126
Rivulariaceae, 129
Rock-building, 23
Rodway, J., 64
Roots, _Calamites_, 342–349
Rosanoff, 187
Rose, C. B., 127, 128
_Rosellinia congregata_, 209
_Rosellinites_, 209
_R. Beyschlagii_, 209
Rosenvinge, L. K., 187
Ross, Sir J., 139
Rothliegendes strata, 46
Rothpletz, A., 120, 147, 148, 154, 160, 175, 177, 192, 387
_Rotularia_, 390
_R. marsileaefolia_, 407
Rotuma Island, 184
Royle, J. F., 411
_Rubeola mineralis_, 341
Rufford, P., 114, 280
Rules for nomenclature, 111–115
Russia, 282
_Saccamina_, 151
Saccardo, P. A., 210
St. Étienne, 137, 390, 399
Salt Lake, 122, 123
_Salvinia_, 355, 390
Sandberger, F., 173
Sanderson, 8
Sandstone, 24
Saporta, Marquis de, 6, 106, 114, 176, 234, 235
_Saprolegnia_, 215
Sardinia, 177
_Sargassum_, 191
Saxony, 79
Scale-insects, 210
Schenk, A., 9, 279
Scheuchzer, J. T., 4, 296, 338, 389
Schimper, W. P., 9, 269, 271, 276, 322, 351, 362, 386, 411
Schimper and Mougeot, 291, 293
Schizomycetes, 121, 132–138
_Schizoneura_, 254–256, 276, 284, 285, 291–294
_S. gondwanensis_, 292, 293
_S. paradoxa_, 292, 293
Schizophyceae, 121–132
Schizophyta, 121–138
_Schizopteris dichotoma_, 232
_S. trichomanoides_, 232
_Schizothrix_, 125
Schlotheim, E. F. von, 5, 297, 332, 389
Schlüter, C., 176
Schmalhausen, J., 282, 283, 286, 289, 290
Schoenlein, J. L. and Schenk, A., 269
Schröter, J., 206
Schulze, C. F., 296
Schulze, F., 87
Schütt, F., 118, 119, 121, 154
Schweinfurth, G., 92
Schwendener, S., 414
Sclerotia, 207
_Sclerotites Salisburiae_, 210
Scott, D. H., 301, 390, 405, 412, 413
Sea sawdust, 122
Secondary thickening, 300
Sections, geological, 27–29
Sedgwick, A., 36, 37, 42
Seeds, fossil, 91
_Selaginella_, 231, 232, 237, 355
_S. Oregana_, 231, 232, 240
_S. rupestris_, 232
_Selaginellites_, 232, 236
_Sequoia_, 16
Sézanne, 70, 235
Shales, 24
Sharpe, S., 69, 227
Shell-boring organisms, 183
Shells, 24
Sheppey fruits, 4
Shrubsole, W. H. and Kitton, F., 153
Siberia, 290
_Sigillaria_, 3, 6, 10, 75, 108
Silurian system, 38, 171, 173, 176, 204
Siphoneae, 125, 156, 157–177, 193
_Siphonema_, 160
Skiddaw slate, 338
Skye, Island of, 151
_Smilax_, 99
Smith, William, 43, 48
Smith, Worthington, 217, 218
Solenhofen plants, 78
_Solenopora_, 189, 190
_S. compacta_, 189
Sollas, W. J., 146
Solms-Laubach, Graf zu, 77, 92, 149, 166–168, 175, 177, 187, 193,
194, 200, 204, 277, 282, 286, 360, 362, 399, 405
Solomon Islands, 65
Sorby, H. C., 120
Spencer, J., 342
_Sphaeriaceae_, 209, 220
_Sphaerites_, 209
_Sphaerocodium_, 160
_S. Bornemanni_, 160, 186
_Sphaerospermum_, 87
Sphagnales, 236
_Sphagnum_, 236, 241
_Sphenophyllales_, 389–414
_Sphenophyllites_, 390
_S. emarginatus_, 407
_Sphenophyllostachys_, 400, 402
_S. Dawsoni_, 402, 403, 405
_S. Römeri_, 405, 413
_Sphenophyllum_, 100, 350, 387–414
_S. antiquum_, 413
_S. cuneifolium_, 401, 402, 405, 408, 413
_S. emarginatum_, 391, 407, 408
_S. furcatum_, 386
_S. insigne_, 394, 396, 397, 400, 413
_S. myriophyllum_, 409
_S. oblongifolium_, 413
_S. osnabrugense_, 407
_S. plurifoliatum_, 394, 396–398, 400
_S. speciosum_, 411
_S. tenerrimum_, 409, 413
_S. Thoni_, 391, 410, 413
_S. trichomatosum_, 398, 408, 409
_S. truncatum_, 407
_Sphenopteris_, 77, 112
_S. Mantelli_, 112
_Sphenothallus angustifolia_, 148
_Spiridens_, 237
_S. longifolius_, 237
_Spirophyton_, 144
Sprengel, A., 7
Sprudelstein, 123
_Stachannularia_, 340, 361
Starch-grains, in fossil cells, 213
Stefani, K. de, 155, 156
Steinhauer, H., 5, 297, 373
_Stenopora crinita_, 215
Stenzel, G., 9, 298
_Stephanopyxis_, 154
Sternberg, Graf C. von, 5, 104, 257, 297, 332, 336, 363, 371, 389
_Sternbergia_, 104
Sterzel, J. T., 341, 378, 408
_Stigmaria_ and Stigmarian appendages, 3, 10, 57, 71, 72, 73, 106,
111, 123, 218, 305, 311, 348, 404
_Stigmatocanna Volkmanniana_, 386
Stokes, C., 80
Stolley, E., 172
Stoneworts, 222–228
Storrie, J., 198, 203, 204
Strata, table of, 31, 32, 33
Stratigraphical Geology, 26
Strickland, H. E., 111, 203
_Stromatopora compacta_, 189
Stur, D., 310, 343, 369, 370, 375, 376, 378, 384, 385, 386, 387, 409
_Stylocalamites_, 367, 374–376
Submerged forests, 59, 60
Suckow, G. A., 296, 297, 302
Surface-soils, 55–60
Swanage, 227
_Sycidium_, 173
_S. melo_, 155, 173
_Synedra_, 153
Tchikatcheff, P. de, 283
_Teleutospora Milloti_, 213
Tenison-Woods, J. E., 279, 291
Teredo, 61, 62
Tertiary Period, 51–53
Thallophyta, 116–228
Thiselton-Dyer, W. T., 87
Thomas, K., 71
van Tieghem, 136
_Tmesipteris_, 390
Torbanehill, 179, 182
Torbanite, 178, 179
Toula paper-coal, 68, 133
_Trametes radiciperda_, 215
Transition series, 40, 44, 413
Travertine, 69, 70, 234, 235
Treub, M., 131
Triassic system, 47, 48, 77, 78, 80, 146, 155, 160, 171, 174, 175,
256, 267, 268, 292–294
_Trichomanes Goebelianum_, 242
_Trigonocarpon_, 91
_Triploporella_, 177
_Tristichia hypnoides_, 231, 232
_Trizygia_, 411, 412
_T. speciosa_, 411
Tuedian series, 43
Tulip tree, 16
Turner, D., 191
_Tylodendron_, 104
_Udotea_, 171, 185, 202
Unconformity, 27
Underclay, 43
Unger, F., 187, 298, 299, 301
_Uteria_, 177
Vaillant, S., 225
Vascular Cryptogams, 242–414
_Vaucheria_, 157, 178
Venation, 99
_Vermiporella_, 172, 176
_Vexillum_, 149
Vinci, Leonardo da, 2
Vogelsang, H., 137
Volcanic rocks, 34, 51, 88, 89, 90
Volkmann, G. A., 296
_Volkmannia_, 350, 360, 361, 362
_V. Binneyi_, 351
_V. Dawsoni_, 401
_V. Ludwigi_, 351
Wallace, A. R., 245
Ward, L., 1
Wealden, 50, 55, 112, 114, 234, 257, 279, 280
Weed, W. H., 126
Weiss, C. E., 290, 343, 344, 351, 357, 358, 360–364, 367, 369, 370,
371, 375, 377, 388, 401, 408
Wenlock series, 38, 124, 200, 203
Wethered, E., 124
Willdenow, K. L., 231
Wille, N., 170
Williamson, W. C., 9, 10, 71, 94, 100, 103, 132, 154, 218, 220, 231,
273, 276, 301, 304, 315, 322, 324–326, 342, 346, 355, 358, 390,
392, 397, 401, 403
Williamson, W. C. and Scott, D. H., 88, 307, 312, 319, 320, 342, 346,
349, 355, 358, 390, 392, 396, 397, 405, 406
Witham, H., 7, 8
_Withamia_, 115
Wood-boring insects, 107
Woodward, J., 3, 10, 34, 71, 296
_Woronina_, 216
Wünsch, 89
Yellowstone Park, 79, 92, 126, 150
Yoredale rocks, 40, 43
Yorkshire coast, 55
Young, G. and Bird, J., 269
Zechstein series, 46
Zeiller, R., 76, 100, 133, 146, 232, 261, 264, 265, 282, 283, 289,
367, 375, 378, 390, 401, 403, 405, 406, 408, 409, 410, 411
Zigno, A. de, 6, 271, 276, 287
_Zonatrichia calcivora_, 130
_Zonatrichites_, 129, 130
_Zygosporites_, 214, 220, 221
CAMBRIDGE: PRINTED BY J. AND C. F. CLAY, AT THE UNIVERSITY PRESS.
FOOTNOTES:
[1] Ward (84).
[2] Göppert (36).
[3] Parkinson (11), vol. I.
[4] For an account of the early views on fossils, _v._ Lyell (67),
Vol. I. _Vide_ also Leonardo da Vinci (83).
[5] Woodward, J. (1695), Preface.
[6] Woodward, J. (1728), p. 59.
[7] Mendes da Costa (1758), p. 232.
[8] Scheuchzer (1723), p. 7, Pl. I. fig. 1.
[9] Parsons (1757), p. 402.
[10] Plot (1705), p. 125, Pl. VI. fig. 2.
[11] _Ibid._ Pl. VI. fig. 2.
[12] Lhwyd (1760).
[13] Artis (25).
[14] Steinhauer (18).
[15] Schlotheim (04).
[16] Sternberg (20).
[17] Brongniart (28) (28²) (49).
[18] Brongniart (39).
[19] Heer (55) (68) (76).
[20] Lesquereux (66) (70) (80) etc.
[21] Zigno (56).
[22] Massalongo (51).
[23] Saporta (72) (73).
[24] Ettingshausen (79). Also numerous papers on fossil plants from
Austria and other countries.
[25] Sprengel (28).
[26] Witham (33).
[27] _ibid._, p. 3.
[28] Witham (33), p. 5.
[29] Nicol (34). See note by Prof. Jameson on p. 157 of the paper
quoted, to the effect that he has long known of this method of
preparing sections.
[30] Limpricht (90) in Rabenhorst, vol. IV. p. 73.
[31] Knowlton (89).
[32] Corda (45).
[33] Binney (68), Introductory remarks.
[34] Williamson (71), etc.
[35] Solms-Laubach (95), p. 442.
[36] Lindley and Hutton (31).
[37] Woodward (1729), Pt. ii. p. 106.
[38] Humboldt (48), vol. I. p. 274.
[39] _Vide_ Hooker, J. D. (81), for references to other writers on
this subject; also Darwin (82), ch. XII.
[40] Darwin (87), vol. III. p. 247.
[41] Brongniart (49), p. 94.
[42] Grand’Eury (77), Potonié (96), Kidston (94), &c.
[43] Ward (92), Knowlton (94), Grand’Eury (90), p. 155.
[44] Old Persian writer, quoted by E. G. Browne in _A Year among the
Persians_, p. 220, London, 1893.
[45] W. R. W. Stephens, _Life of Freeman_, p. 132, London, 1895.
[46] Rothpletz (94).
[47] Heim (78).
[48] Geikie (93), p. 706.
[49] Hughes (79), p. 248.
[50] Whitney and Wadsworth (84).
[51] Murchison (72), p. 5.
[52] Kayser and Lake (95), p. 88.
[53] Kidston (94).
[54] Geikie (93), p. 825.
[55] Woodward, H. B. (87), p. 197.
[56] Hinde and Fox (95), p. 662.
[57] Kidston (94).
[58] _Vide_ Zeiller (92) for a list of species of Coal-Measure plants
found in the pieces of shale included in the core brought up by
the borer.
[59] Jukes-Browne (86), p. 252.
[60] Kayser and Lake (95), p. 196.
[61] Neumayr (83).
[62] Woodward, H. B. (87), p. 255.
[63] Kayser and Lake (95), p. 326.
[64] Huxley (93), p. 27.
[65] Discussed at greater length in vol. II.
[66] Woodward, H. B. (95), Figs. 124 and 133 from photographs by Mr
Strahan.
[67] Buckland (37) Pl. LVII.
[68] Young, Glen, and Kidston (88).
[69] Gardner (87), p. 279.
[70] Treub (88).
[71] Morton (91), p. 228.
[72] Lyell (45), vol. I. p. 180.
[73] Mantell (44), vol. I. p. 168.
[74] Forbes, H. O. (85), p. 254.
[75] Hooker, J. D. (91), p. 477.
[76] Hooker, J. D. (91), p. 1. There are several good specimens of the
black pyritised nipadite fruits in the British Museum and other
collections.
[77] Rodway (95), p. 106.
[78] Bates (63), p. 139.
[79] Bates (63), p. 239.
[80] Lyell (67) vol. II. p. 361.
[81] Lyell (67) vol. I. p. 445.
[82] Darwin (90) p. 443.
[83] Challenger (85), Narrative, vol. I. Pt. ii. p. 679.
[84] Bates (63) p. 389.
[85] Challenger (85), Narrative, vol. I. p. 459.
[86] Zeiller (82) and Renault (95).
[87] Heer (76).
[88] Schimper and Mougeot (44).
[89] Sharpe, S. (68) p. 563.
[90] There are still more perfect casts from Sézanne in Prof.
Munier-Chalmas’ Geological collection in the Sorbonne. The best
examples have not yet been figured.
[91] Saporta (68).
[92] Darwin (90) p. 432.
[93] For figures of fossil plants in amber, _vide_ Göppert and Berendt
(45), Conwentz (90), Conwentz (96) &c.
[94] Thomas (48).
[95] Adamson (88).
[96] Williamson (87) Pl. XV. p. 45. A very fine specimen, similar to
that in the Manchester Museum, has recently been added to the
School of Mines Museum in Berlin; Potonié (90).
[97] The British Museum collection contains a specimen of _Stigmaria_
preserved in the same manner as the example shown in fig. 12.
[98] Lyell (45) vol. I. p. 60.
[99] Lyell (45) vol. I. p. 147.
[100] Warming (96) p. 170.
[101] Bornemann (56), Schenk (67), Zeiller (82).
[102] Solms-Laubach (95²).
[103] Nathorst (86) p. 9. See also Delgado (86).
[104] Parkinson (11) vol. I. p. 431.
[105] The British Museum collection contains many good examples of the
Solenhofen plants.
[106] There is a splendid silicified tree stem from Tasmania of
Tertiary age several feet in height in the National Museum.
[107] Darwin (90) p. 317.
[108] Holmes (80) p. 126, fig. 1.
[109] Marsh (71).
[110] Conwentz (78).
[111] A large piece from one of these South African trees is in the
Fossil-plant Gallery of the British Museum.
[112] Barton (1751) p. 58.
[113] Gardner (84) p. 314.
[114] Stokes (40) p. 207.
[115] Witham (81), Christison (76).
[116] Cole (94), figs. 1 and 3.
[117] Harker (95) p. 233, fig. 56.
[118] I am indebted to Dr Renault of Paris for showing to me several
preparations illustrating this method of petrifaction.
[119] Cash and Hick (78).
[120] Stur (85).
[121] Thiselton-Dyer (72) Pl. VI.
[122] Carruthers (70).
[123] Schultze (55).
[124] I am indebted to Prof. Lebour of the Durham College of Science
for the loan of this letter.
[125] Seward (97).
[126] Williamson and Scott (96) Pl. XXIV. fig. 16.
[127] Bryce (72) p. 126, fig. 23.
[128] An erroneous interpretation of the Arran stems is given in
Lyell’s _Elements of Geology_: Lyell (78) p. 547.
[129] Guillemard (86) p. 322.
[130] Heer (55).
[131] Göppert (36), etc.
[132] Hirschwald (73).
[133] Kuntze (80) p. 8.
[134] Schweinfurth (82).
[135] Solms-Laubach (91), p. 29.
[136] Göppert (57). Some of the large silicified trees mentioned by
Göppert may be seen in the Breslau Botanic gardens.
[137] An example referred to by Carruthers (71) p. 444.
[138] Williamson (71) p. 507.
[139] Dealt with more fully in vol. II.
[140] Bentham (70).
[141] See also Bunbury (83) p. 309.
[142] Seward (96) p. 208.
[143] Darwin (90) p. 416.
[144] Solms-Laubach (91) p. 9.
[145] Balfour (72) p. 5.
[146] Grand’Eury (77) Pt. i., p. 3.
[147] 1 Renault and Zeiller (88) Pl. LX. fig. 1.
[148] Williamson (73) p. 393, Pl. XXVII. Described in detail in vol.
II. See also Solms-Laubach (91) p. 7, fig. 1.
[149] A good example is figured by Newberry (88) Pl. XXV. as a
decorticated coniferous stem of Triassic age.
[150] Potonié (87).
[151] Lindley and Hutton (31) vol. III. p. 4. See also Schenk (88) p.
202.
[152] Saporta (79) (77). _Eopteris_ is included among the ferns
in Schimper and Schenk’s volume of Zittel’s _Handbuch der
Palaeontologie_ (p. 115), and in some other modern works.
[153] Reinsch (81).
[154] Williamson has drawn attention to the occurrence of such borings
and coprolites in Coal-Measure plant tissues. _E.g._ Williamson
(80) Pl. 20, figs. 65 and 66.
[155] Renault (96) p. 437.
[156] Slide No. 1923 in the Williamson collection.
[157] Crépin (81).
[158] Rules for Zoological Nomenclature, drawn up by the late H. E.
Strickland, M.A., F.R.S., London, 1878.
[159] Lhwyd (1699).
[160] Knowlton (96) p. 82.
[161] Thiselton-Dyer (95) p. 846.
[162] Saporta (75) p. 193.
[163] Seward (95) p. 173.
[164] Ward (96) p. 874.
[165] Challenger (85) p. 934.
[166] Ehrenberg (36) p. 117, Pl. I. figs. 1 and 4, and Ehrenberg (54)
Pl. XXXVII. fig. vii.
[167] Schütt (96) p. 22.
[168] Bütschli (83–87) p. 1028.
[169] Challenger Reports (85) p. 939.
[170] Challenger Reports (91) p. 257.
[171] Hensen (92), Schütt (93) p. 44.
[172] Murray, G., and Blackman, V. H. (97).
[173] Dixon and Joly (97).
[174] Sorby (79) p. 78.
[175] Rothpletz (96), p. 909, Pl. XXIII. fig. 4.
[176] Challenger (85) _passim_. Schütt (93).
[177] Phillips W. (93).
[178] Kippis (78) p. 115.
[179] Darwin (90) p. 13.
[180] Rothpletz (92).
[181] Walther (88).
[182] Cohn (62).
[183] Murray, G. (95²).
[184] Thiselton-Dyer (91) p. 226.
[185] Nicholson and Etheridge (80) p. 28, Pl. IX. fig. 24.
[186] Wethered (93) p. 237.
[187] For figures of the sheaths of Cyanophyceous algae, see Murray
(95²), Pl. XIX. fig. 5. Gomont (88) and (92); _etc._
[188] Brown (94) p. 203.
[189] For references to the papers of Wethered and others, see Seward
(94), p. 24.
[190] E. G. Bornemann (87), Pl. II.
[191] Moseley, H. N. (75), p. 321.
[192] Weed (87–88), _vide_ also Tilden (97).
[193] Bornet and Flahault (89²) Pl. XI.
[194] Batters (92).
[195] Quekett (54), fig. 78.
[196] Kölliker (59) and (59²); good figures in the latter paper.
[197] Rose (55), Pl. I.
[198] For other references _vide_ Bornet and Flahault (89²).
[199] Duncan (76) and (76²).
[200] Similar borings are figured by Kölliker (59²), Pl. XVI. 14, in a
scale of _Beryx ornatus_ from the Chalk.
[201] Bornet and Flahault (89²).
[202] E. G. Wedl (59). Good figures are given in this paper.
[203] Bornemann (86), p. 126, Pls. V. and VI.
[204] Renault (96¹) p. 446.
[205] Treub (88).
[206] Williamson (88).
[207] Heer (55) vol. I. p. 21, Pl. IV. fig. 2.
[208] de Bary (87) p. 9. A good account of the Schizomycetes has lately
been written by Migula in Engler and Prantl’s _Pflanzenfamilien_,
Leipzig, 1896.
[209] 1 µ = 0·001 millimetres.
[210] James (98²), translation of a paper by M. Ferry in the _Revue
Mycologique_, 1893.
[211] Zeiller (82).
[212] Renault (95¹), (96¹) p. 478, (96²) p. 106. (Several figures of
the cuticles are given in these publications.)
[213] Renault (96¹) p. 492.
[214] Renault (95²), (96¹), (96²).
[215] Renault and Bertrand (94). See also Renault (95²) p. 3, (96¹) p.
449, Pl. LXXXIX. (96²) p. 94, and (96³) p. 280, fig. 3.
[216] Renault (95²) p. 17, fig. 9, (96¹) p. 460, fig. 102, and (96³) p.
292, fig. 10.
[217] Renault (96³) p. 297, fig. 14.
[218] Van Tieghem (77).
[219] Van Tieghem (79).
[220] Vogelsang (74). _Vide_ also Rutley (92).
[221] I am indebted to Prof. Kanthack for calling my attention to
an interesting account of Bacilli in small stones found in
gall-bladders; a manner of occurrence comparable to that of the
fossil forms in petrified tissues. _Vide_ Naunyn (96) p. 51.
[222] Renault (96³) p. 277.
[223] Hooker, J. D. (44) p. 457. Pls. CLXVII. CLXVIII. and CLXXI. D.
[224] An American writer has recently discussed the literature and
history of _Fucoides_; he gives a list of 85 species. It is very
doubtful if such work as this is worth the labour. (James [93].)
[225] Wille (97) p. 136, also Murray, G. (95) p. 121.
[226] Linnarsson (69) Pl. XI. fig. 3. There are many good specimens of
this fossil in the Geological Survey Museum, Stockholm.
[227] Nathorst (81²), and (96).
[228] Nathorst (81) p. 14.
[229] Mantell (33) p. 166. _Vide_ also Morris (54) p. 6.
[230] Bateson (88).
[231] Nathorst (81), (86) &c. Dawson (88) p. 26 _et seq._ Dawson (90)
Delgado (86) Williamson (85) Hughes (84) Zeiller (84) Saporta
(81) (82) (84) (86) Fuchs (95) Rothpletz (96).
[232] Kinahan (58).
[233] Sollas (86).
[234] Barrois (88). References to other records of this genus may be
found in Barrois’ paper.
[235] Zeiller (84). _Phymatoderma_ is probably a horny sponge (_vide_
p. 154).
[236] Newberry (88) p. 82, Pl. XXI. There are some large specimens of
this supposed alga in the National Museum, Washington; they are
undoubtedly of the nature of rill-marks.
[237] _Vide_ Williamson (85).
[238] Dawson (90) p. 615.
[239] Salter (78) p. 99.
[240] Lapworth (81) p. 176, Pl. VII. fig. 7.
[241] Rothpletz (96).
[242] A term applied to a certain facies of Eocene and Oligocene rocks
in Central Europe.
[243] Hall (47) Pl. LXVIII. 1 and 2, p. 261.
[244] Nathorst (83) p. 453.
[245] Seward (94²) p. 4.
[246] Kidston (83) Pl. XXXII. fig. 2. Specimens of this form may be
seen in the British Museum collection.
[247] _Cf._ Matthew, G. F. (89). Hall called attention in 1852 to the
prevalent habit of describing ‘algae’ from the older strata,
without any evidence for a vegetable origin. (Hall [52] p. 18.)
[248] Credner (87) p. 431.
[249] Saporta (84) p. 45, Pl. VII.
[250] Solms-Laubach (91) p. 51.
[251] A monograph on the Diatomaceae has recently been written by
Schütt for Engler and Prantl’s systematic work. See also Murray,
G. (97) and Pfitzer (71).
[252] Darwin (90) p. 5.
[253] Weed (87).
[254] Wilson (87).
[255] Ehrenberg (54).
[256] Noll (95) p. 248.
[257] Hooker, J. D. (44) vol. I. p. 503.
[258] Murray, J. and Renard (91) p. 208.
[259] Nansen, Daily Chronicle, Nov. 2, 1896.
[260] Schütt (93) p. 10.
[261] Ehrenberg (36) p. 77.
[262] Cayeux (92), (97).
[263] Shrubsole and Kitton (81).
[264] I am indebted to Mr Murton Holmes for specimens of these London
Clay Diatoms.
[265] Rothpletz (96) p. 910, fig. 3, Pl. XXIII. fig. 203.
[266] Schütt (96) p. 62.
[267] Castracane (76).
[268] Heer (76) p. 66, Pl. XXIII. and (53) p. 117, Pl. VI.
[269] Stefani (82) p. 103.
[270] The Chlorophyceae have recently been exhaustively dealt with by
Wille (97) in Engler and Prantl’s _Pflanzenfamilien_.
[271] _Vide_ p. 142.
[272] Murray G. (95) p. 123.
[273] Göppert (60) p. 439. Pl. XXXIV. fig. 8.
[274] Zeiller (84).
[275] Murray G. (92) p. 11; also (95) p. 127.
[276] Damon (88) Pl. XIX. fig. 12.
[277] _Vide_ also Rothpletz (96) p. 894.
[278] Ellis (1755) Pl. XXXIII. α p. 86.
[279] Fuchs (96) Pl. VIII. fig. 3.
[280] Murray G. (95) Pl. III. figs. 1 and 2.
[281] Rothpletz (90), and (91) Pls. XV. and XVI.
[282] Bornemann (87) p. 17, Pl. II. pp. 1–4.
[283] Lamouroux (21) Pl. XXV. fig. 5, p. 23.
[284] Munier-Chalmas (79).
[285] For references to genera of calcareous algae previously referred
to Foraminifera, _vide_ Sherborn (93).
[286] Lamarck (16) p. 193.
[287] Defrance (26) Pl. XLVIII. fig. 2, and Pl. L. fig. 6.
[288] Carpenter (62) p. 179, Pl. XII. figs. 9 and 10.
[289] Michelin (40–47) Pl. XLVI. fig. 24.
[290] Munier-Chalmas (79).
[291] Lamouroux (21) Pl. XXV. fig. 6, p. 23.
[292] Harvey (58) Vol. I. Pl. XXII. fig. 3.
[293] Fuchs (94).
[294] Lamouroux gives a figure of _Acetabularia_, and includes this
genus with several other algae in the animal kingdom (Lamouroux
[21] p. 19), Pl. LXIX.).
[295] Solms-Laubach (95³).
[296] D’Archiac (43) p. 386, Pl. XXV. fig. 8.
[297] Solms-Laubach _loc. cit._ p. 33, Pl. III.
[298] D’Archiac (43) p. 386, Pl. XXV. fig. 8.
[299] Michelin (40) p. 176, Pl. XLVI. fig. 14.
[300] Carpenter (62) p. 137, Pl. XI. figs. 27–82.
[301] Solms-Laubach _loc. cit._ p. 32.
[302] _Ibid._ p. 34, Pl. III. fig. 13.
[303] Andrussow (87).
[304] Solms-Laubach (95³) p. 11.
[305] Carpenter (62) Pl. XI. fig. 32.
[306] Reuss (61) p. 8, figs. 5–8.
[307] Ellis (1755) Pl. XXV. C.
[308] Solms-Laubach (91) p. 38 gives a detailed description with two
figures of a recent species of _Cymopolia_.
[309] Murray G. (95).
[310] Wille (97).
[311] Munier-Chalmas (77).
[312] Cramer (87) (90).
[313] Solms-Laubach (91) (93) (95³).
[314] Church (95).
[315] Gümbel (71).
[316] Benecke (76).
[317] Defrance (26) p. 453.
[318] Munier-Chalmas (77) p. 815.
[319] Munier-Chalmas _ibid._
[320] Defrance (26) Pl. XLVIII. fig. 1.
[321] Stolley (93).
[322] Deecke (83).
[323] Benecke (76) Pl. XXIII.
[324] Solms-Laubach (91) p. 42.
[325] Rothpletz (94) p. 24.
[326] Lamarck (16) p. 188.
[327] Carpenter (62) Pl. X.
[328] Gümbel (71). _Vide_ also Solms-Laubach (91) p. 39.
[329] Stolley (93).
[330] Schlüter (79).
[331] Saporta (91) Pl. XXXII. &c.
[332] Solms-Laubach (93), Pl. IX. figs. 1, 8.
[333] Cramer (90).
[334] Rothpletz (92²) p. 235.
[335] Steinmann (80).
[336] Solms-Laubach (91), p. 40. fig. 3. _Vide_ also Deecke (83) Pl. I.
fig. 12.
[337] Brongniart (28) p. 211.
[338] Bornemann (91) p. 485. Pls. 42 and 43.
[339] Seward (95²) p. 367.
[340] Report of the Trial (62).
[341] Bertrand and Renault (92) p. 29.
[342] Bertrand (93), Bertrand and Renault (92) (94), Bertrand (96),
Renault (96). Additional references may be found in these memoirs.
[343] Batters (92). _Vide_ also Schmitz (97) p. 315.
[344] Hauck (85) in Rabenhorst’s _Kryptogamen Flora_, vol. II.
[345] Kent (93) p. 140.
[346] Agassiz (88) vol. I. p. 82.
[347] Walther (85).
[348] _Ibid._ (88) p. 478.
[349] Philippi (37) p. 387.
[350] Rosanoff(66) Pl. VI. fig. 10.
[351] Rothpletz (91) Pl. XVII. fig. 4.
[352] Früh (90) fig. 12.
[353] Kjellman (83).
[354] Holmes and Batters (90) p. 102.
[355] Hauck (85). Rosanoff (66). Rosenvinge (93) p. 779. Kjellman (83)
p. 88. Solms-Laubach (81).
[356] Unger (58).
[357] A microscopic section of the Vienna Leithakalk is figured in
Nicholson and Lydekker’s _Manual of Palæontology_ (89) vol. II.
p. 1497.
[358] Gümbel (71) Pl. II. fig. 7, p. 41.
[359] Hauck (85) p. 272.
[360] Rothpletz (91) Pl. XVII. fig. 4.
[361] _Vide_ Walther (88) p. 499; also Jukes-Browne and Harrison (91)
_passim_. I am indebted to Mr G. F. Franks, who has studied the
Barbadian reefs, for the opportunity of examining sections of
West-Indian coral-rock.
[362] Brown A. (94).
[363] Brown A. (94) p. 147.
[364] _ibid._ p. 200.
[365] _e.g._ Saporta (82) p. 12.
[366] Turner (11) vol. II. p. 51.
[367] Rothpletz (96).
[368] Penhallow (96) p. 45.
[369] Dawson (59).
[370] _Vide_ ‘Academy’ 1870, p. 16.
[371] Carruthers (72).
[372] Penhallow (89).
[373] Barber (92).
[374] Solms-Laubach (95²).
[375] Penhallow (96).
[376] _loc. cit._ p. 83.
[377] Dawson (59), also (71) p. 17.
[378] Penhallow (89) and (96) p. 46.
[379] Barber (92) p. 336.
[380] Penhallow (89) and (93).
[381] Dawson (81) p. 302.
[382] Barber (92).
[383] Hicks (81) p. 490.
[384] Lake (95) p. 22.
[385] Solms-Laubach (95²).
[386] A similar method of fossilisation has been noted by Rothpletz in
the case of the Lower Devonian alga _Hostinella_. [Rothpletz (96)
p. 896.]
[387] Penhallow (96) p. 47.
[388] Carruthers (72) p. 162 regards this species as identical with _N.
Logani_.
[389] Seward (95³).
[390] Strickland and Hooker (53).
[391] Hicks (81) p. 484.
[392] Dawson (82) p. 104.
[393] Barber (89) and (90).
[394] Murray G. (95³).
[395] Hooker J. D. (89).
[396] _loc. cit._
[397] Solms-Laubach (95²) p. 81.
[398] Seward (94²) p. 4.
[399] An excellent monograph on the Mycetozoa has lately been issued
by the Trustees of the British Museum under the authorship of Mr
A. Lister (94). _Vide_ also Schröter (89) in Engler and Prantl’s
_Natürlichen Pflanzenfamilien_.
[400] Renault (96) p. 422, figs. 75 and 76.
[401] Schröter (89) p. 32, fig. 18 B.
[402] Cash and Hick (78²) Pl. VI. fig. 3.
[403] Göppert and Menge (83) Pl. XIII. fig. 106.
[404] Harper (95).
[405] _e.g._ Ludwig (57) Pl. XVI. fig. 1.
[406] Potonié (93) p. 27, Pl. I. fig. 8.
[407] References are given by Potonié to illustrations by Zeiller (92²)
Pl. XV. fig. 6, Grand’ Eury (77) Pl. XXXIII. fig. 7, and others
in which possible fungi are represented.
[408] Engelhardt (87).
[409] For figures of the Coccineae, see Comstock (88), Maskell (87),
Judeich and Nitsche (95) &c.
[410] Massalongo (59) Pl. I. fig. 1, p. 87.
[411] Meschinelli (92).
[412] James, J. F. (93²).
[413] Lesquereux (87).
[414] Herzer (93).
[415] Conwentz (90) Pl. XII. fig. 5.
[416] Feilden, H. W. (96); Seward (96²) p. 62, appendix to Feilden’s
paper. I am indebted to Dr Bonney for an opportunity of examining
the plant remains from the Feilden collection.
[417] Hartig (78).
[418] Conwentz (80) Pl. V. fig. 17.
[419] Carruthers (70) Pl. XXV. fig. 3.
[420] Renault (96) p. 427, fig. 80, _d_.
[421] _ibid._ p. 427, fig. 80, _a–c_.
[422] p. 127.
[423] Etheridge (92) Pl. VII.
[424] Hartig (78) and (94), Göppert and Menge (83).
[425] Renault (96) p. 425, fig. 78.
[426] Fischer in Rabenhorst, vol. i. (92) p. 144.
[427] Carruthers (76) p. 22, fig. 1.
[428] Smith, W. O. (77) p. 499.
[429] Williamson (81) Pl. XLVIII. p. 301.
[430] Renault (96) p. 439, figs. 88 and 89.
[431] Cash and Hick (78²).
[432] Cash and Hick, Pl. VI. fig. 3.
[433] Felix (94) p. 276, Pl. XIX. fig. 1.
[434] _ibid._ p. 274, Pl. XIX. figs. 5 and 6.
[435] Williamson (78) and (80).
[436] Conwentz (90) p. 119, Pl. XI. pp. 2, 3, Pl. XV. fig. 8.
[437] Migula (90) in Rabenhorst’s _Kryptogamen Flora_, vol. V.
[438] Vaillant (1719) p. 17.
[439] Migula (90) p. 53.
[440] Knowlton (89²).
[441] Meek (73) p. 219.
[442] Saporta (73) p. 214, Pl. IX. figs. 8–11.
[443] Seward (94²) p. 13, fig. 1.
[444] Woodward, H. B. (95) pp, 234, 261, _etc._
[445] Forbes, E. (56) p. 160, Pl. VII.
[446] _Vide_ p. 69, fig. 10.
[447] Lyell (29).
[448] Skertchly (77) p. 60.
[449] Schiffner and Müller in Engler and Prantl (95), Campbell (95),
Dixon and Jameson (96) are among the best of modern writers on
the Bryophyta.
[450] Hooker, J. D. (91) p. 513.
[451] Schiffner (95) p. 140.
[452] Hooker, W. J. (20) Pl. CLXIII.
[453] Bennett and Brown (38), Pl. V.
[454] Bennett and Brown (38) p. 35.
[455] Nathorst (90) Pl. II. fig. 3.
[456] Lindenberg (39) Pl. IX. fig. 1.
[457] Zeiller (92²) Pl. I. figs. 7 and 8.
[458] Fliche and Bleicher (81).
[459] Brongniart (49) p. 12.
[460] Seward (94²) p. 17.
[461] Leckenby (64) Pl. XI. fig. 3.
[462] Seward _loc. cit._ p. 18, Pl. I. fig. 3.
[463] Raciborski (94) p. 10, Pl. VII. figs. 1–3.
[464] Brongniart (49) p. 12.
[465] Saporta (68) p. 308, Pl. I. figs. 1–8. _Vide_ also Watelet (66)
p. 40, Pl. XI. fig. 6.
[466] Göppert and Berendt (45) Pl. VI. and (53).
[467] Gottsche (86).
[468] Schimper (65) Pl. III.
[469] Greville (47) Pl. XII.
[470] Brown, R. (11) Pl. XXIII.
[471] Hooker, W. J. (20) Pl. CLXII.
[472] Limpricht (90) p. 67.
[473] Brongniart (28²) p. 93.
[474] Renault and Zeiller (88) p. 84, Pl. XLI. figs. 2–4.
[475] Solms-Laubach (91) p. 186.
[476] Lesquereux (79) Pl. LXII. fig. 1.
[477] Heer (65) p. 89.
[478] Buckman (50) 1.
[479] Gardner (86) p. 203.
[480] Ludwig (59) p. 165, Pl. LXIII. fig. 9.
[481] Schimper and Schenk (90) p. 75.
[482] _e.g._ the Fern _Trichomanes Goebelianum_ Gies. Giesenhagen (92)
p. 157.
[483] Scott (96) a text-book for elementary students; a full account
is given of _Equisetum_ and other genera of primary importance.
Vines (95) Part iii. Campbell (95), Luerssen (89) in Rabenhorst’s
_Kryptogamen-Flora_, vol. III., Van Tieghem (91), de Bary (84),
Baker (87).
[484] Wallace (86) p. 117.
[485] Baker (87) p. 4. Hooker, W. J. (61) Pl. LXXIV. _Vide_ also Milde
(67) for figures of _Equisetum_.
[486] Seeman (65).
[487] Duval-Jouve (64) Pl. I. fig. 5.
[488] Milde (67) Pl. XIX. fig. 8.
[489] _ibid._ Pl. XXXI. fig. 3.
[490] Cormack (93) p. 71.
[491] Williamson and Scott (94) p. 877. These authors, in referring
to Cormack’s description of the secondary nodal wood of _E.
maximum_, express doubts as to the existence of such secondary
growth in _all_ species of the genus.
[492] Pfitzer (67).
[493] Strasburger (91) p. 443.
[494] Strasburger (91) p. 435.
[495] Bower (94) p. 495.
[496] Sternberg (38) p. 43.
[497] Scott (97). This genus will be described in Volume II.
[498] Potonié (93) Pl. XXV. fig. 1_a_.
[499] _ibid._ p. 179. _Vide_ also Potonié (92).
[500] Zeiller (92²) p. 56, Pl. XII. Other similar leaf-sheaths have
been figured by Germar (44) Pl. X., Schimper (74) Pl. XVII. and
others.
[501] Grand’Eury (90) p. 223, Pl. XV. fig. 16.
[502] Renault (93) Pl. XLII. figs. 6 and 7.
[503] Kidston (92).
[504] Zeiller (95).
[505] Potonié (93) p. 179, Pl. XXV. figs. 2–4.
[506] Renault and Zeiller (88) p. 396, Pl. LVII. fig. 7.
[507] Lindley and Hutton (31) Pl. CXIV.
[508] Schoenlein and Schenk (65) Pl. V. fig. 1.
[509] Schimper and Mougeot (44) p. 58, Pl. XXIX.
[510] Jäger (27).
[511] Schimper (74) Pls. IX–XI.
[512] Schimper and Koechlin-Schlumberger (62).
[513] Schoenlein and Schenk (65) Pls. I–IV.
[514] Brongniart (28) p. 115, Pl. XIII.
[515] Young and Bird (22) p. 185, Pl. III. fig. 3.
[516] König, in Murchison (29) p. 293, Pl. XXXII.
[517] Murchison (29) p. 368.
[518] Bunbury (51) p. 189.
[519] Schimper (69) p. 267.
[520] Zigno (56) Pl. III. fig. 1, p. 45.
[521] Gardner (86) Pl. IX. fig. 3.
[522] Williamson (83) p. 4.
[523] Williamson and Scott (94) p. 889, Pl. LXXIX. fig. 19.
[524] Phillips J. (29) Pl. X. fig. 13.
[525] Lindley and Hutton (31) Pl. CLXXXVI.
[526] Bunbury (51) p. 189.
[527] Zigno (56) Pl. III. fig. 3, p. 46.
[528] Heer (77) p. 43, Pl. IV.
[529] Schimper (69) p. 284. _Vide_ also Nathorst (80) p. 54.
[530] Andrae (53) Pl. VI. figs. 1–5.
[531] Solms-Laubach (91) p. 180.
[532] _cf._ p. 283.
[533] There is a similar specimen in the Oxford Museum.
[534] Since this was written I have found a specimen of _Equisetites
lateralis_ in the Woodwardian Museum, in which a diaphragm
like that in fig. 64, C, occurs in the centre of a flattened
leaf-sheath similar to that of fig. 64, B.
[535] Buckman (50) p. 414.
[536] Schenk (67).
[537] Tenison-Woods (83), Pl. VI. figs. 5 and 6. Specimen no. V. 3358
in the British Museum.
[538] Dunker (46) p. 2, Pl. V. fig. 7.
[539] Seward (94²) p. 30.
[540] Seward (94²) p. 33.
[541] Heer (55) vol. III. p. 158, Pl. CXLV.
[542] Heer (77) p. 99, Pl. XXII.
[543] _Vide_ Saporta (73) p. 227.
[544] The distribution will be dealt with in Volume II.
[545] Brongniart (28) p. 151.
[546] Schmalhausen (79) p. 12, Pl. I. figs. 1–3.
[547] Solms-Laubach (91) p. 181.
[548] Zeiller (96).
[549] Zeiller (96²).
[550] _ibid._ (96).
[551] Letter, July 30, 1897.
[552] On this character of Annularian leaves, _vide_ p. 337.
[553] Göppert (45) p. 379, Pl. XXV. figs. 1, 2.
[554] Schmalhausen (79) p. 12.
[555] McCoy (47) Pl. XI. fig. 7.
[556] Bunbury (61) Pl. XI. fig. 1.
[557] Seward (97²) p. 324, Pl. XXIV. fig. 1.
[558] Feistmantel (81) Pl. IX. A. fig. 7, &c.
[559] _ibid._ (90) Pl. XIV. fig. 5.
[560] Bunbury (61) Pl. XI. fig. 1.
[561] Solms-Laubach (91) p. 181, fig. 17.
[562] Heer (82) p. 9.
[563] Potonié (96²) p. 115, fig. 3.
[564] Zigno (56) Pl. VII. p. 59.
[565] Bunbury (61).
[566] Feistmantel (81), Pl. XII. A.
[567] Brongniart (28) p. 152.
[568] Bunbury (61).
[569] Grand’Eury (90) p. 221.
[570] Etheridge (95).
[571] Weiss (76) p. 88.
[572] Heer (77) p. 43, Pl. IV. (78) p. 4, Pl. I.
[573] Tenison-Woods (83) Pl. IX. fig. 2.
[574] Brongniart (28) p. 128.
[575] Schimper and Mougeot (44) p. 48, Pls. XXIV–XXVI.
[576] Feistmantel (81) p. 59, Pls. I. A–X. A.
[577] _ante_, p. 284.
[578] Schimper and Mougeot (44) p. 50, Pls. XXIV–XXVI.
[579] Seward (97²).
[580] Scheuchzer (1723), p. 19, Pl. IV. fig. 1.
[581] Volkmann (1720), p. 110, Pl. XIII. fig. 7.
[582] Woodward, J. (1728), Pt. II. p. 10.
[583] Schulze, C. F. (1755), Pl. II. fig. 1.
[584] Suckow (1784), p. 363.
[585] Steinhauer (18), Pls. V. and VI.
[586] Martin (09), Pls. VIII. XXV. and XXVI.
[587] Artie (25).
[588] Brongniart (22), p. 218.
[589] Brongniart (28), p. 34.
[590] Lindley and Hutton (31).
[591] Cotta (50). I am indebted to Prof. Stenzel of Breslau for calling
my attention to the fact that Cotta’s work appeared in 1832, but
in 1850 the same work was sold with a new title-page bearing this
date.
[592] Unger (40).
[593] Petzholdt (41).
[594] Unger (44).
[595] Brongniart (49), p. 49.
[596] E.g. _Isoetes_, _Botrychium_, &c.
[597] Mougeot (52).
[598] Göppert (64), p. 183.
[599] ἄρθρον, joint; πίτυς, Pine-tree.
[600] The original specimens described by Göppert are in the rich
palaeobotanical Collection of the Breslau Museum.
[601] Williamson (71³), p. 174.
[602] _vide_ Solms-Laubach (96).
[603] Letter, November 1897.
[604] _Vide_ p. 310.
[605] Hick (94), Pl. IX. fig. 1.
[606] On this point _vide_ Williamson and Scott (94), p. 869.
[607] Williamson and Scott, _loc. cit._ p. 876.
[608] Renault (93), Pl. XLVII. fig. 4.
[609] Stur (87).
[610] The term _primary ray_ may be conveniently restricted to the
truly primary interfascicular tissue, and the term _principal
ray_ may be used for the outward extension of the primary rays by
the cambium [Williamson and Scott (94), p. 878].
[611] Binney (68).
[612] Ettingshausen (55).
[613] The sections of fossil plants described by Binney were presented
to the Woodwardian Museum, Cambridge, by his son (Mr J. Binney).
[614] _Vide_ footnote, p. 311.
[615] Williamson (83²), Pl. XXXIII. fig. 19.
[616] Williamson (78), p. 323, Pl. XX. figs. 14 and 15.
[617] Williamson and Scott (94), p. 888.
[618] Hartig (94), pp. 149, 297, _etc._
[619] Renault (96), p. 91.
[620] Williamson and Scott, _loc. cit._ p. 893. _Vide_ specimens
133*–135* in the Williamson Collection.
[621] _E.g._ specimen 132*** in the Williamson Collection.
[622] _Vide_ Williamson (71), Pl. XXVIII. fig. 38; (71²), Pl. IV. fig.
15; (78), Pl. XXI. figs. 26–28. Williamson and Scott (94), Pl.
LXXII. figs. 5 and 6. Renault (93), Pl. XLV. figs. 4–6, etc.
Felix (96), Pl. IV. figs. 2 and 3.
[623] Strasburger (91), Pl. II. fig. 40.
[624] Williamson (78).
[625] Williamson (71), p. 507.
[626] Williamson (71²), Pl. I. fig. 1; (78), Pl. XXI. fig. 31.
[627] Lyell (55), p. 368.
[628] Williamson (96), p. 194.
[629] _Vide_ specimens 15–17, etc. in the Williamson Collection.
[630] The stem of fig. 83 is an example of _Arthrodendron_, but the
appearance of the secondary xylem agrees with that in some forms
of _Arthropitys_.
[631] For figures of this type of stem _vide_ Göppert (64); Cotta (50),
Pl. XV. (specimens 13787 in the British Museum Collection);
Mougeot (52), Pl. V.; Stur (87), pp. 27–31; Renault (93), Pls.
XLIV. and XLV. etc.
[632] Williamson (71), (71²), (87), fig. 5.
[633] Williamson and Scott (94), p. 879
[634] _Vide_ Williamson (87²). In this paper Williamson compares the
three subgenera of Calamite stems. Renault and Zeiller (88), Pl.
LXXV. Renault (93), Pls. LVIII. and LIX.
[635] Renault (96), p. 125; (93), Pl. LIX. fig. 2.
[636] Lindley and Hutton (31), Pls. CXIV., CXC. etc. Most of the
specimens figured by these authors are in the Newcastle Natural
History Museum. For notes on the type-specimens of Lindley and
Hutton, _vide_ Howse (88) and Kidston (90²).
[637] Weiss (88), Stur (87), _etc._
[638] _Vide_, p. 367.
[639] _Ante_, p. 260.
[640] Hick (95).
[641] Brongniart (22), p. 235.
[642] Schlotheim (20).
[643] Brongniart (28), p. 159.
[644] Lindley and Hutton (31), Pl. CXC.
[645] Ettingshausen (55).
[646] Schimper (69), p. 323.
[647] Grand’Eury (90).
[648] Martin (09), Pl. XX. figs. 4 and 6.
[649] Schlotheim (20), p. 397.
[650] Sternberg (25), p. xxviii.
[651] Brongniart (28), p. 154.
[652] Lindley and Hutton (31), Pl. CXCI.
[653] Ettingshausen (55), p. 28.
[654] Schimper (69), Pls. XXII. and XXVI. fig. 1.
[655] For other lists and synonyms, _vide_ Zeiller (88), p. 368, and
Kidston (86), p. 38 and (93), p. 316, also Potonié (93), p. 162.
[656] Sternberg (20).
[657] Brongniart (28), p. 155.
[658] Lehmann (1756), p. 127. _Vide_ also Volkmanns (1720), Pl. _XV._
p. 113.
[659] Potonié (93), pp. 169 _et seq._, Pl. XXIV.
[660] Dawson (71).
[661] Nicholson (69) Pl. XVIII. _B._ Nicholson’s specimens are in the
Woodwardian Museum, Cambridge.
[662] Schlotheim (20), p. 397.
[663] Sternberg (26), p. xxviii.
[664] Brongniart (28), p. 156.
[665] Lindley and Hutton (31), Pl. CXXIV.
[666] Binney (68), Pl. VI. fig. 3.
[667] Stur (87), Pl. XVI b, and Pls. IV b and XIII.
[668] Scheuchzer (1723), p. 63, Pl. XIII. fig. 3.
[669] Potonié (93), p. 166.
[670] _Vide_ pp. 351 _et seq._
[671] Renault (96), p. 66; (93), Pl. XXVIII.
[672] One of the finest specimens of _Annularia stellata_ is figured
by Stur (87), Pl. XVI b; it is in the Leipzig Museum. VIDE also
Schenk (83), Pl. XXXIX.; Germar (44), Pl. IX.; Renault and
Zeiller (88), Pls. XLV. and XLVI. There are some well-preserved
impressions of _A. stellata_ in the British Museum from Radstock,
Newcastle and elsewhere.
[673] Zenker (33), Pl. V. pp. 6–9.
[674] Heer (65), fig. 6, p. 9, and other authors.
[675] Weiss (76), p. 27, Pl. III. fig. 2.
[676] Lhwyd (1699), Pl. V. fig. 202.
[677] Sterzel (82).
[678] _Vide_ Weiss (76), Pl. III. and Weiss (84), p. 178.
[679] Williamson (71), p. 487, Pls. XXV. and XXVI.
[680] _Ibid._ (78), p. 319, Pl. XIX.
[681] Cash and Hick (81), p. 400.
[682] Williamson (81), _vide_ also Spencer (81).
[683] Spencer (83), p. 459.
[684] Williamson (83), p. 459, Pls. XXVII.–XXX.
[685] Renault (85).
[686] Lindley and Hutton (31), Pls. LXXVIII. and LXXIX. (The specimens
are figured in a reversed position.)
[687] Binney (68), p. 5, fig. 1.
[688] Grand’Eury (77), Pls. I. and II.; (87), Pls. XXVII., XXVIII.
[689] Weiss (84), Pls. II.–IV., VIII. and IX.
[690] Stur (87), Pls. III., VI., VII., _etc._; Zeiller (86), Pl. LIV.
[691] Weiss (76), (84).
[692] For references, _vide_ Kidston (86), p. 58.
[693] Artis (25), Pl. V.
[694] Williamson and Scott (95), p. 694.
[695] Williamson (83²), Pl. XXIX. fig. 7.
[696] Williamson and Scott (95), Pls. XV.–XVII.
[697] For figures _vide_ Williamson, _loc. cit._, Williamson and Scott,
and Renault (85), (93).
[698] _E.g._ Schenk (90) in Zittel’s _Handbuch_, p. 237.
[699] Williamson and Scott, _loc. cit._ p. 689.
[700] de Bary (84), p. 474; van Tieghem (91), p. 720.
[701] Renault (96), pp. 118, 126; (93), Pl. LV.
[702] Carruthers (67), Pl. LXX.
[703] Sternberg (25), Pl. XLVIII. and LI.
[704] Brongniart (49), p. 51.
[705] Binney (68), p. 23, Pls. IV. and V.
[706] Schimper (69), p. 330.
[707] For figures and descriptions of this type of cone _vide_
Williamson (73), (80), (89); Hick (93), (94) and Williamson and
Scott (94).
[708] Weiss (84), Pls. XXII.—XXIV.
[709] Williamson and Scott (94), p. 911, Pls. LXXXI. and LXXXII.
[710] _Vide_ Heinricher (82); Bower (94), p. 495; Campbell (95), pp.
396, 503.
[711] Williamson (81), Pl. LIV.
[712] An excellent figure illustrating the co-existence of heterospory
and secondary thickening is given by Williamson and Scott, _loc.
cit._, Pl. LXXXII. fig. 36.
[713] Weiss (76), p. 103.
[714] Williamson (71²).
[715] _Ibid._ (88²).
[716] Williamson and Scott (94), p. 900.
[717] Weiss (84), Pl. XXI. fig. 4.
[718] Williamson (74), Pl. V. fig. 32.
[719] Williamson (88²), Pl. IX. fig. 20.
[720] _E.g._ Renault (82), Pl. XIX. fig. 1; (96), Pl. XXIX. figs. 1 and
4.
[721] Williamson (88²), Pl. VIII. figs. 1 and 4.
[722] Renault (93), Pl. XXIX. fig. 7.
[723] Solms-Laubach (91), p. 325.
[724] Weiss (84) p. 161. Solms-Laubach, _loc. cit._ p. 326.
[725] _E.g._ _Volkmannia Ludwigi_ Carr., also _Volkmannia elongata_
Presl. [Solms (91), p. 332 and Weiss (76), p. 108].
[726] _E.g._ _Brukmannia Grand’Euryi_ Ren. [Renault (76)].
[727] Weiss (84), p. 190.
[728] Weiss (76), p. 1.
[729] Weiss (84), p. 161.
[730] Renault (82), p. 139; (76).
[731] Solms-Laubach (91), p. 330.
[732] Schenk (88), p. 132; (83), p. 232.
[733] Weiss (84), p. 161.
[734] _E.g._ _Volkmannia gracilis_ Sternb. [Renault (76), Pl. II.].
[735] Schimper (69), p. 332. _Vide_ also Renault and Zeiller (88), p.
420.
[736] Renault (82), p. 120, Pl. XIX.; (93), Pl. XXIX. figs. 8–14; (96),
p. 77.
[737] Weiss (84), p. 98, Pl. XX. etc.
[738] Sterzel (82).
[739] Renault and Zeiller (88), Pl. XLVI. fig. 7.
[740] Kidston (86), p. 47; (93), p. 319. _Vide_ also Renault (93), Pl.
XXVIII.
[741] Renault and Zeiller (88), Pl. XLV.
[742] Solms-Laubach (91), p. 339. Weiss (84), p. 159.
[743] Weiss (84), Pl. XX. fig. 6.
[744] Weiss (84), Pl. XX. fig. 7; Pl. XXI. fig. 4.
[745] _Ibid._ Pls. XIV. and XV. _Cf._ also Stur (87), Pls. VI. and VII
b, and Lesquereux (84), Pl. XC. fig. 1.
[746] Grand’Eury (90), pp. 205, 208. Renault and Zeiller (88), Pl. LI.
[747] _Vide_ Unger (50), p. 63.
[748] Weiss, _loc. cit._
[749] Solms-Laubach (91), p. 328.
[750] Schenk (83), p. 234.
[751] Weiss (76), p. 88; (84), p. 162. Solms-Laubach, _loc. cit._ p.
334, fig. 47.
[752] Renault (96), p. 132.
[753] Zeiller (88), Pl. LIV. fig. 4.
[754] Stur (87), p. 17.
[755] Lindley and Hutton (31), Pl. CXLII B. The original specimen is in
the University College Collection, London.
[756] Nathorst (94), p. 56, Pl. XV. figs. 1 and 2.
[757] Seward (88).
[758] Weiss (84), p. 54.
[759] Zeiller (88), p. 329.
[760] Weiss (76), p. 117; (84), p. 55.
[761] Weiss (84), p. 93, Pl. XI. fig. 1.
[762] _Vide_ Weiss (84), Pl. XXV. fig. 2; Pl. XVI _a_, etc.
[763] Weiss (76), Pl. XVII. fig. 1.
[764] Weiss (84), Pl. I.
[765] Grand’Eury (90), p. 208, and (77), Pl. V.
[766] Lindley and Hutton (31), Pl. CXC.
[767] Stur (87), Pl. V. fig. 1.
[768] Grand’Eury (77), Pl. IV.
[769] Stur (87), Pl. XVII.
[770] Renault (82), Pl. XVII. fig. 2.
[771] Renault and Zeiller (88), Pt. II. p. 434, Pls. LII. and LIII.
[772] Stur (87), p. 37, fig. 17. _Vide_ also Grand’Eury (90), p. 208.
[773] Stur, _loc. cit._
[774] Weiss (84), p. 61.
[775] _Vide_ Grand’Eury (77), Pl. V. fig. 5.
[776] Weiss, _loc. cit._ Pl. XXI. fig. 5.
[777] Stur (87), Pl. XI. fig. 1.
[778] Renault and Zeiller (88), Pt. II. Pl. LI. p. 423.
[779] Lindley and Hutton (31), Pl. CXIV. and Pl. CXC. The original
specimens are in the Natural History Museum, Newcastle-on-Tyne.
[780] _Ibid._ Pl. CXXX. and Feistmantel (75), Pl. I. fig. 8.
[781] Lesquereux (79), Pl. XIII. fig. 14.
[782] Salter (63), figs. 6 and 7. _Vide_ also Carruthers in Woodward,
H. (72), p. 168.
[783] Grand’Eury (69); _vide_ also (77) and (90).
[784] Renault and Zeiller (88), Pt. II. Pls. LII. and CIII.
[785] _Ibid._ Pl. LI.
[786] Sternberg (21), Pl. XII.
[787] Weiss (84), p. 61.
[788] Ettingshausen (55), Pl. I. fig. 4.
[789] Ettingshausen, _loc. cit._
[790] Grand’Eury (69), p. 709.
[791] Feistmantel (75), Pl. I. fig. 8.
[792] Williamson (74), Pl. VII. fig. 45.
[793] Weiss (76), Pl. XVII. figs. 1 and 2.
[794] Weiss (84), Pl. XVI a. figs. 10 and 11.
[795] _Ibid._ Pl. XXV. fig. 2.
[796] Stur (87), Pl. II. etc.
[797] Zeiller (88), p. 363, Pl. LVII. fig. 1.
[798] Steinhauer (18), Pl. VI. fig. 1.
[799] Kidston (93), p. 311, Pl. II.
[800] Kidston (94), p. 248.
[801] Weiss (84), p. 119.
[802] Steinhauer (18), Pl. V. figs. 1 and 2.
[803] Artis (25), Pl. XXIV.
[804] Brongniart (28²), Pls. XV. and XVI.
[805] Lindley and Hutton (31), Pl. LXXIX.
[806] Kidston (93), p. 314; (86), p. 24.
[807] Zeiller (88), p. 333.
[808] Lindley and Hutton (31), Pl. LXXIX.
[809] Weiss (84), Pl. IV. fig. 1.
[810] Renault and Zeiller (88), p. 385.
[811] Grand’Eury (90), p. 214.
[812] Stur (87), p. 160, Pl. IX. fig. 2.
[813] Kidston (94), p. 249.
[814] Grand’Eury (89), p. 1087.
[815] Zeiller (88), p. 355.
[816] Weiss (84), p. 96.
[817] Sterzel (93), p. 66.
[818] Zeiller, _loc. cit._ p. 353.
[819] Sternberg (25), Pl. XLIX. fig. 5.
[820] Brongniart (28²), p. 128, Pl. XIX.
[821] Germar and Kaulfuss (31), p. 221, Pl. XLV. fig. 1.
[822] Lindley and Hutton (31), Pl. CCXVI.
[823] Grand’Eury (77), p. 293.
[824] Zeiller (80), Pl. CLXXIV. (expl. plates) fig. 3.
[825] Weiss (84), pp. 112, 113, 114.
[826] Zeiller (88), p. 353.
[827] Kidston (94), p. 249.
[828] Grand’Eury (90), p. 216 (expl. plates).
[829] Stur (87), p. 68.
[830] Stur (87), Pl. X.
[831] Artis (25), Pl. II.
[832] Weiss, _loc. cit._ Pls. V. VI. and X.
[833] Grand’Eury (77), (90).
[834] _Vide_ Stur (75), _etc._ for remarks on the course of the
vascular strands.
[835] For good figures of the leaves _vide_ Stur (75), Rothpletz (80),
Ettingshausen (66), Solms (96).
[836] Renault (96), p. 80; (93), Pls. XLII. and XLIII. Since the
above was written an account of the internal structure of
_Archaeocalamites_ has been published by Solms-Laubach (97); he
describes the wood as being of the _Arthropitys_ type.
[837] Renault, _loc. cit._ Pl. XLII. figs. 6 and 7.
[838] Stur (75), p. 2, Pls. II.–V.
[839] An examination of the specimens in the Museum of the Austrian
Geology Survey did not enable me to satisfactorily verify the
features of the cone as described by Stur; the impressions are
far from clear.
[840] Kidston (83²).
[841] _Vide_ Paterson (41); Lyell (67), vol. I. p. 149 _etc._
[842] Volkmann (1720), p. 93, Pl. VII. fig. 2.
[843] Schlotheim (20), p. 402, Pl. XXII. fig. 4.
[844] Sternberg (25).
[845] Brongniart (28²), p. 122, Pl. XXVI. figs. 1 and 2.
[846] Paterson (41), Pl. III.
[847] Göppert (52), Pls. _III._, _V._, _VI._, _VIII._, _XXXVIII._
[848] Ettingshausen (66), Pls. I.–IV.
[849] Feistmantel (73), Pl. XIV. fig. 5.
[850] Zeiller (80), p. 17.
[851] Binney (68), p. 7.
[852] Stur (75), p. 3.
[853] Kidston (86), p. 35.
[854] Schimper and Koechlin-Schlumberger (62), Pl. I. The original
specimens of Schimper’s figures are in the Strassburg Museum.
[855] Feistmantel (73), p. 491, Pl. XXIV. figs. 3 and 4.
[856] _Vide_ specimens 20 A, 20 B, 24 in the Williamson Collection.
[857] Stur (75), p. 17.
[858] Rothpletz (80), p. 8.
[859] Weiss (84), p. 56.
[860] Scheuchzer (1723), p. 19, Pl. IV. fig. 1.
[861] Schlotheim (04), Pl. II. fig. 24, p. 57.
[862] Sternberg (25), p. 32.
[863] Brongniart (22), Pl. XIII. fig. 8, p. 234.
[864] _Ibid._ (28), p. 68.
[865] Dawson (66), p. 153, Pl. XII.
[866] For reference _vide_ an excellent monograph by Coemans and Kickx
(64), also Potonié (94).
[867] _e.g._ Newberry (91).
[868] Renault (73), (76²), (96).
[869] Williamson (74), (78).
[870] Williamson and Scott (94), p. 919.
[871] Specimen 929 in the Williamson Cabinet is a longitudinal section
of the French _Sphenophyllum_, as described by Renault (76²).
[872] Williamson and Scott (94), p. 926.
[873] Williamson (91), p. 18.
[874] Williamson and Scott (94), p. 920.
[875] Williamson (91), p. 12.
[876] _Ibid._ (74).
[877] Felix (86), Pl. VI. fig. 2.
[878] For figures _vide_ Renault (82), Pl. XVI. fig. 1, (76²) Pls. VII.
and IX.
[879] Williamson (71²).
[880] Weiss (84), p. 200.
[881] Binney (71).
[882] Williamson (91²).
[883] Zeiller (93).
[884] Williamson (92).
[885] Potonié (94), fig. 1.
[886] For a more complete account of this strobilus _vide_ Zeiller
(93), and Williamson (91²), _etc._
[887] Zeiller (93), p. 37.
[888] Williamson and Scott (94), p. 943.
[889] Scott (97), p. 24.
[890] Solms-Laubach (95⁴).
[891] _Ibid._ Pl. X. fig. 6.
[892] Kidston (90).
[893] I am indebted to my friend Mr Kidston for an opportunity of
examining these specimens.
[894] _Vide_ Renault (77), (96), p. 158. Zeiller (93), p. 34.
Williamson and Scott (94), p. 942.
[895] Brongniart (22), p. 234, Pl. II. fig. 8.
[896] Brongniart (28), p. 68.
[897] Bischoff (28), Pl. XIII. fig. 1.
[898] Römer, F. (62), p. 21, Pl. V. fig. 2.
[899] Sterzel (86), pp. 26, 27, etc.
[900] Kidston (93), p. 333.
[901] Zeiller (88), p. 414.
[902] _Ibid._ p. 411.
[903] Weiss (84), p. 201, Pl. XXI. fig. 12.
[904] Solms-Laubach (95⁴), p. 232.
[905] Geinitz (55), Pl. XX. fig. 7.
[906] Zeiller (88), Pl. LXIV. figs. 3–5, and (93), p. 24, Pl. II. fig.
4.
[907] Stur (75), p. 108.
[908] Stur (87), Pl. XV. and Kidston (90), p. 59, Pl. I.
[909] Zeiller (88), Pl. LXII. figs. 2–4.
[910] Stur (87); Williamson (74); Seward (89), _etc._
[911] Renault (82), p. 84, and Newberry (91).
[912] Kidston (90), p. 62.
[913] Stur (75), p. 114, Pl. VII.
[914] Zeiller (93), p. 32.
[915] Mahr (68), Pl. VIII.
[916] Zeiller (80), p. 34, Pl. CLXI. fig. 9.
[917] Coemans and Kickx (64); Zeiller (80), (88); Schimper (69).
[918] Royle (39), p. 431.
[919] For other figures of this plant, _vide_ Feistmantel (81), Pls.
XI. A and XII. A.
[920] Zeiller (91).
[921] _Vide_ also Zeiller (92²), p. 75.
[922] Feistmantel (81), p. 69.
[923] Scott (97).
[924] Scott (96²), p. 15.
[925] Dawson (61), p. 10, fig. 7.
[926] Kidston (94), p. 250.
[927] Kidston (94), p. 250.
[928] Sterzel (93), p. 143.
[929] Zeiller (94), p. 172.
[930] De Bary (84), p. 499.
[931] Westermaier and Ambronn (81).
[932] Schwendener (74), p. 124. Haberlandt (96), p. 165.
Transcriber’s Notes:
- Text enclosed by underscores is in italics (_italics_).
- Text enclosed by equals is in bold (=bold=).
- Text enclosed by tildes is in antiqua (~antiqua~).
- Blank pages have been removed.
- Obvious typographical errors have been silently corrected.
- Odd-numbered page headings are entered at approximate locations as
sidenotes, except where repeated, or duplicated in headings.
- Errata from Vol. II. applied.
- Some headings and table of contents have been modified to be more
consistent, including removal of section list under Chapter
IX as sections II-IV are in other chapters.
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