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Title: On Molecular and Microscopic Science, Volume 1 (of 2)
Author: Somerville, Mary
Language: English
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ON
MOLECULAR AND MICROSCOPIC
SCIENCE
VOLUME THE FIRST
_By the same Author._
ON the CONNECTION of the PHYSICAL SCIENCES. 9th Edition. Portrait. Post
8vo. 9_s._
PHYSICAL GEOGRAPHY. New Edition, thoroughly revised. Portrait. Post
8vo. [_In the press._
[Illustration:
_Fig. 89, p. 20._
EUCYRTIDIUM CRANOIDES.
[_Frontispiece to Vol. I._
]
ON
MOLECULAR
AND
MICROSCOPIC SCIENCE
BY MARY SOMERVILLE
AUTHOR OF ‘THE MECHANISM OF THE HEAVENS’ ‘PHYSICAL GEOGRAPHY’
‘CONNECTION OF THE PHYSICAL SCIENCES’ ETC.
_Deus magnus in magnis, maximus in minimis_—St. Augustine
IN TWO VOLUMES—VOL. I.
WITH ILLUSTRATIONS
LONDON
JOHN MURRAY, ALBEMARLE STREET
1869
_The right of translation is reserved_
LONDON: PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
PREFACE.
MICROSCOPIC INVESTIGATION of organic and inorganic matter is so
peculiarly characteristic of the actual state of science, that the
Author has ventured to give a sketch of some of the most prominent
discoveries in the life and structure of the lower vegetable and marine
animals in addition to a few of those regarding inert matter.
The Author feels bound to return her best thanks to kind friends—Sir
John Herschel, Mr. Huggins, Mr. Gwyn Jeffrey, Prof. Tyndall, and Mr. T.
Moore of Chelsea, who have aided in revising some of the sheets for
press, and have thus counteracted the disadvantage under which she
labours, of a residence abroad, and at a distance from libraries of
reference.
CONTENTS
OF
THE FIRST VOLUME.
PART I.
ATOMS AND MOLECULES OF MATTER.
SECT. PAGE
I. ELEMENTARY CONSTITUTION OF MATTER 1
II. ON FORCE, AND THE RELATIONS BETWEEN FORCE AND 23
MATTER
III. ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, 93
UTILITY OF WASTE SUBSTANCES—COAL-TAR COLOURS, ETC.
IV. THE SOLAR SPECTRUM, SPECTRUM ANALYSIS, SPECTRA OF 129
GASES AND VOLATILIZED MATTER, INVERSION OF
COLOURED LINES, CONSTITUTION OF SUN AND STARS
PART II.
VEGETABLE ORGANISMS.
I. MICROSCOPIC STRUCTURE OF THE VEGETABLE WORLD 167
II. ALGÆ 179
III. FUNGI 260
IV. LICHENS 298
V. CHARACEÆ 312
VI. HEPATICÆ, OR LIVERWORTS 316
VII. MUSCI, OR MOSSES 323
VIII. FILICES, OR FERNS 335
IX. EQUISETACEÆ, OR HORSETAILS 367
X. MARSILEACEÆ, OR RHIZOSPERMÆ 371
XI. LYCOPODIACEÆ, OR CLUB MOSSES 373
XII. GENERAL STRUCTURE OF FLOWERING PLANTS 378
XIII. MONOCOTYLEDONOUS, OR ENDOGENOUS PLANTS 383
XIV. DICOTYLEDONOUS, OR EXOGENOUS PLANTS 404
ILLUSTRATIONS
TO
THE FIRST VOLUME.
(_The Author is indebted to the works of Dr. Carpenter, Rev. M. J.
Berkeley, Mr.
Gosse, and Mr. Darwin, for the larger part of these Illustrations._)
FIG. PAGE
89. Eucyrtidium cranoides _frontispiece_
1. Form of stratified discharge in a vacuum tube 79
2. Form of stratified discharge in a vacuum tube, as 81
affected by an electro-magnet
3. Development of Ulva 171
4. Vertical section of the cuticle of Iris germanica 173
5. Longitudinal section of stem of Italian reed 176
6. Palmoglœa macrococca 182
7. Protococcus pluvialis 184
8. Volvox globator 189
9. Various species of Staurastrum 192
10. Economy of Closterium Lunula 193
11. Diatoma vulgare and Grammatophora serpentina 197
12. Biddulphia pulchella 198
13. Pleurosigma angulatum 199
14. Actinocyclus undulatus 200
15. Meridion circulare 201
16. Bacillaria paradoxa 203
17. Cell multiplication in Conferva glomerata 207
18. Zoospores 208
19. Threads of Rivularia nitida 215
20. Trichodesmium erythræum 216
21. Conjugation of Zygnema quininum 217
22. Ulva latissima 225
23. Polyides rotundus and Furcellaria fastigiata 229
24. Vertical sections of conceptacles of Gracilaria 230
armata, Grinnelia americana, and Corallina
officinalis
25. Callithamnion corymbosum 232
26. Rhabdonia Coulteri, Sphærococcus coronopifolius, 236
Wrangelia penicillata, and Cruoria pellita
27. Dictyurus purpurascens 242
28. Polyzonia cuneifolia 243
29. Fruit of various species of Ectocarpus 245
30. Dictyota dichotoma 247
31. Vertical section of receptacle of Fucus 254
platycarpus
32. Various species of Pucciniæi 276
33. Puccinia Graminis 280
34. Various species of Mucedines 286
35. Torula Cerevisiæ, showing successive stages of 287
cell-multiplication
36. Various species of Sphæriacei 294
37. Various species of Lichens 299
38. Sporopodium Leprieurii, Coccocarpia smaragdina, 300
and Lecanora affinis
39. Paulia perforata, Calicium tympanellum, and 301
Graphis Leprevostei
40. Nitella flexilis 302
41. Antheridia of Chara fragilis 314
42. Further development of antheridia of Chara 315
fragilis
43. Marchantia polymorpha 317
44. Anatomy of frond of Marchantia polymorpha 318
45. Archegonia of Marchantia polymorpha 319
46. Elater and spores of Marchantia 319
47. Funaria hygrometrica 324
48. Polytrichum commune, group of antheridia 325
49. Polytrichum commune, development of spermatozoids 325
50. Microscopic structure of leaves of mosses 330
51. Development of spores of Pteris serrulata 336
52. Antheridium and spermatozoids of Pteris serrulata 338
53. Archegonium of Pteris serrulata 338
54. Section of footstalk of fern frond 341
55. Pinnule of Polypodium bearing sori 342
56. Sporangia of Polypodiaceous ferns 343
57. Pinnule of Lastrea Filix-mas with sori 346
58. Sorus and indusium of Polystichum or Aspidium 347
59. Pinna of Polystichum Lonchitis 348
60. Sorus and cup-shaped indusium of Deparia prolifera 350
61. Scolopendrium vulgare 351
62. Athyrium Filix-fœmina 353
63. Asplenium Ruta-muraria 353
64. Ceterach officinarum 355
65. Blechnum Spicant 357
66. Pteris aquilina 357
67. Adiantum Capillus-Veneris 359
68. Trichomanes radicans 361
69. Hymenophyllum tunbridgense 362
70. Equisetum giganteum 368
71. Pilularia minuta 371
72. Orchis mascula, side view of flower 389
73. Orchis mascula, front view of flower 390
74. Orchis mascula, pollinium 391
75. Orchis mascula, pollen grains 391
76. Orchis mascula, pollinia 392
77. Orchis pyramidalis, front view of flower 393
78. Orchis pyramidalis, side view of flower 394
79. Orchis pyramidalis, disc with one pollinium 395
80. Orchis pyramidalis, pollinia, attached to disc 395
81. Orchis pyramidalis, pollinia, with disc contracted 395
82. Orchis pyramidalis, pollinia, withdrawn 395
83. Epipactis palustris, side views of flower 397
84. Epipactis palustris, side view and dissection of 398
flower
85. Listera ovata, side view of flower 399
_Errata._
Page 3, _line_ 5 _from bottom, insert_ the _before_ earth
59, _line_ 19, _dele_ the
59, _line_ 21, _dele_ the
59, _lines_ 20 _and_ 22, _for_ part _read_ parts
100, _In the Table of Atomic Weights, read_ Copper 32; Zinc 32·5;
Rubidium 86; Cæsium 133
101, _lines_ 10 _and_ 33, _for_ 32 _read_ 32·5
104, _line_ 12 _from bottom, for_ 29 _read_ 32
MOLECULAR AND MICROSCOPIC
SCIENCE.
PART I.
ATOMS AND MOLECULES OF MATTER.
SECTION I.
ELEMENTARY CONSTITUTION OF MATTER.
THE INVESTIGATIONS which have revealed the most refined and wonderful
relations between light, heat, electricity, and highly elastic media;
the relation of these powers to the particles of solid and liquid
matter, new methods of analysis, and the microscopic examination of that
marvellous creation, animal and vegetable, which is invisible to the
unaided eye of man, have brought a new accession to the indefinitely
small within the limits of modern science.
Wherever the astronomer has penetrated into the depths of space,
luminous points are visible; and since light merely consists in the
undulations of the ethereal medium, matter must exist in every part of
the universe of which man is cognizant, for although the luminiferous
ether is so attenuated that its very existence is almost an hypothesis,
its atoms are not more inconceivably small than those of highly elastic
ponderable matter on earth. Atoms are the ultimate constituents of
homogeneous simple substances; molecules, or groups of heterogeneous
atoms united in definite proportions, constitute such as are compound.
High pressure steam is invisible as it issues from the boiler, yet each
of its molecules contains two atoms of hydrogen and one of oxygen. The
perfume of a flower is a compound invisible substance formed of
molecules.
We know nothing of the forms either of atoms or of those groups of atoms
which we call molecules; but we cannot suppose them otherwise than as
excessively hard, since conceive them how we will, we are sure that an
atom, whatever be its form or nature, is ever the same. It never wears,
it never changes, though it may have formed part of thousands of bodies
and entered into thousands of combinations, organic and inorganic; when
set free by their dissolution, it is ready to enter into a new series;
it is indestructible even by fire, the same now as when created. Nor has
the quantity of matter in our terrestrial abode ever been increased or
diminished; liable to perpetual change of place and combination, the
amount remains the same: the bed of the seas may be changed to dry land,
and the ocean may again cover the lofty mountains, but the absolute
quantity of matter changes not.
All substances, whether solid, liquid, or aëriform, are supposed to
consist of hard separate atoms or particles, and in conformity with that
supposition to be surrounded by the ethereal medium, otherwise they
could not transmit light and heat, which are merely vibrations of that
medium. Even the hardest and most compact substances are capable of
compression, and have been compressed to an enormous degree by the
hydraulic press; but it probably transcends mechanical force to bring
their atoms into contact: in fact, no known substance is impervious to
_both_ light and heat, however thin.
By far the greater number of terrestrial substances consist of
heterogeneous atoms chemically combined into atomic systems or
molecules; but there are sixty-four which have never yielded to chemical
analysis, and are therefore believed to be respectively formed of only
one kind of atoms. Thirty-five of these are metals found either pure or
as ores, and sixteen are metals existing naturally in chemical
combination with alkalies, alkaline earths, or earthy bases, that is as
salts, from which they have been obtained by the analytical power of
electricity or other means. The thirteen remaining simple substances are
non-metallic: some are aëriform, some solid, one liquid.
The alkaline metals are sodium, potassium, lithium, cæsium, rubidium,
and thallium. They are distinguished by their energetic affinities for,
and the simplicity of their compounds with, non-metallic elements. They
are never met with native, and are amongst the most difficult metals to
reduce from their ores, and their spectra are remarkable for simplicity.
Sodium and potassium—which have been such important agents in spectrum
science—were reduced from their alkalies of soda and potash by Sir
Humphry Davy by means of the voltaic battery, a discovery which led the
way to the reduction of many of the others. Lithium is a white metal
which burns brilliantly in air and oxygen; it swims in naphtha, and is
the lightest solid body known. Cæsium is the most energetic of all
metals in its chemical affinities.
The metals of the alkaline earths are barium, strontium, calcium, and
magnesium. They possess, like the preceding, energetic affinities for
the non-metallic elements, and are reduced with difficulty from their
ores. Barium is obtained from earth baryta: it is powerfully alkaline,
and its salts are colourless and poisonous. Calcium is obtained from
limestone, chalk, marble, and gypsum, which are amongst the most
abundant constituents in the crust of the earth; it is a bright ductile
metal of a bronze colour. Magnesium, which is a brilliant silver-white
hard brittle metal, is obtained from magnesium limestone or dolomite.
Although the ores of calcium and magnesium cover vast areas of the
globe, the metals form a very small comparative proportion of them.
The metals derived from non-alkaline earths are glucinum, yttrium,
thorinum, zirconium, and aluminium, which is the only one of any
interest: it is now becoming a very useful metal. It combines readily
with oxygen to form clay. The ruby, sapphire, and oriental topaz are
merely coloured varieties of corundum, which is nothing but crystallised
clay. Rubidium, cæsium, and thallium were discovered by spectrum
analysis.
The avidity of some of these metals for oxygen is quite remarkable:
potassium and rubidium inflame when they touch ice or cold water; they
decompose the water and combine with its oxygen. Calcium becomes
luminous in warm water, and burns with intense light when heated to
redness; but a magnesium wire burns with such intense brilliancy that it
has been employed for photography, and will probably become useful for
household purposes, as two ounces and a half of magnesium wire when
burnt give a light equal to that of twenty pounds’ weight of stearine
candles.
The metals whose oxides are not reducible by heat without the aid of
some form of carbon include nearly all the useful metals. They are all
polyatomic, that is, they combine with other elements in the number of
atoms varying from two to eight, and are divided into seven groups in
regard to this property. For instance, zinc, copper, and cadmium are
diatomic. Zinc is invaluable as a source of electric light and heat in
the voltaic battery, and its vapour burns brilliantly. Copper is one of
the most useful of metals, while cadmium is of no value at all. Nickel,
cobalt, and uranium form the triatomic group; they are remarkable for
their complex spectra. Nickel is usually an ingredient in meteorites;
cobalt is employed in pigments and in sympathetic inks; and the oxide of
uranium is used to stain glass, and gives it some very peculiar
properties, as will be shown. The precious metals have a feeble affinity
for oxygen at any temperature, and their oxides are decomposed by heat
alone, and sometimes even by the undulations of light.
Metals are excellent conductors of heat, but they vary exceedingly in
that respect; both theory and experiment prove that the best conductors
are invariably the worst radiators. In fact those atoms which transfer
the greatest amount of motion to the ethereal medium, that is, which
radiate most powerfully, are the least competent to communicate motion
to each other, that is, to conduct with facility. Silver and copper are
the best conductors of heat, but the worst radiators. These two metals
are the best conductors of electricity, but it is influenced by
temperature; for MM. Matthiessen and Von Bose’s experiments have proved
that all pure metals in a solid state vary in conducting power to the
same extent between zero and 100° Cent., and that the alkaline metals
conduct electricity better when heated than when cold.
All metals are capable of being vaporized, but at very different degrees
of temperature. Platinum requires the heat of the oxy-hydrogen blowpipe,
which by estimation amounts to 8801° Cent. This property makes it
valuable for terminal points to the conducting wires of the voltaic
battery and magneto-electric induction machine where great heat can be
employed without fusing the platinum terminals. Copper is always
employed for the conducting wire on account of its superior conductive
power. The coil of wire in the magneto-electric machine, which is often
miles long, is insulated by a coating generally of green silk thread.
But in experiments of extreme delicacy where magnetism might vitiate the
results, perfectly pure copper wire which is diamagnetic is used for the
conducting wires in the thermo-electric pile of the goniometer, and the
wires are coated with white silk thread, since it was discovered that
the green dye contains some magnetic metal.
The mass of the metals however constitutes comparatively but a small
part of the terrestrial globe, which is formed of chemical combinations
of only thirteen simple elementary substances,—a wonderful manifestation
of creative power that could form a world of such variety and beauty by
means of atoms so little diversified; still more wonderful is it that
four simple elements alone constitute the basis of nearly the whole
organic fabric. The air we breathe, water, the bodies of men and living
creatures, and the vegetation that adorns the earth, are chiefly
combinations of three invisible gases, oxygen, hydrogen and nitrogen,
with carbon, the purest amorphous form of coal.
Oxygen gas forms three-fourths of the superficial crust of the
terrestrial globe, its productions and its inhabitants. At least a third
part of the solid crust of the earth is oxygen in combination; it
constitutes eight parts out of nine in water, and water covers
three-fourths of the surface of the globe; it forms more than twenty
parts out of a hundred of atmospheric air, and in the organic kingdom it
is an essential constituent. Except in the atmosphere, oxygen is never
uncombined, but may be obtained by distilling chlorate of potash, by the
decomposition of water by voltaic electricity, and by other means. When
pure it is a colourless, tasteless, inodorous, invisible gas; it is
incombustible at ordinary temperatures, yet absolutely essential to
combustion; no animal can live long in it, and none can exist without
it. In the atmosphere oxygen is highly magnetic; its magnetism increases
with cold and decreases with heat; hence its intensity varies with night
and day, winter and summer, but its magnetic property vanishes when it
enters into composition.
Oxygen is perfectly quiescent and passive as a gas in the atmosphere,
and as a constituent of water and solid bodies, yet that inactivity
conceals the most intense energy, which only requires to be called into
action. Thus combustion of extreme intensity takes place when ignited
sulphur is put into a vessel containing oxygen gas; the metal potassium
is instantly inflamed by it on touching water; some of its combinations
with chlorine are highly explosive, and phosphorus burns in it with
dazzling splendour. Thus a stupendous amount of energy is latent in
oxygen under the most tranquil appearance.
M. Schönbein of Basle discovered that oxygen exists in another state,
which has neither the extreme quiescence on the one hand, nor the
intense violence on the other, of its ordinary form; and to express that
intermediate condition, in which its activity is less in amount and
different in quality, it has been called by another name, viz. ozone,
from the following peculiarity.
It had long been observed that there is a peculiar smell when an
electric machine is in activity, and when objects are struck by
lightning; that smell Professor Schönbein ascertained to arise from the
change of oxygen into ozone, and actually produced ozone by passing
electric sparks through that gas. Ozone differs from oxygen in having a
strong smell and powerful bleaching property; it purifies tainted air,
changes vegetable colours, and stains starch prepared by iodide of
potassium blue, which thus becomes a test of its presence; yet it
certainly is oxygen in an allotropic or changed state, for it readily
oxidizes or rusts silver and other metals, and when ozonized gas is sent
through a red-hot tube, it comes out pure oxygen. According to the
experiments of Messrs. Tait and Andrews, oxygen gas loses six eighths of
its volume, and becomes four times more dense by the change; it
contracts more readily with obscure electricity than with the spark. The
experiments of Professor Tyndall on the absorption of radiant heat by
gases give reason to believe that ozone is produced by the packing of
the atoms of elementary oxygen into oscillating groups, and that heating
dissolves the bond of union and restores the ozone to the form of
oxygen. Ozone chiefly exists in air that has passed over a great expanse
of sea, and the quantity is increased during the aurora, which alone
might lead to a surmise of that phenomenon being electric.
The change of oxygen into ozone is not the only instance of
Allotropism,—that is to say, the existence of the same substance in two
states differing from each other in every respect,—for ozone itself is
allotropic. Professor Schönbein has discovered that there are two kinds
of ozone standing to one another in the relation of positively and
negatively active oxygen; namely ozone and antozone, which neutralize
each other into common oxygen when brought into contact. In this respect
they are analogous to electricity, and, like electricity too, one kind
cannot be produced without a simultaneous development of the other.
When a metal, such as silver for example, is oxidized or rusts, it gives
polarity to the atoms of oxygen in the atmosphere and divides them into
the opposite states of ozone and antozone; the ozone combines with the
silver and rusts or oxidizes it, at the same time that the antozone is
dissolved in the moisture or aqueous vapour in the air and forms
peroxide of hydrogen. The oxidized or rusted silver, as well as every
other oxidized substance, is an ozonide, while the peroxide of hydrogen
is an antozonide.
Since both kinds of ozone are produced during the decomposition of water
by electricity, and as sea air is always found to contain more or less
free ozone, the ocean is probably an antozonide, for all the antozone
formed by electricity during thunderstorms must be either dissolved in
the sea-water, or carried into it in the form of peroxide of hydrogen by
the rain. Ozone must be exceedingly abundant in the zone of calms and
light breezes near the equator known as the variables, which is subject
to heavy rains and violent thunderstorms, and also in the regions of the
monsoons. On land one of the benefits arising from these formidable
phenomena is the production of ozone, which oxidizes decomposing organic
matter and hastens its decay, while the antozone, which is dissolved in
the atmospheric vapour, forms the peroxide of hydrogen and frees the air
from the antagonist principle.
The peroxide of hydrogen thus produced is a transparent colourless
inodorous liquid with a metallic taste, and contains one equivalent of
hydrogen and two of oxygen. It retains its liquid state under a great
degree of cold, and mixes with water in any proportion. It has a strong
bleaching property, instantly destroying vegetable colour. If exposed
suddenly to a temperature of boiling water it is decomposed with violent
explosion, and readily gives off oxygen at 59° Fahr. The mere touch of
an oxidized metal, as the oxide of silver, completely and
instantaneously decomposes it, and oxygen gas is evolved by the union of
the ozone and antozone so rapidly as to produce a kind of explosion
attended by an intense evolution of heat.
During the combustion of phosphorus in the atmosphere both kinds of
ozone appear, and Professor Schönbein considers the slow combustion of
that substance, which unites with the ozone and sets the antozone free,
as the type of all the slow oxidations which organic and inorganic
bodies undergo in moist atmospheric air; that true oxidation is always
preceded by the appearance of the peroxide of hydrogen, and that this
compound acts an important part in slow oxidations, and is deeply
concerned in animal respiration, and in many other chemical actions
going on in nature.
In confirmation of these views, it is certain that ozone is a powerful
minister in the work of decay. If wood be made explosive like gun-cotton
by a similar process, it becomes pulverulent after a time, and burns
without exploding, though it still retains its shape. In the natural
state of the wood the oxygen is passive and quiescent, for oxygen is a
constituent of wood; in its second state it is explosive, and after a
time that is succeeded by the semi-active state of ozone, which by a
slow imperceptible combustion causes the wood to decay. Mr. Faraday
observes that the force which would have been explosive had it been
concentrated into one effort, expends itself in a long continued
progressive change.
‘The majestic phenomena of combustion bespeak our admiration and rivet
our attention because of their imposing grandeur; yet these are but
spasmodic efforts in the grand economy of the material world,
occurrences of now and then. The slower but continuous progress of the
elements to their appointed resting-place, the silent, tranquil, ever
progressing metamorphic changes involved in the phenomena of
decomposition and decay, these we count for nothing and pass unheeded
by. Yet with all their majesty, with all their brilliancy, all their
development of tremendous energy, what are the phenomena of combustion
in the grand scheme of the universe compared with these? When the loud
crash of the thunder or the lightning’s flash awakens us from our
thoughtless abstractions or our reveries, our feelings become impressed
with the grandeur of Omnipotence and the might of the elements he
wields, yet the whole fury of the thunderstorm—what is that in
comparison with the electric energies which silently and continually
exert themselves in every chemical change? Why, the electric force in a
single drop of water, and disturbed when that water is decomposed, is of
itself greater than in the electricity of a whole thunderstorm. Those of
us who limit our appreciation of the powers of oxygen to the energies
displayed by this element in its feebly active state, form but a very
inadequate idea of the aggregate results accomplished by it in the
economy of the world.’ Oxygen is the only known gas that is allotropic,
and is the only known substance that is doubly allotropic, that is
existing in three different states similar to oxygen, ozone, and
antozone.
Hydrogen when pure is an invisible gas without smell or taste; it is a
constituent of various acids and alkalies, but is itself neither acid
nor alkaline. It is highly inflammable, burning with a pale light, and,
as already mentioned, a combined jet of oxygen and hydrogen produces
heat of 8801°, which is so intense that nothing can withstand it. It is
the lightest substance known. A balloon having the form of a globe ten
feet in diameter, would hold 32-1/2 pounds weight of common air, while
two pounds weight of hydrogen gas would fill it. Associated with this
small quantity of ponderable matter, hydrogen has an enormous power of
combination, but its activity is only called forth by some exterior and
exciting cause. A mixture of two measures of hydrogen and one of oxygen
gas would remain inert for ever, but the instant an electric spark is
sent through it, a bright flash and an explosion takes place, and the
result is water: thus a tremendous force lies quiescent in that bland
element.
Hydrogen gas is introduced into the atmosphere by imperfect combustion,
but it is instantly diffused and becomes harmless, for aëriform fluids
are capable of rapid and perfect diffusion through one another, each
having a capacity peculiar to itself, which under the same circumstances
is greater as its density is less; therefore hydrogen the lightest of
gases not only rises in the air on account of its levity, but is more
quickly and completely diffused than oxygen which is the support of
life. Though hydrogen is inferior in density to every other gas, it
surpasses them all in conducting electricity, just as silver and copper
conduct electricity better than platinum, though far less dense. The
great refrigerating power of hydrogen is owing to its extreme mobility
and consequent rapid convection of heat, in which it surpasses all other
gases. It is as permeable to radiant heat as atmospheric air, has a very
high refractive power, a specific heat of 3·2936, and may be substituted
in many chemical formulæ for a metal, without altering their character:
hence it is sometimes called a metalloid.
The quantity of nitrogen gas or azote that exists in nature is enormous.
It constitutes four-fifths of the atmosphere, whence it may be had in a
pure state, as well as by chemical means. Like oxygen, this gas is
permanently elastic, without smell, taste, or colour; it is neither acid
nor alkaline, it does not change vegetable colours, it neither burns nor
supports combustion, and is incapable when breathed of supporting animal
life. It abounds in organic bodies, in all parts of the animal texture,
in the blood, muscles, nerves, even in the brain; and is either a highly
nutritious or poisonous principle in the vegetable kingdom.
Nitrogen gas is altogether passive; it has no affinity for the metals,
and cannot be liberated from any of its compounds even by electricity.
Excepting boron and titanium, it will not combine directly or
spontaneously with any simple element, even under the highest
temperature, but its indirect combinations are numerous and violent:
those with hydrogen are either noxious or poisonous, those with oxygen
are all deadly poisonous. Had nitrogen combined spontaneously with
either of these gases, especially with oxygen, life would have been
impossible as the organized creation is constituted; its inertness
renders its mixture with oxygen in atmospheric air innocuous. However,
combinations of nitrogen and hydrogen, forming nitrate of ammonia, have
been discovered in the atmosphere by Professor Schönbein, the union of
evaporation, heat and air being the cause; and as evaporation is
continually going on, he concludes that nitrate of ammonia, nitrates and
other salts are generated in the moist air, and are speedily washed down
in our rainy climates into the springs and rivers. He considers the
formation of nitrates out of water as highly important for vegetation,
because each plant becomes a generator of a portion at least of its
azotized food, while the rain furnishes the ground on which it stands
with a supply of the same.
In the atmosphere, nitrogen has all the mechanical properties of common
air, but with a greater refractive power, and its specific gravity is
nearly the same with that of oxygen. Since the atmospheric gases are the
most permeable to radiant heat, the earth is in the most favourable
circumstances for being warmed by the solar rays, and thus the
properties of the elementary gases are admirably adapted for our
comfort, nourishment, safety, and pleasure.
Carbon, which combined with the three elementary gases forms the basis
of the organic creation, is widely distributed throughout the globe, in
enormous coal formations, the vegetation of former ages. Diamond is its
purest crystalline form; and charcoal, which is wood whence the volatile
matters have been driven off by heat, is its purest amorphous state. To
this simple substance and to hydrogen, we are indebted for terrestrial
light and heat, whether our fuel be coal or wood, our light a candle or
a lamp. The products of combustion are carbonic acid gas, whether pure
or mixed with smoke, for ashes are the incombustible earthy matter mixed
with coal or wood, and smoke is unconsumed carbon arising from the bad
construction of our chimneys; so that the waste is enormous in a great
city like London where coal is the only fuel. Light is given out by
incandescent solid particles, which become luminous sooner than gas, for
all gases have a feeble illuminating power, and heat results from the
chemical combination of the carbon with oxygen, a process in which the
chemical force merges into its correlative heat. Mr. Faraday observes,
that had the result of the combination of carbon and oxygen been a gas
only, we should have had very little light, and had it been a permanent
solid, the world would have been buried in its own ashes.
Diamond and pure carbon leave no residuum when consumed; they combine
with the oxygen of our atmosphere into carbonic acid gas, which is
invisible, poisonous, and so heavy, that it may be poured from one
vessel to another like water, thereby showing how much carbon it
contains in an invisible state. The quantity of carbonic acid gas thrown
into the atmosphere in this invisible yet ponderous state is immense,
since six tons weight of atmospheric air rushes hourly through an
average size blast furnace, carrying with it more than half a ton of
carbon in the form of that gas, whose constitution and properties are
always the same, whether it arises from combustion, fermentation, or
respiration, which latter may be regarded as a slow combustion,
consuming us to the bones if not supplied with carbon by means of food.
It has been computed that two thousand million pounds weight of oxygen
gas is daily converted into carbonic acid gas by these operations, and
given into the atmosphere, which would soon be contaminated by its
poison and suffocating quality, were it not for vegetables which
decompose it, assimilate the carbon and set the oxygen free to mingle
with the air and make it again fit for respiration. Carbon has a greater
power of combination than any other simple substance except hydrogen.
Mr. Faraday compressed carbonic acid gas into a liquid by the pressure
of its own elasticity when disengaged from combination in close vessels,
a force equal to the weight of thirty-five times that of our atmosphere;
and the liquid was reduced to a solid by M. Thilorier by rapid
evaporation, during which the heat was given out so quickly by one part
of the liquid, that the remainder was condensed into a substance like
snow, which could be touched with impunity, but when mixed with
sulphuric ether its temperature was reduced to 166° below zero of
Fahrenheit’s thermometer.
Carbon appears naturally under a great variety of forms, and exhibits
one of the most striking instances of allotropism, the same substance
showing the greatest contrast in appearance and physical properties. The
diamond, the most resplendent, transparent, and hardest of gems, is
identical with carbon, which is black, dull, opaque, and brittle. Both
are combustible; carbon is easily ignited, but it requires a heat of
1860° to consume the diamond.
However numerous the crystalline forms assumed by substances either
naturally or artificially may be, they are all capable of being grouped
into geometrical systems; each system possessing its own allied and
derivative forms capable of mutual variations among themselves, but the
forms of one system never assuming those of the other. With that law,
however, carbon and a few other substances are completely at variance.
The diamond crystallizes in octohedrons, while graphite, which is also
carbon, crystallizes in six-sided plates,—two forms that belong to
different systems quite irreconcilable with one another: and thus carbon
possesses the property of being dimorphous.
Sulphur is a simple inflammable mineral abounding in volcanic countries,
either in a crystalline or amorphous state, and forming a constituent in
organic substances, animal and vegetable. It is readily dissolved by
bisulphide of carbon, by benzine, and by a moderate heat; and copper
filings exposed to its vapour spontaneously take fire, the chemical
force of combination merging into light and heat. Sulphuretted hydrogen
gas, a combination of sulphur and hydrogen, forms naturally during the
putrefaction of organic matter, and Mr. Faraday observes with regard to
the affinities of sulphur, ‘so numerous are its relations, so extensive
its range of combinations, that we must consider it to be the very
foundation on which chemical manufacture is built up.’
Though a simple substance, sulphur exhibits the two remarkable phenomena
of dimorphism and the allotropic property. When reduced by heat to
vapour and cooled slowly, it crystallizes in rhombic octohedrons; when
merely melted and allowed to cool slowly, it takes the form of oblique
rhombic prisms. Here the same atoms when in vapour and in a liquid state
are acted upon by different forces; but however that may be, sulphur is
another singular exception to the law of the immutability of the
crystalline systems.
Sulphur becomes allotropic by the continued application of heat; that is
to say, it entirely changes its appearance and character, though it
remains chemically the same. Naturally it is yellow and brittle, but
when fused, it is a colourless pellucid fluid which by continued heat is
changed into a black tenacious substance that becomes like India rubber
or gutta percha when thrown into water. In this allotropic state it is
endowed with properties more powerful, energetic, and exalted; its
tendencies to act chemically being increased like those of ozone. That
this black tenacious substance is chemically the same with common
sulphur there can be no doubt, for when it is exposed to greater heat,
it again becomes a colourless pellucid fluid, which thrown into water
resumes the form of brittle yellow sulphur.
These new arrangements among atoms of the same kind show that the
immutability of matter is not without exceptions.
The animal kingdom is the great reservoir of phosphorus, a simple
substance that is never found uncombined. It is sparingly met with in
the vegetable kingdom, and still less in the mineral, but may be
procured abundantly from calcined bones. When pure it is colourless,
transparent, solid, extremely poisonous, and so inflammable that it must
be kept in water. In air it is in continual combustion with oxygen,
during which ozone is produced. When burnt in a current of air
phosphorus leaves a residuum consisting of two substances, of which one
is an acid, the other is red allotropic phosphorus, which has been
extensively used in the manufacture of lucifer matches, because its
fumes are not deleterious, and because it inflames less easily than
common phosphorus, to which it is reduced by heat or friction, which
generates heat.
Silicon is a simple substance, never found alone, but when forty-eight
parts of it are combined with fifty-two parts of oxygen gas it forms
rock crystal, the purest form of silica or quartz. Silica is so abundant
that it may be said to constitute the basis of the mineral world. The
sand on the sea-shore, which is the debris of quartz rocks, shows how
universally it prevails. It is even abundant in the vegetable kingdom,
giving strength to the stalks and leaves of the grasses, and may be felt
in the harshness of the beards of wheat and barley. Silicon exists in
three different states—the amorphous, which has no form; the graphic,
which takes the form of small hexagonal plates; and that of octohedral
silicon: hence this substance is dimorphous.
A singular analogy obtains between silicon and carbon: the amorphous
form of silicon corresponds to charcoal, the graphic form of silicon
corresponds to the graphic form of carbon, and the octohedral form of
silicon to the diamond; yet the chemical relations between the two
substances are very small.
Silica has hitherto been considered to be insoluble in pure water; at
least M. Bischoff states that only one part of silica dissolves in
769,230 parts of water; but by a method hereafter to be explained,
Professor Graham has actually obtained a limpid solution of silica in
pure water.
Boron is a constituent of boracic acid, a natural production in Thibet
and Monte Corbalo in Tuscany. It is a greenish-brown solid, insoluble in
water, but when heated to about 600° it burns in open air with a vivid
flame.
Fluorine is a constituent of a very beautiful mineral, well known as
fluor spar, which is found in cubic crystals of a green, yellow, or
purple colour. Hydrofluoric acid obtained chemically from the mineral is
highly volatile and extremely corrosive.
Three of the non-metallic simple substances, chlorine, bromine, and
iodine, are connected by the most remarkable analogies. They are marine
productions, for chlorine is obtained from common sea-salt and in
greater purity from rock-salt, both of which are compounds of chlorine
and the metal sodium. When sea-water is evaporated, salt and a substance
called bittern remain, which contains a salt whence bromine is
separated.
Again, when kelp, the ashes of burnt seaweeds, is purified from the
carbonate of soda and the chloride of potassium, a salt is left which is
the iodide of potassium, whence iodine is obtained. Iodine is also found
in sponges, oysters, and other low sea animals, as well as in certain
mineral springs, and sometimes in combination with silver. These three
elemental bodies have little affinity for one another, but they combine
powerfully with other substances.
Chlorine is a yellowish-green gas, twice as heavy as atmospheric air,
with a noxious suffocating smell and astringent taste. It has a powerful
bleaching property, and when combined with water, which absorbs twice
its volume of the gas, it is used for bleaching linen, in
calico-printing, and other arts. The clear solution of chloride of lime
is still more in use for the same purpose, as well as for an antidote
against contagion and unwholesome smells. Carbon does not burn in
chlorine gas, yet it is capable of supporting combustion, for oil of
turpentine, phosphorus, thin leaves of tin and copper, and powdered
antimony, take fire spontaneously in it. This gas shows its power by the
development of intense heat, but not by brilliant light, because the
results of its combustion are mostly vapours, or such gases as have a
feeble illuminating power; so chlorine differs materially from oxygen in
the phenomena of combustion. Mr. Faraday observes, however, that the
bleaching powder is analogous to ozone in being an intermediate state,
for chlorine is pernicious and violently destructive as a gas, perfectly
innocuous and quiescent in common salt and in its other natural
combinations, while in the bleaching substances its energy is subdued by
art, so as to make it an important agent in various manufactures.
Providentially, chlorine is never found free; but in a combined state it
exists in enormous quantities in the salt of the ocean, in salt lakes,
brine springs, and in extensive deposits of rock-salt, as well as in
organic liquids. It has a strong affinity for hydrogen, and forms
muriatic acid. A mixture of these two gases remains inactive in the
dark, but explodes in sunshine.
By chemical means chlorine is made to combine with oxygen so as to
produce four substances, two of which are gases of such unstable
equilibrium and weak affinity that the slightest cause makes them
detonate violently; the other two are more stable, though they contain a
greater quantity of oxygen. The only combination of chlorine with
nitrogen is the most powerful and dangerous explosive compound known.
Chlorine combines naturally with sulphur, and with the metals so as to
form ores.
Common salt affords a remarkable instance of change of volume by
chemical combination. Twenty-four parts in bulk of salt contain 20·7
parts of sodium and 23·3 parts of liquid chlorine; hence by chemical
combination a bulk of 44 is compressed into a bulk of 24, yet that great
compression is consistent with perfect transparency, crystallized salt
being perfectly transparent to light, and more so as regards radiant
heat than any other substance. Thus chemical affinity does what no
mechanical power could accomplish.
At an ordinary temperature and barometric pressure, bromine is an orange
red, extremely volatile fluid, which congeals and becomes brittle at a
temperature a little below the zero of Fahrenheit’s thermometer, and if
combined with water at that degree of cold it crystallizes in octohedral
crystals which are permanent even at 50° Fahr. Bromine is very
poisonous, corrodes the skin, has a disagreeable taste, and a smell
similar to that of chlorine, but more pungent and hurtful. It possesses
a powerful bleaching property, does not conduct electricity, and like
chlorine a taper will not burn in its gas, though it spontaneously sets
fire to phosphorus, and some of the metals. Reasoning from analogy
Professor Schönbein believes that chlorine and bromine are not simple
substances; he considers them to be ozonides analogous to the peroxides
of manganese, lead, &c. He believes chlorine to be the peroxide of
murium, and bromine to be the peroxide of bromium. Professor Tyndall’s
experiments on the absorption and radiation of gases show that the
action of these two substances is very different from that of the simple
gases.
Iodine is a dark purple solid, crystallized in scales or elongated
octohedral plates. It slowly evaporates at ordinary temperatures, and at
that of 350° Fahr. it is volatilized into a beautiful violet coloured
gas which changes starch into a bright blue, and for that reason a
little starch will detect the millionth of a grain of iodine in
composition. Iodine is slightly soluble in water, has a hot acrid taste,
and although used in medicine it is poisonous when taken in large doses.
Its bleaching properties are inferior to those of its congeners, but its
chemical combinations are the same. With hydrogen it forms a highly
explosive compound, which detonates with the slightest pressure.
These three simple substances are analogous in almost every respect.
They all possess a bleaching property, many of their compounds are
exceedingly explosive, combustible substances do not burn in their
gases, while their gases set fire spontaneously to substances generally
reckoned incombustible. Hence, though not combustible, they support
combustion, but in a very different manner from oxygen. Chlorine and the
gases of bromine and iodine diluted with common air, do not transmit
blue and violet light; that is to say, the spectrum of a sunbeam
transmitted through them is deprived of its most refrangible coloured
rays, and that which remains is crossed by more than a hundred
equidistant dark lines; their spectral properties however will be given
hereafter. They resemble oxygen in one respect—that when a current of
electricity is passed continuously through a glass tube filled with any
of these three gases, much attenuated, they slowly combine with the
platinum wire of the negative pole of the battery inserted in the tube.
The electricity by degrees passes in diminished quantity, and at last
ceases altogether, showing that matter, however attenuated, is requisite
to conduct it.
According to the experiments of M. Dumas, the volatility of a compound
is in the inverse ratio of the condensation of the substances composing
it, and simple bodies come under the same law. For example, chlorine is
more volatile than bromine, and bromine is more volatile than iodine;
hence according to that law, chlorine is the least dense of the three,
bromine is intermediate, and iodine is the most dense, which is actually
the case: for chlorine is a gas, bromine a liquid, and iodine a solid at
ordinary temperatures, which proves that there is a sequence in the
intensity of the cohesive forces in this triad.
SECTION II.
ON FORCE, AND THE RELATIONS BETWEEN FORCE AND MATTER.
FORCE is only known to us as a manifestation of divine power which can
neither be created nor destroyed. The store of force or energy in nature
is ever changing its form of action, its amount never. It may be
dispersed in various directions, and subdivided so as to become
evanescent to our perceptions; it may be balanced so as to be in
abeyance, or it may become potential as in static electricity; but the
instant the impediment is removed the power is manifested by motion.
Whatever form force may assume it has invariably a compensation or
equivalent, whether in the heavens or on the earth. The total sum of the
living forces, vis viva, or actual energy of the planets is the same
every time they return to the same relative positions with regard to one
another, to their orbits and to space, whatever may have been their
velocities or mutual disturbances. In the ocean, the energy by which
25,000 cubic miles of water flow over a quarter of the globe in six
hours, is exactly equal to the force or energy that makes it ebb during
the succeeding six hours. A body acquires heat in the exact proportion
that the adjacent substances become cold; and when heat is absorbed by a
body, it becomes an expansive energy at the expense of those around it,
which contract. Chemical action many miles distant from the
electro-magnet, as in telegraphs, is perfectly equivalent to the
dominant chemical action in the battery. The two electricities, positive
and negative, are developed in equal proportions, which may be combined
so as to produce many changes in their respective relations, yet the sum
of the energy of the one kind can never be made in the smallest degree
either to exceed or to come short of the sum of the other.
The mechanical energy of machinery or working power is exhausted by the
very act of working, and cannot be restored except by the action of
other forces. In clockwork, the weight must sink to move the wheel, and
when the weight is down, the store of energy is gone, and can only be
restored by raising the weight through the expenditure of energy in the
human arm, and the expenditure of human energy must be restored by food
and rest. The heat given off from the bodies of men and animals is
restored by the combustion of the oxygen inhaled during respiration and
the carbon of the food, and the light and heat given out by the
combustion of fuel, whether in the form of coal or wood, is compensated
by the light and heat of the sun stored up in living vegetables. It is
this equivalent for force or energy which prevails in every department
of nature that constitutes the universal and invariable law of the
Conservation of Energy, ‘a principle in physics as large and sure as
that of the indestructibility of matter or the invariability of gravity.
No hypothesis should be admitted nor any assertion of a fact credited,
that denies this principle. No view should be inconsistent or
incompatible with it. Many of our hypotheses in the present state of
science may not comprehend it, and may be unable to suggest its
consequences, but none should oppose or contradict it.’[1] Thus, ‘there
is a definite store of energy in the universe, and every natural change
or technical work is produced by a part only of this store, the store
itself being eternal and unchangeable.’[2]
Cohesion is a force which acting at inappreciably small distances unites
atoms and molecules of the same kind into solids, liquids, and aëriform
fluids, exactly according to the law of the conservation of energy; for
it requires the very same amount of force to dissolve their union as to
form it. Cohesion varies with temperature both in simple and compound
bodies, for metals can be fused and vaporized by artificial heat, and
ice becomes water and aqueous vapour as the seasons change from winter
to summer.
In solids the force of cohesion is so strong, that their atoms and
molecules always retain their respective places; that power is so weak
in liquids, that their atoms and molecules are capable of motion among
themselves, and in gases and the ethereal medium the atoms are free and
have no cohesion whatever. The resistance offered by substances to
compression is an equal and contrary force.
The reciprocal attraction between solids and liquids in capillary tubes
is a case of cohesion. If a glass tube of extremely fine bore be plunged
into a glass of water or alcohol, the liquid will immediately rise in
the tube above the level of that in the cup, and the surface of the
little suspended column will be a hollow hemisphere. If on the contrary
mercury be the liquid, it will not rise so high in the glass tube, and
the surface of the little column will be a convex hemisphere. There is a
reciprocal attraction between the glass tube and the liquid, and another
between the particles of the liquid itself; and the effect is produced
by the difference between the two. In the first case the attraction of
the glass is greater than that of the liquid, and in the second it is
less; hence the water rises higher in the tube than the mercury, and its
surface is concave, while that of the mercury is convex. The elevation
or depression of the same liquid in different tubes of the same matter
is in the inverse ratio of their internal diameters, and altogether
independent of their thickness; whence it follows that molecular action
is insensible at sensible distances, for when tubes of the same bore are
wetted throughout their whole extent with water, mercury will rise to
the same height in all of them whatever be their thickness or density,
the film of water being sufficient to intercept the molecular action,
and to supply the place of a tube by its own capillary attraction. The
action of this force is daily seen in the absorption of water by
sponges, sugar, salt and other porous bodies, and it is a most important
agent in the circulation of fluids in animals and vegetables.
Every atom of matter is subject to the force of gravitation, but each
substance has its own peculiar weight of specific gravity, that is to
say, the same bulk of different substances contains different quantities
of matter. Since nothing is known of absolute weight it is necessary to
have some standard of comparison, and for that purpose pure water at the
temperature 39° Fahr. (that of its maximum density) is chosen for solids
and liquids; while for gases and vapours atmospheric air at the
temperature of sixty degrees of Fahrenheit’s thermometer, and a
barometric pressure of thirty inches, is assumed as the unit of specific
gravity.
The foot-pound, which is the unit of mechanical force as established by
Mr. Joule, is the force that would raise one pound of matter to the
height of one foot; or it is the impetus or force generated by a body of
one pound weight falling by its gravitation through the height of one
foot. Now impetus or vis viva is equal to the mass of a body multiplied
by the square of the velocity with which it is moving: it is the true
measure of work or mechanical labour. For if a weight be raised ten
feet, it will require four times the labour to raise an equal weight to
forty feet. If both these weights be allowed to fall freely by their
gravitation, at the end of their descent, their velocities will be as
one to two, that is as the square roots of their heights, but the effect
produced will be as their masses multiplied by one and four; but these
are the squares of their velocities. Hence impetus or vis viva is equal
to the mass multiplied by the square of the velocity. Thus impetus is
the true measure of the labour employed to raise the weights, and of the
effect of their descent, and is entirely independent of time.
It is well known that iron becomes red-hot by percussion or impetus. The
atoms of the iron are thrown into vibration, and these minute motions
communicated to the nerves produce the sensation of heat. Now the
mechanical labour required to raise the hammer to any number of feet is
equal to the weight of the hammer multiplied by that number of feet; but
the impetus or mechanical effect of the fall of the hammer is equal to
its mass multiplied by the square of the velocity, that is to the vis
viva: hence the quantity of heat generated is proportional to the vis
viva. The circumstances being the same, if the mass be doubled the
amount of heat is doubled; and if the velocity be doubled the amount of
heat is quadrupled. If the weight and the perpendicular height through
which a body has fallen be known, the quantity of heat generated may be
determined. The same amount of heat is generated by the same amount of
force, whatever that force may be, whether impetus, friction, or any
other.
Dr. Thomson has put in a strong point of view the quantity of heat that
might be generated by percussion or impetus. He computed that if by any
sudden shock the earth were arrested in its orbit, the heat generated by
the impulse would be equal to 11,200 degrees of the centigrade
thermometer, even if the capacity of our planet for heat were as low as
that of water; it would therefore be mostly reduced to vapour, and
should the earth then fall to the sun as it certainly would do, the
quantity of heat developed by striking on the sun would be 400 times
greater. It is even supposed that the light and heat of the sun are
owing to showers of bodies falling on the surface with impetus
proportionate to his attraction, for had he been in combustion he would
have been burnt out ages ago. The masses of meteoric iron and stone that
occasionally fall on the earth show that matter may be wandering in
space; the vast zone of smaller bodies that in their annual revolutions
round the sun come within the earth’s attraction in August and November,
when thousands of them take fire and are consumed on entering our
atmosphere, show that a great amount of matter of small dimensions
exists within our own system. Much may be beyond it which drawn by the
sun’s attraction may fall on his surface.
When a body is heated, it absorbs one part of the heat; the other part
raises its temperature. The part absorbed increases the bulk or volume
of the body, the expansion being the exact measure, or mechanical
equivalent of the heat absorbed. In fact the coefficient of expansion is
the fractional part of the expansion in length, surface, or volume of
the body when its temperature is raised one degree. When the body is
cooled, its volume is diminished, and then the contraction is an exact
measure, or mechanical equivalent of the heat given out, and thus
expansion and contraction are correlatives with and represent heat and
cold.
Specific heat is the quantity of heat required to raise a given bulk or
a given weight of a body a given number of degrees. In the one case it
is distinguished as the specific heat for a constant volume, in the
other for a constant weight.
Although the specific heat of a substance remains the same, its sensible
and absorbed heat may vary reciprocally to a great extent.
As there can be no direct measurement of heat independent of matter, its
mutations and action on matter are the sole means we have of forming our
judgment concerning its agency in the material world.
Mr. Joule has proved that the quantity of heat requisite to raise the
temperature of a pound of water one degree of the centigrade thermometer
is equivalent to the mechanical work or force that would raise the same
mass of water to the height of 1,389 feet. This is the unit, or
mechanical equivalent of heat.
In fact, for every unit of force expended in percussion, friction, or
raising a weight, a definite quantity of heat is generated; and
conversely, when work is performed by the consumption of heat, for each
unit of force gained, a unit of heat disappears. For since heat is a
dynamical force of mechanical effect, there must be an equivalence
between mechanical work and heat as between cause and effect. That
equivalence is a law of nature. The mechanical force exerted by the
steam engine is exactly in proportion to the consumption of heat,
neither more nor less; for if we could produce a greater quantity than
its equivalent we should have perpetual motion, which is impossible.
When steam is employed to perform any work, the temperature of the steam
is lowered; the heat that disappears is transformed into the force that
performs the work, and is exactly proportional to the work done, and
vice versâ.
The heat which is the motive force in the steam engine is due to the
chemical combination of the carbon of the fuel with the oxygen of the
atmosphere. A pound weight of coal when consumed in one of our best
steam engines produces an effect equal to raising a weight of a million
of pounds a foot high, yet marvellous as that is, the investigations of
recent years have demonstrated the fact, that the mechanical energy
resident in a pound of coal and liberated by its combustion is capable
of raising to the same height ten times that weight.[3] The quantity of
coal existing in the whole globe is believed to be inexhaustible, hence
the energy in abeyance is incalculable. The chemical energy continually
and actually exerted in the great laboratory of nature is greater than
that which maintains the planets in their orbits.
The act of the combination of the atoms of carbon and oxygen in
combustion is ‘now regarded exactly as we regard the clashing of a
falling weight against the earth, and the heat produced in both cases is
referable to the same cause;’[4] so chemical combination in combustion
is only a particular case of falling bodies. Drummond’s light, the most
brilliant of artificial illuminations, is produced by a simultaneous
shower of the atoms of oxygen and hydrogen gas upon lime; and platinum,
the least fusible of metals, is vaporized by a similar shower from the
oxy-hydrogen blowpipe, and thus impetus generates both light and heat,
for although the atoms are too small to admit of an estimation of their
individual vis viva, there can be no doubt that like causes produce like
effects.
In what manner or under what form magnetism and electricity exist when
quiescent in matter we know not, but the compass needles show that
numerous lines of magnetic force, subject to periodic and secular
variations, perpetually traverse the earth and the ocean; and that waves
of magnetic force occasionally sweep rapidly over great tracts of the
globe. These phenomena would seem to stand in some periodic connection
with the solar spots. Professor Lamont of Munich has discovered that a
permanent and regular current of electricity is propagated parallel to
the equator all over the earth, and another similar to it in the
atmosphere. Besides these, there are currents of electricity in the
surface of the earth, sometimes in one direction and sometimes in
another, which decrease with the depth; and M. Lamont conceives that
this electric system is the cause of terrestrial magnetism. Electricity
of intense power and inappreciable quantity certainly exists in abeyance
in the atmosphere and in all terrestrial matter till the equilibrium
between the antagonist forces be disturbed, and then it bursts forth
with terrific violence in the lightning flash and stunning crash of
thunder. Since it requires electricity equivalent to that in activity
during a thunderstorm to form one drop of water, what must that power
have been which the Omnipotent wielded when he created that deep over
the face of which ‘darkness brooded.’
Electricity, though the most formidable power in nature, is made
available to man by the voltaic battery, and by the electro-magnetic
induction apparatus, in the battery of which it is generated by the
chemical action of dilute sulphuric acid on zinc. The positive and
negative electricities thus produced pass in opposite directions through
the two conducting wires of the machine by a continuous transmission of
force or vibration from atom to atom, a circulation that is accompanied
by a continual development of heat in overcoming the resistance it meets
with in the wires. The electricity decreases as the heat increases, and
vice versâ; the action is reciprocal. Thus electricity is merely a
transmission of force. Mr. Joule has proved that the quantity of heat
produced in a unit of time is proportional to the strength of the
current, whatever may be its direction, and that its power to overcome
resistance is as the square of the force of the current. The force is
exactly in proportion to the chemical action which produces it, and that
is measured by the quantity of zinc consumed in the battery. Thus
chemical action produces electricity, and conversely electricity is a
powerful agent in the chemical composition and decomposition of matter.
The light and heat of the electric spark are intense though
instantaneous; but a powerful induction apparatus like Ruhmkorff’s gives
so rapid a succession of sparks that the light and heat are sensibly
continuous and of great intensity. The light and heat, powerful as
lightning itself, are produced by the combined currents of two
batteries, each consisting of fifty Bunsen elements of moderate size.
This formidable united current passes through a circuit of thick copper
wire coated with silk thread, with an intensity of perpetually renewed
heat that no substance can resist. When the copper conducting wires are
fitted with charcoal terminals and brought near to one another, the
dazzling lights emanating from each pole combine in one blaze of
insupportable brilliancy. The most refractory substances, silica,
alumina, iron and platinum, when placed between the poles, immediately
melt like wax, and volatilize. Charcoal is so good a conductor of
electricity that when the terminals are in contact they complete the
circuit, and neither light nor heat appear. Air and glass are
non-conductors, yet the spark has passed through several inches of air
and perforated a mass of glass two inches thick. A long electric spark
combines or decomposes a greater quantity of gas or vapour than a short
one, and for a given induction apparatus and induction current, M.
Perrot has shown that there exists a length of spark corresponding to a
maximum chemical action.
Professor Seebeck of Berlin discovered that electric currents are
produced by the partial application of heat to a circuit formed of two
solid conducting substances as antimony and bismuth soldered
together,—another proof of the correlation of heat and electricity.
There cannot be a doubt that the atoms of a conducting wire are in
motion, and that they successively take definite and momentary positions
during the passage of an electric current, after which they return
successively to their normal state. When electricity is invariably sent
from the same pole of an inductive apparatus through the wire of a
telegraph, in a very short time the wire is torn or divided into small
sections, which destroy its continuity; but when the electricity is sent
from each pole alternately, the conducting wire is not injured. As each
atom of the wire has its own electricity, this seems to indicate that
during the successive transits of the same kind of electricity, the pole
of each individual atom is attracted more and more in the same
direction, till at last they no longer return to their normal state, the
cohesive force is overcome, and a rupture takes place, the more readily
if there be any imperfection in the wire. Since the electricity from the
other pole of the machine would have the same effect, but in the
contrary direction, an alternate motion in the atoms must maintain the
continuity of the wire.
A closed current of electricity or magnetism is accompanied by a
simultaneous current of the opposite force in the tangential direction
equal in quality and intensity. Thus the electric and magnetic currents,
which are merely transmissions of energy, differ by moving at right
angles to one another; their effects are alike, yet they are not
identical.
The amount of the chemical action of light has been determined by
Professor Roscoe to be directly proportional to the intensity of the
light; and when the light is constant the amount of action is exactly
proportional to the time of exposure. It appears that equal volumes of
chlorine and hydrogen explode in sunshine, but combine slowly in shade;
and as the combined gases are absorbed by water as soon as combined, the
gradual diminution of the volume of the mixed gases during the time of
absorption is a measure of the amount of action exerted by the light.
Professor Wm. Thomson has computed, by the aid of Poullet’s data of
solar radiations and Mr. Joule’s mechanical equivalent of heat, that the
mechanical value of the whole energy, active and potential, of the
disturbances kept up on the ethereal medium by the vibrations of the
solar light in a cubic mile of our atmosphere, is equal to 12,050 times
the unit of mechanical force: that is to say, twelve thousand and fifty
times the force that would raise a pound weight of matter to the height
of one foot. The sensible height of the atmosphere is about forty miles,
whence some idea may be formed of the vast amount of force exerted by
the sun’s light within the limits of the terrestrial atmosphere. The
green mantle which clothes the earth proves under a beautiful form the
influence of light on the organic world.
It has been proved that at _any given_ fixed temperature the amount of
light and heat absorbed and that which is emitted remains constant for
_all bodies_. The greater the amount absorbed, the greater the amount
radiated. The molecules or atoms of the bodies in consequence of the law
of resonance emit those ethereal undulatory motions which have been
previously impressed upon them, as a musical instrument resounds in
answer to the note impressed upon it. The whole is referable to
molecular or atomic motion, for in absorption the vibrations of the
ether are communicated to the atoms, and in radiation, the vibrations
are returned again to the ether. This principle is known as the law of
exchange.[5]
Matter has a decomposing and an elective power with regard to both
radiant light and heat; most coloured bodies, such as flowers, green
leaves, dyed cloth, &c., though seen by reflection, owe their colour to
absorption. The light by which they are seen is reflected, but it is not
in reflection that the selection of the rays is made which causes the
objects to appear coloured. When light falls upon red cloth, a small
portion is reflected at the outer surfaces of the fibres, and this
portion, if it could be observed alone, would be found to be colourless.
The greater portion of the light penetrates into the fibres, when it
immediately begins to suffer absorption on the part of the colouring
matter. On arriving at the second surface of the fibre, a portion is
reflected and a portion passes on, to be afterwards reflected from, or
absorbed by, fibres lying more deeply. At each reflection the various
kinds of light are reflected in as nearly as possible the same
proportion, but in passing across the fibres while going and returning
they suffer very unequal absorption on the part of the colouring matter;
so that in the aggregation of the light perceived the different
components of white light are present in proportions widely different
from those they bear to each other in white light itself, and the result
is a vivid colouring.
In certain substances however, as gold and copper, the different
components of white light are reflected with different degrees of
intensity, and the light becomes coloured by these reflections. Gold is
yellow by reflection; red cloth is red by absorption. In the same sense,
physically speaking, in which the red cloth is red, gold is not yellow
but blue or green; such is in fact the colour of gold by transmission
through gold leaf, and therefore gold is greenish blue by absorption. In
this case we see that while the substance copiously reflects and
intensely absorbs rays of all kinds, it more copiously reflects the less
refrangible rays with respect to which it is more intensely opaque. In
general absorption and radiation are independent of colour.
There is a vast diversity in the property which substances possess with
regard to the transmission of radiant light and heat; glass, for
instance, transmits light abundantly, but is impervious to heat from
non-luminous sources; while other substances, which are altogether
opaque to light, transmit heat copiously, as the bisulphide of carbon,
which of all liquids is the most diathermic, while water in all its
forms is almost impervious to heat.
Sir William Herschel discovered that invisible rays of high heating
power exist beyond the red end of the solar spectrum, and Mr. Tyndall
has shown that the reason of a substance being impervious to the light
of the most brilliant flame and at the same time pervious to these extra
red rays is, that the intercepted rays of light are those whose periods
of recurrence coincide with the periods of oscillation possible to the
atoms of the substance in question. The elastic forces which separate
these atoms are such as to compel them to vibrate in definite periods,
and when their periods synchronize with those of the ethereal waves, the
latter are absorbed. Thus transparency in liquids as well as in gases is
synonymous with _discord_, while opacity is synonymous with _accord_
between the periods of the waves of ether and those of the body on which
they impinge. All ordinary transparent and colourless substances owe
their transparency to the discord which exists between the oscillating
periods of their molecules and those of the waves of the whole visible
spectrum. The general discord of the vibrating periods of the molecules
of _compound bodies_ with the light-giving waves of the spectrum may be
inferred from the prevalence of the property of transparency in
compounds, while their greater harmony with the extra red periods is to
be inferred from their opacity to the extra red rays. Water illustrates
this transparency and opacity in the most striking manner. It is highly
transparent to the luminous rays, which demonstrates the incapacity of
its molecules to oscillate in periods which excite vision. It is as
highly opaque to the extra red oscillations, which proves the
synchronism of its periods with more of the longer waves. If, then, to
the radiation from any source water shows itself to be eminently or
perfectly opaque, it is a proof that the molecules whence the radiation
emanates must oscillate in extra red periods.
It has been already mentioned that many substances which transmit
radiant heat freely radiate badly, and vice versâ. Rock-salt is
extremely permeable to radiant heat but radiates feebly; the reason
according to Mr. Tyndall is, that the motion of the molecules of the
salt, instead of being expended on the ether between them and then
communicated to the ether external to the mass, is transmitted freely
from molecule to molecule.
Alum is exactly the reverse. Mr. Balfour Stewart proved that alum is an
excellent radiator, and Mr. Tyndall proved it to be a very bad
conductor, imparting freely and with ease the motion of its molecules to
the external ether, and ‘for that very reason it finds difficulty in
transferring the motion from molecule to molecule. The molecules are so
constituted that when one of them approaches its neighbour, a swell is
produced in the intervening ether; this motion is immediately
communicated to the ether outside, and is thus lost for the purposes of
conduction.’[6]
Melloni had investigated the laws of the radiation and absorption of
radiant heat in solid and liquid matter; but its radiation and
absorption by gases and vapours was unknown previous to the experiments
of Mr. Tyndall.
The apparatus employed was a horizontal brass tube four feet long,
between two and three inches in diameter, polished inside, and closed
air-tight at each end by a plate of rock-salt, which transmits more heat
than any other substance. The air could be pumped out of the tube by one
pipe, and the gas or vapour for the experiment introduced by another.
Close to one end of the brass tube there was a thermo-electric pile
connected with its goniometer. On each side of this arrangement there
was a vessel of water kept at the boiling point. These two vessels were
so placed that when the rays of heat from one of them passed through the
exhausted tube, and fell upon one face of the thermo-electric pile,
their effect was so neutralized or balanced by the rays of heat falling
on the opposite face of the pile from the other, thus the needle of the
goniometer was steadily maintained at zero, and its deflection instantly
showed the absorbent effect produced by any gas or vapour that was
admitted into the exhausted tube.
Since aqueous vapour has a very exalted absorbent power, a gas or vapour
was rendered perfectly dry before its absorbent capacity was determined.
For that purpose the pipe that introduced it into the brass experimental
tube was so constructed that the gas had first to pass over fragments of
pumice-stone wet with strong sulphuric acid, which absorbed its moisture
and dried it. Common atmospheric air, however, was not only dried in
this manner, but it was deprived of its carbonic acid by passing over
caustic potash, and many other precautions were taken to prevent the
possibility of error.
Under the ordinary pressure of the atmosphere, when the experimental
tube was exhausted, the needle of the goniometer stood at zero, but as
soon as pure dry atmospheric air was introduced into the tube its
absorption caused the needle to move from zero to 1°.
The tube was again exhausted; the needle stood at zero, but was
deflected from zero to 1° as soon as the tube was filled with oxygen. A
similar experiment was made with nitrogen and hydrogen with the same
result. Thus, dry air and the elementary gases, oxygen, nitrogen, and
hydrogen, have the same absorptive power, and consequently they all
deflected the needle of the goniometer one degree. The whole amount of
radiant heat that passed through the exhausted tube produced a
deflection of 71° 5ʹ; hence taking as unit of heat the amount that would
deflect the needle one degree, the number of units expressed by 71° 5ʹ
is 308, consequently the absorption of each of these four gases amounts
to 100/308 or 0·3 per cent. The most delicate tests could not show any
difference between the three first, but Professor Tyndall had reason to
believe that hydrogen has the lowest absorptive power of all gases and
vapours, though he was unable to express the amount. The absorptive
power of all four is very much less than that of every other gas or
vapour, and invariably deflects the needle to 1°, which thus becomes the
unit of comparison.
Olefiant gas, the most luminous of the constituents of coal gas,
possesses the highest absorptive power of the permanent gases. When sent
into the exhausted tube it deflected the needle of the goniometer from
0° to 70° 3ʹ, which is equivalent to 290 units. The whole heat that
passed through the exhausted tube before the gas was admitted produced a
deflection of 75° or 360 units, consequently more than 7/10ths or 81 per
cent. of the whole heat was cut off by the olefiant gas. Such opacity to
heat in so transparent a gas is quite marvellous. A current of it was
sent into the open air between the thermo-electric pile and one of the
sources of heat, and although it was perfectly invisible, it instantly
deflected the needle of the goniometer from 0° to 41°.
In order to ascertain the relation between the density of the gas and
the quantity of heat extinguished or absorbed, an ordinary mercurial
gauge was attached to the air-pump. The experimental tube was exhausted,
and the needle of the goniometer stood at zero. Then, from a graduated
glass vessel, measures of olefiant gas, each amounting to the 1/50th of
a cubic inch, were successively sent through the drying pipe into the
exhausted tube. The amount of the heat absorbed and the depression of
the mercurial column corresponding to each measure of gas as it was
introduced, was registered from one to fifteen measures. This experiment
showed that for very small quantities of gas, the absorption is exactly
proportional to the density or tension. One measure of the gas only
produced a depression of the mercurial column amounting to the 1/367th
part of an inch, or about the 1/15th of a millimetre.
In many of the vapours of volatile liquids, the preceding law only
prevails to a certain amount of pressure differing in each case, beyond
which increase of tension produces diminished effects. In sulphuric
ether the change begins at the eleventh term.
In bisulphide of carbon the law changes after the sixth measure, &c.
In order to adapt the apparatus for experiments on coloured gases, a
glass experimental tube 2 ft. 9 in. long, and 2 ft. 4 in. in diameter,
was substituted for the brass tube, and, instead of boiling water,
sources of radiant heat having a constant temperature of 270° Cent. were
adopted.
The following table shows the absorption of a number of gases at a
common pressure or tension of one atmosphere.
Dry air = 1
Oxygen 1
Nitrogen 1
Hydrogen 1
Chlorine 39
Hydrochloric acid 62
Carbonic oxide 90
Carbonic acid 90
Nitrous oxide 35·5
Sulphuretted hydrogen 390
Marsh gas 403
Sulphurous acid 710
Olefiant gas 970
Ammonia 1195
The absorptive power of ammonia is so great, that although as
transparent in the glass tube as if it had been a vacuum, a length of
three feet of it would be perfectly impervious or black to the heat here
employed, yet even this does not express the energy which it exhibits
under one inch of pressure.
When the relative absorptive actions of gases and vapours is compared,
it must be under the same amount of pressure. Hence, for one inch of
tension, the absorptive action of
Dry air = 1
Oxygen 1
Nitrogen 1
Hydrogen 1
Chlorine 60
Bromine 160
Hydrochloric acid 1005
Carbonic oxide 750
Nitric oxide 1590
Nitrous oxide 1860
Sulphide of hydrogen 2100
Ammonia 7260
Olefiant gas 7950
Sulphurous acid 8800
Thus, for a tension of an inch of mercury, the absorption of ammonia
exceeds that of air more than 7000 times; the action of olefiant gas is
7950 times, and that of sulphurous acid 8800 times, greater than the
absorption of air.
The effect produced by 1/30th of an inch of tension of air and the
elementary gases is equivalent to that produced by one inch in the
others, so the unit representing the absorption of these four gases is
only the 1/30th part of the unit in the preceding table.
It appears from the preceding tables of comparative absorption that
chlorine, a highly-coloured gas with a specific gravity of 2·45, has an
absorptive power expressed by 39° under the pressure of one atmosphere,
while, at the same tension, hydrochloric acid, a chemical compound of
chlorine and hydrogen which is perfectly transparent, with a specific
gravity of only 1·26, has an absorptive action amounting to 62, whence
it appears that the chemical change which renders chlorine more
transparent to light, makes it more opaque to obscure heat. Again,
bromine, which is far less permeable to light than chlorine, and has a
specific gravity of 5·54, has an absorptive power of 160 under a tension
of one inch; while hydrobromic acid, which is perfectly transparent to
light, has an absorptive action for obscure heat amounting to 1005. This
is a striking instance of transparency to light and opacity to heat
being produced by the very same chemical art.
The enormous difference between the absorptive power of compound and
simple gases and vapours is ascribed to their atomic structure; in fact
the radiant and absorptive powers augment as the number of atoms in the
compound molecule augments. The three elementary gases are formed of
simple atoms, the compound gases and vapours consist of different kinds
of atoms chemically united into groups. Both are free to receive the
vibratory motions of the ether which constitute heat; but single atoms
must produce a less effect than when a number of them are united into a
molecule. The atoms are loaded by their chemical union, which offers a
greater surface of resistance to the vibrations of heat, and renders the
motion of the molecule more sluggish and more fit to accept the slowly
recurrent waves of the obscure heat that strike upon it.
Thus when atoms of hydrogen and nitrogen are mixed in the proportion of
three to one, the absorption of the mixture is represented by unity; but
when they are chemically united in ammonia, the absorption is 1190 times
as great. Atoms of hydrogen and oxygen mixed in the proportion of two to
one absorb very feebly; when chemically united into a molecule of
aqueous vapour the absorptive power is enormous. The absorptive power of
nitrous oxide, a chemical compound of oxygen and nitrogen, exceeds that
of dry air 250 times; a convincing proof that the atmosphere is a
mixture and not a compound gas. Olefiant gas at five inches of tension
absorbs 1000 times that of its constituent hydrogen. In fact all the
compound gases and vapours far surpass the simple elementary gases and
dry atmospheric air in their capacity for absorption.
Chlorine and bromine, which have so many singular properties in common,
have this peculiarity also, that though simple substances respectively
formed of homogeneous atoms, their absorptive powers are similar to
those of compound substances, for the absorptive power of chlorine is 60
times that of the elementary gases, and that of bromine 160 times. This
high absorptive power is ascribed by Professor Tyndall to their atoms
being united into groups which act powerfully as oscillating systems,
instead of the feeble action of single atoms.
Ozone is an analogous instance of the presumed union of homologous atoms
into oscillating groups. By comparing the absorptive effect of ozonized
oxygen obtained from the electric decomposition of water with that of
the same oxygen deprived of its ozone by passing it over a very strong
solution of iodide of potassium, Professor Tyndall found that ozonized
oxygen possesses an absorption force 136 times greater than that of pure
oxygen. The quantity of ozone producing this astonishing effect was too
small even to admit of estimation, far less of measurement. This result
induced Professor Tyndall to believe that ozone is produced by the
packing of the atoms of elementary oxygen into oscillatory groups; and
that heating dissolves the bond of union and allows the atoms to swing
singly, thus disqualifying them from either intercepting or generating
the motion which as systems they were competent to intercept and
generate.
The indefinitely small and invisible constituents of perfumes of plants
and flowers are proved to be compound bodies by their absorptive and
radiating properties. The dried leaves of a flower or aromatic plant
such as thyme were stuffed into a glass tube 18 inches long and a
quarter of an inch in diameter. It was then inserted between the drying
pipe of the machine and the experimental glass tube, which was
exhausted, and the needle of the goniometer stood at zero. Then when the
air admitted into the drying pipe passed over the thyme and carried its
aroma into the experimental tube, the needle was deflected, and from
thence the absorption of the thyme was computed to be 33 times greater
than that of the air which carried it. By the same process it was found
that the absorption of peppermint was 34 times, spearmint 38 times,
lavender 32 times, and wormwood 41 times greater than that of the dry
air, which was unity as usual. When small equal squares of bibulous
paper rolled into cylinders and moistened with an aromatic oil, were
substituted for the dried herbs, the absorption corresponding to the
deflection of the needle was for dry air, equal to 1,—
Patchouli 30
Sandal wood 32
Geranium 33
Oil of cloves 33·5
Otto of roses 36·5
Bergamot 44
Lavender 60
Lemon 67
Orange 67
Thyme 68
Rosemary 74
Oil of laurel 80
Chamomile flowers 87
Spikenard 355
Anise seed 372
The absorption of thyme and lavender shows how much aroma is lost when
plants are dried. So great is the absorption of heat, that the perfume
of a flower-bed may be more efficacious than the entire oxygen and
nitrogen of the atmosphere above it.
The enormous absorption and consequently radiating power of the perfumes
of plants and flowers is a proof that their constituent parts are
molecules and not simple atoms, incredible as it may seem. The absolute
weight of the substances producing these wonderful effects is unknown,
but there must be great differences: some perfumes are carried to vast
distances, others are less volatile, and that of mignonette was remarked
by Dr. Wollaston to be absolutely so heavy that it was quite as powerful
below a balcony containing a box of that plant, as in the balcony
itself.
The perfumes during the experiments adhered to all parts of the
apparatus so pertinaciously, that after a continued stream of dry air
had been pumped through the tube till the exhaustion seemed to be
complete and the needle stood at zero, after a few minutes’ repose, the
residue of the perfume came out so powerfully from the crannies of the
apparatus as almost to restore the original deflection. ‘The quantities
of those residues must be left to the imagination to conceive. If they
were multiplied by billions they probably would not obtain the density
of the air.’
The absorptive power of the odour of musk was 72 or 74 times that of the
air that conveyed it into the experimental tube; the quantity that
produced it was quite inappreciable, yet the perfume was so persistent
that the pieces of the apparatus through which it had passed had to be
boiled in a solution of soda before they were fit for other experiments.
The absorption of many gases and vapours having been determined, their
radiation was measured by a very simple arrangement. The thermo-electric
pile was raised on a stand with a screen of polished tin in front of it.
A heated copper ball in a perforated ring on a low stand was placed
behind the screen; all direct radiation from the ball was thus cut off,
but the heated air rising in a column above the screen radiated its heat
on the pile and deflected the needle of the goniometer 60° when the ball
was red-hot; but the radiation of the hot air was neutralized by another
source of radiant heat on the opposite side of the pile which kept the
needle steadily at zero. Then a purified gas or vapour conveyed by a
pipe into the perforated ring which held the ball rose mixed with the
heated air above the screen, but the radiation of the gas or vapour
alone was shown by the deflection of the needle, because that of the air
was compensated. With this apparatus Professor Tyndall proved that the
amount of the absorption of each gas and vapour is exactly equal to the
amount of its radiation. He has shown that this result is a necessary
consequence of the dynamical nature of heat. For as no atom or molecule
is capable of existing in vibrating ether without accepting a portion of
the motion, the very same quality whatever it may be that enables it to
do so, must enable it to impart its motion to still ether when plunged
into it. ‘Hence from the existence of absorption we may on theoretic
grounds infallibly infer a capacity for radiation; from the existence of
radiation we may with equal certainty infer a capacity for absorption,
and each of them must be regarded as the measure of the other.’ This
reasoning, founded simply on the mechanical relations of the ether and
the atoms immersed in it, is completely verified by experiment.
Hitherto the absorption and radiation of heat by the same thickness of
different gases and vapours have been compared with each other, but in a
recent series of experiments Mr. Tyndall has compared the action of
different thicknesses of the same gas or vapour on radiant heat. The
experiments extend from a thickness of 0·01 of an inch to that of 49·4
inches. The instrument employed for ascertaining the action of the
smaller thickness was a horizontal hollow cylinder closed at one end by
a plate of rock-salt. A second cylinder was fitted into this with its
end also closed by a plate of rock-salt. This cylinder moved within the
other like a piston, so that the two plates of rock-salt could be
brought into flat contact with one another, or could be separated to any
required distance, and the distance between the plates was measured by a
vernier. At one end of the cylinder there was a source of constant heat,
and the differential goniometer already described at the other. With
this apparatus Mr. Tyndall found that olefiant gas maintains its great
superiority over the other gases in absorptive power at all thicknesses.
A layer of that gas not more than 0·01 of an inch thick intercepted
about one per cent. of the total radiation. This great absorption
corresponded to a deflection of 11° of the needle of the goniometer, and
such was the delicacy of the apparatus that it would be possible to
measure the action of a layer of this gas of less thickness than a sheet
of writing paper. A layer of olefiant gas two inches thick intercepts
nearly 30 per cent. of the entire radiation. A shell of olefiant gas two
inches thick surrounding our globe would offer no appreciable hindrance
to the solar rays in coming to the earth, but it would intercept, and in
great part return, 30 per cent. of the terrestrial radiation; under such
a canopy the surface of the earth would probably be raised to a stifling
temperature.
The apparatus for measuring the action of the greater thicknesses of gas
was a hollow brass cylinder 49·4 inches long, closed at both ends by
plates of rock-salt, and divided internally into two compartments or
chambers by a third plate of rock-salt movable in the interior; the
source of heat being at one end and the differential goniometer at the
other.
Carbonic oxide and carbonic acid are pervious to a vast majority of the
rays of radiant heat. When the cylinder was filled with carbonic oxide
gas and so divided, by moving the internal plate of rock-salt, that a
stratum of the gas 8 inches long was next to the source of heat, and
that 41·4 inches long farthest from it, the 8 inches of gas intercepted
6 per cent. of the whole radiation. But when the plate of rock-salt was
moved till the column 41·4 inches long was next to the source of heat,
and that of 8 inches farthest from that source, or behind the long one,
the absorption of the 8 inches was sensibly zero. In like manner eight
inches of carbonic acid gas when in front of a column of 41·4 inches of
the same gas absorbed 6-1/4 per cent. of the whole radiation, while
placed behind that column the effect was nearly zero. The reason is that
when the 8 inch stratum is in front, it stops the main portion of the
rays which give it its thermal colours,[7] while placed behind these
same rays have been almost wholly withdrawn, and to the remaining 94 per
cent. of the radiation the gases are sensibly permeable.
It is inferred from an extension of this reasoning that the sum of the
absorptions of the two chambers taken separately must always be greater
than the absorption effected by a single column of the gas of a length
equal to the sum of the two chambers; this conclusion is illustrated in
a striking manner by the experiments. It is also found that when the
mean of the sums of the absorptions is divided by the absorption of the
sum, the quotient is sensibly the same for all gases. It may farther be
inferred that the sum of the absorptions must diminish and approximate
to the absorption of the sum as the two chambers become more unequal in
length, and that the sum of the absorptions of the two chambers is a
maximum when the medial plate of rock-salt divides the long tube into
two equal parts.
When air enters an exhausted tube it is heated dynamically by the
collision of its particles on the sides of the tube as it rushes in to
fill the vacuum; and when the tube is exhausted again by the air pump,
chilling is produced by the application of a portion of the heat of the
air to generate vis viva. This dynamic principle occurred in some of the
experiments, and was dexterously adopted and applied to the solution of
a striking and unprecedented problem: ‘To determine the radiation and
absorption of gases and vapours _without any source of heat external to
the gaseous body itself_.’
The two external sources of heat being therefore dispensed with in the
absorptive apparatus, the thermo-electric pile was presented to the cold
glass tube which was exhausted, and the needle of the goniometer stood
at zero. Nitrous acid on entering the exhausted tube became heated and
radiated its heat upon the adjacent face of the pile which deflected the
needle of the goniometer through 28° in the direction that indicates
absorption. As the heat of the gas became gradually exhausted, the
needle returned slowly to zero. The pump was now worked, the rarefied
gas in the tube was chilled, and the adjacent face of the pile gradually
poured its heat on the chilled tube till the temperature of the pile was
so much lowered, that the needle was deflected 20° on the negative side
of zero, that is on the side denoting radiation.
When olefiant gas entered the exhausted tube, the needle showed an
absorption of 67°, and when the gas was pumped out again, the needle
showed a radiation amounting to 41°. When the gas was then pumped out,
very dry atmospheric air was introduced into the tube,—the needle
pointed to 59° indicating absorption; and when it was pumped out again
the needle swung to nearly 40° on the other side of zero, indicating
radiation. Remembering that the radiation and absorption of dry air only
produce a deflection of 1°, it is evident that the preceding great
deflection of the needle is entirely owing to the action of the small
residue of olefiant gas that remained in the exhausted tube. In order to
ascertain how much the quantity of a gas or vapour might be reduced
before its action became insensible, the vapour of boracic ether, which
has the greatest absorptive energy, was chosen.
The mercurial gauge for measuring the pressure or tension of the vapour
already mentioned remained attached to the apparatus. When one-tenth of
an inch of the vapour of boracic ether was admitted into the exhausted
tube, the barometer stood at 30 inches: hence the tension of the vapour
within the tube was the 1/300th part of an atmosphere. Dynamically
heated by dry air the radiation of the vapour produced a deflection of
56°. Again the tube was exhausted to 0·2 of an inch and the quantity of
vapour was thereby reduced to 1/150th of its first amount; the needle
was allowed to come to zero, and the residue of the vapour produced a
deflection of 42°. The pump was again worked till a vacuum of 0·2 of an
inch was obtained, this residue containing of course the 1/150th of the
quantity of ether present in the tube; and on dynamically heating the
residue, its radiation produced a deflection of 20°.
Thus it is evident that the tension of the ether in these experiments
was continually diminished by the 0·2 of an inch, consequently its
quantity was continually diminished by its 1/150th part, accompanied by
a corresponding decrease in the deflections of the needle. The final
result of this process showed that the radiation of an amount of vapour
in the tube possessing a tension of less than the thousand millionth of
an atmosphere is perfectly measurable. The temperature imparted to this
infinitesimal quantity of matter did not exceed 0·75 of a centesimal
degree. The molecules which constituted this intensely attenuated
vapour, though inconceivable, had as true an existence as the suns which
constitute the star-dust of the nebulæ. ‘A platinum wire raised to
whiteness in a vacuum by an electric current, becomes comparatively cold
in a second after the current has been interrupted; yet that wire, while
ignited, was the repository of an immense amount of mechanical force.
What has become of this? It has been conveyed away by a substance so
attenuated that its very existence must for ever remain an hypothesis.
But here is matter that we can weigh, measure, taste, and smell; that we
can reduce to a tenuity which, though expressible by numbers, defeats
the imagination to conceive of it. Still we see it competent to arrest
and originate quantities of force which on comparison with its own mass
are almost infinite, a small fraction of this force causing the double
needle of the galvanometer to swing through considerable arcs. When we
find ponderable matter producing these effects, we have less difficulty
in investing the luminiferous ether with those mechanical properties
which have long excited the interest and wonder of all who have
reflected upon the circumstances involved in the undulatory theory of
light.’
The dynamical principle was next applied to determine the radiation of a
gas through itself; or through any other gas having the same period of
vibration. For that purpose Mr. Tyndall made use of the hollow cylinder
49·4 inches long already mentioned, closed at both ends by plates of
rock-salt, and divided internally into two chambers by a movable plate
of the same substance. All sources of heat being dispensed with, the
chamber next the voltaic pile contained the gas which was to act as an
absorber, and the more remote as a radiator.
Heat is evolved in air when its motion is arrested; on entering an
exhausted tube, the more rapid the motion the greater the heat. Both
chambers of the cylinder were at first filled with the vapour to be
examined, the usual pressure being the 1/60 part of an atmosphere. But
the vapour entered so slowly, and the quantity was so small that the
radiation due to the warming of the vapour by its own collision was
insensible. The needle of the goniometer being at zero, dry air was
allowed to enter the chamber most distant from the pile; this air became
heated dynamically by the collision of its particles against the sides
of the tube, communicated its heat to the vapour, and the vapour
immediately discharged the heat thus communicated to it against the
pile. This case not only resembles, but is actually of the same
mechanical character as, that in which a vibrating tuning fork is
brought into contact with a surface of some extent. The fork, which
before was inaudible, becomes at once a copious source of sound. What
the sounding board is to the fork, the compound molecule is to the
elementary atom. The tuning fork vibrating alone is in the condition of
the atom radiating alone; the sound of the one and the heat of the other
being insensible. But in association with sulphuric or acetic ether
vapour the elementary atom is in the condition of the tuning fork
applied to its sounding board, communicating motion to the luminiferous
ether through the molecules, as the fork through the board communicates
its motion to the air.
Mr. Tyndall’s experiments show the great opacity of a gas to radiations
from the same gas, and may likewise show the remarkable influence of
attenuation in the case of vapour. The individual molecules of a vapour
may be powerful absorbers and radiators, but in their strata they
constitute an open sieve through which a great quantity of radiant heat
may pass. In such thin strata, therefore, the vapours as used in the
experiments were generally found far less energetic than the gases,
while in thick strata the same vapours showed an energy greatly superior
to the same gases, but the gases were always employed at a pressure of
one atmosphere.
Lastly Mr. Tyndall examined the diathermancy of the liquids from which
his vapours were derived, and the result leaves not a doubt that both
absorption and radiation are phenomena irrespective of aggregation. If
any vapour is a strong absorber and radiator, the liquid from whence it
comes is also a strong absorber and radiator.
Perfectly dry pure air is as pervious to light and heat as a vacuum
itself; consequently, if the atmosphere was quite pure and dry, the rays
of the sun would fall on the earth with unmitigated force during the
day, and would be radiated back again and dissipated in space during the
night to the destruction of vegetation. But the earth is protected from
these extremes by the absorptive power of aqueous vapour, which is
always present more or less in the atmosphere; even when the air is so
transparent that distant objects seem to be near, it is loaded with
vapour in an elastic invisible state, which a change of temperature may
condense into cloud or precipitate in rain.
The absorptive power of aqueous vapour was determined by placing tubes
containing fragments of glass moistened with water between the drying
apparatus and the experimental glass tube of the instrument, so that
perfectly pure dry air in passing over the wet fragments of glass
carried a portion of aqueous vapour with it into the exhausted
experimental tube, and the deflection of the needle of the goniometer
showed that the absorptive power of the aqueous vapour exceeded that of
the dry air 80 times. Now since in the atmosphere there is one molecule
of aqueous vapour with an absorptive power of 80 for every 200 atoms of
oxygen and nitrogen whose absorptive power is 1 like that of one of its
constituent atoms, it follows by comparison that the absorptive power of
the molecule is 16,000 times greater than that of an atom of either
oxygen or nitrogen. From this enormous opacity to obscure heat ‘it is
certain that more than ten per cent. of the terrestrial radiation from
the soil of England is stopped within ten feet of the surface of the
soil; remove for a single summer night the aqueous vapour from the air
which overspreads the country, and you would assuredly destroy every
plant capable of being destroyed by a freezing temperature.’
The quantity of vapour in each place varies with the latitude, the
season, and other circumstances; but whenever the amount of heat
radiated from the earth surpasses the absorption, the remainder passes
through the vapour into space, and for the same reason the residue of
that coming from the sun passes through the vapour and comes to the
earth, so that whatever may be the local differences it has been
decidedly proved with regard to the whole globe, that the quantity of
heat annually received from the sun is annually radiated into space; the
latter is a force lost to the earth, nevertheless it does not interfere
with the law of the conservation of force which extends to the universe.
By observations made during ten scientific ascents in a balloon to very
great altitudes, Mr. Glaisher has proved, that the theory of the uniform
decrease of temperature with increase of elevation is no longer tenable.
Since the absorptive force of aqueous vapour is 16,000 times that of dry
air, the whole of the heat radiated by the full moon is intercepted by
our atmosphere. It raises the temperature of the higher regions,
dissolves the vapour, dissipates the clouds, prevents the formation of
more, and allows the heat radiated from the earth to pass freely into
space: thus confirming the common, and almost universal, belief that the
full moon dispels the clouds. The absorptive power of aqueous vapour is
so enormous that even the planet Mercury may be habitable should his
atmosphere contain a sufficient quantity of it to mitigate the heat of
the sun.
No doubt all the heat from the stars must be absorbed by the atmosphere,
but their photographs show that it is pervious to the chemical rays.
Those from Sirius, the nearest and brightest of the stars, travelling
through 180 millions of millions of miles and decreasing in quantity
inversely as the square of the distance, still have sufficient energy to
give a perfect photographic impression of its spectrum; but Sirius is
sixty times larger than the sun, and is many times more luminous. A
photograph of the spectrum of Capella has been taken, though three times
more distant than Sirius. Photographs of double stars of the sixth and
seventh magnitude show that actinic rays from immeasurable distances in
space have power sufficient to decompose matter in unstable equilibrium
on the surface of the earth.
The chemical power of the moon’s light only surpasses that of Jupiter in
the ratio of 6 to 4 or 5, and Jupiter’s light has twelve times more
actinic energy than that of Saturn. For such comparisons a standard of
photographic intensity is requisite.
A paper coated with chloride of silver can be prepared which has a
constant degree of sensitiveness, and Dr. Roscoe has proved that a
constant dark tint is produced on this standard paper by a constant
quantity of light, the tint being the same, whether light of the
intensity represented by 1 acts for the time represented by 50, or light
represented by 50 acts for the time represented by 1; or in other words
the amount of the chemical action of light is directly proportional to
the intensity of the light, and when the light is constant, the amount
of action is exactly proportional to the time of exposure.
The ratio of the chemical action of the rays of light falling directly
from the sun to the chemical action of the light diffused over the whole
sky can be determined by means of an instrument, in which the shadow of
a little ball is made to fall on a sensitive paper so as to intercept
the direct rays of the sun, and allow it to be impressed by an action of
the light diffused over sky alone; this compared with a similar paper,
on which both the direct and indirect light has fallen, gives the ratio
required. From this it appears, that the relative amount of chemically
active light which comes directly from the sun, is very much less than
the amount of his direct visible light. For while Professor Roscoe was
making experiments at Manchester on the maximum effect of the chemical
action of light, he found when the sun had an altitude of 20°, that of
100 chemical rays which fell on a piece of standard paper, only about 8
came from the direct light of the sun; while on the contrary, of 100
rays of visible light, 66 came directly from the sun, and only 40 from
the light diffused over the whole sky, so that the diffused light is
richer in chemical rays than the direct solar beam, ‘a startling
result,’ but borne out by observations not only made at Manchester and
in its vicinity, but at Kew, Heidelberg, and at Pará on the Amazon
nearly under the equator.
On account of the increasing rarity of the atmosphere, the greater the
height above the level of the sea, the less the amount of diffused light
and consequently of actinic power. Hence photographers have to expose
their plates for a much longer time to the light on the snowy peaks of
the Alps and other great heights than in England or at the level of the
sea. During Mr. Glaisher’s tenth balloon ascent simultaneous
observations were made at Greenwich Observatory and in the balloon, when
at more than three miles above the surface of the earth, the standard
paper exposed to the full rays of the sun was not as much coloured in
half an hour as the corresponding paper at Greenwich in one minute.
By a series of observations at Heidelberg, Kew, and Manchester, it has
been proved that the very small relative chemical action of the sun’s
direct light decreases rapidly with his altitude, and at these three
places of observation, it has frequently happened when the sun’s
altitude was very low, as at 12°, that his direct light made no
impression on a sensitive paper. ‘The sun’s light had been robbed of its
chemical power in passing through the air.’ This singular result is
ascribed by Professor Roscoe to what he calls the opalescence of the
atmosphere.
Opalescent glass, slightly milky liquids, pure water with particles of
sulphur floating in it, are impervious to the chemical rays, whence
Professor Roscoe infers that the atmosphere, more especially its lower
regions, possesses that property in consequence of multitudes of solid
particles floating in it. What they are is unknown, but infinitesimal
particles of soda seem to be everywhere, and no doubt particles of other
substances mixed with them may be often seen as motes dancing in the
sunbeams. Besides, it is clearly proved that myriads of the eggs and
germs of organized beings, though invisible to the naked eye, are
continually floating in the air, and that they are more abundant in the
lower than in the higher strata of the atmosphere. Since opalescent
matter reflects the blue rays of light and transmits the red, Professor
Roscoe ascribes the blue colour of the sky and the bright tints at
sunrise and sunset to the opalescent property of the air.
The atmosphere is permeable to every kind of chemical rays, which is far
from being the case with bodies on earth, some of which though
transparent to all the visible rays, vary greatly in their transparency
to the chemical rays.
The atoms and molecules of matter not only have the power of turning the
rays of the solar beam out of their rectilinear path, but of changing
their refrangibility.
The myriads of ethereal waves or rays of light that constitute the seven
colours of the solar spectrum, decrease in refrangibility and increase
in rapidity of vibration and length of wave from the extreme violet to
the end of the red; each ray having its own rate of vibration, its own
length of wave, and its own colour. From the middle of the yellow, which
is the luminous part of the spectrum, the chemical spectrum extends
invisibly, but with increasing refrangibility and increasing velocity of
vibration, to a point far beyond the violet. On the contrary, the heat
spectrum, which may also be said to begin in the yellow light, extends
invisibly but with decreasing refrangibility, and decreasing velocity of
vibration to some distance beyond the visible red.
The rays of heat are absorbed by the humours of the eye, but were they
to reach the retina we should see that they differ from one another as
much as those of the luminous spectrum; the chemical spectrum from its
greater length is still more diversified.
The whole of the solar spectrum, visible and invisible, is crossed at
right angles to its length by innumerable dark rayless lines, differing
in breadth and intensity. Sir John Herschel discovered vacant spaces in
the extra-luminous part of the heat spectrum, and more recently M.
Edouard Becquerel, by throwing the solar spectrum upon a daguerreotype
plate, discovered that the chemical spectrum given by a glass prism,
from its beginning in the yellow to its extreme point beyond the violet,
is crossed by rayless lines, and that the lines in the part passing
through the visible spectrum coincide exactly with the rayless lines in
the luminous part. This coincidence was confirmed by the independent
researches of Dr. Draper at New York. By means of the rayless spaces or
black lines in the visible spectrum, M. Kirchhoff has proved that
thirteen terrestrial substances are constituents of the sun’s
atmosphere.
The length of the undulations of the ether which produce the impression
of the extreme violet rays of the solar spectrum on our eyes, is the
seventeen millionth part of an inch; the length of the ethereal
undulation that produces the sensation of the extreme red is the
twenty-six millionth part of an inch; the ethereal undulations beyond
these limits are invisible to human eyes. Nevertheless certain
substances have the power of increasing the length of the vibrations,
and reducing the rays of the spectrum to a lower grade in the scale of
refrangibility, so that the invisible rays of the chemical spectrum have
thus been brought within the limit of human vision.
For example, the chemical rays shine as visible light when they fall on
glass tinged with the oxide of uranium. When these dark rays fall upon
the glass, they put the whole of its molecules into vibrations, the same
with their own, while at the same time they give a more rapid vibration
to a certain number of the same molecules. The whole of the molecules
restore their vibrations to the surrounding ether. Those having the same
velocity with the chemical rays make no sensible impression on our eyes;
but the more rapid vibrations come within the limits of the visible
spectrum; they have consequently a lower refrangibility, and shine as
visible light. It is called degraded light on account of its lower
position in the prismatic scale, but more frequently fluorescent light,
because fluor spar was the first solid known to possess the property. A
number of substances are fluorescent, both solid and liquid, organic and
inorganic.
If in a dark room a non-fluorescent body be illuminated by a sunbeam
passing through glass stained deep blue by cobalt, it will reflect blue
light; but it will appear to be perfectly black if it be viewed through
glass tinged yellow by silver; while a piece of canary glass, which is
highly fluorescent, will shine with a vivid light under the same
circumstances. All the molecules of the canary glass give back to the
ether the undulations that have been impressed on them by the blue
light; while a certain number of them possess the power of receiving and
giving back more rapid vibrations to the ether. The yellow glass held
before the eye is impervious to the undulations of the blue rays, but
transmits those of the fluorescent light, which emanate from the smaller
number of molecules, and which thus become in reality new centres of
light, different from the sun’s light, though dependent upon it: the one
terrestrial, the other celestial. Since the vibrations of the
fluorescent light are more rapid than those of the blue light their
colour is lower in the prismatic scale. The vibrations of the molecules
in a fluorescent substance are analogous to those of a musical cord,
which give the fundamental note or pitch and its harmonics, for the
whole of the musical cord while vibrating the fundamental note divides
itself spontaneously into parts having more rapid vibrations, which give
the harmonics. Professor Stokes of Cambridge, who made this beautiful
experiment, computed that the vibrations which produced the fluorescent
light were a major or minor third below the pitch or vibrations of the
blue light.
One of the first discoveries of fluorescence was made by Sir John
Herschel—certainly the first who observed the property in a liquid. He
found that the blue light which emanates from all parts of a solution of
the sulphate of quinine, especially from its surface, is fluorescent,
and that the light transmitted through the liquid, though sensibly like
the incident white light, is no longer capable of producing
fluorescence; it has been deprived of its chemical rays by absorption.
The chemical rays having been rendered visible by an increase in the
length of the periods of vibration, unsuccessful attempts have been made
to change the periods of the rays of heat beyond the red end of the
spectrum so as to bring them within the limits of vision. The idea of
effecting such a change by employing a substance opaque to light, but
pervious to heat, is due to Dr. Akin; but it has since been accomplished
by Dr. Tyndall, who, in the course of his experiments on radiant heat,
found that a solution of iodine in the bisulphide of carbon excludes the
most dazzling light, but transmits the rays of heat freely. He employed
a mirror, lined in front with silver, to concentrate the rays emitted
from the charcoal points of the electric lamp, and interposed a vessel
containing the solution in question, so that the rays of heat alone were
brought to a focus almost undiminished. When the solar spectrum was
examined, the point of maximum heat was found to be as far beyond the
extreme red on one side as the green rays on the other. In the spectrum
of the electrical light the point of maximum heat was also found to lie
beyond the extreme red, but the augmentation of intensity was so sudden
and enormous as far to exceed the maximum heat of the sun previously
determined by Professor Müller. Aqueous vapour powerfully absorbs
radiant heat; so a solar spectrum beyond the earth’s atmosphere might
probably exhibit as great intensity as the electrical light. With the
apparatus described oxidizable substances burst into the flame of common
combustion when put into the focus; but when the chemical action of the
oxygen of the atmosphere was excluded by igniting substances in vacuo by
the invisible rays of heat, their periods of vibration were so changed
as to bring them within the limits of vision. When the electric light is
very powerful, a plate of platinized platinum in vacuo is raised to
white heat at the focus of invisible rays; and when the incandescent
platinum is looked at through a prism, its light yields a complete and
brilliant spectrum. ‘In all these cases we have a perfectly invisible
image of the charcoal points formed by the mirror; and no experiment
illustrates the change of heat into light’ more strongly than the
following:—When the plate of platinum or one of charcoal is placed in
the focus, the invisible image raises it to incandescence, and thus
prints itself visibly on the plate. On drawing the coal points of the
lamp apart, or causing them to approach each other, the thermograph
follows their motion. By cutting the plate of carbon along the boundary
of the thermograph, a second pair of coal points may be formed of the
same shape as the original ones, but turned upside down; and thus by the
rays of the one pair of coal points which are incompetent to excite
vision, we may cause a second pair to emit all the rays of the spectrum.
Fluorescence and calorescence act in contrary directions. Fluorescence
causes the molecules of a fluorescent substance to oscillate in slower
periods than the incident light, while calorescence causes the molecules
of a substance to oscillate in longer periods than the incident light.
The refrangibility of the rays is lowered in the first case, and raised
in the second.
Substances differ as much in their transmission of the chemical rays as
those of light and heat. Glass is impervious to the most highly
refrangible chemical rays, while rock crystal transmits them with the
greatest facility; and on that account the absolute length of the
spectrum was not known till the light was refracted by prisms of rock
crystal. Besides, the number, position, and intensity of the chemical
rays vary with the source of light. Some flames have scarcely any
chemical rays; that of the oxy-hydrogen blowpipe, though intensely hot,
has very few, and even the solar light is inferior in that respect to
electricity. The electric spark from the prime conductor of a common
electrifying machine, or the discharge of a Leyden jar, emits rays of
very high refrangibility, far surpassing those which emanate from the
sun. For, when the electric light from a highly charged Leyden jar was
refracted by two quartz prisms and thrown by Professor Stokes on a plate
of uranium glass, the chemical spectrum was highly luminous, and six or
eight times as long as the visible spectrum. An equally extensive
spectrum was obtained from the voltaic arc taken between copper points;
it consisted entirely of bright lines. The long spectrum also appeared
on the uranium glass when the spark refracted by quartz prisms was
obtained from the secondary terminals of an induction coil in connection
with the coatings of a Leyden jar. It consisted of bright lines, but was
not so luminous as that from a powerful voltaic battery. On changing the
metals of the points between which the sparks passed, the bright lines
were changed, which showed that they were due to the particular metals.
The heat of the electric spark volatilizes the metals which form the
points of the conducting wires; and all volatilized metals give
characteristic spectra, both visible and chemical. The visible part
differs from that of the solar spectrum in being crossed by bright lines
instead of dark ones; but the number, intensity, and position of both
the visible and invisible lines change with each metal. The changes in
the invisible part under consideration may be readily observed by
throwing the spectra either on a fluorescent or collodion plate. For
example: in the spectrum from the spark between thallium points thrown
on the latter, Dr. Miller found that there were two strong groups of
lines in the least refrangible part of the spectrum; at a little
distance from these there were three groups, the two first less intense
than the third; several rows of feeble dots followed, and the chemical
spectrum terminated rather abruptly with four nearly equidistant groups.
This spectrum bears a resemblance to those of zinc and cadmium, less
strongly to that of lead. Dr. Miller found that the photographic spectra
of iron, cobalt, and nickel, also have a strong analogy, but that the
metals arsenic, antimony, and tin showed as great a difference in the
invisible as in the visible part of their spectrum.
The fluorescent spectra of seventeen metals were examined by Professor
Stokes of Cambridge; several of them showed luminous lines of
extraordinary strength, especially zinc, cadmium, magnesium, aluminium,
and lead, which in a spectrum not generally remarkable contains one line
surpassing perhaps all other metals in brilliancy. Some other metals
exhibit in certain parts of their spectra lines that are both bright and
numerous; on the whole some parts of the spectra are strong and
tolerably continuous, while in others they are weak. This grouping of
the lines is most remarkable in copper, nickel, cobalt, iron, and tin.
Of all the metals examined, magnesium gave the shortest spectrum, ending
in a very bright line, beyond which however excessively faint light
extended to a distance equal to that of the long spectra. Aluminium, on
the other hand exceeded all the other metals in richness of the rays of
the very highest refrangibility. All the strong lines mentioned lie in
that part of the spectra.
In the course of these experiments Professor Stokes observed that even
quartz of a certain thickness is not transparent to invisible lines of
the highest refrangibility, for the highest aluminium line, which is
double, could only be seen by rays passing through the edge of the
prism. This leads to another branch of the subject, namely, the
absorption of the invisible rays by solids, liquids and gases. Mr. Wm.
Allen Miller has shown from his own experiments that bodies pervious to
the chemical rays in the solid form, are so also in the liquid and
gaseous form; that colourless transparent solids which absorb the
photographic rays, absorb them more or less also in their liquid and
gaseous states. He has moreover found that the following substances have
the same maximum transparency:—rock crystal, ice, and fluor spar among
solids, water among liquids, the three elementary gases and carbonic
acid among gaseous substances. The most opaque to the invisible rays
are, nitrate of potash, bisulphide of carbon, and sulphuretted hydrogen.
It appears that a thin plate of mica is intensely opaque to all the
invisible rays except a small portion of them of the lowest
refrangibility.
The absorptive property however is partial: an absorptive substance
either cuts off a portion of the light of a fluorescent spectrum or
stripes it with dark lines: each substance absorbs rays peculiar to
itself. Those employed by Professor Stokes were the alkaloids and
glucosides, and he assumed the spectrum of tin for their examination
because it has a long interval of continuity.
The fluorescent property of yellow uranite was discovered by Professor
Stokes some years ago, and now he has added another fluorescent mineral
in adularia or moonstone; from its natural faces and planes of cleavage
alike a beautiful blue fluorescence emanated under the induction spark.
As the same was observed in colourless felspars generally, Professor
Stokes concluded that fluorescence is an inherent property in the
silicate of alumina and potash constituting the crystal of moonstone.
The blue fluorescence extended to a very sensible though small depth
within the substance.
A particular variety of fluor spar found at Alston Moor in Cumberland,
which is very pale by transmitted light, shows a strong blue
fluorescence, and is eminently phosphorescent on exposure to the
electric spark. It is the same kind of crystal in which Sir David
Brewster originally discovered the property of fluorescence. On
presenting such a crystal to the spark passing between aluminium
terminals, besides the usual blue fluorescence, there was another of a
reddish colour extending not near so far into the crystal, produced by
the rays belonging to the strong lines of aluminium of extreme
refrangibility.
The cube of fluor spar which showed these effects was externally
colourless to the depth of 1/20 of an inch; then came one or two strata
parallel to the faces of the cube showing the ruddy fluorescence, while
the blue fluorescence extended to a much greater depth and had a
stratified appearance. This crystal was eminently phosphorescent, its
blue phosphorescence being arranged in strata parallel to the face of
the cube like the blue fluorescence, but it was not perceptible beyond a
very moderate distance below the surface at which the exciting cause
entered, that cause being the photographic rays of extremely high
refrangibility of the electric spark—taken, as in all these experiments,
between the secondary terminals of an induction coil in connection with
the coatings of a Leyden jar, and refracted by quartz prisms.
Mr. Stokes has employed fluorescence as a means of tracing substances in
impure chemical solutions. When a pure fluorescent substance is examined
in a pure spectrum it is found that on passing from the extreme red to
the violet and beyond, the fluorescence commences at a certain point of
the spectrum, varying from one substance to another, and continues from
thence onwards more or less strongly in one part or another according to
the particular substance. The colour of the fluorescent light is found
to be nearly constant throughout the spectrum. ‘Hence when in a solution
examined in a pure spectrum we notice the fluorescence taking as it were
a fresh start _with a different colour_, we may be pretty sure that we
have to deal with a mixture of two fluorescent substances.’
Experience as well as theory shows that rapid absorption is accompanied
by copious fluorescence. But experience has hitherto also shown what
could not have been predicted, and may not be universally true, that
conversely absorption is accompanied in the case of a fluorescent
substance by fluorescence.
The phosphorescent light of insects, fish, and plants is owing to
chemical action, which produces many luminous phenomena; but a great
number of inorganic and organic substances shine in the dark with a
phosphorescence which is nearly allied to fluorescence. It is produced
by exposure to the sun, by heat, electricity, insulation, cleavage,
friction, and motion. For if a bottle containing nitrate of uranium be
shaken, it shines spontaneously with a vivid light; even the hand shows
phosphorescence in the dark after being exposed to the sun.
The essential difference between fluorescence and phosphorescence
consists in the time during which the light lasts. Fluorescence ceases
almost immediately after the exciting cause is withdrawn, while a
phosphorescent body whether excited by heat, solar light, or
electricity, lasts a much longer time; besides, the fluorescent rays are
generally of lower refrangibility. Light and heat are temporarily
absorbed and given out again by every body on the surface of the earth,
more or less, that are exposed to the sun’s light. The nights would be
much darker even when illuminated by the stars were it not for earth
light, for the molecules restore to the ether, in the form of
phosphorescence, the undulations they have received from the sun’s light
during the day. The snow and ice blink of the sailors is a striking
instance; generally, however, it is of much shorter duration. The
phosphorescent property is nearly allied to electricity, for bodies that
are bad conductors are apt to become phosphorescent, while good
conductors of electricity rarely if ever show it. Ozone must be
phosphorescent, for oxygen exhibits persistent light when electric
discharges are sent through it, and Mr. Faraday saw a flash of lightning
leave a luminous trace on a cloud which lasted for a short time.
In the solar spectrum the chemical or actinic rays produce
phosphorescence, which the red rays have the power to extinguish. M.
Nièpce de St-Victor found that solar light impresses its vibrations so
strongly on substances exposed for a short time to its influence that
they not only shine in the dark, but that the phosphorescent light they
radiate has chemical energy enough to decompose substances in unstable
equilibrium, and leave daguerreotype impressions of great delicacy and
beauty.
The polarization of light and heat affords a remarkable instance of the
elective power of matter. Light and heat are said to be polarized,
which, having been once reflected, are rendered incapable of being again
reflected at certain angles. For example, a ray incident on a plate of
flint glass at an angle of 57° is rendered totally incapable of being
reflected at that same angle from another plate of flint glass in a
plane at right angles to the first. At the incidence of 57° the whole of
the ray is polarized: it is the maximum of polarization for flint glass,
but there is a partial polarization for every other angle; the portion
of the ray polarized increases gradually up to the maximum, as the
incidence approaches to 57°. All reflecting surfaces are capable of
polarizing light and heat, but the angle of incidence at which the ray
is totally polarized is different in each substance. Thus, the angle of
incidence for the maximum polarization of crown glass is 56° 55ʹ, and no
ray can be totally polarized by reflection from the surface of water
unless the angle of incidence is 53° 11ʹ. As each substance has its own
maximum polarizing angle, the effect is evidently owing to the action of
the molecules of matter, and not to any peculiarity in the light or
heat.[8]
Light and heat are also polarized by refraction, for certain substances,
especially irregularly crystallised minerals like Iceland spar, possess
the property of dividing a ray of light or heat passing through them in
certain directions into two pencils, namely, the ordinary and
extraordinary rays. The first of these is refracted according to the
same law as in glass or water, never quitting the plane perpendicular to
the refracting surface, while the second does quit that plane, being
refracted according to a different and more complicated law. Hence, if a
crystal of Iceland spar be held to the eye, two images of the same
object will generally be seen of equal brightness. But when they are
viewed through a plate of tourmaline it will be found that while the
spar remains in the same position the images vary in relative brightness
as the tourmaline is made to revolve in the same plane; one increases in
intensity till it arrives at a maximum, at the same time that the other
diminishes till it vanishes, and so on alternately at each quarter
revolution of the tourmaline, proving both rays to be polarized. For in
one position the tourmaline transmits the ordinary ray and reflects the
extraordinary, and after revolving 90°, the extraordinary ray is
transmitted and the ordinary ray is reflected.
The undulations of the ethereal medium which produce the sensation of
common light, are performed in every plane at right angles to the
direction in which the ray is moving, but the case is very different
after the ray has been polarized by passing through a substance like
Iceland spar, for the light then proceeds in two parallel pencils whose
undulations are still indeed transverse to the direction of the rays,
but they are accomplished in planes at right angles to one another. The
ray of common light is like a round rod, whereas the parallel polarized
rays resemble two long flat rulers, one of which lies on its broad
surface, and the other on its edge. By a simple mechanical law, each
vibratory motion of the common light is resolved into vibratory motions
at right angles to one another.
The polarization of light and heat by refraction is not owing to the
chemical composition, but to a want of homogeneity in the molecular
structure of the substances through which they pass; for regular
crystals and substances which are throughout of the same temperature,
density, and structure, are incapable of double refraction. The effect
of molecular structure is strikingly exhibited by the circular
polarisation in the dimorphic crystals of quartz. In one form the plane
of polarization revolves from right to left, and in the other that plane
revolves from left to right, although the crystals themselves differ
apparently by a very slight and often almost imperceptible variety of
forms.
Thus polarization forms the most admirable connection between light,
heat, and crystalline structure; showing peculiar arrangements of the
molecules in regions otherwise unapproachable, and too refined for our
perceptions. Besides, the gorgeously coloured images displayed by
depolarization are splendid examples of the power of matter in
decomposing light.
The perfect correspondence of the properties of the symmetrical,
elastic, and optical axes of crystals with light and heat is another
instance of the connection between the latter and crystalline form.
The axis of symmetry is that direction or imaginary line within a
crystal, round which all the parts or particles are symmetrically
arranged. A medium is said to be elastic which returns to its original
form with a resilient force after being relieved from compression, and
the axis of elasticity of a crystal is that direction in which it is
most elastic. The optic axis is that line or direction through which
light passes in one beam according to the law of ordinary refraction.
Crystals may have one, two or more optical axes according to their form.
Doubly refracting crystals such as Iceland spar have only one principal
optic axis in which the whole beam passes according to the ordinary law;
in every other direction the beam of light is divided into two polarized
rays, one of which called the ordinary ray passes according to the
ordinary law, while the other, known as the extraordinary ray, traverses
the crystal in a different direction, with more rapidity and according
to a different and more complicated law. The velocity of this
extraordinary ray is a maximum when at right angles to the principal
optical axis, and a minimum when parallel to it.
In perfectly regular crystals like the cube or die, the octohedron, &c.,
there are three axes of symmetry and of equal elasticity at right angles
to one another. In these regular crystals all the axes are optical, so
that they have no double refraction.
Right square prisms have two equal rectangular axes of symmetry, two
axes of equal elasticity, and one optical axis.
All crystals of the pyramidal and rhomboidal systems have one axis of
symmetry, two axes of elasticity, one optical axis; and form coloured
circular rings traversed by a black cross when viewed by depolarized
light.
Lastly oblique prismatic crystals which have three unequal axes of
symmetry have three axes of unequal elasticity, two optical axes; and by
depolarization give coloured lamnescata, that is coloured figures having
the form of the figure 8 which are traversed by a black cross in two
opposite quadrants, and when the crystal is made to revolve, the same
figure, but in the complementary colours and traversed by a white cross,
appears in the other two quadrants.
The right and left-handed circular polarization of quartz, according as
certain facettes of the crystal are turned to the right or left, and the
property of double refraction being exclusively possessed by crystals of
the rhomboidal form, are striking instances of the connection between
the geometrical arrangement of the molecules of matter and the optical
and thermal forces, for the polarization of heat and all its
consequences are in every respect analogous to those of light, and
similar phenomena would be seen were heat visible.
Heat changes the position of the optical axes of crystals. When applied
to a crystal of sulphate of lime, the two optical axes gradually
approach to each other and at last coincide; if the heat be continued
and increased, the axes open again, but in a direction at right angles
to their former position. Thus the force of heat throws every molecule
in the body into correlative motion. The angles of all crystals that are
not of the octohedral group are changed by heat and vary with the
intensity; the difference between the length of the greatest and least
optic axes in such crystals diminishes as the temperature is raised,
increases when it is lowered, and is constant at a given heat. In
Iceland spar heat indirectly affects the doubly refracting power, for
the expansion of the crystal in the direction of its axis is accompanied
by contraction at right angles to it, which brings the crystal nearer to
the cubical form, and consequently diminishes its doubly refracting
power.
According to the researches of M. Angström, in crystals with different
axes of elasticity the velocity of the molecular vibrations is different
in different directions when they are heated. In rock crystal and
tourmaline the heat radiates from a surface cut parallel to the axis of
the crystal; in felspar the radiating surface is at right angles to the
symmetrical axis.
The optical axes of crystals are also affected by pressure. Doubly
refracting crystals with one principal axis acquire two when the
pressure is perpendicular to it. The new principal axis coincides with
the line of pressure or is at right angles to it according as the
crystal is positive or negative, that is, according as the extraordinary
ray is refracted to or from the optic axis of the crystal. The colours
produced by polarization are affected by compression and dilatation
according as the crystal is positive or negative.
Sir David Brewster is of opinion that all the properties of double
refraction and the gorgeous phenomena of polarization, whether by
crystals or produced in various substances permanently or transiently by
heat, cold, rapid cooling, compression, dilatation, and induration, are
wholly the result of the forces by which the atoms are held together;
but these phenomena may rather be said to depend upon a reciprocal
action between an irregular molecular structure and the agency of light
and heat: which indeed seems to be confirmed by the transit of these two
forces through right and left-handed quartz, for there is no reason to
believe that there is any difference in the form of the particles in
these two crystalline substances.
The experiments of M. Becquerel show that electricity is a power which
makes the atoms of matter aggregate in crystalline forms; for he has
succeeded in forming crystals of gold, silver, cobalt, nickel, platinum,
and a variety of the gems undistinguishable from those in nature, by
exposing saturated solutions of these substances for a very long time to
feeble voltaic electricity; and crystals of earthy matter have been
obtained in the same manner. The electric and magnetic state of mineral
veins in mines which contain a vast variety of crystals, metallic and
non-metallic, strongly favours this view of the origin of crystalline
form.
M. Regnault has proved that the ratio between the specific heat and the
weight of the atoms of matter is intimately connected with the mode of
their aggregation; and indeed if it be considered that the atoms have
not only specific heat and weight, but specific affinity, electricity,
magnetism, consequently polarity, and probably specific forms, these
peculiar forces must necessarily influence the structure of crystals
according as they combine with or oppose the natural or artificial
forces acting upon them, or upon their dissimilar faces, and this may be
the cause of the great variety of forms that matter appears under.
Carbonate of lime alone assumes more than 1,200 different modifications
of its primitive type, but whatever be the variety of forms which any
one substance may take, they are found to be all compatible with and
derivative from a common type. The circumstances which have caused
dimorphous crystals to deviate from the general law have not yet been
explained.
It is very singular that when chlorate of soda is dissolved in water the
solution does not possess the property of circular polarization, but
when evaporated and allowed to crystallise, some of the crystals turn
light to the right, and others to the left. Now if all the crystals that
have the same property be picked out and dissolved in water a second
time, the liquid will still have no circular polarization, but when
allowed to crystallise, some of the crystals make light revolve through
them to the right and others to the left as before. From this it is
supposed that the atoms of liquids, which are free to move in every
direction, already possess part of the characters which the change to
solidity renders evident and permanent.
Although the relations between the force of magnetism and the atoms of
matter do not exhibit such brilliant phenomena as light does, they are
nevertheless most interesting and wonderful. Mr. Faraday discovered that
all substances, whether solid, liquid, or aëriform, are either magnetic
like iron, or diamagnetic like bismuth, the latter being by far the most
numerous. Thus if a bar of iron be freely suspended between the poles of
an extremely powerful magnet or electro-magnet, it will be attracted by
both poles and will rest or sit axially, that is, with its length
between the poles or in the line of magnetic force; whereas an equal and
similar bar of bismuth so suspended will be repelled by both poles and
will rest or sit equatorially, that is with its length perpendicular to
the line of magnetic force. Magnetism and diamagnetism are both dual
forces, but they are in complete antithesis to one another, which is
strikingly illustrated by their action on crystalline matter.
A sphere of amorphous substance freely suspended under magnetic
influence is indifferent, that is to say it has no tendency to set one
way more than another; but a sphere cut out of a crystal whether
magnetic or diamagnetic, is more powerfully attracted or repelled in one
direction than in any other, which shows a connection between the
magnetic forces and crystalline structure.
Crystals of carbonate of iron and carbonate of lime are isomorphous,
that is, they have exactly the same crystalline form, but the carbonate
of iron being highly magnetic is most powerfully attracted in the
direction of its greatest optical axis which therefore sets axially,
that is, in the line of magnetic force; while the principal optic axis
of the carbonate of lime, which is diamagnetic, is most powerfully
repelled and therefore sets equatorially. In both cases the antithetic
forces follow the same law of decrease in intensity from the greatest
optical axis to the least.
A bar of soft iron sets with its longest dimensions axially, but a bar
of highly compressed iron-dust, whose shortest dimensions coincide with
the line of pressure, sets equatorially, because it is most powerfully
attracted in the line of greatest density. A bar of bismuth sets
equatorially, but a bar of highly compressed bismuth dust, whose
shortest dimensions coincide with the line of pressure, sets with its
length axially, because it is most strongly repelled in the direction of
its greatest density. Hence the action of magnets upon matter is most
powerful in the line of maximum density, the force being attractive or
repulsive according to the kind of magnetism possessed by the atoms. It
follows therefore that the density is greatest in the line of the
principal optical axis, and gradually decreases to the least optical
axis, where it is a minimum.
The position which crystals take with regard to the magnetic force
depends also upon their natural joints of cleavages, and upon their
power of transmitting electricity. The diamagnetic force is inversely as
the conducting power of bodies, and the conducting power of crystals is
a maximum in the planes of their principal natural joints. Hence the
action of the diamagnetic power is least in the natural joints, and
conversely the magnetic force is greatest. In fact, the magnetic
phenomena of crystals depends upon unequal conductibility in different
directions, and their set is determined by the difference between the
forces of attraction and repulsion of the poles, for one pole of the
magne-crystallic axis is attracted and the other repelled. It is
unnecessary to give more examples to show the action of the magnetic
forces upon the atomic structure of crystals.[9]
Magnetism changes the relations and distances between the ultimate atoms
of matter, a circumstance which probably depends upon their polarity. It
changes steel permanently, iron temporarily, and it elongates a bar of
iron, which loses in breadth what it gains in length; and as heat is
developed in one direction and absorbed in the other, the temperature of
the bar remains the same. Heat being an expansive force, diminishes the
magnetism of iron and nickel in proportion as it increases the distance
between their atoms, till at length they lose their cohesive force
altogether. But there seems to be a temperature at which the magnetic
force is a maximum, above and below which temperature it diminishes.
Thus the magnetism of cobalt increases with the temperature up to a
certain point; it then decreases as the temperature increases, and it
loses its magnetism altogether when the heat amounts to 1996°.
Sir Humphry Davy and M. Arago noticed that the voltaic arc takes a
rotatory motion on the approach of a magnet; and the effect of magnetism
on the stratified appearance of the electric light in highly rarefied
air shows how powerful its action is. In the year 1858, Mr. Gassiot
published a series of observations on stratified light; subsequently
various publications appeared on the subject both by Mr. Gassiot and by
Professor Plücker, who made a series of very interesting observations on
the nature of the stratifications, but more especially on the effects
produced when they are under the influences of magnetism. Since that
time, Mr. Gassiot has published several papers on the subject, and still
continues his experiments on the stratifications of electric light,
which give a visible proof of the connection between electricity and
magnetism. He first showed that the stratified character of the electric
discharge through highly attenuated media is remarkably developed in the
Torricellian vacuum; latterly he has made his experiments by passing
electricity through closed glass tubes of various lengths and internal
diameters, filled with highly attenuated gases and vapours.[10] Two
among the many brilliant experiments of this gentleman may be selected
as illustrations of the property of electric light.
One of these closed glass tubes containing a highly attenuated gas was
38 inches long with an internal diameter of about an inch, and had the
extremities of two platinum wires fused into the same side 32 inches
apart. When these wires were put in connection with the wires of an
induction battery and brought into contact, and the electricity passed
through the tube, the luminous appearances at the extremities or poles
of the platinum wires were very different, but simultaneous. A glow
surrounded the negative pole, and in close approximation to the glow, a
well defined black space appeared, while from the positive pole there
issued in rapid succession a series of alternate dark and brilliantly
luminous curved strata, which formed a column of stratified light, the
concavities of the strata being turned to the positive pole. The
stratifications do not extend to the black band round the negative wire
or ball, which is quite different to the dark intervening space between
the stratified discharge and the luminous negative glow. On making and
breaking the electric circuit, the stratified discharge emanates from
each pole alternately, the concavities of the strata turning alternately
in different directions; in fact the whole phenomena are reversed, but
not changed. ‘The stratified discharge arises from the impulses of a
force acting on highly attenuated but resisting media,’ a new proof of
the wonderful power inherent in highly attenuated gases; the number of
stratifications given out at each discharge, depending upon the
intensity of the electricity and rarity of the gas.
[Illustration: Fig. 1.]
Fig. 1 represents the form which the stratified discharge assumes in a
vacuum tube one inch diameter and 38 inches in length, + and -
representing platinum wires attached to the terminals of a Ruhmkorff’s
induction coil.
When the tube, with its stratifications just described, was laid
horizontally on the pole of a magnet, the stratified column showed a
tendency to rotate as a whole round it. According to the theory of
Ampère, the polarity of a magnet is owing to a superficial current of
electricity perpetually circulating in a direction perpendicular to its
axis; and he also showed that currents of electricity flowing in the
same direction attract one another, while currents flowing in opposite
directions repel each other. Hence, since the currents of electricity in
the magnet and tube were flowing in the same direction on one side of
the magnet, and in opposite directions on the other side, the stratified
column was attracted at one end and repelled at the other, so as to take
the form [sideways S], in consequence of its tendency to rotate as a
whole round the pole of the magnet.
When narrow bands of tin foil wrapped round the glass tube near the
platinum wires were put in communication with the poles of the induction
battery, brilliant stratifications filled the whole tube between the tin
coatings every time the electric circuit was broken or renewed; and when
the tube was placed horizontally on the pole of a magnet, the
stratifications no longer showed a tendency to rotate as a whole, they
were divided into two parts tending to rotate in opposite directions;
when the tube was placed between the poles of a powerful electro-magnet,
one half of the stratifications were repelled and the other half
attracted. When the tube was placed on the north pole, the divided
stratifications arranged themselves on each side of the tube, changing
their respective positions when placed on the south pole, but in every
case each half was concave in opposite directions.
Fig. 2 (p. 81) represents the form which the induced stratified
discharges assume when the vacuum tube is placed on or between the poles
of a powerful electro-magnet—the tin foil coatings C+ C- being attached
by wires to the terminals of an induction coil.
[Illustration: Fig. 2.]
If a vacuum tube with or without wires or tin coatings be laid upon the
induction coil of a battery, or upon the prime conductor of an
electrifying machine, stratifications are produced by induction which
are divided by a magnet. Thus there are two distinct forms of the
stratified discharge, one direct, the other induced.
When Professor Plücker of Bonn sent an induced current of electricity
from Ruhmkorff’s coil through a vacuum tube having a platinum wire fused
into each extremity, and extending a little way into the interior of the
tube, electric light radiated from every point of the negative wire, and
when exposed to the action of an electro-magnet the whole tube was
filled with a luminous atmosphere. But when all the negative platinum
wire except its extreme point was insulated by a coating of glass, the
rays of electric light which radiated from the point were united into
one single and perfectly regular magnetic curve, upon the approach of an
electro-magnet; when the negative platinum wire was partially insulated
by glass coating, electric light emanated from every exposed part, and
assumed the form of magnetic curves under electro-magnetic action.
Whence Professor Plücker concluded that the luminous atmosphere in the
first experiment was the locus of an infinite number of magnetic curves,
and consequently that magnetic light emanates from the negative or
warmth pole, and electric light from the positive or light pole. These
magnetic curves of light are precisely similar to those assumed by iron
filings from magnetic action.
The most remarkable of these experiments is the absolute extinction of a
powerful electric discharge by magnetic action. Mr. Gassiot sent a
discharge from a voltaic water battery, containing 3,520 insulated
cells, into a tube filled with attenuated carbonic acid gas. The
discharge was so strong that it was capable of passing through more than
six inches of the gas, yet, on the approach of a very powerful
electro-magnet, the stratifications were arrested as soon as they
appeared, as if blown out, and finally extinguished. A stratified
discharge, in vacuo, from 400 insulated cells of a nitric acid battery,
was extinguished by the large electro-magnet of the Royal Institution;
the luminous strata rushed from the positive pole of the battery, but
under the magnetic force they retreated; cloud followed after cloud with
deliberate motion, appearing as if swallowed up by the positive
terminal. The amount of electricity that passed through the tube
appeared to be materially increased by exciting the electro-magnet; the
discharge was so intense on one occasion as to fuse half an inch of the
positive terminal. A very powerful magnet is also capable of
extinguishing a stratified discharge. In fact, according to the law of
the reciprocal action of magnetism, the forces are equal in intensity
and opposite in direction.
The electric discharge from an induction coil is discontinuous, or
eruptive sparks of high tension are given out producing stratified
discharges.
The discharge of the voltaic battery had hitherto been considered
absolutely continuous; and so it is for chemical action, whether of
analysis or combination; nevertheless certain phenomena gave reason to
doubt its continuity. Mr. Gassiot has proved that the tension of a
single cell of a galvanic battery increases in force according to the
chemical energy of the exciting liquid, and in all his experiments he
found that ‘the higher the chemical affinities of the elements used, the
greater was the development of evidence of tension.’ These observations
induced him to institute a series of experiments with galvanic batteries
of different chemical affinities, and to compare the resulting phenomena
with those produced by the induction coil, whose sparks are of high
tension. The same carbonic acid vacuum tubes were made use of in all the
experiments; a copper wire formed the positive terminal, and a copper
plate was fixed at the extremity of the negative terminal. In other
tubes platinum terminals extended into the interior, coated with glass,
except the points, to which charcoal balls were fixed. One end of the
tubes was of small diameter and contained caustic potash.
When a discharge from an induction coil was sent through these tubes,
there were either minute luminous spots, narrow stratifications, or a
well defined cloud-like discharge at the positive pole, according to the
size and structure of the terminal, but the characteristic phenomenon in
all the tubes was a large cloud-like luminosity or circular glow on the
brass plate or charcoal ball at the negative terminal.
With 512 insulated cells of copper and zinc of Daniell’s constant
battery, the exciting liquid being dilute sulphuric acid, a brilliant
glow appeared round the charcoal ball of the negative terminal on the
passage of the electric discharge through the tube, with very trifling
luminosity of the positive pole.
Two copper plates that could be separated or closed by a screw, were
placed between the poles of a nitric acid battery, so that the circuit
could be made or broken gradually, and spark discharges were obtained
between them. The vacuum tubes were placed between one of these plates
and a pole of the battery; one of these tubes was 24 inches long, 18 in
circumference, and had a circular copper disc 4 inches in diameter on
its negative terminal. On completing the circuit, the discharge of the
battery passed with a display of magnificent strata of dazzling
brightness; on separating the plates by the screw, the luminous
discharges presented the same appearance as when taken from an induction
coil, but brighter. On the copper disc within the vacuum tube, there was
a white layer, then a dark space about an inch broad, and then a bluish
atmosphere curved like the disc, evidently three negative envelopes on a
great scale. When the disc was made the positive pole, the effect was
feeble.
In vacuum tubes 6 inches long and 1 inch diameter, with carbon balls on
the terminals, the discharge of the nitric acid battery elicits extreme
heat. In one of these the discharge presented a stream of light of
intolerable brightness, but when viewed through a plate of green glass
strata could be seen. This soon changed to a sphere of light on the
positive ball, which became red hot, the negative being surrounded by
magnificent envelopes; with a horse-shoe magnet the positive light was
drawn out into strata. The needle of a galvanometer in circuit was
violently deflected and the polarity reversed. When the caustic potash
was heated, the discharge burst into a sunlike flame, subsequently
subsiding into three or four large strata of a cloud-like shape, but
intensely bright. This is called the arc discharge: it occurs in vacuum
tubes with charcoal balls; when the potash is heated intensely, dazzling
stratifications suddenly emanate from the positive ball, and powerful
chemical action takes place in the battery, after which the discharge
ceases.
This process facilitates the discharge and assists the disintegration of
the carbon particles, and these in a minute state of division are
subsequently found attached to the sides of the glass. It is these
particles which produce the arc discharge with its intense vivid light
so suddenly observed with far more brilliant effects than the usual
stratified discharge. During its passage the conducting power of the
vacuum tube is greatly enhanced.
It was already mentioned that a stratified discharge was obtained from
3,520 insulated cells of a water battery, which differs but little in
intensity from 400 cells of the nitric acid battery. On one occasion the
electricity seemed to pass through the vacuum tubes in a continuous
stream, but when examined with Mr. Wheatstone’s revolving mirror it was
decidedly stratified. Mr. Gassiot never could obtain a continuous
discharge in air, whether between the points or metallic plates of the
water battery. The discharge was invariably in the form of minute
clearly defined and separate sparks.
Thus it was proved by the preceding experiments that a spark discharge
could be obtained in air from both the nitric acid and water battery;
and that when these discharges were passed through the highly attenuated
matter contained in carbonic acid vacua, the same luminous and
stratified appearance was produced as by an induction coil; a proof that
whatever may be the cause of the phenomena it could not arise from any
peculiar action of that apparatus.
Mr. Gassiot finally concludes that the cause of the stratified discharge
arising from the impulses of a force acting upon highly attenuated but
resisting media is also applicable to the discharge of the voltaic
battery in vacuo; while the fact of this discharge even in its full
intensity having been now ascertained to be also stratified leads to the
conclusion that the ordinary discharge of the voltaic battery, under
every condition, is not continuous but intermittent, that it consists of
a series of pulsations or vibrations of greater or less velocity,
according to the resistance in the chemical or metallic elements of the
battery or the conducting media through which the discharge passes.
Caustic potash absorbs the carbonic acid gas by degrees, and at last so
completely exhausts a vacuum tube that electricity cannot be conducted.
Air is a non-conductor, and an electric discharge that will not pass
through an inch of air, will pass through more than 30 or 40 inches of
attenuated gas.
It has already been mentioned that the stratified discharge can be
obtained by a single discharge of the primary current of an inductive
coil, however long may be the vacuum tube through which the discharge is
passed. If no addition be made to the battery and no alteration be made
in the arrangement of the coil so as to increase or diminish the
intensity of the discharge, the stratifications will always present the
same appearance and form, occupying the same spaces and positions in the
vacuum tube; but if any change be made so as to alter the intensity,
then a corresponding alteration will appear in the discharge, the striæ
assuming a different shape, and the bright and dark divisions occupying
different positions.
In order to try what effect a change of intensity would produce, three
separate insulated voltaic batteries, in which the exciting liquid was
brine, formed an electric circuit which was completed by two long wires.
It was so arranged that the discharge of one, two, or all the three
batteries could be separately employed. In order to vary the resistance
at pleasure, two tubes 18 inches long containing distilled water and
connected at their base were introduced into the circuit. By varying the
depth to which the terminal wires of the circuit were plunged into the
water, the resistance could be regulated at pleasure, and it was
immaterial in what part of the circuit the vacuum tube was introduced
provided the circuit was completed.
The first experiments were made with a carbonic acid vacuum tube 20
inches long and 4 inches in diameter. The negative terminal at one
extremity of the tube was of aluminium, cup-shaped, about 3 inches in
diameter; the positive terminal was a wire of the same metal fused into
the other extremity of the tube; the point of the wire and cup were
about four inches and a half apart. With this tube and 2,240 cells of
the battery the discharge when the resistance was introduced had the
appearance of a positive and negative discharge, impinging on and
intermingling with each other, without any dark space intervening.
Around the negative terminal the luminosity extends to the sides of the
tube and tapers to the point of the positive wire. The light round the
negative terminal becomes brighter, a dark space appears next to it when
the resistance is diminished, and increases as the resistance decreases,
by the rolling back of the light in bright clouds to the point of the
positive terminal. These changes can be perfectly regulated by the
resistance, and various luminous phenomena occur at each stage.
With 2,240 cells distinct sounds were heard in the tube; with the whole
battery of 3,360 series the sounds were not heard till a magnet was
applied to the striæ, when they again became audible and the striæ were
spread over the surface of the tube.
A carbonic acid vacuum tube with platinum terminals fused into the same
side far apart was now put into the circuit, the part of the wires that
penetrated within the tube being coated with glass up to the carbon
balls in which they terminated. When a discharge from all the three
batteries passed through the tube, changes occurred in the form and
number of the striæ corresponding to the greater or less amount of the
resistance offered in the circuit.
At the commencement of the experiments there were 18 inches of water in
each of the tubes, which formed the maximum resistance. The wires
attached to the terminal wires of the battery were placed inside of
these tubes, and as soon as they touched the surface of the water a
faint luminous discharge was seen at each ball in the vacuum tube. As
the wire attached to the negative end of the battery was slowly
depressed, the two luminous discharges appeared to travel towards or
attract each other, and at times a portion of the positive luminosity
passed over and mingled with the negative; in this state the discharge
was extinguished by a magnet.
When the wire was pressed farther into the water a dark space about an
inch in length divided the light into two parts, the positive glow being
sharply defined, the negative glow having an irregular edge. When the
wire had been about three inches deep in the water, the positive and
negative glows became more brilliant, and a single clearly defined
luminous disc burst from the positive side and occupied the middle of
the dark space. When the wire was pressed down till 13 inches of it were
in the water, a second luminous disc travelled from the positive side,
and then the two luminous discs or striæ occupied the dark space at a
little distance from one another. As the wire was pressed more into the
water, three parallel luminous striæ appeared, then four, then five, and
so on till as many as thirteen or fourteen striped the dark central
space. With the full power of the battery, the adjacent disc impinged on
the glow that surrounded the negative ball. This disc was of a pale
green, those adjacent were reddish, while the negative glow was of a
bluish white; minute bright scintillations emanated from the negative
ball, while distinct luminous flash discharges took place through the
striæ. Thus by the amount of resistance introduced into the circuit, the
number of striæ can be regulated, their position fixed, separating or
closing up the dark space between the luminous glows round the balls.
In these experiments there is indication of a force emanating from the
negative wire. The actual disruption of the particles from the negative
terminal also indicates a force, and the disruption is as freely
obtained by the continuous discharge of the battery as it is by the
intermittent discharge of the induction coil. Besides, when Mr. Gassiot
sent discharges from the induction coil through Torricellian vacua, he
several times observed that while a cloud-like discharge issued from the
positive terminal, a long tongue of the most brilliant blue
phosphorescent light emanated from the upper part of the negative
terminal, and a brilliant white tongue of light was also seen close to
the negative wire: so there is reason to believe that force emanates
from both terminals.
Some of the preceding striated discharges ‘present an appearance
somewhat analogous with the stationary undulations (or nodes) which
exist in a column of air when isochronous progressive undulations meet
one another from opposite directions, and on the surface of water by
mechanical impulses similarly interfering with each other.’[11]
‘May not the dark bands be the nodes of undulations arising from similar
impulses proceeding from positive and negative discharges? or can the
luminous stratifications which we obtain in a close circuit of the
secondary coil of an induction apparatus, and in the circuit of a
voltaic battery, be the representations of pulsations which pass along
the wire of the former, and through the battery of the latter, impulses
probably generated by the action of the discharge along the wires?’
The action of magnetism and electricity on light is similarly
illustrated by the rotation of the plane of polarization. Sir John
Herschel was the first who tried to rotate the plane of polarization of
a ray of light by surrounding it with a spiral wire electrized by the
great battery of two enormous plates of copper and zinc at the London
Institution, but he obtained no evidence of any such action. Long
afterward Professor Faraday succeeded by sending a ray of light through
a piece of silico-borate of lead, which formed the core of a magnetic
helix. The silico-borate took on a quasi-crystallised state during the
passage of the electric current round it, giving it for the moment the
property of circular polarization, analogous to that of glass in a state
of tension or compression.
Substances vary exceedingly in the facility with which they transmit
electricity; even the same substance under another form differs
remarkably in that property: charcoal, which next to the metals is the
best conductor known, when under the form of diamond is quite impervious
to electricity. In general, substances that are the best conductors of
heat are also the best conductors of electricity, as for example the
metals, which however, possess the transmissive property in very
different degrees. Silver and copper are the best conductors, lead one
of the worst; its resistance to the passage of electricity is twelve
times greater than that of silver and copper, consequently it becomes
twelve times as hot, for when a current of electricity is impeded it is
changed into heat. So great is the resistance offered by a fine platinum
wire, that the heat amounts to 3280° and the wire is melted, a striking
instance of the correlation of electricity and heat, and of the power of
the cohesive force.
When electricity is passing through conducting substances or when it is
static, it induces an electric state in bodies at a distance by
transmission through non-conducting substances or air, for it gives
polarity and tension to the adjacent atoms, and these to the next, and
the next in succession, throughout the whole intervening mass,—a strong
proof of the individuality and polarity of the atoms of matter.
Motion, which is the result of all the physical powers, has itself a
strong action upon the ultimate elements of matter; in cases of unstable
equilibrium it accelerates and even determines their chemical union.
Some substances will remain merely mixed as long as they are at rest,
but no sooner is their inertia disturbed by a slight motion than they
rush into permanent combination. In newly sublimed iodide of mercury the
vibration impressed by the scratch of a pin is so rapidly transmitted
through the mass that its colour is immediately changed from yellow to
bright red. By a new arrangement of the molecules their action on light
is altered.
Catalysis or the chemical decomposition and composition of substances by
the contact of a foreign body, is well illustrated by the chloride of
nitrogen, that explodes when touched by substances which at ordinary
temperatures would neither combine with the chlorine nor with the
nitrogen. The iodide of nitrogen explodes if touched by a feather, and
M. Becquerel decomposed the iodide of nitrogen by the vibrations of
sound. When substances only exist in consequence of the inertia of their
atoms, the instability of their chemical attractions and repulsions is
only increased by an external agent, so that a great effect is produced
by a slight cause, as in an avalanche, the snowy mass is on the point of
falling, and the smallest motion, a breath of wind, hurls it down. In
such cases the potential energy of the unstable mass is in a moment
changed into vis viva or impetus. Daguerreotype impression shows the
power of the chemical rays on substances in unsteady equilibrium, and
the length of time required to make the impression under the same
circumstances is a measure of the instability.
Most of the fulminates are compounds of nitrogen; of that the fulminate
of aniline is a recent instance, since it is formed by the slow action
of nitrous acid on aniline. Explosion takes place on the sudden
evolution of gas, or the sudden change of a solid into vapour. In these
cases fire or percussion are the foreign causes of change. They are all
particular instances of the general principle of catalysis, which is the
chemical combination of heterogeneous atoms by the action of a substance
that does not participate in the change. Thus it has long been known
that when platinum is plunged into a mixture of oxygen and hydrogen it
combines these gases into water. Acids in some cases seem to have the
same effect; for when rags or starch are dissolved in an acid the starch
is changed to dextrine and the liquid has acquired the power of turning
the plane of polarised light to the right. The acid has undergone no
alteration, but it has changed the properties of the starch though not
its chemical composition. After a time, a second transformation takes
place, the liquid ceases by degrees to turn the plane of polarisation to
the right, and ends by turning it to the left. The acid is still
unchanged, but the dextrine has now disappeared: it has combined with
the water and is transformed into glucose or sugar of grapes.
The quantity of the physical powers, active and latent, is inappreciably
great. The quantity of heat or potential energy generated by chemical
combination alone is enormous.
SECTION III.
ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, UTILITY OF WASTE
SUBSTANCES—COAL-TAR COLOURS, ETC.
THE chemical combination which forms the infinite variety of substances
in the organic and inorganic creation consists in an intimate union of
their ultimate atoms which produces substances differing from their
constituent parts in every respect except gravitation, the sum of the
weights of their constituent parts being invariably equal to the weight
of the resulting substance. Thus the chemical union of oxygen and
hydrogen forms water, and the weight of the water so formed is exactly
equal to the sum of the weights of the two gases.
All chemical changes whether of analysis or composition are subject to
definite unalterable laws of weight, measure and number; nothing is by
chance or casual, the relative weights of the invisible atoms of matter,
and their combination in definite proportions reveal the laws which
prevailed in the primeval structure of created things. By the wonderful
discovery of these laws Dr. Dalton has placed chemistry on a strictly
numerical basis.
The chemical union of different kinds of atoms and volumes of matter in
the definite proportions of whole numbers entirely changes their
character and properties, as for example the chemical combination of one
atom of hydrogen and one atom of oxygen into water. The condensation is
often unexpected and wonderful; two different liquids are often
condensed into a solid, and the result of the chemical combination of
two different gases or vapours in quantitative proportions may be solid,
liquid or aëriform, a fact which could only have been discovered by
experiment. The powers of the atoms are changed and often highly exalted
by chemical union as in ammonia, a chemical compound of three atoms of
hydrogen and one of nitrogen, which absorbs 1,195 times more radiant
heat than its constituents whether simple or mixed. During chemical
combination light and electricity are often evolved, heat always. The
quantity given out is exactly proportional to the energy of the chemical
action, and is often so great and so rapidly evolved as to produce an
explosion by the sudden expansion of the air around. Whatever the
temperature may be, which is given out during the union of the atoms,
the very same quantity of heat is requisite to dissolve their union, and
the atoms are separated in the same definite proportions in which they
were combined.
Voltaic electricity both combines and resolves substances into their
component parts, strictly according to the law of definite proportions.
It combines eight parts by weight of oxygen and one part by weight of
hydrogen into water; and again when it decomposes water, one part by
weight of hydrogen is given out at the negative pole of the battery, and
eight parts by weight at the positive or zinc pole. For an electric
current weakens or neutralizes the force of affinity in one direction
and strengthens it in the other, so that the heterogeneous atoms of the
substance under its influence have a tendency to go in different
directions and appear at opposite poles. Mr. Faraday has established as
a general law, that the quantity of electricity requisite to unite the
atoms of matter, is precisely equal to the quantity requisite to
separate the same atoms again. Electro-chemical action, or the power of
electricity to combine and separate the heterogeneous atoms of matter,
is in direct proportion to the absolute quantity of electricity that
passes in the current. Hence the superior analytical power of voltaic
over static electricity, which has enormous intensity, but is very small
in quantity. The electric current separates molecular combinations which
yield to no other means: it is the most powerful instrument of analysis;
light is the most delicate.
Two simple substances are only capable of a certain number of chemical
combinations, which form a regular series of new substances; as for
example oxygen and nitrogen. Two measures of nitrogen gas will unite
with one measure of oxygen to form the protoxide of nitrogen; with two
measures of oxygen it unites to form the binoxide of nitrogen; with
three measures of oxygen it forms the hyponitrous acid; with four it
forms nitrous oxide; and with five measures of oxygen it forms nitric
acid. Thus there are five compounds of nitrogen and oxygen, no more.
Affinity of _kind_ is merely the attraction of one element or atom of
matter for another; affinity of _degree_ consists in the grades and
limits of combination; the preceding series is of the fifth degree; the
limit is the last term, for no further combination of these two gases
can take place, and these are accomplished by art. All the five
substances are deleterious, most of them deadly poisons, for the
protoxide of nitrogen, which is the laughing gas, could not be long
inhaled with impunity. For a long time the middle term of the preceding
series was wanting, but Gay-Lussac formed it by attending to the laws of
definite proportion and sequence.
The atoms of different kinds of matter possess an affinity, or
attractive force, which binds them together chemically in different and
very unequal degrees. Two substances may unite and form a third
differing from both, as water does from oxygen and hydrogen; but if a
new substance be added which has a greater attraction for one of the
substances than for the other, it will dissolve their union, combine
with that for which it has the strongest attraction, and set the other
free. Thus the metal potassium, which has a greater attraction for
oxygen than it has for hydrogen, decomposes water, combines with the
oxygen, and sets the hydrogen free. Both chlorine and ozone have the
property of liberating the iodine in a weak solution of the iodide of
potassium; the liquid stains starch blue, a proof of the free iodine.
The facility with which acids and alkalies combine affords the means of
eliminating either the one or the other from a compound so as to
liberate what remains.
The constituents of compound substances may be separated from one
another by a variety of means depending upon their greater or less
fusibility, volatility, and other properties. Water, acids, alcohols and
other liquids hot or cold, different degrees of temperature,
sublimation, solution, distillation, evaporation, together with static
and voltaic electricity, are the most powerful means of analysis.
But the animal and vegetable creation rear their fabrics by a synthetic
process. A plant after having absorbed carbonic acid and water,
decomposes the carbonic acid, returns the oxygen to the atmosphere, and
combines the carbon and water into wood, leaves, and a variety of
organic substances. Now MM. Berthelot, Wöhler, and other distinguished
chemists, by following this example of nature, have established a system
of synthetic chemistry, by which they have produced from the chemical
combination of the three elementary gases and carbon alone more than
1,000 complete organic substances, precisely the same with those formed
within the living plants and animals. Yet we are as far as ever from any
explanation of the mystery of life, whether animal or vegetable.
Carbon and hydrogen will not combine at any artificial heat however
great; but when the electric arc between highly purified charcoal
terminals passes through hydrogen gas, acetylene, a new carburet of
hydrogen, is formed, consisting of four equivalents of carbon and two of
hydrogen. This substance, which no organized being is capable to form,
was discovered by M. Berthelot, and being assumed as a base, yielded an
extensive series of organic substances. Thus when two atoms of carbon
are added to acetylene it becomes olefiant gas; when two equivalents of
oxygen are added to olefiant gas, the result is alcohol, which is
transformed into acetic acid by the addition of two atoms of oxygen, and
from this by a similar process have been obtained the malic, tartaric,
succinic, and the other acids; glycerine also, which is the sweet
principle of the oils, wax, essential oils, the perfumes of fruit and
flowers, the principle of the balms, the essential oil of mustard, and
numerous other organic substances, simply from carbon, oxygen and
hydrogen; but nitrogen was introduced by combining alcohol with ammonia,
an inorganic substance consisting of three equivalents of hydrogen and
one of nitrogen, from whence a vast number of nitrogenized substances
were derived, both animal and vegetable.
Chemical combination, which has from the beginning of created things,
and still is, building up organic and inorganic matter in the earth, in
the air, and the ocean, exerts forces of transcendent power, though
silent, unperceived, and for the most part unknown. Professor Tyndall
has given a striking instance of this in water, the most simple compound
of oxygen and hydrogen, a constituent alike of organic and inorganic
nature. ‘In the combustion of the two gases to form a gallon of water
weighing ten pounds, an energy is expended, the atoms clash together
with a force, equal to that of a ton weight let fall from a height of
23,757 feet; and in the change from the state of vapour to water, an
energy is exerted equal to that of a ton weight falling from a height of
3,700 feet, or of a hundredweight falling from a height of 74,000 feet.
The moving force of the stone avalanches of the Alps is but as that of
snowflakes compared with the energy involved in the formation of a
cloud. In passing finally from the liquid to the solid state,’ that is
from water to ice, ‘the atoms of ten pounds exercise an energy equal to
that of a ton weight falling down a precipice of 550 feet of
perpendicular height.’
From Mr. Joule’s investigation of the relation existing between chemical
affinity and mechanical force, it appears that when affinity is feeble
it can be overcome mechanically. He formed amalgams of different metals,
that is he combined them with mercury, by electricity. The affinity of
iron for mercury is so feeble that the amalgam is speedily decomposed
when left undisturbed by the pressure of the atmosphere, and if a
greater pressure be added, almost all the mercury is driven out. The
efficacy of mechanical force to overcome feeble chemical affinities is
strikingly illustrated by the amalgam of tin, out of which nearly the
whole of the mercury is driven by long continued pressure. In these
cases the force of affinity did not amount to chemical equivalency,
otherwise the mercury could not have been driven out by so small a
force. Instances from the weakest to the strongest affinity show that it
is only when the power reaches a definite point that the law of chemical
equivalents comes in. The intense energy which then begins to be exerted
has just been shown.
It is vain to hope for a knowledge of the _absolute_ weight of the
ultimate atoms of matter, and nothing seems to be more beyond the power
of man than to determine even their relative weights; yet the definite
proportions in which they combine have enabled him to do so. Thus, an
atom of oxygen unites with an atom of hydrogen to form water; but as
every drop of water, however small, contains eight parts by weight of
oxygen, and one part by weight of hydrogen, it follows that an atom of
oxygen is eight times heavier than an atom of hydrogen. Now, since
hydrogen gas is the lightest body known, its atom has been assumed as
the unit of comparison. Hence, if the unit of hydrogen be represented by
1, that of oxygen may be represented by 8. Again, carbonic acid gas
contains six parts by weight of carbon, and eight parts by weight of
oxygen, and as an atom of oxygen is eight times heavier than an atom of
hydrogen, therefore an atom of carbon is six times heavier than an atom
of hydrogen, and consequently may be represented by 6. In this manner
the relative weights of many substances have been determined. But the
property of isomorphism also affords the means of ascertaining the
atomic weights of certain substances with unerring certainty. It is
exactly the contrary of dimorphism, for in the latter substances are
chemically the same under different forms; whereas isomorphic bodies are
chemically different under the same form. Now the peroxide of manganese
contains one atom of oxygen for one atom of metal; but in 100 parts of
the protoxide there are 21·94 parts of oxygen and 78·06 of manganese.
Comparing these numbers with 8 the atomic weight of oxygen, the result
is 28 the weight of an atom of manganese. The same number is obtained
from two other isomorphic compounds of oxygen and manganese, which
proves the accuracy of this result. The atomic weights of many bodies
have been determined, of which the following are the most important.
_Atomic Weights, an Atom of Hydrogen being the Unit._
Hydrogen 1
Carbon 6
Oxygen 8
Nitrogen 14
Sulphur } 16
Phosphorus }
Sodium 23
Iron }
Nickel } 28
Manganese }
Bromine 80
Copper 32
Zinc 32·5
Chlorine 35·5
Potassium 39
Rubidium 86
Cæsium 133
Iodine 127
Thallium 204
In the determination of atomic weights a few cases have occurred of
fractional numbers, and although it cannot yet be affirmed that no such
cases exist, yet it seems to be established by the new and more perfect
analyses of MM. Dumas, Isidore, Williamson, and others, that the atomic
weights of substances compared with an atom of hydrogen are in whole
numbers.
This law leads to very important results. For example, the equivalent
weights of the chemical elements of bodies derived from their specific
gravities are either identical with, or simple multiples or
sub-multiples of, their relative weights. Thus the specific gravity of
hydrogen is 0·0693, and that of oxygen is 1·111; hence taking hydrogen
as the unit of comparison, it is easy to see that 0·0693 : 1·111 :: 1 :
16, the simple multiple of 8, the relative atomic weight of oxygen. In
fact since each substance has its own specific gravity or weight, that
weight must depend upon the weight of its atoms, so that the weights of
equal bulks of different substances are proportional to the weights of
their atoms, and thus a relation is established between the atomic
weights and specific gravities of bodies, so that one being given the
other may be found.
Atoms like their substances have many different capacities for heat and
electricity. It was proved by MM. Petit and Dulong, that specific heat,
or the quantity of heat required to raise a simple substance to a given
temperature, is inversely as the weight of its atoms, so that the
specific heat or repulsive force of simple substances multiplied by
their atomic weights is a constant quantity. Such is the condition
requisite for the equilibrium or equality of force; or the law may be
thus expressed: A given quantity of heat will raise to the same number
of degrees a portion of every simple substance represented by its atomic
weight. For instance, the atomic weight of sulphur is 16, that of zinc
32·5; hence it requires twice as much heat to raise a pound of sulphur
ten degrees as it does a pound of zinc. It has also been proved that the
atoms of compound bodies of analogous composition are endowed with the
same capacity for heat, so that there is a perfect correspondence
between the weight of atoms and their specific heat. The numbers
representing the atomic weights derived from the specific heat of bodies
are connected with their equivalent atomic weights by the simple ratios
of equality, multiples or sub-multiples.
Mr. J. Croll has made experiments showing that the specific heat of
compound gases and liquids is generally less, and those of solids more,
than that of their component elements, which is contrary to the hitherto
received opinion. Moreover it appears that the changes in the specific
heat of bodies which occur during combination are not only due to
chemical action, but also to molecular changes; the real specific heat
of a simple atom probably remaining the same under all conditions.
Mr. Faraday has proved that the specific electricity of different
substances is also in proportion to their atomic weights, that is to
say, a given quantity of electricity will separate combined substances
into parts represented by their atomic weights. For example, 32·5 parts
of zinc will generate voltaic electricity enough to separate nine parts
of water into eight parts of oxygen and one part of hydrogen gas. The
weights thus derived from decomposition are exactly the same with those
determined by composition, and thus the atomic weights derived from
electro-decomposition accord exactly with those obtained from chemical
composition. Moreover, Mr. Faraday, as already mentioned, proved that
the very same quantity of electricity necessary to decompose a body into
its elementary atoms, is requisite to unite them again. The analysis and
synthesis of compound matter, solid or fluid, show a constant and
definite proportion of the component elements expressed by number, and
by an equivalent or multiple ratio of parts in every chemical change.
The atomic theory unites, by a common bond, specific gravity, chemical
affinity, heat, and electricity. Taking atmospheric air at the
temperature of 60° Fahr. and a barometric pressure at 30 inches as the
standard unit of specific gravity; the quantity of heat required to
raise a volume of water 1° Fahr. as the unit of specific heat; hydrogen
gas as the unit of atomic weight; and atomic electro-chemical
electricity as the unit of specific electricity, the following numbers
have been established:
+----------+----------+------------+----------+-------------+
| | Specific | Equivalent | Specific | Specific |
| | gravity | atomic | heat | electricity |
| | | weight | | |
+----------+----------+------------+----------+-------------+
| Hydrogen | 0·0693 | 1 | 0·2936 | 1000 |
| Oxygen | 1·111 | 8 | 0·2361 | 125 |
| Carbon | 13·2 | 6 | 0·2631 | |
| Nitrogen | 0·978 | 14 | 0·2750 | |
+----------+----------+------------+----------+-------------+
The distances between the atoms of the gases are equal, hence the atomic
weights of simple gases are proportional to their densities; and for the
same reason, equal volumes of the same fluid contain an equal number of
atoms, and the number of atoms in the same volume of different fluids is
in the simple ratio of one to one, one to two, one to three, &c.
It follows from the atomic theory that the number of atoms in equal
weights of any two solid substances, is in the inverse ratio to the
weights of these atoms. Now since the bodies that have the greatest
specific gravities are the heaviest, if the specific gravities and
atomic weights of equal bulks of two simple substances be known, the
relative number of atoms they contain may be found. For the density
divided by the atomic weight of the one, is to the density divided by
the atomic weight of the other, as the number of atoms in the first to
the number of atoms in the second. By the preceding law it is found that
in equal bulks of the three metals, sodium, platinum, and potassium,
platinum contains five times as many atoms as sodium, and ten times as
many as potassium. When substances which have strong analogous qualities
are compared in this manner, the results are either equality, or a
simple ratio.
It has already been mentioned that the protoxides of iron, copper, zinc,
nickel and manganese, have the same form, and contain the same quantity
of oxygen, but differ in the respective metals that are combined with
it; and by the preceding law it appears that equal bulks of these
isomorphous bodies contain also the same number of atoms.
Mr. Hermann Kopp has proved that the atomic weight of a substance
divided by the specific gravity, that is to say, its atomic volume, is
the same for all isomorphic bodies simple and compound, and as a general
law that the atoms of isomorphous substances are not only the same in
form, but equal in dimensions. It follows, therefore, that any one of
the preceding metals might be substituted for any other in the
respective protoxides, and on that account, according to the modern
theory, they are the chemical equivalents of each other, for that
expression is used now in a different sense from what it formerly had.
Chemical equivalency between two or more substances consists in their
capacity for being exchanged one for the other. Direct or indirect
substitution forms the basis of the modern doctrine of chemical
equivalents.
Substances which are capable of replacing one another in compounds, and
which are endowed with qualities mutually analogous, are said to be
isomeric. Many isomeric compounds are formed of the same materials, in
the same proportions, and yet differ essentially both in their physical
and chemical properties; whence M. Daniel observes, that a specific and
definite arrangement of the constituent molecules in space appears to be
no less essential to the individual constitution of bodies than a
certain proportion between their heterogeneous ingredients.
Successive substitution in isomeric bodies does not alter the character
of the chemical formulæ of these bodies; thus chlorine, bromine and
iodine, are chemically equivalent with an atom of hydrogen, for they may
be put for one or more atoms of hydrogen in various compounds without
changing the character of the chemical formulæ of these compounds. The
peroxide of hydrogen consists of one atom of hydrogen and two of oxygen;
hence if 32·5 parts of zinc, 28 of manganese, and 32 of copper be
successively put for the atom of hydrogen, the result will be the
peroxides of zinc, manganese and copper respectively. Here the character
of the chemical formula of the original compound remains the same, and
the three metals are chemically equivalent to one another, and to the
atom of hydrogen. In many compounds organic and inorganic, one or more
atoms of hydrogen may be replaced by an equal number of atoms of sodium,
potassium, zinc, &c., without altering the character of the chemical
formula of the compound.
Olefiant gas, olefiant oil and paraffin, form an isomeric series of a
gas, a liquid and a solid, consisting of carbon and hydrogen. The gas
contains 86 parts in 100 of carbon, and forms the most luminous part of
coal gas.
M. Dumas has proved it to be a general law, that when three isomeric
bodies are arranged in the sequence of their chemical properties, there
will also be a sequence in their respective atomic numbers, and that
whenever this symmetry of chemical properties and atomic weights
obtains, any one of these substances may be substituted for the other
without changing the chemical character of the formula.
Sulphur, selenium, and tellurium, form an isomeric group; that is, they
form a sequence, with analogous qualities, for sulphur is the most
volatile; selenium, a simple substance found in iron pyrites in Sweden,
is less volatile; and tellurium is the least volatile and with regard to
their atomic sequence, the atomic weight of sulphur is 16, that of
tellurium is 64, and half the sum of these numbers is 40, the atomic
weight of selenium, the mean term. Hence selenium might be put in any
compound for the sulphur, and the tellurium for the selenium, without
changing the chemical character of its formula.
The metallic group of calcium, strontium, and barium, are endowed with
analogous properties, perfect harmony in their chemical qualities, and
in the numbers expressing their atomic weights. That of calcium is 20,
that of barium is 68, and the half sum is 44, the atomic weight of
strontium. So calcium might be put for strontium, and strontium for
barium, in any compound without altering the character of its formula.
Professors Johnson and Allen have shown that the new metalloids cæsium
and rubidium form an isomeric triad with potassium, for the atomic
weight of cæsium is 133, that of rubidium 86, and that of potassium 39.
Transmutations of one isomeric substance for another may also be made in
organic bodies, but chlorine, bromine, and iodine form an exception to
M. Dumas’s law, because the arithmetical relation is wanting.
There are certain groups of substances, especially among the metals,
whose atomic weights are in regular arithmetical series, as those of
titanium, tin, and tantalum, which are 25, 59, and 92, the common
difference being 34.
Certain groups of combined atoms called compound radicles are much more
important than the preceding. They unite chemically with one another,
and with other substances in definite proportions, precisely as if they
were ultimate atoms. They are even capable of being substituted one for
the other, forming groups of infinitely varied properties, and thus
chemical equivalency extends to them.
Cyanogen, amidogen, and the peroxide of hydrogen are compound radicles
which combine with other substances and with simple atoms as if they
themselves were simple elements; though the first is a chemical compound
of two atoms of carbon and one of nitrogen, the second a chemical
compound of one atom of nitrogen and two of hydrogen, and the peroxide
contains as before mentioned two atoms of oxygen and one of hydrogen.
All three are capable of replacing hydrogen, chlorine, and metals by
equivalent substitutions. For example, the chlorate of potash consists
of one atom of potash, an atom of chlorine, and five atoms of oxygen; if
then an atom of cyanogen whose weight is 26, be put for the atom of
chlorine, the result would be the cyanate of potash.
Cyanogen, formed by passing nitrogen over red-hot carbon, consists of
two equivalents of carbon and one of nitrogen. It is a frequent
constituent of organic and inorganic compounds, and travels in the
voltaic circuit as if it were a simple substance.
Ammonia consists of three equivalents of hydrogen and one of nitrogen;
now, when the radical phenyle, which consists of twelve equivalents of
carbon and five of hydrogen, is put in the ammonia for one equivalent of
hydrogen, the result is aniline, whence most of the coal tar colours are
obtained. In like manner carbazotic acid, a beautiful yellow dye from
coal tar, is carbolic acid, three of whose equivalents of hydrogen have
been replaced by three equivalents of an oxide of nitrogen.
Compound radicles, consisting of carbon and the three elementary gases,
have been discovered which enter into combination in definite
proportions as simple atoms, and all compound radicles travel in the
galvanic circuit as equivalents to the elementary substances. Hitherto
they have been regarded as representatives or equivalents of one atom of
hydrogen. Now it is generally admitted that each has the property of
replacing two, three, or more atoms of hydrogen by equivalent
substitution. This multiple equivalency among compound radicals forms
the basis of what is called the polyatomic theory, now so much employed
by MM. Hofmann, Berthelot, and other great modern chemists.
Water is the most common radicle both in the inorganic and organic
world. Though a compound of oxygen and hydrogen, it enters, according to
the law of definite proportion, into the composition of various
amorphous bodies in a dry state, that is in the form and proportion of
its gases. It is an essential element in the greater number of crystals,
and abounds in organic matter. In certain cases the same substance
crystallizes at different temperatures, unites with different quantities
of water under the form of oxygen and hydrogen, and assumes
corresponding forms. For example, the seleniate of zinc unites with
three different portions of water and takes three different forms,
according as its temperature is hot, lukewarm, or cold. Thus each
particle of water, containing one atom of oxygen and one of hydrogen,
combines with one atom of zinc in three different proportions as if it
were a simple atom.
The water of crystallization may be driven off from many substances by
heat, as from the hydrates of lime, iron, copper, &c., but when combined
with the oxides of certain metals, potassium for instance, it cannot be
driven off by any means whatever. In general a heat of 212° Fahr. is
sufficient, but some crystals lose their water of crystallization at the
ordinary atmospheric temperature.
Crystals whose atoms are in unstable equilibrium, are readily altered
both externally and internally by a very moderate degree of heat.
Arragonite and calcareous spar are isomeric, that is, they are
chemically the same but differ in form and hardness, which shows that
their molecules are grouped differently. When the arragonite is heated,
the inertia of its atoms is overcome, the crystal explodes with force,
and becomes a mass of crystals of calcareous spar. The expansive force
of the heat suddenly overcoming the force of cohesion causes the
explosion, and at the same time disturbs the unstable repose of the
atoms, which immediately obey their natural attractions and assume the
stable form of calcareous spar.
Dialysis is a method of separating and analysing substances by means of
their diffusion in alcohol or water. If a wide-mouthed vial nearly full
of a solution of common salt be placed in a jar of water, after a few
days it will be found that the particles of salt have come out of the
vial and have diffused themselves through the superincumbent water, even
to its surface. Now Professor Graham, Master of the Mint, with whom this
subject originated, made three arrangements precisely like that
described; the three vials were exactly similar and equal, the three
jars exactly the same in size and form, and contained the same quantity
of water; but the first vial contained a solution of gum arabic, the
second a solution of Epsom salt, and the third a solution of common
salt. After fourteen days the diffusion of the gum had risen through one
half of the superincumbent water, while the particles of both the salts
had risen to the surface. However the common salt would have risen much
higher, for when the strata of water at the two surfaces were drawn off
by a siphon and evaporated to dryness, there was fifteen times as much
common salt as Epsom salt. The three solutions are heavier than water,
yet they rise notwithstanding their gravitation, whence Mr. Graham
thinks that there is probably an attraction between the particles of the
dissolved substances and those of the water. The force of molecular
attraction is more powerful than gravitation, hence the particles must
rise by the difference of the two forces.
After many comparative experiments the professor concluded that most
substances differ in diffusibility, and that crystalloids or crystalline
substances such as salts, sugars, &c., are much more diffusible than
colloids or amorphous sticky bodies, such as gum, caramel, jellies, and
substances that combine with the hydrogen of the water to form
gelatinous hydrates.
The partial decomposition of definite chemical compounds may be effected
by diffusion. Alum, which is a double sulphate of the two metals
potassium and aluminium, furnishes an example; when allowed to diffuse
itself from its aqueous solution, the diffusive tendency of potassium
compounds is so much greater than the diffusive property of aluminium
compounds, that a portion of the sulphate of potassium actually breaks
away from the sulphate of aluminium with which it was combined, in order
to diffuse itself in the superincumbent external water more freely than
the sulphate of alumina can do.
Common salt diffuses itself in a solid mass of jelly almost as easily
and extensively as in the same bulk of free water. Thus colloid bodies
do not interfere with the diffusion of crystalloids such as salts, but
they almost entirely arrest the diffusion of one another. Solutions of
salts, sugars, and other crystalloids pass freely through colloid
substances, such as parchment-paper, vellum, and membrane into water,
although they have no pores, because the particles of the crystals unite
diffusively with the water combined in these substances, which solutions
of gum, caramel, and other colloids cannot do. These colloid substances
are permeable to solutions of crystalloids, impermeable to solutions of
colloids. This constitutes Dialysis.
The instrument used by Mr. Graham was a little tray formed of vellum or
membrane stretched tightly over a hoop of gutta-percha and capable of
holding a liquid and floating on water. When a mixed solution of equal
parts of salt and gum is put into the tray, after a time all the salt
will have passed into the water below, leaving nothing in the tray but
an aqueous solution of gum.
The following is one of the most extraordinary results of dialysis. Mr.
Graham took a silicate of soda, a soluble crystalline salt formed by
fusing quartz with carbonate of soda at a red heat, which diffuses
readily. He acidulated the aqueous solution of the salt with
hydrochloric acid, which changes the constituent silica from being a
crystalloid substance into a colloid form. When the liquid was poured
into the tray floating on water, after four days, the whole of the acid
and the chloride of sodium had been diffused in the water and nothing
remained in the tray but an aqueous solution of quartz. There remained
in fact, a solution of sand in water, a substance so hard that no pure
aqueous solution of it had ever been obtained. Many other crystalline
substances besides quartz can exist both in the colloid and crystalloid
states.
All colloid substances are characterized by non-crystalline habits, low
diffusibility, chemical inertness, high atomic weight, and above all by
their mutability. The aqueous solution of quartz is limpid and liquid,
even if it contains 14 per cent. of silica, but after a time it becomes
opalescent, viscous, and ultimately sets into a firm insoluble jelly,
capable however of solution by chemical means. This jelly gradually
shrinks, exudes pure water, and when perfectly dry it forms a glassy,
transparent, but not anhydrous substance, and the residue left by
ignition has a specific gravity of 2·2, that of crystallized silica
being 2·6.
Mr. Graham has obtained many pure aqueous solutions of organic and
inorganic matter, most of them being unstable. Ice near or at its
melting point is believed to be a colloid body, consequently it is
unstable and resembles a firm jelly, having a tendency to rend and
recombine. ‘The constant intervention of colloid septa in so many of the
phenomena of animal and vegetable life gives to the subject of dialysis
a high physiological interest, and it will doubtless exercise an
important influence on the progress of physiological research.’[12]
Subsequently to these researches Mr. Graham published a memoir on a new
method of analysing gases which he had called atmolysis. The memoir may
be regarded as consisting of four parts, the first of which is
preliminary, being on the reciprocal diffusion of gases through porous
plates. The next three parts relate to effusion, or the passage of gases
under constant pressure through a minute opening in a very thin plate
into a vacuum; transpiration, or the passage of gases through capillary
tubes into vacuo; and lastly atmolysis, which is the partial separation
of a mixture of gases and vapours of different degrees of diffusibility
by permitting them to diffuse themselves through a porous plate into a
vacuum: a new kind of analysis, which possesses a practical character of
extensive application.
The diffusing instrument employed by Mr. Graham was a cylindrical glass
tube about an inch in diameter, ten inches long, with one end closed by
a very thin porous disc of compressed artificial graphite fixed by a
resinous cement. While the tube was being filled with hydrogen gas over
a trough of mercury, the escape of the gas was prevented by covering the
graphite very carefully with a thin sheet of gutta percha. As soon as
the gutta percha was removed, the reciprocal diffusion of the gases
began, and in from forty to sixty minutes the whole of the hydrogen had
escaped from the tube, and a quantity of atmospheric air amounting to
about one fourth of the volume of hydrogen had entered the tube and
taken its place, according to the ordinary law of the diffusion of
gases. During this time the mercury rises in the tube so as to form a
column several inches high, a fact which is a striking demonstration of
the intensity of the force with which the reciprocal penetration of
different gases effected.
Natural plumbago or graphite has little or no porosity and cannot be
used in these experiments, but the pores of artificial graphite of which
pencils are made, appear to be so minute that only isolated molecules of
gas are able to pass, without however being at all impeded by friction;
for the smallest pores that we can suppose to exist in the graphite must
be real tunnels compared with the minuteness of the ultimate atoms or
molecules of a gaseous body. The cause of motion appears to reside
solely in that internal movement of molecules which is now generally
admitted as an essential condition of matter in a gaseous state. The
molecules and atoms are assumed to be perfectly elastic and to move in
all directions with different velocities according to the nature of the
gas. Enclosed in a porous vessel the moving atoms constantly strike
against its walls and against one another, but in consequence of their
perfect elasticity, no loss of movement results from the collision. When
the gases inside and outside of the tube are of the same density and
molecular movement, an exchange takes place without any perceptible
change of volume; but when the two gases are of different densities and
molecular velocities, then the reciprocal penetration ceases to be equal
on the two sides. Reciprocal diffusion of gases is accelerated by heat
and retarded by cold; the tension of the gases is increased in the first
case, and diminished in the second.
In Mr. Graham’s experiments relating to effusion, a gas under a constant
pressure was on one side of a minute opening in a very thin plate, and a
vacuum on the other. The rapidity with which air or gases enter the
vacuum depends upon their specific gravity. A gas rushes into a vacuum
with the speed acquired by a heavy body in falling from the height of an
atmosphere of the gas in question supposed to be everywhere of the same
density. The height of this uniform atmosphere will be in an inverse
ratio to the density of the gas. An atmosphere of hydrogen, for example,
will be 16 times higher than one of oxygen. But the velocity acquired by
a heavy body not being in direct proportion to the height, but to the
square root of the height, it follows that the rate of flow of different
gases into a vacuum will be in an inverse ratio to the square root of
their respective densities. The rate of flow of oxygen being represented
by 1, that of hydrogen will be represented by 4 the square root of 16.
This law has been verified by experiment, and is quite analogous to that
which regulates molecular diffusion, but the phenomena are essentially
different. It is the gas _en masse_ which partakes of the movements of
effusion, whilst only the molecules or atoms of a gas are affected by
the movements of diffusion. For that reason the swiftness of the
effusion of a gas is many thousand times greater than that of diffusion.
The swiftness of the efflux of atmospheric air is as rapid as the
velocity of sound.
The rate of the flow of different gases under constant pressure through
capillary tubes into a vacuum, constitutes the capillary transpiration
of gases. These rates bear a constant proportion to one another, but
they are singularly unlike the rates of effusion. They are independent
of the material of the tube; they are not governed by specific gravity;
and ‘they appear to be in constant relation with no other known property
of the same gases; and they form a class of phenomena remarkably
isolated from all else at present known of gases.’
The pores of graphite are so fine that it is incapable either of
effusion or transpiration, but it is readily penetrated by means of the
molecular or diffusive movements of gases, as appears on comparing the
time requisite for the passage of equal volumes of different gases under
constant pressure into a vacuum. For oxygen, hydrogen and carbonic acid
gas, the times are nearly as the square roots of their densities.
The atmolysis or partial separation of mixed gases and vapours of
unequal diffusibility, can be effected by allowing the mixture to
penetrate through a graphite plate into a vacuum. The amount of
separation is in proportion to the pressure, and attains its maximum
when the gases pass into a perfect vacuum. One of the results of
atmolysis was the concentration of oxygen in atmospheric air. When a
portion of air confined in a vessel was allowed to penetrate into a
vacuum through graphite or unglazed earthenware, the nitrogen passed
more rapidly than the oxygen in the ratio of 1·0668 to 1, and the
portion of oxygen is proportionally increased in the air left behind in
the vessel. The increase of oxygen actually observed when the air in the
vessel was reduced from 1 volume to 0·5 was 0·48 per cent. The diffusion
was continued till the air in the vessel was reduced to 0·0625 and the
concentration of the oxygen in it amounted to 2·02 per cent. The
molecular or diffusive mobility exercises a certain influence on the
heating of gases by contact with heated liquid or solid substances. The
more rapid the molecular movement of a gas is, the more frequent will be
the contact of the molecules and the quicker will be the communication
of heat. The greater cooling power of hydrogen compared with that of
oxygen or air is probably owing to that cause. ‘Oxygen and hydrogen gas
have the same specific heat for equal volumes; but a hot object placed
in hydrogen is really touched 3·8 times more frequently than it would be
if placed in oxygen gas. Dalton had already ascribed this peculiarity of
hydrogen to the high mobility of the gas.’[13]
It appears that isomorphic substances such as chloride, bromide, and
iodide of sodium, have a similar diffusibility, another of the many
analogies between these singular marine substances.
Modem chemistry is essentially experimental; the unprecedented magnitude
to which British manufactures have risen is chiefly owing to experiments
conducted with consummate skill and dexterity. In these investigations,
accidental circumstances have sometimes occurred which led to other
researches quite different from that originally in view, which have had
unexpected and invaluable results. Although the simple elements are few,
they are capable of an infinite variety of combinations, so that by
analysis and new combinations, the most useful and valuable materials
are now obtained from obnoxious or useless substances, formerly thrown
away. The instances are numerous; but sawdust may be mentioned as one of
the most remarkable. It was not even fit for fuel, but now oxalic acid,
a bleaching principle most extensively used in the various processes of
calico printing, is procured from it; the quantity required may be
imagined, since the cotton cloth annually printed in Great Britain
previous to the American war, would surround the earth’s equator
nineteen times. Oxalic acid, which is a vegetable substance, found
combined with potash in wood sorrel or Oxalis acetosella, used to be
made from sugar or starch, by the action of nitric acid. Now starch,
sugar, and woody fibre or fibrine, all contain twelve parts of carbon
and different portions of oxygen and hydrogen, always in the proportions
that form water; hence the name of carbohydrates. Their composition is
so similar that the one may be changed into the other by the addition or
subtraction of one or two atoms of water under its atomic form; thus
when fruits ripen, the starch they contain is changed into sugar by the
addition of one atom of water under its dry form.
Now sawdust is woody fibre, and might be changed by nitric acid into
oxalic acid like the others. But a less expensive method is actually
employed.
When sawdust, mixed with two equivalents of the hydrate of soda and one
equivalent of the hydrate of potash, is exposed to a heat of 400° for a
few hours, the substances are fused, and when raised to a still higher
temperature the hydrates are decomposed: hydrogen is evolved, and the
carbon combines with the oxygen to form the oxalate of soda and the
oxalate of potash. In order to separate these oxalates they are put into
a filter, a solution of carbonate of soda is passed through it; the
oxalate of soda remains in the filter, the carbonate of potash passes
through it; and when lime is added to the oxalate of soda, the soda is
liberated, passes through the filter, and the oxalate of lime remains.
Sulphuric acid is then added to the oxalate of lime, sulphate of lime is
formed, and oxalic acid mixed with water remains, and by evaporation
forms into beautiful crystals of oxalic acid. This is an instance of a
complicated chemical process; nevertheless it is carried on to a vast
extent in Manchester, nine tons a week being furnished by one
manufactory alone. Two pounds of sawdust yield one pound of oxalic acid.
In ordinary distillation a volatile substance such as water, by
absorbing the heat applied to it, becomes converted into vapour; by
abstracting the absorbed heat from the vapour, it is reconverted into
the original substance. Destructive distillation, on the contrary,
consists of an entire destruction of the original substance and a
simultaneous production of new substances. Of this the destructive
distillation of coal furnishes the most interesting illustration, and
shows at the same time the success of modern chemistry in utilizing
waste substances.
Coal had been distilled for years to furnish gas for the illumination of
our cities before it was discovered that the refuse contained principles
of the greatest value. The products of the distillation are threefold:
gas, coal water, and coal tar.
Coal gas is a combination of various gases, whose illuminating
properties depend upon, and are exactly in proportion to, the quantity
of carbon they contain. The particles of carbon raised to a white heat
give the light, for the gaseous part has a feeble flame, and requires a
higher temperature than solid matter, which becomes luminous at about
700° in the dark, and at from 1000° to 2000° in bright daylight. Coal
gas consists of a combination of illuminants: olefiant gas, which
contains 86 per cent. of carbon, carburetted hydrogen or marsh gas,
which contains 75 per cent., carbonic oxide, carbonic acid gas,
hydrogen, sulphuretted hydrogen, and a very small quantity of nitrogen,
besides the bisulphide of carbon, and benzol, a pure hydro-carbon,
consisting of 12 equivalents of carbon and 6 of hydrogen.
The poisonous quality of coal gas is owing to the carbonic oxide, which
is fatal to life, and its explosive quality to carburetted hydrogen,
which also is generated by decomposition of vegetable matter in stagnant
pools and marshes; and in the firedamp of mines it still bears testimony
to the vegetable origin of coal. That fatal gas increases in explosive
force as it mixes with atmospheric air, and is at a maximum when it
amounts to 12 per cent. Hydrogen, carburetted hydrogen, and carbonic
oxide do not add much to the light, on account of the feeble flame of
hydrogen and the small quantity of carbon they contain, but they force
the chief illuminating gases out of the iron retorts in which the coal
is distilled before the heat has had time to decompose them, and they
also enable them to burn without smell or smoke.
Carbonic acid, bisulphide of carbon, and sulphuretted hydrogen are
impurities from which coal gas is freed before it is fit for use. By
passing the gas over lime, the lime absorbs both the carbonic acid and
the sulphuretted hydrogen; one per cent. of carbonic acid diminishes the
illuminating power six per cent., and the sulphuretted hydrogen has an
abominable smell.
The bisulphide of carbon, consisting of one equivalent of carbon and two
of sulphur, is got rid of by passing the gas over hot lime. The water of
the lime is decomposed, and carbonic oxide and sulphuretted hydrogen are
produced; but the latter may be absorbed by passing the gas again over
lime, or through a mixture of sawdust and the oxide of iron. The oxide
of iron decomposes the sulphuretted hydrogen, forms water and sulphide
of iron, then the air restores the sulphide to oxide, and the sulphur is
deposited in the mixture. After passing the gas through it till none of
that impurity remains, the gas is fit for use. The test is the
nitro-prusside of sodium, which the gas stains purple if any of the
impurity remains.
Paraffin, already mentioned as isomeric, is a pure hydrocarbon,
colourless, transparent, and of crystalline texture. It melts at a heat
of 120° or 130°, burns like wax without smell or smoke, and makes
beautiful candles, which give a brilliant light on account of the 86 per
cent. of carbon they contain. Paraffin oil is much used for lamps; the
manufacture of these two substances at Bathgate is one of the largest
chemical establishments in the world.
The black fœtid gas water resulting from the distillation of coals,
formerly thrown away, is so rich in the salts of ammonia, that it has
become the chief source from which these materials so important in the
arts are obtained.
Ammonia is well known to be a colourless gas, with an acrid pungent
smell, consisting of one equivalent of nitrogen and three of hydrogen.
It has an alkaline character, combining with acids, and is extremely
soluble in water.
Now the gas water contains carbonate of ammonia and sulphide of
ammonium, and when any acid strong enough to decompose these substances
is put into the liquid, the carbonic acid and sulphuretted hydrogen
being volatile are driven off, and the acid combines with the ammonia to
form a salt. For example, when muriatic acid is put into the liquid, it
drives off the volatile gases and combines with the ammonia in solution
to form muriate of ammonia, which is dissolved in water and evaporated
till it crystallises; then it is vaporized and sublimed to free it from
impurities.
When ammonia and muriatic acid are separately vaporized, the two
colourless transparent vapours, when mixed, combine into solid muriate
of ammonia, a result so unexpected that as Mr. Playfair justly observes,
it could only have been taught by experiment. About 4,000 tons of
muriate of ammonia are annually made from gas water in England for
soldering, and for making alum.
Sulphate of ammonia to the extent of 5,000 tons is annually made by
adding oil of vitriol to the liquid. It is also used for making alum, as
well as for manure; it supplies our grain with nitrogen, an important
article of vegetable food. To these may be added 2,000 tons of carbonate
of ammonia, so that a substance that was considered to be good for
nothing yields 11,000 tons of valuable materials, but even this quantity
forms only part of the enormous amount annually consumed in the
manufactures of Great Britain.
Coal tar is of complicated nature, containing a variety of substances,
many of which are more or less volatile. When it is distilled by sending
a current of steam through it, the steam collects the volatile parts,
condenses them into naphtha; the first product is condensed steam or
water with naphtha swimming on its surface, the next product is dead
oil, and the remainder is pitch.
By the aid of the crude naphtha thus produced, Indian rubber is
dissolved and waterproof clothes are made. When purified by sulphuric
acid, it forms a substance like tar which is thrown away, and the
remaining products when clarified are acid oils and neutral
hydro-carbons. The carbolic and cressylic acids are the most important
of these acid oils. The carbolic acid, which has the property of
arresting the putrefaction and decay of organic matter, consists of 12
equivalents of carbon, 6 of hydrogen, and 2 of oxygen. The cressylic
acid only differs from the preceding by having two more equivalents of
hydrogen and two of oxygen in its chemical composition.
Creosote is a mixture of these two acids. Those vast beams of wood that
are driven as piles into the sand or mud at the bottom of the sea, as
well as the timbers that form marine superstructures, are saturated with
it to a certain depth to preserve them from the attacks of marine
insects, especially Limnoria terebrans, an isopod crustacean, which is
so destructive in some of our harbours. The wood is deprived of its air
by heat and the creosote easily enters.
Carbolic acid is liquid, but becomes solid when purified and dried; and
as already mentioned the brilliant yellow dye, carbazotic acid, one of
the coal tar colours, is a compound radical, in which the peroxide of
nitrogen has replaced three equivalents of hydrogen. The other coal tar
colours are obtained from the neutral hydro-carbons, that is to say,
compounds of hydrogen and carbon, such as benzol, toluol, and other
analogous substances.
Benzol, which consists of 12 equivalents of carbon and 6 of hydrogen, is
very volatile, boiling at 117° Fahr., and when acted upon by nitric
acid, it forms a compound radicle in which one equivalent of oxide of
nitrogen takes the place of one of hydrogen. It smells strongly of
bitter almonds, and may be used with safety instead of them. When water
and iron are mixed with nitro-benzol, the iron combines with the oxygen
and forms oxide of iron, and the result is rusted iron and aniline,
which is the origin and foundation of the coal tar colours. Now aniline
consists of 12 equivalents of carbon, 7 of hydrogen, and 1 of nitrogen.
It is a compound radical: it is ammonia in which one equivalent of
hydrogen has been replaced by the radical phenyle, consisting of 12
equivalents of carbon and 5 of hydrogen. It may be remarked that in all
these chemical operations the quantity of carbon has remained the same.
Aniline is a colourless liquid, and, being an analogue of ammonia, it
readily combines with the different acids to form the beautiful coal tar
dyes, for which the world is indebted to the brilliant researches of Dr.
Hofmann, professor of chemistry.
By combining a solution of the chloride of lime with the colourless
liquid aniline, he obtained the beautiful colour mauve, but it could not
be used as a dye till it was rendered permanent by his pupil, Mr.
Perkins. His next discovery was the rich crimson crystalline dye
magenta, which M. Verguin first introduced into trade at Lyons as a
dyeing agent. It may be produced by mixing the anhydrous bichloride of
tin with aniline and then driving off the excess of aniline by heat.
Other metallic chlorides, nitrates, and many oxidizing agents, have the
power of converting aniline into magenta; as for example when the two
colourless liquids acetic acid and aniline are mixed and heated, a
chemical combination takes place in which three atoms of ammonia have
coalesced into one, a salt is formed which is the acetate of aniline or
magenta. Here two liquids unite to form a solid and as in many other
instances the resulting substance has the power of decomposing light
which neither of its constituents can do. Magenta has a redder tint than
mauve, and on that account it is sometimes called aniline red. Professor
Hofmann has discovered quite recently that pure aniline has not the
property of producing these colours, but that they originate in an
impurity of the aniline called toluidine.
Rosaniline or roseine, a white substance, is the base of aniline. It is
a powerful alkali, readily combining with acids to form highly coloured
salts, many of which have a tendency to crystallize, like magenta. This
base is most easily extracted from the acetate of aniline. The boiling
solution of that salt decomposed by a large excess of ammonia, yields a
crystalline precipitate of a reddish colour, and when the colourless
liquid is separated by filtration from the precipitate, it deposits on
cooling perfectly white needles and tablets of pure rosaniline. This
substance unites to acids in three different proportions forming three
kinds of salts. The salts that contain one equivalent of acid are
extremely stable compounds; for the most part they have a green metallic
reflection like some insects’ wings; by transmitted light they are red,
and their solutions in alcohol have the magnificent crimson colour of
magenta.
A bright purple dye is furnished by mixing equal weights of magenta and
aniline. When this mixture is kept at the temperature of 329° for some
hours and then mixed with water and hydrochloric acid to remove any
excess of magenta or aniline, the result is an insoluble purple residuum
or precipitate, but which when well washed with water becomes soluble in
alcohol and boiling water slightly acidulated with acetic acid. When the
insoluble purple residue is boiled several times with dilute
hydrochloric acid, a fine blue dye is formed; azuline, the most
beautiful of the blue dyes, which resists the action of the strongest
acids, and which is produced by oxidizing aniline under high pressure.
It was first prepared at Lyons from phenic acid, a product of the
distillation of coal; when pure it appears under the form of copper
bronze-coloured crystals soluble in alcohol, to which they communicate a
magnificent blue colour tinged with red; but most of the blue dyes are
derived from carbolic acid and from creosote. A blood-red colour is the
direct result of mixing the muriatic and phenic acids. Aniline, the
great source of the coal tar colours, yields also a fine yellow. A vast
deal of talent has been employed in the research of colouring dyes both
at home and abroad, in which the manufacturers themselves have shown
great scientific knowledge.
Attempts have been unsuccessfully made to obtain a green dye from
chlorophyll, the green colouring matter of plants. The want was for a
short time supplied by Lo-hao, a Chinese dye, but being unstable it was
given up. However the very same substance has been procured from the
Rhamnus cathartica (Buckthorn), one of the commonest European trees. M.
Charwin of Lyons, who made the discovery, has utilized a waste
substance, and rendered it permanent as a dye. It is the only known
substance which with proper reagents is capable of producing all the
seven colours of the spectrum.[14]
The coal tar colours have nearly superseded those from lichens which
incrust rocks, walls and stems of aged trees with brilliant colours,
which do not however furnish dyes directly; they yield a colourless
crystalline substance which combines with alkalies to furnish very
beautiful dyes; it is exactly the opposite of rosaniline, which is a
base. The Variolaria dealbata yields litmus or orchil, from which the
beautiful French purple is made. The Rocella tinctoria and fusiformis
give blue and purple, and the pale yellow lichen, Parmelia parcolerina
furnishes a bright yellow dye, which a little ammonia changes to a rich
red, inclining to purple. Mauve was first made from orchil, but was not
permanent. The fine dyes, alizarine blue, Turkey red and garancine, are
still much in use. They are derived from madder, the dried roots of the
Rubia tinctorum; the madder dyes most extensively employed are alizarine
and flower of madder. Mauve and other dyes are derived from guano, the
offal of seabirds, which is imported in large quantities for manure.
The coal tar colours are manufactured on a highly scientific plan and
most extensive scale in Great Britain, to supply the enormous quantity
annually consumed in dyeing silk and printing cotton. In general, animal
substances such as silk and wool can be permanently dyed at once,
because they have a strong affinity or attraction for coloured dyes. If
silk is destined to be a moiré, the silk before it is woven undergoes a
chemical process in order to introduce fatty matter into it which gives
a softness to the silk when woven and renders it fit to receive the
moiré by intense pressure.
Cotton cloth has no affinity for dyes, which are washed out at once if
not fixed by art, because cotton fibre consists of minute tubes
generally open at the extremity, which imbibe the dye by capillary
attraction, but cannot retain it unless fixed by a mordant, such as the
white of a raw egg, which readily absorbs any dye that is mixed with it,
and being then laid on the cloth in any pattern it is absorbed by the
tubular fibres, and when coagulated by steam or any other application of
heat it is immovably fixed. Both animal and vegetable substances afford
a variety of mordants. Caseine or cheese, the curd of milk, which may
also be obtained from pease and beans, is the mordant most used by
calico printers; for if caseine be dissolved in twice the quantity of
alkali necessary for its solution, it coagulates like white of egg and
may be used in the same manner. Skimmed milk cheese from Scotland and
Holland when purified is extensively used in calico printing. The
quantity of mordants required is very great, for of all the cotton that
was imported into Britain before the late American civil war, one
seventh only was manufactured into muslin and printed calico, yet as
already mentioned that was sufficient to envelope the earth’s equator
nineteen times, and twenty-seven millions of pieces were exported
annually. Atmospheric electricity and ozone affect the process of
dyeing, and east wind has a retarding and injurious effect. The Lyons
manufacturers, not less celebrated for their scientific skill and taste
than for the brilliancy of the colours, have an advantage in their fine
climate and bright sun.
It is a singular circumstance that petroleum has existed in enormous
quantities throughout the North American States and a great part of
Canada, unnoticed and neglected till the year 1859, when its value was
discovered, and it almost immediately formed a new and extensive branch
of commerce, for during the succeeding year at least 1,000 wells were
dug, some of which enriched the proprietors; others were a failure.
Petroleum from the fountains of Is, on the banks of the Euphrates about
120 miles from Babylon, furnished the asphaltic mortar for building
Nineveh 2,000 years before the Christian era. There are many sources of
naphtha, petroleum, and asphalt in Europe and Asia, which like those in
Trinidad and Venezuela occur for the most part in rocks of the newer,
secondary and tertiary formations, though sometimes in the lower. But in
the northern part of the United States and Canada these substances occur
in rocks of all ages from the lower silurian to the tertiary period
inclusive; they are usually found in the limestones and more rarely in
the sandstones and shales. Petroleum collects in the fissures of the
rocks, chiefly in those that have a tendency downwards; in wells dug for
it near one another, an abundant supply is furnished at all depths from
70 to 300 feet. In some parts of Ohio and Canada the ground is saturated
with petroleum, so that it is believed there is enough in North America
to supply the world for ages. In 1861 no less than 42,000,000 gallons of
petroleum were sent to England. The wells are not without danger, for
when they pass through the coal strata, the petroleum is accompanied by
a highly inflammable gas which on one occasion was accidentally set on
fire; it ignited the petroleum, which was forced out as from the mouth
of a volcano, and covered the ground with liquid fire far around; at the
same time the burning gas formed an incandescent atmosphere which
extended to a still greater distance.
The distillation of petroleum yields substances for the most part
identical with those arising from the distillation of coal. The crude
petroleum is put into an iron retort connected with a coil of iron pipes
surrounded by cold water, called the condenser. Heat is applied to the
retort, and from the open extremity of the condenser, a pale coloured
liquid with a strong smell flows, which is very volatile and explosive
naphtha. After the naphtha has passed over, an oil of excellent
illuminating quality is distilled over. Steam is then forced into the
retort, and a heavy oil is driven over, and there remains a black, oily,
tarry matter, and a black cake used for fuel. After the naphtha has been
repeatedly distilled, benzol is formed, and when the heavy oil is cooled
to 30° Fahr., crystals of paraffin appear, which are separated from the
oil by pressure, and when they are purified by alternate pressure and
agitation in a melted state, they are moulded into candles. This
paraffin is identical with that from coal. Among the products of the
distillation of petroleum are naphthalin whence aniline is obtained,
which yields mauve, magenta, and the other coal tar colours, also
solferino which yields dianthine and other dyes and has been proposed as
a substitute for chloroform and ether. Many other substances have been
separated from petroleum which like some from coal have not yet been
chemically examined. Most of the substances obtained from petroleum and
the distillation of coal are common also to distilled peat, and now it
is proposed to utilize sea weeds, in which the northern coasts of
Scotland and Ireland are so rich. They were burnt for many years chiefly
to furnish soda, but as that substance is obtained at a cheaper rate
from salt, kelp or sea weed ashes has only been made lately to obtain
iodine for medical purposes, and more than one half is wasted in the
process. Besides iodine and six other substances generally procured from
kelp, Mr. Stanford has discovered that it contains naphtha, paraffin oil
and volatile oil rich in benzol, which yields aniline and magenta dyes
and shows that marine vegetation as well as terrestrial abounds in
colouring matter.
Every substance is now of use, no substance is without its value, but it
would be a vain attempt to mention the innumerable discoveries made by
experimental chemistry, which is daily extending its empire over the
three kingdoms of organic and inorganic nature.
_Composition of some of the preceding Substances._
Acetylene C_{2}H_{2}
Olefiant gas C_{2}H_{4}
Ammonia H_{3}N
Benzol C_{12}H_{6}
Phenyle C_{12}H_{5}N
Aniline C_{12}H_{7}N_{3}
Rosaniline C_{40}H_{9}N_{3}
Carbolic acid C_{12}H_{6}O_{2}
Cressylic acid C_{14}HO_{2}
SECTION IV.
THE SOLAR SPECTRUM, SPECTRUM ANALYSIS, SPECTRA OF GASES AND VOLATILIZED
MATTER, INVERSION OF COLOURED LINES, CONSTITUTION OF SUN AND STARS.
TO the unrivalled genius of Sir Isaac Newton we owe the solar spectrum,
and the laws of coloured rings, by aid of which, Dr. Thomas Young proved
and established the undulatory theory which forms the basis of the whole
science of light. The visible part of the solar spectrum forming a band
of seven colours was supposed to be continuous till the year 1802, when
Dr. Wollaston looking with a prism whose axis was parallel to a narrow
slit in a window shutter, at a sunbeam passing through it, discovered
seven dark lines crossing the coloured band, at right angles to its
length.
Twelve years afterwards, Fraunhofer of Munich, a celebrated optician,
magnified the spectrum of a vertical line of light passing through an
upright prism by receiving it upon the object glass of a telescope, and
discovered 600 dark lines. Having ascertained that the position of the
lines in the spectrum, and their distances from one another, are
invariable under every circumstance, he determined their places
accurately and drew the diagram known as Fraunhofer’s lines, which is
universally referred to as a standard of comparison. For that purpose,
the principal lines are designated by letters; thus the dark line A is
in the red near the least refrangible end of the spectrum, B and C are
in the orange, the very remarkable double line D is in the yellow, _b_
and E are in the green, F is at the limit between the green and the
blue, G is in the blue, and the double line H is in the violet.
The instrument used by MM. Bunsen and Kirchhoff, though more
complicated, is constructed on the same principle as the preceding. A
sunbeam transmitted by a very narrow vertical slit passes through four
prisms, which disperse it so much, that if drawn on the scale seen with
the magnifying telescope which receives it, the spectrum would extend
over twenty feet. By means of a micrometer screw, the telescope can be
turned round a vertical axis, and as the dark lines come successively
under the cross wires in its eye-glass they are seen to pass over a
graduated scale, so that the distances between two thousand of them have
been measured in millimetres with unerring accuracy, but that is only a
small part of the whole. When viewed through the telescope, the retina
of the eye is the screen on which this wonderful spectrum falls, crossed
by innumerable dark rayless lines of various breadths and intensities.
Black bands given by the inferior refraction of one prism are here
resolved into numerous dark lines as fine as a spider’s thread.
Mr. Glaisher during his tenth scientific balloon ascent devoted his
attention for a time almost entirely to the dark lines on the solar
spectrum. At a height of about four miles and a half, they were almost
innumerable; all he had seen on the earth were there, and many more. The
nebulous lines H were both seen, the spectrum was a good deal lengthened
at the violet end, and at the red end the line A was visible. The light
from the sky near the sun gave a shorter spectrum; the lines were only
visible from B to G.
Besides these cosmical or permanent lines, Sir David Brewster observed
that certain dark bands and lines in the red and green parts of the
spectrum are only visible when the sun is near the horizon, whence he
concluded that they are occasioned by the absorption of the solar light
while traversing a thicker stratum of air than when the sun is in the
zenith. Various groups of these absorption bands are to be seen at times
on the solar spectrum, especially a remarkable one near Fraunhofer’s
line D, and Dr. Miller observed that temporary dark lines appeared
during a heavy shower, which vanished when the rain ceased.
When the sun was high, M. Kirchhoff mentions that he had noticed traces
of lines and nebulous bands in different parts of the spectrum, which he
thinks might be resolved by a greater number of prisms than those in his
apparatus.
Sir David Brewster was led to his discovery of atmospheric bands by
observing that the brownish red vapour of nitrous oxide has the property
of absorbing solar light, resolving the spectrum into a series of bright
and dark bands, alternating. Professors Daniel and Miller found that
bromine, iodine, and chlorous acid do the same, and Sir John Herschel
observed a multitude of similar bands in the flame of cyanogen; but Dr.
W. A. Miller, who has particularly studied the phenomena of absorption
bands, has proved that the colour of a vapour does not necessarily
determine the position or even the existence of dark bands. He has shown
that some simple substances which do not occasion dark bands produce
them abundantly by the absorptive power they acquire when in
composition, while lines that are produced by a simple vapour, vanish
when it is in combination. Dr. W. A. Miller has proved also that none of
the preceding vapours exist in the atmosphere. He computed that if free
bromine constituted only one in a thousand million parts of atmospheric
air, it would betray its presence by absorptive bands; nevertheless he
suspects that there may be some substance in the air that occasions
certain unaccountable changes. Possibly ozone, so intimately connected
with atmospheric electricity, may produce some unknown effect.
The spectra from glowing solids and liquids, such as Drummond’s light,
which is incandescent lime, the still more brilliant flame of the
electric arc between charcoal points, glowing solid and fused metals,
and coal-gas flame, are continuous; the spectra exhibit the seven
colours, but they are not crossed by dark rayless lines, because such
incandescent substances give off light of all refrangibility. But solids
and liquids reduced to glowing vapours, and incandescent gases, only
give out rays of certain refrangibilities, which cross their spectra at
right angles, as bright lines of various colours and intensities. Each
glowing vapour and gas has bright lines on its spectrum peculiar to
itself.
In order to compare these bright lines with Fraunhofer’s dark lines,
solar light is transmitted through one half of the vertical slit in
Kirchhoff’s apparatus, and the light of the luminous vapour or gas
through the other half. Then by prismatic refraction two spectra are
seen in looking through the telescope, the gaseous one immediately below
the solar one, and only divided from it by an almost imperceptible dark
line. So that the bright lines appear to be continuations of the dark
lines if they occupy the same position in the two spectra; if not, the
deviation is at once visible. The coincidence or deviation of the bright
lines on the spectra of two volatilized substances may be determined by
the same method.
The coloured light that has so beautiful an effect in fire-works is
owing to the combustion of the salts of different metals: as soda, or
common salt, which gives a perfectly pure homogeneous yellow; potash
gives a violet light, strontia red, baryta green. The colour is given
out by the glowing atoms of the vaporized metals sodium, potassium,
strontium, and barium in a state of violent ignition; for as the salt
and its metal give the same colour and the same spectrum when ignited,
it is evident that the colour is independent of the oxygen of the
alkali.
Sir David Brewster appears to have been the first who analysed coloured
light with a prism; and in 1822 Sir John Herschel, besides having made a
series of observations on coloured flames, had determined the spectra of
the muriates of strontia and lime, the chlorides and nitrate of copper
and boracic acid; and observes that ‘the colours thus communicated by
different bases to flame afford, in many cases, a ready and neat way of
detecting extremely minute quantities of them.’[15]
The same opinion was afterwards formed by Mr. Fox Talbot, who after many
experiments on metallic salts, says in his paper,[16] that a glance at
the prismatic spectrum of a flame may show it to contain substances
which it would otherwise require a laborious chemical analysis to
effect. In that paper this gentleman noticed that the glowing salts of
lithium and strontium give a crimson or red colour to flame so exactly
of the same tint that if these metals were in combination it would be
impossible to decide to which metal the colour is due. But when he
passed their respective lights through a prism, he found that the bright
lines on their spectra are entirely different. ‘The strontia flame,’ he
observes, ‘exhibits a great number of red rays well separated from each
other by dark intervals, not to mention an orange, and a very definite
bright blue ray. The lithia exhibits one single red ray,’ Whence Mr. Fox
Talbot observes, ‘I hesitate not to say that optical analysis can
distinguish the minutest portions of these two substances from each
other with as much certainty, if not more, than any other known method.’
Thus Sir John Herschel and Mr. Fox Talbot laid the foundation of a
spectrum analysis of unrivalled delicacy and beauty, since carried to
perfection by Messrs. Bunsen, Kirchhoff and other experimenters,
presently to be mentioned.
M. Bunsen detected the characteristic crimson lithium line in the
spectra of numerous substances; in granite, in the earliest geological
strata, in meteoric stones, in the ashes of most land plants, in blood
and other animal matter; so that instead of being one of the rarest
metals, it exists in all the three kingdoms of nature. In the year 1857
Mr. Swan gave an instance of the extreme minuteness of spectrum
analysis, by detecting the 1/2,300,000th part of a grain of salt by its
yellow light; but by the same reaction M. Bunsen not only recognised the
180 millionth part of a grain of sodium, but found that there is hardly
any substance that does not contain it. It exists in the dust on our
clothes and furniture, particles of it float in the air we breathe, so
that while examining the spectra of other incandescent substances,
flashes of yellow light appear as these atoms are volatilized and
instantly burnt up, which shows that common salt is perhaps more
universally diffused than any other kind of matter.
By spectrum analysis, M. Bunsen has discovered the two new metals,
rubidium and cæsium. While examining with a prism the spectrum of the
hundredth part of a grain of an alkaline substance separated from the
residuum of the Durckheim mineral water, he saw coloured lines, which he
had never seen before on the spectrum of any other alkali, and at once
concluded that they belonged to a new metal; and having obtained about
200 grains of the substance by the evaporation of forty tons of the
water, he found that they contained the chlorides of the two new metals
in question. Moreover he perceived that these metallic chlorides
resemble the chloride of potassium so nearly in spectrum and chemical
character, that a refined prismatic analysis could alone determine the
difference. He thus ascertained that the spectra of all the three have
two red lines in the red part of their spectrum, and two violet lines in
the indigo, while the middle part is occupied by a continuous diffused
light. The only difference is that the two red lines in the rubidium
spectrum are less refrangible than the red lines in the potassium
spectrum, and that the cæsium spectrum is distinguished by two bright
blue lines in the diffuse middle part. Rubidium received its name from
rubidus, on account of the dark red of its lines, and cæsium from its
sky-coloured blue lines.
M. Bunsen thinks that there can hardly be a doubt of rubidium having
been mistaken for potassium, but he has shown that they may be
distinguished by the difference in the solubility of the double salts
which the chlorides of these two metals form with the chloride of
platinum. An aqueous solution of the bichloride of platinum and
potassium gives an insoluble yellow precipitate, consisting of the
bichlorides of platinum and potassium. An aqueous solution of the
bichlorides of platinum and rubidium gives an insoluble yellow
precipitate of the bichlorides of platinum and rubidium. These two
precipitates are undistinguishable to the eye. Now if a solution of
platinum be added to the first, no further precipitate can take place,
but if a solution of rubidium be added to it, a yellow precipitate is
formed consisting of the bichloride of rubidium and potassium, because
the chloride of rubidium resolves the precipitate, combines with the
chloride of potassium, and sets the chloride of platinum free. Thus the
precipitate of the bichloride of rubidium and potassium is the least
soluble of the two. The yellow colour is evidently due to the potassium.
Cæsium may be distinguished from potassium by the same process. The
carbonates, hydrates, and other salts of the two metals were determined;
their carbonates were shown to be readily separated, because the
carbonate of cæsium is soluble in alcohol, which the carbonate of
rubidium is not, and finally the metal rubidium was separated. It has an
extreme avidity for oxygen, and burns in water like potassium, and
possesses many other analogous qualities. It melts at the temperature of
38·5° Cent., and has a specific gravity of 1·516. Rubidium is abundant
in the mineral lepidolite in many parts of Europe and North America, and
M. Grandeau has detected it in the ashes of beetroot, tobacco, coffee,
tea, and grapes by spectrum analysis. It exists in various mineral
waters, and in fact is very general. Traces of various metals are met
with in the same vegetable; thus the spectrum of tobacco gives lines
characteristic of lithium, potassium, rubidium, and lime.
Mr. W. Crookes discovered the new metal thallium by means of its
spectrum, which differs from every other in having one bright green line
upon a dark ground. He obtained its various salts, and the metal itself,
which he describes as being heavy, dense, and very like lead, but of
greater specific gravity. Its fresh surface has a bright metallic
lustre, not so blue as that of lead, but it tarnishes more easily. It is
so soft that it can be indented by the nail, yet it can be drawn into
wire, and in chemical properties it resembles mercury, lead, and
bismuth. Altogether it is more like a metal than a metalloid, perhaps
something between the two. Thallium is completely volatilized at a
temperature below red heat, whether single or in composition. If the
quantity be small, the green line appears in a sudden flash, lasting but
the fraction of a second. If a larger quantity of the metal be gradually
put into the flame, it lasts a little longer, appearing as a single
green line of extraordinary purity and intensity, sharply defined on a
black ground. With respect to volatility, thallium is analogous to the
non-metallic element selenium, which is so volatile that its beautiful
blue light only lasts a few seconds. The green light of thallium comes
out more rapidly, and with less of the substance, than the blue light of
selenium, a quantitative distinction which accords with Dr. Miller’s
observation that the rapidity with which a result is obtained, and the
minuteness of the quantity required for the examination, gives this
method a superiority over every other for the qualitative analysis of
the alkalies and alkaline earths. Thallium has been detected in mineral
waters, wine, treacle, tobacco, and chicory.
Drs. Reich and Richter discovered a fourth new metal in the zinc-blende
at Freiberg in Saxony, which has been called indium, from two beautiful
indigo-blue lines in its spectrum, which have a greater refrangibility
than the blue lines in strontium. The chemical relations of indium
resemble those of zinc, with which it is associated in nature. The metal
can be reduced before the blowpipe into a bead, which marks paper and
has the colour of tin.
The practical importance of spectrum science has been beautifully
illustrated by Professor Roscoe by its application to overcome a
difficulty in Bessemer’s process for the manufacture of steel. According
to that process, steel is made by sending a blast of air through a
quantity of melted iron; the difficulty was when to stop the blast, for
if stopped too soon, the metal retains so much carbon that it crumbles
under the hammer; if continued a few minutes too long, the molten metal
is so viscid that it cannot be poured into the moulds. Experience had
hitherto enabled the manufacturer to judge of the right time from the
appearance of the flame which issued from the mouth of the converting
vessel, but now Professor Roscoe has determined the exact moment for
cutting off the blast by a spectral examination of the flame, the light
of which is most intense. The flame spectrum in its various phases
revealed complicated masses of dark absorption bands and bright lines,
showing that a variety of substances were present in the flame in a
state of incandescent gas; and by a simultaneous comparison of these
with well-known spectra of certain elementary bodies, Mr. Roscoe
ascertained the presence of sodium, potassium, lithium, iron, carbon,
phosphorus, hydrogen, and nitrogen in the flame.
Both Dr. Wollaston and Fraunhofer noticed that the spectrum of the
electric spark was crossed by bright-coloured lines; and in the year
1835, Professor Wheatstone determined the spectra of the electric spark
taken from fused zinc, cadmium, tin, bismuth, lead, and from mercury,
and found that each is crossed by bright lines differing in number,
position, and colour, but which are the same whether the electric spark
be from a static, voltaic, or magneto-electric machine. Having given a
plate showing the colours of these bright lines on the respective
spectra, he proved that they are not owing to the electricity, but to
the incandescent atoms of the metals, for by using different metals as
terminals to the conducting wires, he determined the spectra of these
metals in vacuo, which proved that they were due alone to the
volatilization of the metallic terminals, and concluded that any one
metal may be distinguished from another by the appearance of the spark.
Wheatstone discontinued his spectrum researches, for he had invented the
electric telegraph, and was busy in extending the first telegraphic wire
that ever carried the thought of man to man between London and
Manchester. Soon after he laid the first aquatic line across the Thames,
and he has lived to see his telegraphic lines spread over the surface of
the earth and the bottom of the ocean.
Mr. Wheatstone had perceived that the bright lines on the spectra of the
metals are different and more complicated when taken in air than in
vacuo, and Professor Angström made the important remark that the
electric spark gives two superposed spectra, one due to any metal that
may be under examination, the other to the incandescence of the air
through which the spark passes. Hence the importance of the spectrum
analysis of gaseous substances, especially of those which constitute our
atmosphere, a subject that has been ably and successfully investigated
by Professor Plücker. For that purpose he made use of the Geissler or
vacuum tubes, similar to those he used in his experiments on the
stratification of electric light. When electricity was sent through a
tube containing oxygen gas, the gas combined so rapidly with the
platinum of the negative terminal of the battery that there was little
time to examine the spectrum. The electral light in the tube was too red
at first, but as the attenuated gas gradually disappeared it changed
through flesh-colour to green, then through blue to reddish-violet, and
at length there was too little gas to convey the electricity. However,
the oxygen spectrum has a remarkably bright red band at its red
extremity, two bright orange lines divided by a black one in the orange,
and some bright bands in the green.
The electric light of attenuated hydrogen is red, and almost the whole
light in its spectrum is concentrated into six bright bands of nearly
equal breadths. There is a dazzling red band near the red end of the
spectrum, which, however, does not coincide with the oxygen band; then
comes a very beautiful yellow band, in which the whole of the yellow
rays seem to be concentrated, followed by a grey interval which
separates the yellow from three bright lines in the green, the first of
which is yellowish green, the last a beautiful greenish blue; a black
and a dark space separates the latter from the violet in which there is
a bright line. The electric light in a tube containing highly rarefied
aqueous vapour is red, the vapour is resolved into its simple elements
by the electricity, the oxygen combines with the platinum of the
negative or heat pole, and the spectrum is that of pure hydrogen with
the three most prominent bands only.
The nitrogen spectrum is brilliant with all the seven colours; there are
no broad dark spaces like those which divide the bright bands in the
hydrogen spectrum, but it is crossed by numerous very fine black and
grey lines. Fifteen of the latter stripe the red and orange; the green
is separated from the yellow by a black narrow band; it is terminated by
two bright blue lines, and very fine dark lines cross it and the rest of
the spectrum. The tube light is yellowish red.
The spectrum of highly rarefied atmospheric air is chiefly that of
nitrogen, for the oxygen combines with the platinum of the negative
terminal, and is in too small a quantity to transmit the electricity
through the tube.
The rarefied vapours of chlorine, bromine, and iodine are so rapidly
combined with the platinum of the negative terminal, that it is
difficult to determine their spectra; but they have peculiarities in
common, which distinguish them from all other spectra. The bright lines
that cross them are first at rest, but soon become flickering. In the
iodine spectrum, five of those lines of flickering light of great beauty
are in the green, two of them close together. The bromine spectrum shows
a greater number, which extend across the colours of its middle part,
accompanied by dark lines; and in the chlorine spectrum there are many
lines, both of flickering light and darkness. New lines are brought out
in the iodine spectrum by increase of temperature. At a low heat it is
crossed by a number of dark lines, but with a higher temperature the
vapour has a greenish hue, which is resolved by the prism into green
lines at some distance from one another, and fainter blue light, crossed
by groups of luminous bands.
Rarefied compound gases are resolved by the electricity into their
component parts, and the result is superposed spectra, one belonging to
each element. M. Seguin considers the aspect of the electric spark to be
a sure indication of chemical action, for while the decomposition is in
progress, the electric spark is encompassed by a halo, and the bright
lines of the double spectrum are less distinct; but when the reaction is
finished, the spark becomes slender, and the spectrum bands distinct. In
the decomposition of highly carburetted and attenuated hydrogen gas, the
spark resembles a flame, and the spectrum is like that of white light.
When the gas is decomposed, the hydrogen is disengaged, and the carbon
deposited on the extremities of the conducting wires; the spark becomes
slender, and then the lines of the hydrogen, the lines belonging to the
hydro-carbon and to carbon itself may be seen on the spectrum.[17]
The bright and coloured lines on the spectra of the gases, and the
vapours of a great number of the metals and metallic salts, were known
before MM. Bunsen and Kirchhoff began their systematic researches,
during which they added many more, some so difficult and analogous, that
it required all their skill and experience to make them out.
Of all the spectra that have been determined, those of sodium and iron
are the most important and interesting. In that of sodium, the only
light is of the purest yellow condensed into a double line of intense
brilliancy on a dark ground. The iron spectrum on the contrary is
crossed by bright lines of all intensities and colours in such
multitudes, that their number has not been ascertained. The calcium
spectrum has one very bright green band in the orange, a red line in the
yellow, and a well-defined yellow line in the indigo. As already
mentioned, the red and orange parts of the strontium are crossed by many
red lines separated by dark intervals; there is a bright blue line
between the orange and yellow, and an orange line in the blue. One
intense crimson band in the orange characterises the lithium spectrum.
Seven broad green bands stripe the yellow and a part of the green, in
the barium spectrum, and that of magnesium has many green bands and
lines.
All of these were determined by the heat of white coal gas flame, which
amounts to 2350° Cent., and at the time MM. Bunsen and Kirchhoff were
not aware that by an increase of temperature new bright lines were added
to some of the spectra. That discovery was made by Professor Tyndall,
while examining the spectrum of chloride of lithium, which with the low
temperature has only one crimson band in the orange, but with the hotter
flame of hydrogen gas, amounting to 3259° Cent., an orange line appeared
in the yellow, and when Mr. Tyndall employed the electric lamp,[18] the
spectrum acquired a broad brilliant blue band between the orange and
yellow, while the crimson band remained unchanged. Professors Roscoe and
Clifton confirmed Tyndall’s discovery, and upon comparing the spectra of
strontium and lithium, they found where only one prism was employed that
the blue line of lithium appeared to coincide with the blue line, delta,
of strontium; but with an apparatus having several prisms like that of
Kirchhoff, they saw that the two blue lines differed by one division of
the measuring scale, the lithium line being the most refrangible. A
great change was produced on the strontium spectrum by increased
electric temperature: three of the red bands vanished, and new bright
lines appeared, that were not coincident with those they replaced; the
blue line was not affected, but four new violet lines were added. With
the intense heat of the electric spark, the broad green band of the
calcium spectrum is replaced by five green lines of less refrangibility,
the well-defined yellow line vanishes, and instead of the red band three
red or orange lines appear, of greater refrangibility than those that
have vanished. Six of the bright green bands in the spectrum of barium
entirely vanish, and bright new non-coincident lines appear. Thus, not
only new lines appear at very high temperature, but the broad bands,
characteristic of the metal or metallic compound at a low temperature of
the flame or a weak spark, totally disappear at the higher temperature.
The new bright lines, which supply the part of the broad bands, are
generally not coincident with any part of the band, sometimes being less
and sometimes being more refrangible. The gentlemen who made these
experiments add, that possibly the cause of the disappearance of the
broad bands and the production of the bright lines may be, that at the
lower temperature of the flame or weak spark, the spectrum observed is
produced by the glowing vapour of some compound, probably the oxide of
the difficultly reducible metal, whereas, at the enormously high
temperature of the intense electric spark, these compounds are split up,
and the true spectrum is obtained, namely, the narrow bright lines. No
such changes take place in the easily reducible metals, potassium,
sodium, or lithium, which remain unaltered by change of temperature. In
these experiments, a bead of the metallic salt on a platinum wire was
placed between the platinum terminals, from which the spark of a
powerful inductive coil could be passed, but in order to have a more
intensely hot spark the coating of a Leyden jar was placed in
communication with the terminals of the secondary current respectively.
By this addition of static electricity, the intensity of the current was
increased four-fold, and must have been beyond estimation.
By high temperature the cæsium spectrum has been so changed, that for
number, colour and distinctness of its lines, it is the most beautiful
of those of the alkaline and earthy metals, for besides its
characteristic blue lines, it has six red and an orange-red line in the
red part of its spectrum, a fine yellow line, and nine green lines, the
last coinciding with Fraunhofer’s E. The thallium spectrum also acquires
more lines when evaporated by electricity, for besides the remarkable
green line in the green, it acquires a faint one in the orange, two of
nearly equal intensity in the green, a third fainter, and a fifth in the
blue.
MM. Plücker and Hittorf, in recent experiments, proved that many
non-metallic bodies, such as nitrogen and sulphur, give two distinctly
different spectra on change of temperature, and that the transition from
one spectrum to the other is sudden. The change is particularly striking
in sulphur, for at the moment the first spectrum attains its maximum
brightness, it disappears, and gives place to the second or high
temperature spectrum, which is one of the richest in brilliant rays
known. When the temperature is lowered the first spectrum reappears.
These changes M. Plücker ascribes to the existence of the elements in
two allotropic conditions. M. Plücker has also found that each metalloid
possesses a peculiar and characteristic spectrum: as hydrogen, which has
three bright lines, all of which are coincident with dark solar lines,
and nitrogen, which exhibits a complicated series of bands.
The experiments of the Rev. Dr. Robinson on a variety of gases and
vapours, inclosed in glass tubes, show that a greater change is produced
by pressure than by heat. At the ordinary atmospheric pressure, the
spectra show a number of bright lines on a coloured ground, the light of
which is, in general, stronger towards the red than the violet end, and
strongest in the green. In some the ground is so bright as to efface all
but the most luminous lines. This is especially the case with hydrogen.
On gradually exhausting the tube in which the vapour is contained, the
spectra rather suddenly fade away, leaving only a suspicion of one or
two lines, but upon exhausting the tube still more, these transition
spectra become bright again, fresh lines appear, and they are changed
into new spectra which are never so bright as those at ordinary
pressure. Fewer lines are visible in the rarefied spectra, and of these
four-tenths are not found in the spectra of atmospheric pressure. The
difference between the common pressure spectra, the transition, and the
rarefied spectra shows, that the character and even the existence of
certain lines depend upon the mere density of the media, the chemical
circumstances remaining unchanged. Dr. Robinson also observed that
spectra are not superposed without a change; the spectrum of atmospheric
air does not always exhibit all the lines of oxygen and nitrogen, and
occasionally there are some lines not visible in either of them. It
appears also that for certain lines the actions of bodies may be
antagonistic.
Metals do not always give the same spectrum, whatever may be the
combinations in which they are found. Among various instances M.
Mitscherlich mentions that the spectra of copper and the chloride and
iodide of copper present essential differences, and Mr. Roscoe has found
that a similar difference prevails in the spectra of carbon compounds
when in a state of incandescent gas, which have hitherto been supposed
to yield the same spectrum. ‘The spectrum obtained from the flame of
olefiant gas is different from that obtained by the electric discharge
through a vacuum of the same gas; while the spark passing through a
cyanogen vacuum produces a spectrum identical with that of the olefiant
gas flame, and through the carbonic oxide vacuum a spectrum coincident
with that of the spark through olefiant gas vacuum.’
The chlorides, bromides, and iodides are the most easily vaporized of
all the metallic salts, and give the most brilliant flames and the most
intense spectra, especially the chlorides. A small piece of the chloride
of barium volatilized by a colourless gas flame tinges the flame green,
and the red and green lines on the spectrum stand out with extreme
brilliancy. The scattered yellow light on the spectrum of the chloride
of sodium is comparatively dark by contrast with the bright lines, and
upon shading off the more luminous part of it, traces of lines are
visible in the more refrangible portion.
Chloride of lithium gives the red and orange lines on its spectrum; the
brilliant blue band discovered by Mr. Tyndall, and another more
refrangible blue line is seen when the ignition is at its greatest
intensity. Chloride of calcium gives a blue band very brightly, and
several other lines. The light of the chloride of copper is very vivid,
and its spectrum is remarkable for changing its appearance with the
decomposition of the chloride. The chlorides of lead and cadmium, also,
give very bright and definite spectra, and chloride of bismuth shows
numerous brilliant red and blue rays which quickly disappear. Thus the
chlorides give spectra with lines, such as the blue lithium and
strontium lines, hitherto only brought out by an intense electric
spark.[19]
M. Bunsen produced a beautiful effect by vaporizing a mixture of equal
parts of the chlorides of sodium, potassium, lithium, calcium,
strontium, and barium, and passing the light through the slit of his
apparatus. For on looking through the telescope the spectrum of each
substance with its characteristic coloured lines in all their brilliancy
came successively into view, and gradually faded away as each substance
was volatilized and driven off. The sequence showed the time required to
vaporize each metal, and by spectrum analysis each metal could be
recognized, although the mixture only contained the 1/1000 part of a
grain of each chloride.
The position, colour, and nature of the bright lines on the spectra of
more than thirty metals have been determined, besides those of the
elementary gases and that of the electric spark. To these M. Louis
Grandeau has added the spectrum of lightning. By a particular
arrangement the light passed at once through the slit in the instrument,
and a glass tube containing nitrogen and the vapour of water. The
general appearance of the lightning spectrum at first recalled that of
the electric spark, but on a closer examination, M. Grandeau noticed in
the spectrum of almost every flash the coincidence of a certain number
of the rays of the lightning spectrum with those of the spectra of
nitrogen and hydrogen. M. Grandeau remarks that this result is not
surprising, since all admit the production of ammonia and nitric acid
under the influence of electrical discharges. Besides the rays of
nitrogen and hydrogen, the lightning spectrum contains the ubiquitous
yellow ray of sodium.
Fraunhofer had noticed a coincidence between the double yellow sodium
line and the double dark line D of the solar spectrum, though he was not
aware to what it was due. This coincidence, observed by M. Kirchhoff
many years afterwards, was fully appreciated by him, and became the
foundation of one of the most brilliant discoveries of modern times.
During a systematic comparison between the spectra of volatilized
substances and the solar spectrum, he discovered a perfect coincidence
between Fraunhofer’s dark lines and all the bright and coloured lines on
the spectra of the volatilized substances, sodium, calcium, magnesium,
chromium, iron, and nickel. To these M. Angström has added aluminium and
manganese, and M. Plücker has very recently found that all the three
bright lines in the hydrogen spectrum are coincident with dark solar
lines, and that none of the potassium lines correspond with any solar
lines.
Drawings have been made of Fraunhofer’s spectrum placed above the
spectra of the principal metals and metallic salts, in which the
coincidence of the bright and dark lines is shown from the line A in the
extreme red to the line G in the indigo, and as the length of an
undulation of the extreme violet light of the solar spectrum is the
17/1,000,000 of an inch, and the length of an undulation of the extreme
red is the 26/1,000,000 of an inch, the length of the undulations of the
intermediate rays can be computed by the undulatory theory of light. The
length of the waves corresponding to Fraunhofer’s seven principal lines
and many of the intermediate ones have been computed, so that when a
bright or coloured line is coincident with any of these, the length of
its waves is at once known. There are other tables of Fraunhofer’s
lines, and the coincident bright ones in which each dark line is marked
by its own number, as the two principal lines in the double line D,
which are expressed by the numbers 1002·8 and 1006·8, and so with the
others; thus the coincidence of the spectra of volatilized substances
with the solar line forms a regular system.
Professor J. P. Cooke, junior, has recently constructed a spectroscope
which shows that the lines of the solar spectrum are as innumerable as
the stars of heaven, that at least ten times as many are distinctly seen
as are given by Kirchhoff in his chart, besides an infinitude of
nebulous bands just on the point of being resolved. Yet even with this
greatly increased power, the coincidences between the bright lines of
the metallic spectra and the dark lines of the solar spectrum remain
perfect. M. Kirchhoff had seen a fine yellow line between the double
lines D of the sodium spectrum. M. Merz of Munich found four additional
lines, but Professor Cooke has discovered that there are in all seven
intermediate lines and a nebulous band. Although the two members of the
sodium line D could be spread so far apart that the 1/2000 part of the
intermediate space could be readily distinguished, yet the coincidence
with the two dark Fraunhofer lines was absolute. The spectroscope ‘shows
that many of the bands of the metallic spectra are broad coloured spaces
crossed themselves by bright lines. This is the case with the orange
band of the strontium spectrum, and with the whole of the calcium and
barium spectra to a remarkable extent.’[20]
As early as the year 1849, M. Foucault discovered that the sun’s light
when shining through the electric light gives black bands on that part
of the spectrum where the electric light alone would have produced
bright bands, so that the black and bright bands could be produced
alternately by admitting or excluding the solar light; whence he
concluded that the electric arc emits the same lines which it absorbs
when they come from another luminous source. M. Angström also observed
that the bright lines on the spectra of volatilized metals could be
reversed by a stronger light shining through their flames. Neither of
these gentlemen was aware of the importance of a discovery which enabled
M. Kirchhoff to apply his delicate and refined analysis of terrestrial
matter to the sun and stars.
He had already determined the coincidence of the double yellow sodium
line with Fraunhofer’s dark line D, but while looking with a prism at a
bright solar beam passing through a yellow sodium flame, he was
surprised to see a strong and well-defined double dark line instead of
the double yellow sodium line which he expected. He obtained the very
same result, more strongly, with Drummond’s lime light, which is
brighter than the flame of any volatilized metal, and as he found that
he could produce the dark and yellow lines alternately, by admitting and
shutting out the brighter light, he concluded that the sodium flame is
subject to the law of exchange, in consequence of which it absorbs rays
of the same refrangibility with those that it emits. In fact, the soda
flame is pervious to all the rays in solar light and Drummond’s flame,
except those of the same refrangibility with its own; these it absorbs
and it may be supposed changes them into heat. Hence M. Kirchhoff came
finally to the conclusion, that the double dark line in the solar
spectrum is the reverse or negative of the double yellow line seen on
the spectrum of the sodium flame.
Quite recently, M. Fizeau has discovered that the spectrum of sodium
burning in air is reversed during the combustion. At first it is black,
with the usual double yellow line; at last, when the light is at its
maximum, the double yellow line becomes black on a continuous spectrum
with all the seven colours.
After M. Kirchhoff had ascertained that the bright lines in the spectra
of calcium, chromium, magnesium, iron and nickel coincide with dark
lines in the solar spectrum, he reversed them by sending Drummond’s
light through their respective flames, thus proving that the coloured
flames of these six metals are subject, like the sodium light, to the
law of exchanges.
M. Kirchhoff infers by analogy that the vapours of all these six metals
exist in the luminous atmosphere of the sun, and that they absorb and
change into heat such rays of the continuous light of the incandescent
solar globe as have the same refrangibility with their own, so that the
corresponding dark rayless lines on the solar spectrum are the reverses
of the bright lines in the spectra which these vapours would give were
it not for the brighter light of the sun shining through his luminous
atmosphere.
The dazzling white light of the incandescent body of the sun containing
rays of all refrangibilities would give a continuous spectrum shaded
with all the seven colours, but for his luminous absorbent atmosphere,
which comes like a veil between him and the earth, and crosses his
spectrum with thousands of dark lines, which are the reverses or
negatives of the bright lines in the spectra of the innumerable vapours
it contains, all of which must doubtless be the gases of substances
existing in the solar mass itself and vaporized by his intense heat.
Every metal, and almost every elementary substance in a state of gaseous
combustion, gives its own peculiar luminous lines to its spectrum, but
no volatilized matter can be proved to exist in the sun’s atmosphere
except such as have bright lines in their spectra coincident with some
of its dark lines.
The bright lines in the spectrum of iron, coincident with the dark lines
of the solar spectrum, are so numerous that many yet remain unknown. M.
Kirchhoff counted seventy in the small space between Fraunhofer’s lines
D and F, in which the coincidence extends even to shade, the deepest
dark lines corresponding to the most brilliant bright ones, and he
computed that the chances are as 1 to the ninth power of 10, that the
coincidence of these seventy lines is not fortuitous, but owing to a
definite cause, whence he concluded that the presence of iron vapour in
the solar atmosphere is proved with as much certainty as can be attained
in any question of natural science.
In a later publication, M. Angström observes that, although the
coincident iron lines between D and F are not so numerous as M.
Kirchhoff affirmed, they are quite sufficient to establish beyond a
doubt the presence of iron in the solar atmosphere. The iron lines are
the most characteristic in the whole solar spectrum, and if a magnifying
power be used, or if the light be refracted through several prisms,
these lines, or at any rate the stronger ones among them, appear to be
perfectly black. M. Angström noticed that on a careful examination of
the solar spectrum, certain lines can be discovered, imbedded in a mass
of fainter ones, which, with increased illumination, seem to withdraw
themselves and disappear, while the first mentioned lines, on the
contrary, only stand out in a stronger relief. These are metallic lines
of high fusion temperature; the most remarkable among them almost
invariably belong to iron.
The substances common on earth that have their vapours in the atmosphere
of the sun, though they have fewer bright lines in their spectra than
that of iron, are quite as characteristic, and quite as distinctly
coincident with their reverses, whether they be single, in groups, or
double, as the sodium line, which is brighter and its reverses darker
than that of any other substance, because volatilized sodium gives out a
greater quantity of light, and consequently absorbs a greater quantity.
M. Angström has added aluminium and manganese to the seven metals whose
vapours M. Kirchhoff has shown to exist in the atmosphere of the sun,
but he thinks it doubtful whether barium, zinc, or copper are solar
metals, for although their brighter lines correspond with distinct dark
solar lines, their weaker lines do not. Strontium is doubtful also, for
one of its strongest bright lines is not coincident with any dark line.
Though both iron and nickel are decidedly solar metals, yet as cobalt is
doubtful, it cannot be presumed that meteorites are of solar origin.
The spectrum of luminous magnesium has many green lines perfectly
coincident with those in the solar spectrum, so there is no doubt of
that metal being a constituent of the sun’s atmosphere. But there are
magnesium rays as well as some of iron of such high refrangibility that
in Mr. Stokes’s long spectrum they are situated ten times as far from H
as the whole length of the visible spectrum from A to H. These highly
refrangible rays only become visible at the exalted temperature of the
electric spark, and as they are not found in the solar spectrum, it is
inferred that the heat of the sun is inferior to that of the electric
spark.[21] Mr. Roscoe observes that this conclusion would only be
legitimate if we knew that these rays of high refrangibility are not
absorbed in passing through the atmosphere.
These are some of the most striking results of the numerous
investigations that have been made since M. Kirchhoff published his
discoveries, for the subject is anything but exhausted.
The intensely vivid light of a magnesium flame is rich in violet and
extra-violet rays, partly due to the incandescent vapour of magnesium,
and partly to the intensely heated magnesia formed by the combustion.
The properties of this light having been examined and compared with
those of the sun by Professors Roscoe and Bunsen, with a view to
photographic purposes, they came to the conclusion that ‘the steady and
equable light evolved by magnesium wire, burning in the air, and the
immense chemical action thus produced, render this source of light
valuable as a simple means of obtaining a given amount of chemical
illumination, and that the combustion of this metal constitutes a
definite and simple source of light for the purpose of photochemical
measurement.’
Bright lines of two different metals sometimes coincide with the same
black line, that is, they appear to have the same reverse as an iron and
a magnesian line, an iron and a nickel line, and some others; but it is
not known whether the coincidence be real or apparent.
M. Kirchhoff has proved that neither gold, silver, tin, lead, antimony,
arsenic, mercury, lithium, cadmium, and some others are constituents of
the sun, because none of their bright lines are coincident with any of
the dark lines of the solar spectrum. This negative discovery does no
less honour to M. Kirchhoff than the proof of so many substances being
common to the earth and sun.
Since all incandescent solid and liquid bodies give a continuous
spectrum which exhibits no dark lines, M. Kirchhoff conceives that the
sun consists of a solid or liquid nucleus, heated to the temperature of
the most dazzling whiteness, and that it is surrounded by a luminous
gaseous atmosphere of somewhat lower temperature, endowed with the law
of exchanges. The spectra of Arcturus, Capella, and many other fixed
stars are crossed by dark lines similar to, and often coincident with,
the dark lines in the solar spectrum; therefore, it may be concluded
that their structure is to a certain extent the same with that of the
sun.
Numerous observations have been made on the spectra of the fixed stars,
both in Britain and on the Continent. In England, Mr. Huggins and
Professor W. A. Miller have published tables of the measures of about
ninety dark lines in the spectrum of Aldebaran, nearly eighty in that of
α Orionis or Betelgeux, and fifteen in that of β Pegasi, with diagrams
of the two first which include the results of a comparison of the
spectra of various terrestrial elements with those of the stars. Thus
coloured lines of sodium, magnesium, calcium, hydrogen, iron, bismuth,
tellurium, antimony and mercury were found to be coincident with some of
the dark lines in the spectrum of Aldebaran, and besides these there are
numerous lines in the spectrum of this star which are probably due to
forms of matter unknown to us. Coloured lines of sodium, magnesium,
calcium, iron and bismuth, coincided with dark lines in the spectrum of
α Orionis; and β Pegasi had a spectrum closely resembling that of α
Orionis, but much fainter.
Between forty and fifty stars were examined, and it was observed that
the solar lines C and F corresponding to hydrogen, which are present in
the spectra of nearly all the stars, are wanting in those of α Orionis
and β Pegasi. With a few exceptions, the terrestrial elements hydrogen,
sodium, magnesium, and iron, which appear to be most widely diffused
through the stars, are precisely those which with the exception of
magnesium are essential to life as it exists upon the earth. Besides,
the elements hydrogen, sodium, and magnesium, represent the ocean, which
is an essential part of a world similar to the earth. Should any planets
revolve round α Orionis and β Pegasi, they probably would have no
hydrogen, consequently, no ocean and no water: therefore, they could not
be inhabited by beings constituted as we are.
Padre Secchi, the Roman astronomer, divides the stars into three types;
the first and most dominant type includes Sirius, α Lyræ, and other
white stars, which invariably contain hydrogen of high temperature, and
are denoted by a black line in their spectra, which coincides with the
solar line F; and there is another band also probably due to hydrogen in
the violet half of the stars visible to the naked eye belonging to this
group. A singular modification of this group, however, occurs in the
stars of the constellation Orion, which so rarely show any deviation
from one type, that, with the exception of α Orionis or Betelgeux, they
may be said to form a family distinguished from all the other stars in
the sky; their spectra are crossed by fine lines, faint in the violet,
with a band more or less visible in F. γ Cassiopeiæ and β Lyræ differ
from the stars of the first type in having a bright band near the solar
line F, instead of a black one.[22]
Padre Secchi’s second type includes α Orionis, α Tauri, Antares, β
Pegasi, &c., which have coloured bands in the red and orange. According
to M. Secchi, the most remarkable star in this section is α Herculis. It
gives a spectrum which has the appearance of columns illuminated on one
side; ‘the stereoscopic effect of the convexity of these bands due to
the shading is so surprising, that it cannot be beheld without
astonishment.’ The spectrum of the star δ^2 Lyræ has a similar
appearance, only instead of convex it has concave bands.
The third type consists of stars whose spectra are crossed by fine
lines, as Arcturus, Capella and our own sun.
The colours of the stars are produced by vapours existing in their
atmospheres, one colour predominating over the others, which are
absorbed by the number of dark lines.
Messrs. Huggins and Miller obtained extraordinary results from the
examination of temporary and periodic stars. Temporary stars suddenly
shine forth with great brilliancy and soon vanish or nearly vanish. A
temporary star which suddenly appeared on the night of May 12, 1866,
when examined with a spectroscope, had two spectra, showing that its
light emanated from two distinct sources. One spectrum, analogous to
that of the sun, was formed by the light of an incandescent solid or
liquid photosphere, which suffered absorption by the vapours of an
envelope cooler than itself. The second spectrum consisted of a few
bright lines, indicating that the light by which it was formed was
emitted by luminous gas: the position of some of the lines denoted
hydrogen; whence the observers believed the phenomena to result from the
burning of hydrogen with some other element, and that the photosphere
was heated to incandescence by the resulting temperature.
The variation in the brightness of periodic stars has by some been
supposed to be due to an opaque body periodically obscuring the light.
Should that body be surrounded by an atmosphere like our planet’s, its
presence would be revealed by the absence or presence of additional
lines of absorption in the spectrum of the star. Now three lines
determined in the spectrum of Betelgeux were no longer found when the
star arrived at its maximum of brightness, indicating it may be the
presence of an atmosphere round the opaque body.
With regard to our own planets, Jupiter has lines in his spectrum which
indicate the existence of an absorptive atmosphere; one band indicates
the presence of vapours similar to those existing in our atmosphere,
another band has no counterpart among the lines of absorption of the
earth’s atmosphere, and tells of some gas which it does not contain.
In the feeble spectrum of Saturn there are lines similar to those in the
spectrum of Jupiter. These lines are less strongly marked in the ansæ of
the rings, and show that the absorptive power of the atmosphere about
the rings is less than that of the atmosphere which surrounds the ball.
M. Jansen has found lines denoting aqueous vapour in the atmospheres of
both Jupiter and Saturn. Some very remarkable lines have been seen in
the more refrangible part of the spectrum of Mars supposed to be
connected with his red colour. Though the spectrum of Venus is
brilliant, and the dark lines distinct, no additional lines indicate the
existence of an atmosphere differing from our own.
The phenomena resulting from an examination of the nebulæ are most
wonderful; their light is very feeble, even that of the brightest. ‘The
total light of the whole nebula in Orion, the largest and brightest of
them, makes so small an impression on the naked eye, that you may look
twenty times at its place and not perceive any nebulous light at
all.’[23] Besides, the brightness of a surface cannot be increased by a
telescope, however good. Notwithstanding difficulties which seem to be
almost insurmountable, Mr. Huggins in England, and Padre Secchi at Rome,
have been, and still are, engaged in these researches.
The planetary nebulæ are beautiful objects; they are like planets with a
round or oval disc, equable, slightly mottled and of enormous magnitude;
one near γ Aquarii is twenty seconds, and another is twelve seconds in
diameter. Sir John Herschel computed that if these objects be as far
from us as the nearest of the fixed stars, their magnitude, on the
lowest estimation, would fill the orbit of Uranus. He discovered
twenty-eight or twenty-nine of them, some of a beautiful blue tint, in
the southern hemisphere; and from the uniformity of the discs in both
hemispheres, and their apparent want of condensation, he presumed that
they may be hollow shells emitting a feeble light from their surfaces
only. The spectrum analysis of that light, by Mr. Huggins, in six of the
planetary nebulæ, showed that their structure is utterly unlike anything
else in creation,[24] for instead of an ordinary spectrum he found, to
his infinite surprise, that the spectra of the feeble light of these
bodies consist only of three bright lines, such as those which proceed
from an intensely heated gas, and that the lines exhibited some of those
of the hydrogen and nitrogen spectra and an unknown gaseous substance:
whence he draws the astounding conclusion, that planetary nebulæ are
probably composed of hydrogen, nitrogen, and some unknown gas, without
any solid nucleus whatever.
The annular nebula in Lyra, which is probably nearest to the earth, and
the dumb-bell nebula, gave a spectrum indicating matter in a gaseous
form. The annular nebula appears to be a hollow elliptical ring of
nebulous matter of enormous magnitude. The interior opening of the ring
is not entirely dark, but filled with a faint hazy light, like fine
gauze stretched over a hoop. The dumb-bell nebula in the constellation
Vulpecula is like an hour-glass of bright matter surrounded by a thin
hazy atmosphere, which gives the whole the form of an oblate spheroid.
Both of these nebulæ when viewed with a very high telescopic power seem
to consist of minute clustering stars, but the spectra of these two
nebulæ have one bright line, the structure of both being of the same
gaseous constitution.
The great nebula on the sword handle of Orion was then examined. The
spectrum of the light from the brightest parts of this nebula, near the
trapezium, was crossed by three bright lines, in all respects similar to
those on the spectra of the planetary and other nebulæ. Other portions
of the great nebula were then brought successively under examination,
but the spectra of the whole of those portions which still were
sufficiently bright for this method of observation remained unchanged,
and exhibited the three bright lines only. The whole of the great
nebula, as far as it lay within the power of Mr. Huggins’ instrument,
emits light which is identical in its characters; the light from one
part differs from the light from another part in intensity alone. The
brighter portions of this nebula have been to a certain extent resolved
into stars, by the powerful telescopes of Lord Rosse and Professor Bond,
of the United States of America; the whole, or the greater part, of the
light from that portion of the nebula must therefore be regarded as the
united radiation of numerous stellar points. The spectrum of this
radiation being crossed by the three bright lines reveals its gaseous
source; Mr. Huggins therefore infers that at least some of these stellar
points are merely denser parts of a gaseous matter, and that the nebulæ
which he examined are enormous gaseous systems.
The spectrum of the great nebula in Orion was subsequently examined by
Padre Secchi. He describes the light of the spectrum as of a uniform
green, crossed by three bright lines; one tolerably wide and perfectly
sharp, a very slender one close to it, and the third at a little
distance from the latter. This spectrum afforded a striking contrast to
the spectra of the small stars in the brighter parts of the nebula. As
soon as the light from one of these stars entered the slit of the
instrument, its continuous spectrum was seen to flash across the field
of vision in a long coloured band. This shows that the mass of matter in
this immense nebula is in a different state from that of the stars
themselves, as Mr. Huggins had already observed. Padre Secchi does not
draw any inference from his observations as to the structure of nebulæ
in general, probably thinking it premature, but he expresses
astonishment at their results.
Since the preceding lines were written, Mr. Huggins and Professor W. A.
Miller have continued their researches on the constitution of the
celestial bodies by a method of direct simultaneous comparison of the
lines in their spectra with the lines in the spectra of many of the
terrestrial elements. The spectra for comparison were obtained from the
spark of the induction coil taken between points of various metals; and
sometimes a platinum wire was used, surrounded with cotton, moistened
with a solution of the substance required. The telescope of the
instrument was mounted equatorially, and followed the star by clockwork.
By this arrangement the spectrum of the star, and the spectrum of the
metal compared with it, are seen in juxtaposition; and the coincidence
or relative position of a dark line in the stellar spectrum with a
bright line in the metallic spectrum can be determined with great
precision.
It was found that Jupiter’s atmosphere has a much greater absorptive
power than the terrestrial atmosphere; that they have some gases or
vapours in common, but that they are not identical.
Some of the lines seen in the atmosphere of Saturn appear to be
identical with those seen in the spectrum of Jupiter.
‘The lines characterizing the atmospheres of Jupiter and Saturn are not
present in the spectrum of Mars. Groups of lines appear in the blue
portion of the spectrum; and these, by causing the predominance of the
red rays, may be the cause of the red colour which distinguishes the
light of this planet.’[25]
All the stronger lines of the solar spectrum were seen in the brilliant
light of Venus; but no additional lines indicating an absorptive action
of the planet’s atmosphere.
The authors are of the opinion that in most of the planets the light is
probably reflected from clouds floating at some distance from the
surface, so that it is not subject to the strong absorptive action of
the lower and denser strata of the planet’s atmosphere, which, like our
own, are most effective in producing atmospheric lines.
The results of the observations on the fixed stars are exceedingly
interesting, for they show that their elementary constituents are
similar, but not identical; and that although they contain many of the
sixty-five terrestrial elements, there are probably new unknown
substances also.
When seventy dark lines on the spectrum of the star Aldebaran, and
eighty on that of α Orionis (Betelgeux) were compared with the bright
lines on the spectra of the vapours of a variety of the terrestrial
simple elements, it was found that Aldebaran contained nine terrestrial
substances and α Orionis five: that is, there were only nine out of
seventy of the dark lines of Aldebaran coincident with bright lines, and
five out of eighty of those of α Orionis. Yet the seventy and eighty
dark lines that were compared represented some of the strongest only of
the numerous lines which were seen on the spectra of these stars. Some
of those remaining were probably due to the vapours of other terrestrial
elements which were not compared with these stars, but Mr. Huggins
concludes that many of those dark lines are due to new unknown elements
existing in these stars, and that we cannot assume that the sixty-five
simple terrestrial elements constitute the entire primary material of
the universe. A community of matter, however, exists throughout the
visible creation; for the stars contain many of the elements common to
the sun and earth. ‘It is remarkable that the elements most widely
diffused through the host of stars are some of those most closely
connected with the living organism of our globe, including hydrogen,
sodium, magnesium and iron. May it not be that, at least, the brighter
stars are like our sun, the upholding and energizing centres of systems
of worlds adapted to the abode of living beings?’
With regard to the nebulæ Mr. Huggins’s observations show that nine are
gaseous, the spectra of six exhibiting three bright lines, one shows an
additional faint line also, while the spectra of the dumb-bell nebula
and the annular nebula in Lyra show the brightness of three green lines
only. The spectra of eight other nebulæ were continuous, showing that
their light has not undergone any modification on its way to us.
Mr. Huggins has been able to discriminate between the light of the
nucleus of a comet and that of its tail. The nucleus is self-luminous,
and its substance is in the form of ignited gas. The coma shines by
reflected light as clouds do, and observations of the spectra give
reason to believe that comets chiefly consist of nitrogen and another
elementary body different from nitrogen combined with it.
The terrestrial elements found in the fixed stars show that, like the
sun, they have an intensely luminous nucleus: but if it be taken for
granted that highly heated gases are non-luminous internally, the
planetary nebula and the great nebula in Orion itself being thus
considered to be gaseous, must emit their feeble light from their
surfaces alone. All the true clusters of stars which are resolved by the
telescope into distinct bright points of light, give a spectrum which
does not consist of separate bright lines, but is apparently continuous
in its light. The great nebula in Andromeda, which is visible to the
naked eye, has an apparently continuous spectrum, but the whole of the
red and orange part is wanting, and the brighter parts have a mottled
appearance. The easily resolvable cluster in Hercules has a similar
spectrum; Lord Rosse discovered dark streaks or lines in both.
There is a striking correspondence between the results of prismatic and
telescopic observations; half of the nebulæ which have a continuous
spectrum have been resolved into stars, while none of the gaseous nebulæ
have been resolved even by Lord Rosse’s telescope. Thus it appears
probable that primordial nebulous matter does exist, according to the
theories of Sir William Herschel and La Place.
The structure of the sun himself, which forms one amidst the multitude
of stars which constitute the Milky Way; and the maintenance of his
light and heat without apparent waste, are still in various respects
involved in mystery.
The luminous gaseous atmosphere of the sun is of great extent and of
lower temperature, at least in its upper regions, than the photosphere
on which it rests. Mr. De la Rue’s photographs of the sun show that the
light from the border of the solar disc is less intense than that from
the equator, on account of the greater depth of solar atmosphere it has
to pass through before it reaches the earth, by which a larger portion
of the light is absorbed.
The photosphere of the sun has a mottled appearance, exhibiting minute
masses, which must be of enormous magnitude to be visible at such a
distance. They have been examined with a very high telescopic power by
Mr. Nasmyth, who describes them as lens-shaped bodies of wonderful
uniformity, and likens them to willow leaves crossing each other in all
directions, and moving irregularly among themselves. Mr. De la Rue and
Padre Secchi say they have seen something similar, and others liken them
to rice grains. Sir John Herschel[26] is of opinion that they consist of
incandescent matter sustained at a level corresponding to their density
in the solar atmosphere, an atmosphere which he considers as varying
from a liquid state below to the highest tenuity of a rarefied gas
above. In a memoir read at the Institute of Paris,[27] by M. Faye,
something of the same kind is suggested.
There are comparatively brighter waves of the sun’s disc, called faculæ,
which are portions of the sun’s photosphere thrown up into the higher
regions of his atmosphere; for Mr. De la Rue took a stereoscopic
impression of a solar spot and some faculæ, in which the spot appeared
to be a hollow and the faculæ elevated ridges. Being elevated above the
photosphere, their light is less absorbed by the sun’s atmosphere, and
by contrast they are brighter at the less luminous border of the solar
disc than at the equator.
It appears that the red flames and protuberances seen round the edge of
the sun during a total eclipse are gaseous or vaporous luminous bodies
which certainly belong to the sun; for during the total eclipse in 1860
it was observed, that as the moon moved over the sun’s disc, the red
flames and part of the corona discovered themselves at the side which
she had left, and were covered by her disc at the side towards which she
was approaching. Besides, the illuminating effect of the red light of
these flames is so inferior to its photographic power, that Mr. De la
Rue photographed one of the protuberances, although it was invisible to
the naked eye.
The sun spots which are situated in that region of the sun which lies
below the photosphere consist of a central darkness or umbra, surrounded
by a penumbra which is less dark. Professor Wilson, of Glasgow, proved
that the spots are cavities, of which the umbra or darkest part forms
the bottom, and the penumbra the sloping sides, by observing that the
umbra encroaches on that side of the penumbra which is next to the
visual centre of the sun. Hence the umbra of a spot is at a lower level
than the penumbra; and since luminous ridges and sometimes detached
portions of luminous matter cross over the spots, it is concluded that
the whole phenomenon is below the surface. The spots have an apparent
motion from east to west, due to the rotation of the sun; and Mr.
Carrington discovered that they have a proper motion also from east to
west, those nearest the solar equator moving fastest. They are confined
to the equatorial regions.
No reason has yet been assigned for the periodicity of the spots, which
go through a cycle of maxima and minima every ten years nearly. They are
singularly connected with terrestrial magnetism; the maximum of the
spots coincides with the period of the greatest disturbance of
terrestrial magnetism. The spots seem to be influenced by the planet
Venus in such a manner that when a spot comes round by rotation to the
ecliptical neighbourhood of this planet, it has a tendency to dissolve;
and, on the other hand, as the sun’s surface recedes from the planet it
has a tendency to break out into spots.[28]
PART II.
VEGETABLE ORGANISMS.
SECTION I.
MICROSCOPIC STRUCTURE OF THE VEGETABLE WORLD.
THE STUDY of the indefinitely small in the vegetable and animal
creation, is as interesting as the relation between the powers of nature
and the particles of matter.
The intimate organic structure of the vegetable world consists of a
great variety of different textures indeterminable by the naked eye, and
for the most part requiring a very high magnifying power to
discriminate. But ultimate analysis has shown that vegetables are
chemical combinations of a few very simple substances. Carbon and the
three elementary gases constitute the bases of all. No part contains
fewer than three of these universal elements, hence the great uniformity
observed in the chemical structure of vegetables. The elements unite
according to the same laws within the living plant as in the inorganic
creation, and the chemical laws acting upon them are the same. For, as
already mentioned, M. Berthelot having combined carbon and hydrogen into
acetylene, which no plant is capable of doing, he assumed it as a base
from which he deduced, by the common laws of synthetic chemistry,
hundreds of substances precisely similar to those produced by
vegetables. Although it may be inferred from this that chemical action
is the same within the vegetable as it is in the inorganic world, yet it
is accomplished within the plant under the control of the occult
principle of plant-life. No mere physical powers are capable of forming
directly out of inorganic elements, the living organism whose passage
through the cycle of germination, growth, reproduction and decay, serves
so pre-eminently to distinguish between it and inert matter. Plants,
indeed, borrow materials from the inorganic, and powers from the
physical world, to mould them into living structures, but both are
returned at death to the great storehouse of nature.
All other circumstances being the same, the vigour and richness of
vegetation are proportionate to the quantity of light and heat received.
The functions of light and heat are different, but their combined and
continued action is indispensable for the perfect development of
vegetation. Light enables plants to decompose, change into living
matter, and consolidate, the inorganic elements of carbonic acid gas,
water, and ammonia, which are absorbed by the leaves and roots, from the
atmosphere and the earth; the quantity of carbon consolidated being
exactly in proportion to the intensity of the illumination, which
accounts for the darker green tint of the tropical forests. Light acting
in its chemical character is a deoxidizing principle, by which the
numerous neutral compounds common to vegetables are formed. It is the
principal agent in preparing the food of plants, and in all the
combinations and decompositions the law of definite quantitative
proportions is maintained. It is during these chemical changes that the
specific heat of plants is slowly evolved, which, though generally
feeble, is sometimes very sensible, especially when the flowers and
fruit are forming, on account of the increase of chemical energy at that
time. To the same cause, the phosphorescence of certain flowering plants
and a few fungi, is supposed to be due.
The action of heat is manifested through the whole course of vegetable
life, but its manifestations take various forms suited to the period and
circumstances of growth. Upon it depends the formation of protein and
nitrogenous substances, which abound in the seeds, buds, the points of
the roots, and all those organs of plants which are either in a state of
activity, or are destined to future development. The heat received,
acting throughout the entire organism of a plant, may augment its
structure to an indefinite extent, and thus supply new instruments for
the chemical agency of light, and the production of new organic
compounds. The whole energy of vegetable life is manifested in this
production, and, in effecting it, each organ is not only drawing
materials, but power, from the universe around it. The organizing power
of plants bears a relation of equivalence to the light and heat which
act upon them. The same annual plant from germination to the maturation
of its seed receives about the same amount of light and heat, whatever
be the latitude, its rate of growth being in an inverse ratio to the
amount it receives in any given time. For one of the same species, the
more rapid the growth, the shorter the life.
The living medium which possesses the marvellous property of being
roused into energy by the action of light and heat, and which either
forms the whole or the greatest part of every plant, is in its simplest
form a minute globe consisting of two colourless transparent concentric
cells in the closest contact, yet differing essentially in character and
properties. The external one, which is the strongest, is formed of one
or more concentric globular layers of cellulose, a substance nearly
allied to starch, being a chemical compound of carbon, hydrogen, and
oxygen in the proportions of 12, 10, and 10, respectively.[29] It forms
the universal framework or skeleton of the vegetable world, but it has
no share whatever in the vital functions of vegetation. It only serves
as a protection to the globular cell within it, which is called the
primordial cell because it is first formed, and because it pre-eminently
constitutes the living part, since the whole phenomena of growth and
reproduction depend upon it. In its earliest stage the primordial cell
is a globular mass of an azotized colourless organizable liquid, called
protoplasm, the life blood of vegetation, containing albuminous matter
and dextrine or starch-gum. It is sufficiently viscid to maintain its
globular form, but its surface becomes slightly consolidated into a
delicate soft film. The viscid albuminous liquid within it is mixed with
highly coloured semi-transparent particles containing starch; besides
cavities or vacuoles full of a watery vegetable sap of highly refractive
power are imbedded in it. By degrees the coloured particles become more
and more condensed within a globule of mucus, which constitutes the
nucleus of the primordial cell. The watery sap in the cavities increases
so much as ultimately to fill nearly the whole of the cell at the
expense of the viscid protoplasm, which then merely forms a lining to
the cell either coloured or hyaline. The primordial cell then secretes
and envelopes itself with the strong protecting coats of cellulose
already described. On account of its high colour, which is chiefly
green, the whole contents of the primordial cell are called the
endochrome. The minute globular nucleus contains a liquid of high
refractive power, and is coated with a delicate film. Its structure,
which is best seen in the hairs and young parts of plants, is not always
the same, nor is it always in the centre of the primordial cell, being
sometimes attached to the internal cell wall.[30] On the minute but
complicated organ, the primordial cell, vegetable life depends.
It will be shown afterwards that the primordial cell sometimes
constitutes the whole plant, with or without its cellular coat. By its
continual bisection when so coated, linear plants, such as the confervæ,
are formed and lengthened (fig. 3). When bisection is about to take
place, the cell increases in length; the nucleus, which always plays an
important part in cell formation, spontaneously divides into halves; at
the same time the cell wall becomes constricted in the middle and
gradually folds between them, and divides the original cell into two new
ones, in which the nuclei become perfect and assume their normal
position. The terminal cell may undergo the same process, so that the
plant may be lengthened indefinitely.
[Illustration: Fig. 3. Development of Ulva:—A, isolated cells; B, C,
clustered subdivided cells; D, E, confervoid filaments; F, G, frond-like
expansions.]
Plants which spread in two directions are formed and increased by the
successive division of the cells into four equal parts, as in some of
the fuci, and the solid vegetable mass is formed and augmented upon the
same principle, so that it consists of a congeries of primordial cells
or globules coated with cellulose, which by mutual pressure take a
many-sided cellular form. Six or eight sides are most common; when
six-sided, a section of the solid is like honeycomb, but it frequently
resembles a very irregular fine lace or network. The form of the cells,
which not only depends upon the number of sides but on the direction of
the pressure, varies exceedingly in different plants, and in different
parts of the same plant. The size of the cells averages from the three
to the five hundredth part of an inch in diameter. Some are very large,
as in the pulp of the orange and lemon; but in the pollen of flowering
plants, and other cases, they are not more than the thousandth part of
an inch in diameter, consequently invisible to the naked eye.
Occasionally the cells are elongated in the direction of least pressure,
as in the stems and hairs of plants, or sometimes they have a stellar
form. In the looser and fleshy parts they retain their globular form and
only touch one another, leaving triangular spaces between them filled
with air in water plants; but in general the cells are held together by
a viscid liquid. When these intercellular spaces, whether left by
globular or polyangular cells, are placed the one over the other for
some distance, they constitute intercellular passages or channels, and
sometimes they form lacunæ or large empty spaces.
Notwithstanding its great variety of forms, this solid congeries of
cells, called cellular tissue, is the universal basis of vegetable
structure; it forms the principal part of all plants, and the entire
mass of many. Though often highly coloured, as in flowers, green leaves
and young shoots, it is frequently hyaline and colourless. The dark
cells in fig. 4 represent the green part of a leaf, the white ones are
those of the colourless skin. Since the primordial cell is the medium in
which light and heat act, cellular tissue is present wherever growth is
in progress, for all the vital operations take place within its cells.
All the organs of plants in their earliest stage consist entirely of
cellular tissue, and even in full grown trees the bark and pith of the
stem, as well as the soft parts of leaves and flowers, are generally
composed of the cells of this tissue, which though assuming a great
variety of forms never deviates far from the original type. Every
important change in the structure of the cell diminishes or destroys its
power of contributing to the nourishment of the plant, as appears in all
the tissues derived from it, and which, according to M. von Mohl, is a
necessary consequence of the disappearance of the vital part of the
primordial cell from those parts of the cellular tissue destined to
undergo the change.
[Illustration: Fig. 4. Vertical section of the cuticle of Iris
germanica:—_a_, cells of the cuticle; _b_, cells at the sides of the
stomata; _c_, small green cells placed within these; _d_, openings of
the stomata; _e_, lacunæ of the parenchyma; _f_, cells of the
parenchyma.]
The fibro-vascular bundles which constitute the wood of trees, and form
consecutive cylinders round the stem between the pith and the bark,
consist of vascular ducts and woody fibre. The vascular ducts are formed
of large wide cells each standing upon the other’s flattened end, their
cavities being thus separated from each other by septa or partitions
directed at right angles to their longitudinal axis. This vascular
tissue when young conveys the sap from the roots through the stem and
branches to the leaves. It forms part of the stems of all climbing and
quick-growing plants, in which the circulation of the sap is rapid, and
the perspiration great. The sap in this crude state passes freely
through the partitions, being probably a dialysable liquid; but in the
autumn, when the sap ceases to rise, the septa are either absorbed or
destroyed, as appears from the fragments of them that sometimes remain,
and then the ducts become filled with air, which they convey to mature
the sap in the leaves and all parts of the plant. Some of the vascular
ducts have very narrow parallel fibres of a bluish colour twisted in a
more or less elastic spiral from end to end of their internal surface,
which in by far the greater number of cases turns in the same direction
as a left-handed screw. In some ducts they merely cross the inner wall
of the cell at regular distances as circles. The reticulated form from
the crossing of right and left-handed spirals is still more frequent
than the simple spiral; there is scarcely a plant, from the mosses
upwards, in which that structure cannot be found.
In a vast majority of cases the secondary internal membrane of some of
the ducts is perforated by orifices of numerous forms, sometimes
irregularly, and sometimes in a regular pattern like a sieve, and on
that account they are called the pitted tissue.
In the stem of a tree the vascular cylinders alternate with cylinders of
woody fibre, as may be seen in a section perpendicular to the axis, in
which the two tissues form a series of alternate rings. The woody or
ligneous tissue, which gives strength and solidity to all vegetable
structures, consists of bundles of nearly parallel spindle-shaped
tubular fibres, having their attenuated extremities applied end to end
to the extremities of those above and below them, so that they form
groups of nearly straight lines; but although the ends of these tubular
fibres overlap each other they do not prevent a free circulation of the
sap. The different layers of these combined tissues which form the wood
do not convey the rising sap in equal quantity. The outermost layers
that are nearest the bark, which are always the last formed or youngest,
convey it in greatest quantity, and on that account are called the
sap-wood: the older the layers the less they convey, because the
interior walls of the cells of both tissues are coated with successive
layers of a mucilaginous substance which is the colouring matter of the
wood (lignin), and is called sclerogen, which becomes hard, is
ultimately united to the cell walls, and fills or nearly fills the
tubular fibres and vascular ducts, so that those nearest the centre of
the tree lose or nearly lose the power of conducting the sap, as in hard
wood like the oak, though in softer wood, as the lime tree, it is not
entirely lost. Ligneous tissue forms the chief part of the stems,
branches, and shoots of trees and shrubs; it gives firmness to leaves,
flowers, and all their parts, and strength to the stems and skins of
herbaceous plants; it is found in the bark of all trees, and constitutes
the strong fibre of hemp, flax, the agave, and many other plants, whence
linen, canvas, and cordage are made. Cells lined with sclerogen form the
shells of nuts, cocoa-nuts, and walnuts, as well as cherry, peach, and
plum stones, the brown coat of apple and pear seeds, the gritty
particles in the heart of the pear, the white coats of the pips of the
orange and lemon, the husks of peas, &c.
All the tissues are represented in fig. 5, which is a longitudinal
section of the Italian reed, much magnified. It consists of three parts:
at _a_ the cellular tissue of the pith is represented; _b_ is a
fibro-vascular bundle containing annular ducts (1), spiral ducts (2), a
pitted duct (3), besides the long spindle-shaped threads of woody fibre;
_c_ is the exterior part of the reed, which consists of cellular tissue,
the two surface rows being rather compressed and filled with coloured
particles.
Besides the spiral vessels that are attached to the interior walls of
the vascular ducts, there are groups of independent spiral vessels of
great beauty and elasticity, of which the seeds of the wild clary afford
a remarkable instance. They consist of cylindrical tubes with conical
extremities twisted into a right or left-handed screw, which can be
unrolled without breaking. They are found in the leaves of almost all
plants, in the petals and stamens of flowers, in the stalks of all
fruits, even in the minutest seeds; large parallel bundles of them
imbedded in hexagonal cellular tissue may be seen in the veins of the
kernel of the hazel nut, and they constitute the medullary sheath which
surrounds the pith in trees. They are all hollow, and capable of
conducting liquids.
[Illustration: Fig. 5. Longitudinal section of stem of Italian
reed:—_a_, pith; _b_, fibro-vascular bundles; _c_, cuticle.]
The laticiferous vessels or vasa propria, those which contain the proper
juices of plants whether milky or coloured, are exceedingly varied in
their forms and arrangement in different plants, and in different parts
of the same plant. In the leaves they generally form a delicate
capillary network, in the bark they constitute a system of long vascular
ducts forming an elongated irregular network pervious to the proper
juices throughout; sometimes they are formed of cells joined end to end,
and frequently they are thin branching flexuous tubes, meandering
through the passages or interstices of the cellular tissue, and
occasionally filling the lacunæ.[31]
Every one of the preceding tissues may be found in many of the highest
class of vegetables—those which are distinguished by having seeds with
two lobes or seed leaves, such as our common trees, shrubs, and most of
the herbaceous plants. Palms, the cereals, grasses, canes, and all
plants having seeds with but one lobe, which form the second class,
consist of cellular tissue mixed with fibro-vascular bundles; whilst in
the third or flowerless spore-bearing class, there is a general tendency
to a more and more simple structure, from the tree fern to the lichens
and algæ, which last consist of cellular tissue alone, and contain the
lowest germs of vegetable life.
In seeds the miniature plant is enclosed between the two lobes, as in
peas and beans, or in a cavity of a lobe, as in a grain of wheat or
barley; and all the parts of the embryo are merely developed into the
perfect plant during the progress of vegetation. A spore, on the
contrary, which is the seed of a Cryptogam, or flowerless class of
plants, is a most minute globular cell, full of granular matter, in
which no embryo has yet been discovered, so that the parts of the future
plant are supposed to be formed during the progress of vegetation,
instead of being developed. Seeds, and spores also, sometimes produce
new varieties, while buds and offsets only transmit the parent plant,
with all its peculiarities. In the higher classes, the organs of
nutrition and reproduction are always separate; in the lowest grades of
vegetable life they are often the same. Seeds bear no proportion to
spores either in size or number; the latter are often so extremely small
that they are invisible to the unaided eye, and are not to be counted
even by thousands. It appears that beings, whether animal or vegetable,
are prolific in the inverse ratio of their size. The incredible
multitudes of the lowest grades of vegetable life, the rapidity of their
growth, the shortness of their existence, and their enormous
fruitfulness, make them powerful agents in preparing soil for the higher
classes which are nourished by their decay. But no sooner do even the
monarchs of the forest fall than the work of destruction begins; the
light and heat which in their chemical form brought them to maturity,
now in their physical character accelerate their decay; the moss and the
lichen resume their empire, and live at the expense of the dying and the
dead, a cycle which perpetuates the green mantle of the earth.
Notwithstanding the important part these inferior beings perform in the
economy of nature, they were imperfectly known till they became a test
for the power of the microscope. Then indeed not only were the most
wonderful organisms discovered in the ostensible tribes of the
Cryptogamia, but a new and unseen creation was brought under mortal eye,
so varied, astonishing, and inexhaustible, that no limit can be assigned
to it. This invisible creation teems in the earth, in the air, and in
the waters, innumerable as the sand on the seashore. These beings have a
beauty of their own, and are adorned and finished with as much care as
the creatures of a higher order. The deeper the research, the more does
the inexpressible perfection of God’s works appear, whether in the
majesty of the heavens, or in the infinitesimal beings on the earth.
SECTION II.
ALGÆ.
THE principal objects in the study of plant-life are the organs by means
of which they obtain and assimilate substances that are essential for
their nourishment and growth, and those by which the perpetuity of their
race is maintained and their type transmitted from age to age. In the
lowest group of plants, represented by the Algæ, which come first into
consideration, the two properties are combined; in the highest they are
distinctly different, but the progress from one to the other may be
traced through an ascending series of vegetable structure. In the simple
grades of vegetables, the primordial cell frequently constitutes the
whole plant; it appears first, and then envelopes itself with a coat
either of cellulose or of a gelatinous substance.
Many instances of this are to be found amongst the Algæ, which are all
aquatic plants, and are found growing either attached to other bodies,
or floating independently, and live, some species in fresh water, and
others in the sea and its estuaries. The Algæ absorb carbonic acid and
give out oxygen, under the influence of sun-light, exactly as do the
flowering plants; and the quantity of oxygen disengaged by them is said
to be enormous.
Before proceeding to trace the structure and development of the Algæ, it
may be desirable to indicate something of the classification of this
curious group of plants. As already stated, they are without exception
aquatic plants. They comprise three distinct orders, the Chlorospermeæ,
having green spores; the Rhodospermeæ, having red spores; and the
Melanospermeæ, having olive-coloured spores. These groups embrace all
the varied plants known as sea-weeds, as well as the cellular plants
which are developed in fresh water.
The Chlorospermeæ are separable into three groups, namely, those which
are simply cellular, including the Palmelleæ, the green Desmidiaceæ, and
the yellow-brown silicious-coated Diatomaceæ; those which are
filamentous, called generally confervas, and including the true
Confervaceæ, in which the threads have no compound axis, the
Batrachospermeæ, in which the threads are partially incorporated with an
axis, the Nostochineæ, in which the slender moniliform threads are
invested with a mucous or gelatinous mass, the Oscillatoriæ, and some
others; and those which are foliaceous, comprising the Ulvaceæ. All
these are monœcious plants, whose reproductive bodies are zoospores
provided with ciliary appendages, or motionless cysts filled with
endochrome, true spermatozoids being rarely present.
The Rhodospermeæ divide primarily into two groups defined by the nature
and position of their spores: one having the spores indefinite, produced
within mother cells; the other having the spores single in the upper
joints of the threads of the nucleus. The first group includes the
Ceramiaceæ, which are filiform articulate plants, with the nucleus
naked, and the Rhodymeniaceæ, which are compound inarticulate plants,
with the spores generated within the cells of moniliform threads. The
second group includes, amongst others, the Rhodomeliaceæ and the
Laurenciaceæ, the former articulate, the latter inarticulate, and both
bearing terminal spores, and having the nucleus conceptacular. To this
group also belong the calcareous Corallinaceæ and the cartilaginous or
membranaceous Sphærococcoideæ. The plants of this group are diœcious,
with two kinds of fruit, spores and tetraspores, and they bear
antheridia filled with active spermatozoids.
The Melanospermeæ divide into two series, the articulate and
inarticulate. The former comprise the Ectocarpeæ, which are filiform
plants with external cysts, and the Chordariæ, which are interlaced
cylindrical plants with immersed cysts. The latter include the
Laminariæ, flat, often strap-shaped, sometimes gigantic plants, having
the spores superficial and indefinite, and the Fucaceæ, which constitute
a large proportion of the shore-weeds of our seas and estuaries, and
which bear their spores in elliptic or spherical conceptacles sunk in
the frond. The Melanospermeæ are either monœcious or diœcious, and
spermatozoids are general amongst them, though occasionally propagation
is effected by means of zoospores resembling the spermatozoids.
Having thus indicated the several groups of the great Algal family,
their structure and development will now be traced, commencing with the
most simple forms, which occur among the CHLOROSPERMEÆ.
Spring water absorbs oxygen, nitrogen, and a large proportion of
carbonic acid gas from the earth and the atmosphere, without losing its
limpidity, but notwithstanding this apparent purity, if exposed for a
time to the sun, green slime appears, and this the microscope shows to
be full of globules or vesicles filled with green matter—the primordial
cell in its earliest form. No green slime is formed in spring water if
kept in darkness, so solar light is the principal agent in this growth,
which is by no means a spontaneous birth; it is merely the development
of one or more of the many kinds of germs, invisible to the naked eye,
that exist in the earth, air, and water in myriads, waiting till
favourable circumstances enable them to germinate.
[Illustration: Fig. 6. Palmoglœa macrococca:—A, full grown cell; B-E,
successive stages of binary division; F, row of cells produced by a
succession of subdivisions; G-I, cells treated by iodine; K-M, cells in
conjugation.]
The slime that covers damp walls or stones, and moist cliffs or rocks in
the sea, also the slime or mucus that sometimes swims on the surface of
water, are said by M. Bory de St. Vincent to be provisional creations
waiting to be organized. Of this the conferva, Palmoglœa macrococca
(fig. 6), is an example. It is a green slime covering damp places,
consisting of microscopic primordial cells, each of which is surrounded
by a gelatinous envelope, and filled with green granular matter
occasionally concentrated into a nucleus. This singular plant is
propagated in two different ways. The endochrome or green matter within
the cell spontaneously divides into two equal parts, the thin coat of
the cell bends round the two ends, separates them, then each half takes
a globular or ovoid form, and secretes a gelatinous substance round
itself which completes the separation, so that they form two distinct
and independent plants, in every respect similar to that from whence
they were derived. After a little time, each of these plants undergoes a
similar bisection, so that four new plants are formed with their
gelatinous envelopes; by the same process eight are produced and so on
indefinitely, the organ of nutrition being the same with that of
reproduction. Again, the membrane or film that covers each of these
primordial cells is so thin and soft, that occasionally two adjacent
cells of the series unite into one mass by a fusion of their sides and
internal matter, which is then coated by a membrane, and after various
internal changes becomes a spore which terminates a generation. By and
by the spore germinates, produces a green primordial cell which secretes
a gelatinous coat, and becomes by the process of bisection the parent of
a new generation, which terminates by the union of two adjacent cells to
produce a spore, a cycle of alternate modes of reproduction that may be
continued till ended by some external circumstance, as the cold of
winter.
When the matter in two adjacent cells joins to form a spore, it becomes
granular, and mixed with minute particles of oil, which unite in a drop;
and the spore, which is at first green, gradually assumes a yellow brown
colour; conversely when the spore begins to grow, the oil disappears,
and the green matter takes its place. This is a frequent occurrence
during the formation of spores in this class of plants, for the
endochrome or internal matter,—which consists of a small variety of
elements probably in a state of unstable equilibrium or change,—is
easily decomposed and recombined into new substances by chemical action,
but the bisection of the cells of the Palmoglœa so as to form new
individuals is probably owing to heat alone. There is no apparent
difference between the cells selected to produce spores by their union,
and the others. It seems that in every plant certain cells are reserved
for certain purposes. Professor Karsten conceives the nucleated cells to
be reserved for reproduction, while those destitute of nuclei are
designed for secretion.
The Protococcus pluvialis (fig. 7), one of the unicellular Confervæ, is
frequently met with in rain-water cisterns. The spore of the plant (fig.
7 A) is a globular primordial cell invested with a double coat of
cellulose, sometimes separated by an aqueous fluid, sometimes not. The
cell is filled with protoplasm, a colourless watery liquid in which red
and green particles are scattered. When this spore begins to grow, the
endochrome, or solid matter in the primordial cell, divides
spontaneously into two similar and equal parts, round at one end, and
tapering to a point or beak at the other, each being coated by a very
thin film of the transparent colourless protoplasm.
[Illustration: Fig. 7. Protococcus pluvialis:—A, encysted cell; B, C,
cells divided into two; D, cell divided into four; E, cell divided into
eight; F, cell divided into thirty-two; G, escaped motile gonidia; H-L,
primordial utricles furnished with cilia.]
After various changes, the green matter with several red spots is
condensed into the thick round half, while the tapering beaked part is
left transparent, being only filled with the watery liquid. Both bodies
are then coated with cellulose, and two vibratile filaments called
cilia, from their resemblance to eye-lashes, proceed from a point near
the beak. The whole of these changes take place while the two bodies are
still within the common cellulose covering; the moment they come out of
it, by a rupture in the cell-wall, they swim about with the greatest
velocity by means of their cilia, which lash the water so rapidly that
they are invisible even with a microscope. The activity of these
zoospores, as they are called, continues for about an hour and a half;
the motion then becomes gradually less rapid; the cilia may now be seen,
and soon fall off; then the bodies acquire a firmer coat of cellulose,
and sink to the bottom of the water, where they remain at rest as still,
or winter spores. There is great variety in the Protococcus, for the
matter in the primordial cell sometimes divides not only into two equal
and similar parts, but into 4, 8, 16, 32 equal and similar parts
consecutively; each brood is developed into zoospores, which ultimately
become resting spores.
When a spore is to be formed in a primordial cell, the starch and green
matter condense into a nucleus in its centre, and a membrane envelopes
the liquid and the nucleus within it, so that a spore in its first stage
is a free and independent cell containing azotized matter swimming in a
formative liquid. If the spore is to be motile it remains of a green
colour, and gets cilia; but if it is to be a winter spore, the internal
matter forms into granules, mixed with particles of red oil, which
coalesce into a drop, and it generally undergoes the same
transformations as those which take place after the conjugation or union
of two adjacent cells into one as already described. The zoospores may
lose their cilia, fall to the bottom of the water as green spores, and
reproduce a facsimile of the parent plant as buds do, or they may
acquire a cellulose coat, undergo the transformations and change of
colour mentioned, and sink to the bottom of the water as red winter
spores.
Under certain circumstances which do not seem to be perfectly known, it
happens that during the formation of some of the zoospores the green
matter is gradually changed into a red oily substance; they lose their
cilia, acquire by secretion their cell-walls and a mucous envelope, and
float on the water as winter spores. Should they be left dry, they may
remain in that state for an indefinite length of time without losing
their vitality, and as they are extremely small, they are carried by
currents of air into the atmosphere, from whence they are brought down
in the rain, and having fallen occasionally in places where they were
never seen before, have given rise to the idea of spontaneous
generation.
Many cycles may be accomplished from the still cell to the zoospores,
and back again, producing numerous generations from the same plant
before it returns to the red thick-walled cell, which may again be
dormant for an unlimited time. These cycles, however, do not finish the
history of the plant, for there can be little doubt that, in some stage
of its existence, a conjugation of two cells occurs, as in the
Palmoglœa.
Sometimes when the division of the endochrome of the spore of the
Protococcus is successively divided into sixteen parts, or even sooner,
the new cells thus produced get two long cilia, as in fig. 7 H, and are
liberated before they acquire their cellulose coat. This motile
primordial cell soon acquires a bag-like investment (fig. 7 I, K, L,) of
cellulose, through which the cilia pass, and thread-like extensions of
the protoplasm are not unfrequently seen to radiate from the primordial
cell to the surrounding bag, as in fig. 7 I, showing that the
transparent space is only occupied by a watery liquid. The varieties of
this plant are very numerous, and all related to one another. Sometimes
the whole of the matter within the primordial cell of the spore divides
at once into 4, 8, or 16 parts, giving rise to as many minute primordial
cells.
The cilia are extensions of the colourless transparent film which covers
the zoospores, and their vibrations are generally believed to be a
consequence of the vital contractibility of that film, and intimately
related to the changes taking place in the cell on which they are borne.
The persistence of their motions after a cell is detached from a
compound body covered with them, being like the persistence of the
contractibility of muscle fibre after being detached from a living
animal, proves that we must look to a contractile energy in the film of
protoplasm for the maintenance of these curious operations.
It appears that a cell cannot perform two functions at the same time,
and that one must either precede or follow the other. Thus, the
zoospores have two distinct periods of action; the first is that of
mechanical motion alone, which is followed by one of growth and
multiplication, manifestations which, though very dissimilar, are really
modes of action of the same vital energy that formed these bodies while
they were yet in their parent cell. In fact, it seems to be a general
law, that each cell is endowed, altogether or for a time, with its own
mode of action, and is incapable of any other.
Of the Volvocineæ, by some regarded as fresh-water microscopic plants,
the Stephanosphæra pluvialis may be taken as a type. This plant consists
of a colourless transparent globe not more than the 48/1000th of an inch
in diameter, containing eight green primordial cells arranged in a
circle in its equator. Each primordial cell is furnished with a pair of
cilia; these 16 cilia pierce through the hyaline globe, and by their
vibrations, they make it rotate about an axis perpendicular to the plane
of its equator, and move actively through the water. Each of the
primordial cells, which are green with a spot of red in the centre,
secretes a cellular covering, and they swim about in the interior of the
globe as free cells. Eventually they escape either by fissure of the
globe, or by its gradual dissolution. After swimming about for a short
time they become motionless, lose their cilia, and sink to the bottom as
green still spores.
If, after being dried, water be poured on one of these green still
spores, it takes up the water, its contents become closely granular, and
fill the whole membrane of the spore. Then it divides, first into
halves, then into quadrants or heart-shaped segments, meeting in a point
in the centre of the membrane. These quadrants are ultimately divided
into 8 wedge-shaped segments, whose contour lines, like the spokes of a
wheel, meet in the centre, and each gets a pair of cilia. The coloured
matter is driven back in each individual towards the thick end of the
wedge as if by centrifugal force, and a colourless plasm remains in the
points or beak. These disappear, a cavity is formed in the centre of the
disc, the eight bodies assume the form of a wreath in close contact, and
the original cilia, which continue to vibrate, cause the rotatory and
progressive motion of the whole organism.
Sometimes the eight globular bodies have been seen to divide into a
number of extremely minute motile cells, while yet within the parent
globe. These gonidia, as they are called, are, with a few exceptions
which may reproduce the plant, believed to perish when they come into
the water.
The division of the primordial cell of this plant is confined to a
certain time of day; it begins towards evening, and is completed the
following morning, and according to Mr. F. Currey, the exact time is the
same in Lapland, where there is no night, and at Berlin in spring when
the day and night are almost equal. The fertility is enormous. It is
calculated that in eight days, under favourable circumstances,
16,777,216 families of the Stephanosphæra pluvialis may be formed from
one resting spore.
The transmutation of chlorophyll in the Protococcus and Volvocineæ, from
green to red and vice versâ, which so frequently occurs in the lowest
class of plants, shows that its molecules must be united by very feeble
affinities, and easily converted into new combinations either by direct
chemical action, or by other substances also in a state of change.
[Illustration: Fig. 8. Volvox globator.]
The Volvocineæ consist of various species according as the internal
matter of the primordial cell divides into 2, 4, 8, 32, or a greater
number of equal parts, forming respectively as many free cells which
ultimately become ciliated spores, by means of which the globe either
rotates on the spot, or in straight lines. The Volvox globator found in
fresh-water pools is one of the most remarkable of these, both for its
peculiarity and beauty of structure and for its comparatively large
size, since in some lights it is visible to the naked eye while swimming
in a drop of water. When viewed with a microscope, it is a pellucid
sphere whose surface is studded with green spots, often connected by
green threads; each of the spots has two cilia, so that the surface is
bristled with these filaments, whose vibrations give the sphere either a
rolling or smooth motion, or make it spin like a top in the same place.
In the interior of the sphere there are from two to twenty dark green
globes of different sizes; the smaller are attached to the internal
surface, while the larger rotate freely by their cilia in the internal
cavity. After a time the sphere bursts open and its inhabitants swim
forth, and soon assume the form and character of that which gave them
birth.
The growth and development of the Volvox globator are peculiar, for in
the primordial cell the red and green endochrome breaks up into numerous
angular masses, and a central globe rather larger than the rest. The
angular masses are connected by green threads, the interstices between
all the bodies are filled with a hyaline substance secreted from their
surfaces, and the whole is enclosed in a distinctly membranous globular
envelope.
As this young Volvox increases gradually in size, the hyaline matter is
increased, the green threads lengthen, and the angular masses assume the
form of a flask the 1/3000th of an inch in diameter exactly as in the
Protococcus; for the green matter with a few red spots is collected in
the thick end, while the hyaline beak is turned towards the
circumference of the sphere, which is pierced by their long cilia. Each
of them is invested with a pellucid envelope of considerable thickness,
the borders of which are flattened against those of similar envelopes.
While these ciliated bodies are approaching maturity their endochrome
exhibits vacuoles or apparently empty cavities of a spherical form about
one-third of its own diameter. Mr. G. Busk discovered that these
vacuoles expand and contract at regular intervals of about forty
seconds. The contraction, which almost obliterates the cavity of the
vacuole, is rapid and sudden; the dilatation is slow and gradual. This
action ceases when the body comes to maturity.
When this mass of zoospores connected by green threads is immature and
begins to expand into a hollow sphere, then the central globe is
continually bisected so as to form 4, 8, 16, 32, 64, or a greater number
of equal and similar parts, each of which is ultimately developed into a
zoospore exactly the same with the matured green zoospores on the
surface of the primary sphere, so that the ‘Volvox globator is a
composite fabric made up of a repetition of organisms in all respects
similar to each other,’ which Professor Ehrenberg the first to discover,
though he did not investigate the development of the plant.
It appears that certain spheres of the Volvox are monœcious, that is,
each sphere contains male and female cells, though the greater number of
cells are neutral. The germ or female cells are larger and of a deeper
green than the others; the male cells resemble them, but the endochrome
within them breaks up symmetrically into a multitude of linear particles
aggregated into discoid bundles beset with vibratile cilia, which move
about within their cells and soon become decomposed into their component
corpuscles. Each of these corpuscles has a linear body, thicker at its
posterior end, and furnished with two long cilia. The female cell, when
fertilized, gets a smooth envelope, and then a thicker one, beset with
conical-pointed processes, and the contained chlorophyll gives place, as
in Palmoglœa, to starch and a red or orange coloured oil. It appears
that the Volvox stellatus and V. aureus are only phases of the Volvox
globator.
The Desmidiaceæ are minute green algæ inhabiting fresh-water pools or
slow running streams, never those that are muddy. They are free
unicellular plants, sometimes triangular, sometimes cylindrical,
crescent or bow-shaped, smooth or spined. So varied are their
microscopic forms that a description would be tedious. In plants of such
extreme minuteness, the only means of ascertaining the nature of their
component materials is by chemical tests. A solution of iodine turns
starch blue, and cellulose brown, and thus it is found that the interior
of the Desmidiaceæ is occupied by a mass of starch granules, covered
with chlorophyll, and mixed with a formative fluid. This mass, enclosed
in a delicate membrane, constitutes the primordial cell; it has an
exterior coat of firm cellulose, and the whole is more or less enveloped
in a gelatinous substance. Like other plants, when in bright sunshine,
the Desmidiaceæ decompose carbonic acid gas, give off the oxygen, and
assimilate the carbon into chlorophyll.
[Illustration: Fig. 9. Various species of Staurastrum:—A, vestitum; B,
aculeatum; C, paradoxum; D, E, brachiatum.]
These plants are frequently distinguished by projections from their
cellulose coat above their surface, these being sometimes short and
conspicuous, but often projected in spines, which form a beautiful
symmetrical hyaline border round the green internal cell, as shown in
fig. 9. Another peculiarity of the Desmidiaceæ is the appearance of
their being divided into two symmetrical parts by a satural line, as the
name implies, though there is no real division.
Many of the Desmidiaceæ, but more especially the genus Closterium, are
remarkable for having a double circulation of the internal fluid in
opposite directions, maintained by a vital contractile energy. One
current flows between the cellulose horny coat, and the thin film
covering the chlorophyll, while the other spreads in a broad stream in
the contrary direction between the thin film and the chlorophyll mass,
carrying from the latter some of its coloured particles to the
extremities of the frond, where there seems to be a connection between
the two streams.
[Illustration: Fig. 10. Economy of Closterium Lunula:—A, frond showing
central separation; D, frond in a state of self-division.]
The type of the Desmidiaceæ is continued by various modes of bisection,
depending upon the genus and species of the plant. In the Closterium
Lunula, which has an elongated crescent shape, as in fig. 10 A, the
endochrome or internal matter divides into two equal parts, which
retreat from one another at the middle line; and a constriction of the
cellulose coat takes place between them, which increases till it closes
entirely round the extremities, as in fig. 10 D; then one of the halves
remains at rest while the other moves from side to side, and finally
detaches itself from the other with a jerk. In each of these halves a
constriction of the endochrome may be seen, dividing it into an obtuse
and an elongated part, and for some time the circulating fluid flows
round the obtuse end, but the latter gradually assumes the form of the
elongated end, the regular circulation of the fluid is established, and
in five or six hours after the separation, two young desmids are formed
precisely similar to their parent, the Closterium Lunula.
The Cosmarium, another Desmid, consists of a cell of two lobes united by
a narrow isthmus. When about to multiply, the isthmus swells into two
globular expansions, separated from each other and from the two lobes of
the cell, by a narrow neck. These enlargements increase and assume the
appearance of half segments of the original cell. In this state the
plant consists of four segments lying end to end, the two old ones
forming the extremes, with the two new ones in the middle. At last, each
of the middle segments gets a new half, which soon acquires the full
size and characteristics of the old one. This process, which is
accomplished in twenty-four hours, is repeated ere long, and being
continued indefinitely, the extreme lobes of the row are thrust farther
and farther asunder, and the whole constricted thread or chain of
Cosmaria is enclosed in a gelatinous sheath. The last two central lobes
contain no portion of the original frond or plant, and may thus be
considered to be entirely new individuals.
Many of the Desmidiaceæ multiply by the subdivision of their endochrome
into a multitude of granular particles called gonidia, which are set
free by the rupture of the cell wall, and of which every one may develop
itself into a new cell. The gonidia may be zoospores with cilia and
active locomotion, or they may be enclosed in a firm envelope, and
become resting spores. The movement of the zoospores at first within the
cavity of the cell which gave them their origin, and afterwards
externally to it, has frequently been observed in the varied species of
the genus Cosmarium, and has been described under the name of the
‘swarming of the granules,’ from the resemblance of the moving mass to a
swarm of bees. Their subsequent history is unknown.
In the Pediastrum, a plant consisting of a cluster of cells, the
zoospores are not emitted separately, but those formed by the
subdivision of the endochrome of one cell into 4, 8, 16, 32, or 64
parts, escape from the parent plant still enclosed in the inner tunic of
the cell, and it is within this that they develop themselves into a
cluster resembling that in which they originated.
Mr. Thwaites discovered that the Desmidiaceæ are also propagated by
conjugation, which would be impossible if the hard coat of the adjacent
cells about to unite did not split open; then the whole endochrome in
one cell passes into and blends with that in the other cell, so as to
form one mass, which soon acquires a delicate membranaceous envelope. At
first the mass consists of granular green matter, but when the membrane
becomes thicker, it changes to brown or red. This body, which is called
a sporangium, is sometimes smooth, sometimes granular, covered with
tubercles or rough with spines, according to the nature of the original
plants. The filamental species are propagated by conjugation, but the
subsequent history of the produce is still obscure, though there is
reason to believe that they give rise to plants of different forms,
while all the other modes of increase only reproduce a facsimile of the
parent.
Desmidiaceæ exist in America, but their distribution is little known. In
Europe, their maximum seems to be in the south of England. They abound
in small shallow pools that do not dry up in summer, and also on boggy
moors. The larger kinds are spread out as a thin gelatinous stratum at
the bottom of water, or collected in little tufts; others form a dirty
cloud upon the stems and leaves of aquatic plants. They have been found
in a fossil state in flint, their spores have been discovered in the
grey chalk at Folkestone, and the cells of various species of Closterium
and Euastrum are imbedded in the marls of the United States of North
America.
The Diatomaceæ, or Brittleworts, are unicellular microscopic plants so
numerous that there is hardly a spot on the face of the earth, from
Spitzbergen to Victoria Land, where they may not be found. They abound
in the ocean, in still and running fresh water, and even on the surface
of the bare ground. They extend in latitude beyond the limits of all
other plants, and can endure extremes of temperature, being able to
exist in thermal springs, and in the pancake ice in the south polar
latitudes. Though much too small to be visible to the naked eye, they
occur in such countless myriads as to stain the berg and pancake ice
wherever they are washed by the swell of the sea; and when enclosed in
the congealing surface of the water they impart to the brash and pancake
ice a pale ochreous colour.
Although the diatoms have a vast variety of forms, they all consist of a
simple primordial cell whose external coat of cellulose is so deeply
interpenetrated with silex that it is indestructible, a structure which
constitutes the peculiar characteristic of the tribe. This primordial
cell, as in other plants, contains organizable liquid or protoplasm,
through which golden-brown granules are pretty regularly distributed,
except in the centre, where they are collected into a nucleus. Round
this nucleus they commonly form a ring from which radiating lines of
granules diverge to the interior wall of the cell. In each of these
there is a double current of granules, similar to the circulation in the
Desmidiaceæ; it was discovered by Professor Smith in some of the
comparatively large diatoms. At times oil globules are seen in the
protoplasm. The golden-brown matter is supposed to be chlorophyll, whose
green tint has been changed by the presence of iron, which is
assimilated in this group. Such is the internal structure of a race of
plants altogether invisible to the naked eye. Their external forms,
reproduction and movements, are no less wonderful.
The silicious envelope of the simple cell of a Diatom or frustule, as a
single plant is usually called, consists of two valves or plates,
commonly of the most perfect symmetry, closely applied to each other
along a line of junction like the two valves of a bivalve shell, and
each valve being more or less concavo-convex, a cavity is left between
the two which is occupied by the golden-brown cell described above. The
form of the cavity differs greatly, for sometimes each valve is
hemispherical, so that the cavity is globular; sometimes it is a small
segment of a sphere, resembling a watch-glass, so that the cavity is
lenticular; in short, the form of the cavity depends upon that of the
valves, which may be heart-shaped, or much elongated, square,
triangular, boat-shaped, or furnished with outgrowths, which, however,
is rare.
[Illustration:
Fig. 11.—A, Diatoma vulgare:—_a_, side view of frustule; _b_, frustule
undergoing self-division.
B. Grammatophora serpentina:—_a_, front and side view of single
frustule; _b_, front and end view of divided frustule; _c_, frustule
about to undergo division; _d_, frustule completely divided.
]
The diatom or frustule is considered to present its front view when the
joint or suture of the valves is turned to the eye, as in fig. 11 B,
_b_, whilst the side view is seen when the centre of either valve is
directly beneath the eye, as in fig. 11 A, _a_. When the diatoms are
young the valves are in close contact, but as they increase in size by a
secretion round their edges, the valves separate from one another, and
the cell membrane which is left exposed is immediately consolidated by
silex, and forms a kind of hoop between the valves, as in fig. 12. This
hoop increases in breadth as the cell increases in length. When the two
valves are circular discs, they are separated by a circular hoop, round
the edges of which water is admitted to nourish the plant; but when the
diatom has an elongated form, the water enters through depressed points
in its extremities which are free from silex.
[Illustration: Fig. 12. Biddulphia pulchella.]
[Illustration: Fig. 13. Pleurosigma angulatum.—A, entire frustule; B,
its hexagonal areolation; C, the same more highly magnified.]
Numerous as these plants are, the valves of each genus have their own
peculiar ornaments, consisting of the most beautiful and symmetrical
designs, which are impressed upon the young valves when they are in a
plastic state. The genus Navicula and others are marked with the finest
striæ, some diagonally, others transversely. Rows of round or oval spots
disposed in parallel lines are peculiar to some; the valves of others
are covered with hexagonal forms of the most perfect structure, as those
of the Pleurosigma angulatum, fig. 13, where A is the magnified diatom,
and B and C its hexagonal areolations, seen under higher and higher
microscopic powers; but the figures on the discoid genera are the most
beautiful of all. There is generally a small ornamented circular space
in the centre of the valves, from whence rays extend to the
circumference, dividing the surface of the valves into eight, ten, or
more equal parts, the alternate segments being differently and highly
ornamented, as in the Actinocyclus undulatus (fig. 14), where A is the
side view, and B is the front view. The Arachnoidiscus Ehrenbergii takes
its name from the likeness of the figures on its circular valves to a
spider’s web. According to the observations of Mr. Shadbolt, each valve
is formed of two superposed layers; on the uppermost of these, which is
a thin horny transparent substance, the spider’s web is engraven; and
the undermost silicious layer, which forms the supporting frame-work, is
like a circular Gothic window. The genus Triceratium, nearly allied to
the preceding in general characters, though differing in having a
triangular shape, has many species in a fossil state, while others are
still existing in the ocean, and in tidal rivers. The Triceratium favus,
one of the largest and most beautifully marked, occurs in the mud of the
Thames, and that of the estuaries of other rivers on our coasts; it is
also frequently found on the surface of uncleaned shells.[32] From the
few examples given, a faint idea only can be formed of the variety and
beauty of the engravings on the diatoms. It had long been doubted
whether those on the valves of Coscinodiscus, Triceratium and others,
were elevations or depressions, but Professor Rood of New York, United
States, has proved them to be depressions by an optical arrangement
which will be useful for the investigation of microscopic forms.
[Illustration: Fig. 14. Actinocyclus undulatus.]
Diatoms increase by spontaneous bisection, by conjugation, and by the
resolution of their endochrome into minute spores, called gonidia. When
bisection is about to take place the cell elongates, the hoop increases
in breadth, the endochrome divides into two equal parts, and the coating
of the cell bends in between them, which gives the diatom the appearance
of an hour-glass. At last they separate, and upon each of the new
surfaces a new silicious half is formed, usually the exact counterpart
of the old one, so that there are two diatoms instead of one; and the
process may be continued indefinitely. In most cases, the new diatoms
thus produced are free and independent. Sometimes, however, they adhere
to one another by a fragment or connecting membrane, and if they happen
to be slender and rectangular, and attached side by side, they form a
slender filament, or if attached by alternate angles they form a zigzag
chain, as in fig. 11.
The Meridion circulare (fig. 15) is a diatom of exquisite beauty,
millions and millions of which cover every submerged stone, twig, or
blade of grass, and even form the mud at the bottom of the streams at
West Point, in the United States of North America. Its frustule or
single diatom is long, slender, and rectilinear, but being broader at
one end than at the other, by continued bisection and adhering to one
another they form a circular, spiral, or flattened helical screw of
several turns. The individual frustules of some marine diatoms have a
precisely similar form, being rectilinear and broader at one end than
the other, but each frustule is attached by its narrow end to the
extremity of branching cellulose stems fixed to sea-weeds or stones, and
by a continuous subdivision of which the stem does not partake, they are
spread out at their free ends like a fan.
[Illustration: Fig. 15. Meridion circulare.]
By continual bisection a diatom is propagated through many generations,
but at some stage or other, owing to an unknown cause, propagation by
conjugation takes place. When two frustules are near to each other, two
little swellings arise in one, which meet two little swellings in the
other opposite to it. These soon unite and elongate, the septum or
division between them is absorbed so that they form two tubes in which
the endochrome of the two frustules becomes mixed, and a spore is formed
in each of the two connecting tubes, which increase in size and change
in form till they resemble in every respect the parent, except in being
much larger. As these young diatoms swell, they split the two parent
frustules, become free, and lay the foundation of a twin series of
generations. In the Fragillaria only a single spore is formed.[33]
In Surirella and Epithemia the manner of conjugation is somewhat
different. In the former the valves of two free adjacent frustules
separate from each other at the suture or line of junction and the two
endochromes are discharged; they coalesce and form a single mass, which
becomes enclosed in a gelatinous envelope, and in time this mass shapes
itself into a frustule resembling that of its parent, but larger. In
Epithemia, however, the endochrome of each of the conjugating frustules
divides at the time of its discharge into two halves; each half of the
one coalesces with each half of the other, and two frustules are formed
which become invested with a gelatinous envelope and gradually assume
the form and markings of the parent frustules, but grow to a much larger
size, for the spore masses have the power of self-increase up to the
time that their envelopes are consolidated. This double conjugation
seems to be the ordinary type of the process among the diatoms.[34] But
these plants multiply also by gonidia. It is thought probable that as
long as the vegetative processes are in full activity diatoms multiply
by self-bisection, but when a deficiency of warmth, of moisture, or of
some other condition, gives a check to these, that they increase by
gonidia, some of which becoming encysted, possess a greater power of
resisting unfavourable circumstances, and thus the species is maintained
in a dormant state till a change enables them to germinate. It is even
thought they may be the origin of distinct species.
A peculiar spontaneous locomotion is exhibited by some diatoms of a long
narrow form, as the Naviculæ, which by a succession of jerks in the
direction of their length, go to a certain distance, and then return
nearly by the same path. The motion of the Bacillaria cursoria is still
more unprecedented. The frustules, which are narrow, lanceolate, and
acute, are joined end to end in a long line by some highly elastic
invisible medium. One of the terminal frustules remains at rest while
all the others slide over it till the line is so much stretched that
they are nearly detached from one another; then they all slide back
again in the same manner, and this alternate motion is continued
indefinitely at regular intervals of time. The velocity of the diatoms
at the free end of the row is very considerable; in the Bacillaria
paradoxa it is 1/200th of an inch in a second; the impetus of one has
been observed to upset and even to push aside a plant as much as three
times its size which obstructed its path. If the frustule at the free
end gets entangled, the fixed frustule takes the lead and continues the
motion till the other is free. Minute particles in the vicinity are
sometimes attracted and dragged after the frustules, sometimes they are
repelled, possibly by some invisible organs; but the whole motion of the
diatoms themselves may perhaps be attributed to the action of light and
heat upon the highly contractile substance, whatever it may be, which
connects their frustules, since their motion is exactly in proportion to
the quantity of light and heat received, for it ceases during darkness,
and is renewed on the return of light; ultimately it may disperse the
individual frustules, which are not more than between the 28/10,000th
and the 84/10,000th of an inch in length and the 4/10,000th of an inch
in breadth.
[Illustration: Fig. 16. Bacillaria paradoxa]
This Bacillaria paradoxa (fig. 16) differs from the preceding species in
its motion; each half of the row of frustules moves in an opposite
direction on each side of a central stationary frustule, and the
alternate motion is so regular as to time, that if in advancing, the
frustules meet with an impediment, they wait till the proper time comes
for their retreat. The jerking motions of the Naviculæ are ascribed by
Prof. W. Smith to forces acting within the plants, originating in the
vital operations of growth, by which the surrounding water is drawn in
at one end of the frustule, and expelled at the other.
Some species of diatoms are so universal that they are found in every
region of the globe; others are local, but the same species does not
inhabit both fresh and salt water, though some are found in brackish
pools. The ocean teems with them. Though invisible as individuals to the
naked eye, the living masses of the pelagic diatoms form coloured
fringes on larger plants, and cover stones or rocks in cushion-like
tufts; they spread over the surface as delicate velvet, in filamental
strata on the sand, or mixed with the scum of living or decayed
vegetable matter floating on the surface of the sea; and they exist in
immense profusion in the open ocean as free forms. The numbers in which
they exist in all latitudes, at all seasons, and at all depths—extending
from an inch to the lowest limit to which the most attenuated ray of
light can penetrate, or at which the pressure permits—are immeasurably
in excess of what we have been in the habit of assuming. Temperature has
little to do with the distribution of diatoms in the tropics; it
decreases with the depth at a tolerably fixed rate till it becomes
stationary. It increases in the polar regions with the depth, and
approaches the standard, which is probably universal, near the bed of
the ocean.
Nothing can exceed the vividness of colour or massiveness of the
endochrome or soft internal matter of the floating diatoms, that matter
which diminishes their specific gravity and makes the plant buoyant
which otherwise would be weighed down by its silicious coat. At those
periods in which the structural and reproductive phenomena proceed most
vigorously, their position in depth must be fluctuating; hence they
approach and vanish from the surface. Their growth is perfected by the
heat and light which penetrates the sea in calm weather.
Diatoms are social plants crowded together in vast multitudes. Dr.
Wallich met with an enormous assemblage of a filamental species of
Rhizoselenia, which is from six to twenty times as long as it is broad,
aggregated in tufted yellow masses, which covered the sea to the depth
of some feet, and extended with little interruption throughout six
degrees of longitude in the Indian Ocean. They were mixed with
glistening yellow cylindrical species of such comparatively gigantic
size as to be visible to the naked eye.
Other genera constitute the only vegetation in the high latitudes of the
Antarctic Ocean. Dr. Hooker observes, that without the universal
diffusion of diatoms in the South Polar Ocean, there would neither be
food for the aquatic animals, nor would the water be purified from the
carbonic acid which animal respiration, and the decomposition of matter,
produce. These small plants afford an abundant supply of food to the
herbivorous mollusca and other inhabitants of the sea, for they have
been found in the stomachs of oysters, whelks, crabs, lobsters,
scallops, &c. Even the Noctiluci, those luminous specks that make the
wake of a boat shine like silver in a warm summer night, live on the
floating pelagic diatoms, and countless myriads are devoured by the
enormous shoals of salpi and other social marine animals.
The silicious shells of the diatoms form extensive fossil deposits in
various parts of the globe, containing species which have long ceased to
exist, and others that are identical with those still alive even in
their most minute and delicate engravings. The polishing slate of Bilin
in Bohemia, which occurs in beds 14 feet thick, and the Tripoli and
Phonolite stones on the Rhine consist entirely of the silicious coats of
diatoms, while the city of Richmond in Virginia stands upon a marine
deposit of the debris of diatoms 13 feet thick, and of unknown extent.
Near the Mediterranean, very extensive strata, consisting almost
entirely of marine Diatomaceæ, alternate with calcareous strata chiefly
formed of Foraminifera, the latter being a race of microscopic mollusca.
The fossil Diatomaceæ at Oran in Algeria are particularly perfect and
beautiful. In many of these deposits existing species are found.
The trade winds bring over large quantities of dust mixed with diatoms,
which sinks through the upper into the lower current, blowing over
America, and at last falls in Europe. Professor Ehrenberg found that
this dust contained chiefly true American species, many of which were
identical with forms existing at the bottom of the Antarctic Ocean,
where an area of 4,800 square miles was discovered by Sir James Ross
skirting the volcanic coast of Victoria Land, consisting of the remains
of these microscopic plants, which have deposited their silicious valves
at death for countless generations, producing geological changes of
enormous magnitude; while a still greater area of sea-bed in the North
Atlantic is the perpetual grave-yard of myriads of microscopic mollusca.
Thus the Supreme Being, whose power is stupendously manifested in the
motions of the celestial bodies, creates generations of infinitesimal
creatures, adorns them with exquisite beauty, and makes them His agents
to form future continents.
The Confervaceæ are a numerous tribe of pretty little plants, usually of
a green colour, growing in fresh and salt water, on moist ground, wet
rocks, and thermal springs. There is scarcely a gently running stream in
which they may not be seen, like bunches of green threads, attached to
stones and waving in the current; some are so soft as to become almost a
mass of jelly when taken out of the water. They are sometimes branched,
but more frequently simple, formed of cylindrical cells, joined in a
single long row by their flat ends, and they increase in length by the
bisection of their terminal cells.
[Illustration: Cell multiplication in Conferva glomerata:—A, portion of
filament with incomplete separation at _a_, complete partition at _b_;
B, the separation completed; C. formation of additional layers of
cellulose wall.]
In unicellular plants bisection is an act of reproduction; in the
multicellular Confervæ it is an act of growth and extension which is
accomplished as follows:—The terminal cell of the plant grows to twice
its length, the matter within the primordial cell spontaneously divides
into two equal parts, and both the film and cellulose coat which cover
it, bend round, and form a double layer or cellulose division between
them. This cellulose layer extends over the whole exterior of the
primordial cell, so that the new cellulose division or septum becomes
continuous with a new layer which is formed throughout the interior of
the cellulose wall of the original cell. In this manner two perfect
cells are formed out of one, and as the extreme cell may undergo the
same process, the growth of the plant may be continued indefinitely.
Branches are sometimes formed by buds springing from any part of the
stem; though apparently so different, it results from the subdivision of
the cell which produces the bud.
[Illustration: Fig. 18. Zoospores.]
The Confervaceæ are generally reproduced by zoospores. In most cases the
endochrome within a cell divides itself into numerous segments, each of
which becomes a minute zoospore, and escapes into the water through a
rupture in the cell wall. This is the case in a very graceful genus of
Confervæ, of which the Chætophora elegans is a species. It consists of
filamental strings of cells, ending in a capillary bristle, with lateral
branches like narrow fronds. It is reproduced by zoospores. One half of
each zoospore is round, opaque, and full of matter; the other half
hyaline, and tapering to a beak furnished with four cilia. It frequently
happens in this genus of Confervaceæ, where the filaments are divided at
equal distances into little joints or compartments, that the zoospores
issue from the terminal cell first, then from the next, and so on in
succession till the upper part of the branch is left empty, while the
lower part is still forming zoospores. After moving in water for a time,
the zoospores retreat to a shady place, fix themselves to some
substance, and begin to grow. These plants rapidly cover a large surface
of water; for each individual cell may produce 100 zoospores, and as the
development and dissemination of them continues during the whole summer,
one plant may yield an enormous number.
The Sphæroplea annulina is a rare and very remarkable Conferva, whose
cinnabar-coloured spores make the surface of the water in which it
floats, like a pool of blood. It has no root, being merely a filament
with capillary extremities, formed of elongated cells joined end to end.
The spores only grow on the filaments that are exposed to the sun and
air; the filaments that are below the water are green and barren. The
spores are filled with red matter, grains of starch, and red oil, the
outer or cellulose coat being so plaited, that the spore looks like a
red star with white rays.
When a spore germinates, it produces a minute cell ending in capillary
fibres, which increases in length by the continual bisection of its
central cells, while the other Confervæ grow by the bisection of those
that are terminal. During this growth, the red contents of the spore are
so changed by a remarkable succession of chemical processes, that the
primordial cells in the filament of the young plant are filled with a
colourless viscous matter, an aqueous liquid, granules of starch, and
chlorophyll. In some of the cells the starch disappears, while the green
matter and the other materials arrange themselves into a series of
rings, alternating with empty spaces or vacuoles. After a time, the
green changes to a yellowish red, and then each ring in succession
resolves itself into a multitude of minute active particles, which move
with incredible velocity in the void spaces of the cell, till at last
the whole cell swarms with them. They are analogous to the pollen of
flowering plants, and thence are called spermatozoids. Their form is
cylindrical, thick, broad, and yellow at one end, sharp at the other,
with a colourless beak, and long cilia. The parent cell is at last
pierced by their united efforts, and out they rush in great confusion
into the water; some whirl round their centres, others swim in a circle,
many describe cycloidal curves by a series of leaps, and a few swim in
straight lines.
During the preceding changes another process is in progress, within what
may be called female cells. In these the starch, mixed with green matter
and a plastic substance, arrange themselves also into green and vacant
rings, and after various and complicated changes, each green ring forms
itself into a kind of plastic primordial free cell, which, after being
fertilized by the moving bodies, gets a stronger coat. The green matter
becomes first of a red-brown colour, then red; and after leaving the
parent cell it is invested with a plaited cellulose coat, and becomes a
star-like resting spore which may produce a new plant. No cryptogamic
plant exhibits a greater variety in the modes of action of the vital
forces, none more activity in the motile powers. In some of these
Confervæ, the moving male filaments, or spermatozoids, instead of
escaping singly from their prison cell in confusion into the water, are
discharged in a mass enclosed in a capsule furnished with cilia, which
moves with its lively burden like a zoospore, till a lid falls off which
sets them free.
Some pretty plants allied to the Confervæ are called Batrachospermeæ,
from the resemblance which their beaded filaments bear to the spawn of a
frog. They are all inhabitants of fresh water, chiefly of gently flowing
streams, and are so flexible that they yield to every movement of the
water, and when taken out of it are like a mass of jelly. Their colour
is usually a brownish-green, but sometimes it is of a reddish or bluish
purple. The central stem of the plant, though originally formed of a
single row of large cylindrical cells placed end to end, gets an
investment of cells, or rather branches, which ultimately becomes a
thick cylindrical stem, bearing, at nearly regular intervals, whorls of
short radiating branches, each composed of rounded cells, arranged in a
bead-like row, and sometimes branching again. Some of the radiating
branches grow out into transparent points, which may possibly be
antheridia, and contain motile bodies; for within certain cells in other
branches resting spores are found, which are agglomerated and form the
large dark globular masses that are seen in the midst of the whorls.
The Hydrodictyon utriculatum is another allied plant of singular
structure, which grows in fresh-water pools in the midland and southern
counties of England. It resembles a regularly reticulated green purse,
from four to six inches long, and is composed of a vast number of
tubular cylindrical cells, which adhere to one another by their rounded
extremities, the points of junction corresponding to the knots or
intersections of the network. Each of these cells may form within itself
from 7,000 to 20,000 gonidia, which at a certain stage of their
development are observed to be in active motion in its interior;
subsequently, by mutual adhesion, they form into groups which lay the
foundation of new net-plants, when set free by the dissolution of their
envelope. Besides these groups, there are certain cells which produce
from 30,000 to 100,000 more minute bodies of a longer shape, each of
which is furnished with four long cilia, and a red spot. These escape
from their cell in a swarm, move freely in the water for a time, then
come to rest, and sink to the bottom, where they remain, heaped together
in green masses. Their future fate is unknown, but they are believed to
be male filaments similar to those described, and are generally called
spermatozoids.
The Nostochineæ are either an assemblage of cells loosely united into
numerous green chaplets, or distinctly beaded filaments, generally
twisted, and occasionally branched; they are imbedded in a firm
gelatinous frond of different form, sometimes globular, sometimes
spreading in branched masses, often of considerable size. They are
frequently seen on damp shady walks in gardens: they shrink to a film in
dry weather, and reappear so suddenly in rain that they have been called
fallen stars. They are reproduced by spontaneous division of their
filaments; the segments escape from the gelatinous mass, move slowly in
the direction of their length, after a time come to rest, secrete a
gelatinous envelope, and not only grow in length by transverse
bisection, but split longitudinally into new filaments which are
separated by their gelatinous secretions. These movements, discovered by
M. Thuret, are evidently intended to disperse the plant.
Vesicular cells, destitute of endochrome, sometimes furnished with
cilia, and of a larger size than the others, are occasionally seen at
the end or middle of a filament of the Nostocs, sometimes situated at
intervals along their length; and near to these are sporangial cells, a
little larger than the ordinary cells. From analogy, it is believed that
the vesicular cells are antheridia, and that the sporangial cells
contain germs which, after being fertilized by the spermatozoids, are
set free and become resting spores. In some species, the sporangial
cells are oblong, and contain vividly green matter; in others, the cells
are elliptical and brown.
The species are widely distributed. Hormosiphon arcticus, a species
consisting of a modification of cellulose, abounds to such a degree in
the herbless polar regions, that it affords a welcome variety of food.
Each plant lies on a small depression of the snow, which covers the soft
and almost boggy slopes bordering the arctic seas, but it is carried by
the winds in every direction, rolling over the snow and ice to a
distance of several miles. Two northern species of Nostoc were found by
Dr. Hooker in Kerguelen’s Land, growing on wet rocks near the sea; one
of them was the common Nostoc commune. Other species occur in the warm
springs in India, as well as in the arctic and antarctic regions, and an
aquatic species is much used in China as a wholesome food. The genus
Monormia forms floating masses of jelly on the surface of brackish
water. The necklaces are of vast length, and, together with the jelly in
which they are imbedded, wave with the slightest motion of the water.
Floating masses grow on large ponds or lakes, which give the water a
green tint.
The structure of the Oscillatoriæ is microscopic. They are minute
filiform plants closely allied to the Nostocs; and consist of
transparent colourless tubular filaments containing colour cells of
various forms, more or less separated from each other, and visible
through their transparent tubes; the colour is usually some shade of
green, yellowish, or purple. In the genus Rivularia these tubular
filaments have a globular transparent cell at the base, and are closely
packed into little balls, either forming small groups, as in the
Rivularia nitida, or singly attached to stones and rocks. In Rivularia
nitida, the filaments radiate from a centre. Some Oscillatoriæ form
velvety cushion-like patches upon rocks, others are attached in tufts as
parasites to other sea weeds, while many are arranged in free or
attached stratified bundles. Lingbya furnishes a beautiful specimen of
the latter. The filaments in the stratified group are usually much
twisted and interwoven, and some of them exhibit singular oscillating
motions, as the Oscillatoria littoralis and spiralis, Spirulina
tenuissima and others; one end of the filaments remains at rest, while
the other extremity is in constant vibration. With a microscope the
movement in some species is seen to be from side to side like a
pendulum, in others it is spiral or twisting, and when a fragment of the
plant is set free when vibrating the movement is progressive. If a
fragment be put into a glass of water, its edge in a little time becomes
fringed with short filaments radiating from central points with their
tips outwards. They soon detach themselves from the fragment by their
oscillations, and as their vibrations continue after they are free, they
swim with a spiral motion to the edge of the water, and even ascend the
glass till arrested by the dry part above.[35] During these motions
there is a corresponding alteration in the form of the filamental tubes
believed to arise from rhythmical periods of vital contractibility,
which are affected by light and heat, because the motions are more rapid
in sunshine than in shade; besides, they are checked by strong chemical
agents. Some of the species have a tuft of delicate cilia at the
extremities of their filaments.
The free stratified bundles contain the simplest form of the
Oscillatoriæ. Each filament is a straight or slightly curved chain of
cells, full of coloured matter, and enclosed in a common transparent
colourless tube. Multiplication takes place in these by division; when
about to multiply, two adjacent coloured cells, or the two halves of a
divided cell, recede from one another, and the outer tube contracts at
the point of division, and separates them into two distinctly new
filaments. Sometimes the transparent outer tube does not yield, so that
the divided parts retain their places in the tube, which dilates when
these new parts are again divided. The manner of division varies with
the species, and the generic characters of the Oscillatoriæ depend upon
the different conditions of the external tube, and the form and
arrangement of the coloured cells within it. The tube often contracts to
the finest point during division, and frequently consists of distinct
coats, the number of which increases upwards, sometimes with such
regularity as to produce a beautiful streaked effect. Like their allies,
the Oscillatoriæ are reproduced by zoospores. While these parts are
growing, but especially during their dissolution, the endochrome
undergoes various changes of colour, staining the water they die in, and
rendering it putrid; some of the common kinds emit a strong odour of
sulphuretted hydrogen.
In the compound gelatinous Oscillatoriæ, the jelly is of very different
degrees of tenacity. The mass of the Dasyglœa is so slippery that it can
scarcely be taken hold of; Rivularia nitida (fig. 19) is equally so, its
tubes being so thick and tender. Many species of the genus Rivularia
have a peculiar mode of oblique alternate branching; species of that
genus grow on the stems of aquatic plants, on rocks in rapid streams, on
cliffs when washed by cataracts, or sometimes in calcareous water, in
consequence of which crystals of carbonate of lime are deposited on
their substance. The Rivularia nitida occurs among Algæ exposed at low
tides, and a species of another genus floats on fresh-water lakes like
green stars.
[Illustration: Fig. 19. Threads of Rivularia nitida.]
The Oscillatoriæ are found in every part of the world, most abundantly
in the temperate zones. They chiefly inhabit fresh water, but these
minute plants attain their greatest size in the sea. Numerous species
grow in warm springs, and one species, Trichodesmium erythræum (fig.
20), spreads for many square miles over the surface of the Indian seas
in faggots of red-brown threads, like fragments of chopped hay; the same
species is said to abound in the Red Sea also.[36]
The Conjugatæ are fresh-water plants of numerous species, which have
almost the same structure as the Confervæ, but the green endochrome
within the cells of their articulated threads is more highly organized,
and the manner of reproduction is altogether different and very
peculiar.
These plants consist of strings of cylindrical cells joined end to end
by their flat ends, and generally float freely on or near the surface of
still water, especially when buoyed up by the bubbles of gas which are
liberated from them by the heat and light of the sun. In the early stage
of their life, while as yet the cells are undergoing multiplication by
self-division, the endochrome is diffused pretty uniformly in each cell;
but as the plant approaches towards maturity, it undergoes various
modifications, according to the species. In some it consists of large
granules disposed in rows; in others it is formed into broad spiral
bands with large granules in binary or stellar groups placed at
intervals on it; and, in the œdogonium capillare and others, the
granules are united in spiral lines which cross one another and form a
network.[37]
[Illustration: Fig. 20. Trichodesmium erythræum.]
The act of conjugation by which spores are formed, usually takes place
between the cells of two distinct parallel filaments which happen to be
adjacent to each other, and all the cells of the two filaments generally
take part in it at once. The cells that are opposite to one another put
out little protuberances, which come into contact with each other; the
intervening partitions disappear, so that a tube is formed which
establishes a free communication or passage between the cavities of the
conjugating cells. In the genus Mesocarpus and others, the conjugating
cells pour their endochromes into a dilatation of the passage that has
been established between them, and it is there that the matter mingles
to form a spore or embryo cell. But in the Zygnema (fig. 21), which is
the commonest form of these plants, the endochrome of one cell passes
entirely over into the cavity of the other, and within the latter the
two endochromes coalesce into a single mass, round which a firm coat is
developed, and it becomes a spore. All the cells of one filament are
thus left empty, while spores are formed in all the cells of the
other.[38] Sometimes cells in the same filament conjugate, and
occasionally the endochrome in a cell divides into two parts, each of
which becomes a spore.
[Illustration: Fig. 21. Conjugation of Zygnema quininum:—A. two
filaments in the first stage of conjugation; B, completion of the act of
conjugation.]
Some of the spores are quiescent, others have cilia and are motile, but
both after a time become attached at one end by two or three root-like
fibres, and grow into filaments by repeated bisection. According to the
observations of M. Itzigsohn, the endochrome in certain filaments of
Spirogyra breaks up before conjugation into little spherical
aggregations, which are gradually converted into nearly colourless
spiral filaments, having an active spontaneous motion, and therefore
corresponding precisely to antherozoids. With the exception of South
America, the Conjugatæ are widely dispersed in warm and temperate
climates.
The genus Vaucheria may be assumed as a type of the Siphoneæ, whose
essential character is, that the plant consists of one single tubular
cell, however branched and complicated its form may be. The Vaucherias
form tufted masses of branching tubes, filled with bright green granular
matter, on mud and damp soil; they abound in fresh-water pools, and some
grow in the sea. When about to produce fruit, the extremities of some of
the tubes swell out in the shape of a club, in which a portion of the
green matter collects, takes a darker hue, and is separated from the
rest by a transparent space and a new envelope. After various changes,
the darker green matter forms itself into a zoospore, which is so active
that it breaks open the top of its club-shaped cell, and comes into the
water; sometimes several come, one after another. They are egg-shaped,
with a colourless beak, and as their whole body is bristled with cilia,
they leave a long current in their wake when they swim, which they do
with such impetus that they are flattened against any obstacle they meet
with, even to the discharge of their green endochrome. They escape from
their cell about eight in the morning, move for two hours, then come to
rest, and begin to grow into a new plant.
M. Pringsheim discovered another mode of reproduction in the Vaucherias,
which are monœcious plants, that is to say, the same plant produces
snake-like fertilizing spermatozoids and female germ cells. For example,
the Vaucheria sessilis consists of one long branched cell; on the same
side of it two swellings appear near to each other, one of which
elongates, curls round like a horn, and is soon filled with snake-shaped
filaments having long cilia at their thin end, with which they move
rapidly both within the horn, and after they come out of it into the
water. They are perfectly colourless, and correspond to the pollen of
flowering plants. The other protrusion which swells into a globose germ
cell, and which corresponds to the pistil of a flower, contains a mass
of green endochrome, which, after being fertilized by the snake-like
filaments, becomes a primordial cell which has no motion, but after
having secreted a strong coating of cellulose, it sinks to the bottom of
the water, becomes a winter or resting spore, and lays the foundation
for a new generation of plants. The resting spores produce new forms,
while the zoospores, like buds, only multiply the type of the individual
plant with all its peculiarities.
The marine genus Bryopsis grows in New Zealand, the Falkland Islands,
and the seas about Cape Horn. The species are mostly parasites on other
Algæ, and produce innumerable zoospores. The genus Codium is found in
high latitudes, and appears under four different forms on the British
coasts; one of these inhabits turfy banks exposed to the spray of the
sea, the others grow in deep water, or on rocks never uncovered but at
spring tides. Species of this genus are found as far south as
Kerguelen’s Land, and in most of the intervening latitudes. The
Caulerpas inhabit the warmer districts in the northern hemisphere, and
furnish five species in New Zealand. The numerous species afford almost
the whole food of turtles on many coasts, and other genera furnish
nutriment to a host of smaller animals.[39]
The Achlya prolifera is also a unicellular plant, much smaller than the
Vaucheria, but whether an Alga or a Fungus is not very clearly settled.
To the naked eye it appears as a cluster of colourless threads on dead
flies floating in water, on the gills of fishes, and sometimes on frogs.
With a microscope the tufts are seen to consist of tubes extending in
all directions, filled with a nearly colourless granular matter, the
particles of which are seen to move slowly in streams along the walls of
the tubes, the currents sometimes anastomosing with each other. When the
plant is about thirty-six hours old, the endochrome begins to accumulate
in the dilated ends of the tubes, and is cut off from the remainder by a
transverse division, the motion of the particles being still visible in
the part cut off. The endochrome breaks up into a number of long masses,
each of which acquires a cell wall and two cilia, and begins to move
about within the parent cell; when mature they are set free by the
rupture in its wall, and germinate, and produce a facsimile of the
parent. It appears that, in some species, the transverse dividing film
becomes convex as soon as the motile bodies are discharged, a new
fertile articulation is formed and new motile spores are set free, and
this process is continued till the vital powers of the plant are
exhausted. The Achlya has resting spores, which may remain long in the
water without change, but if a dead insect be put into it, they fix on
it and germinate immediately. It is supposed that these resting spores
are fertilized by filamental bodies. The Achlya prolifera goes through
all its changes in an hour and a half or two hours. It is found in the
thermal springs at Vichy, Nevis, and Vaux, where it contains an alkaline
iodide.
The whole of the plants which have been described in the preceding pages
belong to the group of green Algæ, although many are inhabitants of
fresh water. The structure of the marine Algæ is entirely cellular.
Deprived of vascular tubes, they can have no circulation of sap,
consequently they derive their nourishment by absorption throughout
their whole surface from the medium in which they live, for their root,
or rather fulcrum, only serves to fix them to the rocks and stones to
prevent them from being buffeted by the waves. Since solar light and
heat decrease rapidly with the depth, each family of Algæ has a zone
peculiar to itself. The first zone extends from high to low water mark,
and is inhabited by plants periodically exposed to the atmosphere, to
the direct light and heat of the sun, and occasionally to rain. Some of
the Algæ that are long left dry are believed to derive some nourishment
from the substances to which they are fixed. The second zone, which
extends from low water mark to a depth of fifteen fathoms, is the region
of the great marine forests which encircle the globe in both
hemispheres. Other two zones follow at greater and greater depths, but
all are divided into various minor regions, below the last of which the
Algæ decrease as the depth increases, till, as far as we know,
vegetation ceases altogether; that depth, however, must be very great,
as diatoms are sometimes found, and in great quantities, three hundred
fathoms deep.
The marine Confervas, like those growing in fresh water, are
slender-jointed filaments formed of one series of cells joined end to
end. The cells become more or less flattened on the surface of contact,
while the side walls retain their natural curvature, which may be
cylindrical or oval. The filament may, therefore, be cylindrical or
beaded. The cells are almost always longer than they are broad, and for
the most part equal and similar in the same plant, although there are
exceptions to uniformity of size. The cells contain a transparent liquid
through which minute solid particles of various shades of green are
pretty evenly scattered. The conversion of these particles into
zoospores has already been described. Since these Algæ have no roots,
and the cell wall no opening, each cell of a Conferva elaborates
independently the nutriment it absorbs from the water. Some species form
a fleecy layer over rocks, and on the bottoms of salt-water pools and
estuaries, others extend in bundles in salt-water ditches, and some are
found on rocks, between tide marks, rising in long, straight, stiff, and
wiry tufts, from three to eight or twelve inches high.
The genus Hormotrichum, which forms tufts several inches long, of bright
grass green, differs from the Confervas in being soft and gelatinous,
and even more by its mode of increase, which, however, is still by
zoospores. The H. collabens may be taken as the type of this genus. It
forms a long and large tuft of soft gelatinous and slippery filaments of
glossy green. The joints of the filaments are once, or once and a half,
longer than they are broad, and the green granular matter within them is
collected into a round sac or sporidium in the centre of each, and after
being converted into zoospores, the sac comes through a rupture in the
joint into the water, opens, and sets the zoospores free.
The genus Cladophora, which has twenty-five species in the British seas
alone, forms tufts of jointed filaments from four to eight, ten, or even
twenty inches high. In some species the filaments are rigid, bristly,
and wiry; in others they are soft and silky; but they are always richly,
variously, and sometimes densely, branched and rebranched. In some the
branches and branchlets are forked, in others tripartite; the Cladophora
pellucida, which is a rigid, wiry plant, combines both these forms.
The genus Bangia consists of purple filamentous jointed and unbranched
Algæ, which are distinguished from all others by the microscopic
arrangement of their endochrome, which is enclosed in little cells
placed according to a definite plan within the transparent and tubular
joints of the filaments. In the Bangia fuscopurpurea, whose blackish
purple tufts, several inches long, cling closely to the rocks near
high-water mark, the tubular joints contain rows of minute colour cells
radiating from a centre. In the narrow filaments there is but one colour
cell in a joint, but in the broader filaments there are from three to
five, forming a tesselated line across it. In this plant one spore is
produced in each joint. The Bangia ciliaris forms a scarcely perceptible
rosy pink fringe of hair-like jointed filaments on the Zostera marina,
and also on other Algæ. The filaments are not more than the tenth or
fifth of an inch long, consequently their joints are most minute, yet
the microscope shows that they contain from two to three colour cells
set as if radiating from a centre, and that the granular endochrome in
each cell is converted into two zoospores. The Bangia ceramicola, which
forms purplish pink tufts on small Algæ in rock pools, differs from both
of the preceding. The joints of its filaments are once or twice as long
as they are broad, and contain colour cells like long upright lines. By
aided vision zoospores are seen to be formed within the linear colour
cells, then the cells run together into a globular mass, which bursts
through the cell wall, leaving the joint empty. The whole genus is soft
and sometimes gelatinous.
The Enteromorpha genus is characterized by a cylindrical and tubular
stem and branches. These plants form two groups, one whose filaments and
branches swell from a narrow base upwards and terminate in a blunt
extremity, while in the other group the tips of the branches are
pointed. The Enteromorpha intestinalis, which is an inhabitant of many
seas, has a thin membranous, tubular, cylindrical, and unbranched stem,
inflated upwards into a broad round head, being more or less wrinkled
and curled throughout. Downwards it tapers to a fine thread, and
although attached at first, at last it becomes floating. Several of
these plants rise from the same root, sometimes to the height of two
feet, at others not more than an inch, and they are of every width, from
the tenth of an inch to three inches, their colour being grass green.
The typical form of the other group is much branched, and all the
branchlets are finely pointed.
The three genera Codium, Bryopsis, and the marine Vaucherias are all
soft plants characterized by their filaments being tubular, however much
they may be branched. They agree also in being reproduced by zoospores
developed from the green matter within little sacs attached to the
exterior of their filaments. The species of the genus Codium differ
much, although formed of similar elements. In the C. tomentosum, which
is from three to twelve inches long, the dark green stem is thicker than
a crow’s quill and much branched; while Codium Bursa, on the contrary,
is a dark green round spongy lump of tubular filaments, densely
interwoven and matted together. These masses, which are from one to
eight inches in diameter, become hollow when old, and different sizes
and ages grow together in a group.
The Bryopsis is a yellowish green tubular plant, from two to four inches
high, plumed like a feather, and sometimes replumed. It is a rare plant
in England, and grows on the larger Algæ in deep water.
The Vaucheria marina forms soft limp tufts of hair-like filament filled
with bright green matter, which often runs partially out. It is from one
to three inches high, and has a few long upright branches, to which are
attached small stalked pear-shaped sacs containing zoospores. Both this
plant and the Vaucheria velutina, grow on muddy shores.
[Illustration: Fig. 22. Ulva latissima: _a_, portion of ordinary frond;
_b_, cells in which the endochrome is beginning to break up; _c_, cells
from the boundary between the coloured and colourless portion, some
containing zoospores; _d_, ciliated zoospores; _e_, development of
zoospores.]
The Ulvas, which are the grass green layers seen on all our coasts,
originate in the simple vegetable cell, whatever form their foliaceous
fronds may ultimately assume. When the cell is divided in one direction
only, a confervoid filament is the result; and if the filament should
increase in breadth as well as length, according to a determinate law, a
ribbon-shaped frond may be produced; but when the original cell is
divided into four cells, and each of these four and all their successors
undergo similar division, the increase being as the series 1, 4, 16, 64,
&c., a membranous expansion is formed, in which all the cells are firmly
attached to one another, and every portion is the exact counterpart of
another. The cells of the Ulvas frequently exhibit an imperfect
separation of the granular endochrome into four parts preparatory to
multiplication by double division, and the entire frond or leaf shows
the groups of cells arranged in clusters containing some multiple form
of four, as in fig. 3, page 171.
The frond membrane of the true Ulvas, as that of the Ulva lactuca, is
formed of but one layer of cells; the frond itself is thin as cambric
paper, almost transparent, and of a pretty light green. When young it is
a puckered inflated bag, which afterwards bursts and opens into a flat,
ribless, wavy, more or less rounded expansion, three to six inches long,
and as many broad. This plant, which is attached to the rocks between
the tide marks on our shores, is rare in the Mediterranean; nor is it so
common in Britain as the Ulva latissima (fig. 22), which is cosmopolite,
and abundant everywhere. It is found as a ribless irregular expansion of
a full bright green in deep water, and of a yellow apple green when in
shallow water, and exposed to the light. The base and stem are very
short, and the frond, which the microscope shows to be formed of two
layers of cell membrane, spreads so rapidly into crisp wide-lobed
foliations, that the parts often overlap each other in stiff bulging
folds. It is from six inches to a foot in height, and from three to
twelve inches wide. The frond of the Ulva Linza is also formed of two
layers of cells, but so small and so closely pressed together that the
two layers can only be detected by the microscope. This plant, which is
from six inches to two feet long, is a ribless, narrow, ribbon-shaped
expansion with curled wavy edges tapering to a base, and either blunt or
pointed at the top. Its colour is the same as that of the Ulva lactuca.
In the Ulvas, which are multilocular plants, some cells are selected to
bear fruit, and others not. The granular endochrome of these chosen
cells divides into several parts, which are at first in close contact
and at rest; then they become restless, acquire four or a greater number
of cilia, and pass through a fracture in the cell wall into the water,
in which they swim freely as zoospores. After a time they come to rest,
attach themselves to some object, and begin to grow. The walls of the
cells which have thus discharged their endochrome in the form of
zoospores, remain as colourless spots on the frond. The whole colouring
matter of a portion of the frond may escape as zoospores, leaving behind
it nothing but a white membrane. With a microscope, this process may
sometimes be observed in all the different stages of its progress.
Every full-grown Ulva has its own precise and definite form, but
whatever that may be, the young plants on their first appearance from
the shore are in all respects similar to Confervas; the top cells soon
divide, and a plane or sac-like frond is formed.
Certain Ulvas, which have a yellow tint, produce small zoospores with
only two cilia, but in the Ulva bullosa and the Ulva latissima four
zoospores are produced in the same cell, each having four cilia. The
same fructification prevails also in the purple Ulvas—Porphyra laciniata
and vulgaris. The latter is seen in winter and the early spring,
covering the rocks near high water mark, with its tiny bright purple
lanceolate leaves. Later in the season it grows into a flat narrow
ribless frond with a pointed end, and about two feet long, the margin of
the frond becoming waved and plaited as the plant increases in growth.
At a later period, it is seen mixed with the Porphyra laciniata, which
is a ribless flat frond of a dull purple; sometimes it is very thin,
divided or torn, and occasionally growing in a circle round its root.
Both forms are sold as laver.[40]
The RHODOSPERMEÆ, Florideæ, or Rosetangles, are the most beautiful of
the marine vegetation. No sea plant surpasses them in delicacy and grace
of form or richness of colouring, but the most beautiful are seldom
seen, because they grow below the line of ebb tides, or under the
shelter of other sea weeds in the rock pools left at low water, their
crimson tints being deepest when sheltered from strong light. The
Rhodosperms, which have representatives in every sea, are much more
numerous than the green Algæ both in genera and species. Thirteen
orders, comprising sixty-seven genera, inhabit the British coasts. Many
are exceedingly minute, forming patches and velvety cushions on rocks
and other Algæ; a vast number have jointed filamentous fronds, while
others consist of tubular filaments, and many exhibit a shrub-like
collection of firm branches; some are flat and foliaceous expansions
without a midrib, either thin and delicate, or thick and strong, while a
very brilliant group of both narrow and spreading fronds possess a
midrib as a distinguishing character. The structure of the frond varies
from a simple membranous to a cartilaginous or even horny substance,
caused by a greater development of the cellular tissue, which in the
higher kinds of Florideæ divides the epidermal layer or skin from the
parenchyme or spongy matter within.
The mode of reproduction by tetraspores, as well as by simple spores,
distinguishes the Rhodosperms from the other two great divisions of the
marine Algæ. These bodies are produced by the division of the red or
crimson endochrome into four parts, which remain in the cells till they
acquire an envelope; their form, which is much varied, depends upon that
of the endochrome. Some are produced by the breaking up of a globe of
endochrome from the centre into four pyramidal segments; or should the
endochrome be elliptical, by dividing it into four by three parallel
segments, or a mass may be divided into four by horizontal and vertical
sections. Some of these are represented, greatly magnified, in fig. 23.
The tetraspores are lodged in wart-like excrescences, immersed either
partially or wholly in some part of the frond.
[Illustration:
Fig. 23. A, Polyides rotundus:—_a_, thin slice showing the
wedge-shaped spores; _b_, tetraspores.
B, Furcellaria fastigiata:—_c_, thin slice showing a nucleus with the
dividing spores; _d_, one of the large cells; _e_, a tetraspore.
]
The simple spores are produced within colourless tubercules called
nuclei, variously situated upon the plant, as at fig. 23 _a_, _c_. These
nuclei contain many microscopic spores. Sometimes the nuclei are
enclosed in conceptacles or ovate sacs, which are either perforate or
not at the apex. These contain many microscopic strings of cells like
jointed threads, and the endochrome in each joint of these threads is
converted into a spore successively from the summit downward. Sometimes
the endochrome in one or two joints only, becomes a spore whether
terminal or central, and when the spores break through the joint wall
and fall off from the threads, they are collected without any definite
order into a mass within the nuclei. Sometimes new joints or cells are
produced on the threads when the old ones have yielded their fruit.
Occasionally a globose nucleus contains several secondary nucleoli full
of spores. In every instance, the perfect spore is a dense grumous mass
surrounded by a hyaline sub-gelatinous coat consisting of at least two
membranes. The situation, mode of growth, and structure of the nuclei
vary almost infinitely, and together with the structure of the frond
afford the distinctive marks by which the genera are separated from each
other.
The spores and tetraspores are equally capable, like buds, of
reproducing their species; but the spores are believed to be in some
cases fertilized by spindle-shaped particles, and consequently are
considered to be the true fruit. Antheridia, or sacs containing these
particles, have been discovered in various genera of Rhodosperms.
Although, as a rule, the red Algæ have two modes of vegetative
reproduction, yet there are various species in which tetraspores only
have hitherto been met with.
[Illustration: Fig. 24. Vertical sections of conceptacles:—_a_,
Gracilaria armata; _b_, Grinnelia americana; _c_, Corallina officinalis,
the membrane of which, more highly magnified, is shown at _d_.]
A large proportion of the higher Rhodosperms is distinguished from those
possessing the preceding mode of fructification by the internal
structure of their reproductive nuclei. In some of these Algæ the nuclei
are divided into two equal chambers by a fibro-cellular substance to
which the spores are attached; in others, pear-shaped spores radiate
from a fibro-cellular substance at the base of the nucleus. There are,
moreover, Algæ which have nuclei containing conical spores whose broad
bases radiate from the centre, and other arrangements occur.
The Rhodosperms are comparatively small plants. Some which form velvety
cushions on stones, or minute tufts on small Algæ, are only the fraction
of an inch high, but the larger kinds range from one to four, six, ten,
or twenty inches; probably none exceed two feet. In thickness, some
fronds are fine, like jointed and branched hairs, while others are
thick, like hog’s bristles or crow quills. Numerous as the forms are,
the simple jointed filamentous frond is connected by a series of forms
with the highest order of the class.
A great portion of the Rhodosperms on the British coasts is composed of
the exquisitely beautiful order of the Ceramiaceæ. They abound in every
rocky pool, on every piece of wood that has been long exposed to the
waves, on rocks and stones, and, above all, they fringe the Zostera
marina, or sea wrack, as well as the firmer Algæ, with every shade of
red from bright crimson to purple. They are articulated filiform plants,
approaching in simplicity of form to the Confervas. The genus
Callithamnion, which has thirty species in the British seas, consists of
cylindrical jointed threads more or less profusely branched, and
distinguished by having the divisions between the joints opaque and of
various shades of red and purple, while the joints themselves are
transparent and colourless, so that the stem and branches appear to be
striped across by alternately white and coloured bands which are often
visible to the naked eye, notwithstanding the smallness of the plants
and the delicacy of their filaments, as the C. sparsum,—which is a soft
purple tuft of jointed threads scarcely one-tenth of an inch high.
The Callithamnion corymbosum has a soft jointed filamentous stem,
hair-like below, fine as a cobweb above, and excessively branched, with
dichotomous branches. In fig. 25 _a_ represents a thread of this plant
with tetraspores, much magnified; _b_, a portion of the same, more
highly magnified; _c_, a thread with naked nuclei, gongylospermous, that
is, filled with a mass of spores, magnified; and _d_, a spore, magnified
more highly. The nuclei are naked in all the Ceramiaceæ.
[Illustration: Fig. 25. Callithamnion corymbosum.]
The genus Ceramium, some species of which have spinulose branchlets, is
characterized by the tips of the forks of its terminal branchlets being
hooked inwards, and by the stems and branches being striped by alternate
hyaline and coloured bands as in the preceding genus, though the
arrangement of the colours is somewhat different. The Ceramium ciliatum,
which is a dense tuft of capillary jointed filaments, from two to six
inches long, repeatedly and regularly forked, has the tips of the last
forks so much hooked inwards, that the extremities of the branchlets
look as if they were heart-shaped. It has minute spores in globular
nuclei, sessile on the branches with two or three branch-like hairs
beneath them, and tetraspores set in the coloured parts of the joints
with a thorn between each, for in this plant the centre of the joint is
hyaline, the rest coloured.
The genus Griffithsia contains various species of bright rose-coloured
plants, which become bleached when put into freshwater, and form a
circle when spread out. Soft, tender, and gelatinous, they form dense
tufts of jointed and branched filaments on rocks at low water mark. The
filaments are slender below, capillary and forked above, and the joints
contain one linear upright rose-coloured tube, which is seen throughout
their transparent walls: a distinguished mark of the genus. Tetraspores
are borne on the hair-like jointed ramuli, and spores are amassed in
coated roundish sessile nuclei, surrounded by minute hair-like fibres.
Several species once called Griffithsia differ so much from the others,
that they are by some referred to Halurus, which has the stems and
branches thickened by overlapping whorls of tiny forked jointed and
curved ramuli. They are propagated by spores, enclosed in clusters of
nuclei borne on the tips of short branches, with a mass of curved ramuli
folding over them, and by tetraspores attached to the inside of another
set of curved ramuli. Antheridia have been discovered in several genera
of the group Ceramiaceæ, especially in Ceramium, Callithamnion,
Griffithsia, and Halurus. They consist of little clusters of cells
variously arranged, in which the active particles known as spermatozoids
are generated.
The Polysiphonia are Algæ, seen in tufts from ten to twelve inches long,
of usually much branched jointed filaments, on rocks, corallines, and
the smaller Algæ at low water mark. The joints of the filaments contain
upright tubes, full of purple or reddish brown matter, which is seen
through their transparent walls. The number of these colour tubes vary
from four to ten, eighteen, or even twenty, and form the characteristic
of the genus. Thus there is a similarity of structure between the
Polysiphonia, a genus of the highest order amongst Rhodosperms, and the
Griffithsia, which is one of the lowest. The Polysiphonia elongata,
which is from six to twelve inches high, has four primary and several
secondary colour tubes in the transparent joints of its filaments. Like
many of its congeners, this plant does not come to perfection or bear
fruit till the second spring. In its youth, it resembles the full grown
plant but is smaller, and the colour tubes are not formed in the
capillary threads of the tufts, which with many of its branchlets are
deciduous, leaving the plant in its naked winter state. With returning
warmth, it assumes its perfect form, and in March and April bears fruit,
which consists of nuclei in conceptacles, sessile on the branches,
either clustered or scattered. The spores are at the top of jointed
threads rising from a substance at the base of the nuclei. In some
species of this genus, tetraspores only have been found.
The Cryptonemiaceæ are the most numerous and diversified of all the
orders of the Rhodosperms. Thirty-five genera are widely dispersed
throughout the world, chiefly in the northern hemisphere; twenty-four
genera at least occur on the east coast of North America; and fifteen
genera have representatives in the British seas. This multitude of
generic forms is divided into two groups of gelatinous structure, the
one having inarticulate fronds composed of articulate threads closely
incorporated, the other membranaceous, formed of cells closely
incorporated into a foliaceous expansion. Most of these plants have a
stratum of cellular tissue, interposed between a spongy matter in the
interior of the frond, and the epiderm or external skin, which for the
most part consists of a simple layer of minute cells firmly united by
their sides, generally forming a mere film; but it may be thin and
flexible, thick, tough, or leathery, according to circumstances.
The Furcellaria fastigiata (fig. 23 B) has an intermediate layer of
cellular tissue between its skin, and a pulpy interior. The frond is
cylindrical, smooth, strong, and opaque, repeatedly forked with long
narrow forkings. The root is fibrous, and the stem short and tapering.
Masses of spores nestle under the skin and swell out the upper forkings,
and oblong tetraspores are deeply imbedded in the same.
In the Dumontia filiformis the simple undivided stem and branches are
filled with a watery jelly.
The stem of the Chylocladia kaliformis is a cylindrical tube, from four
to eighteen inches high, constricted at intervals of half an inch or
more into long hollow joints; branches of the very same construction but
smaller spring from each constriction either opposite to one another or
in whorls; these again have lesser branches, all tapering more or less
to each end. The plant, which is of a pink colour fading to greenish
yellow, is propagated by tetraspores imbedded in the branches, and by
transparent conceptacles sessile on the branchlets, enclosing nuclei
containing pyramidal spores. We neither possess the Constantinea rosa
marina, nor the C. sitchensis, some of the largest and finest plants of
the group, both being inhabitants of high latitudes, but there are some
very pretty species on the British coasts. They are supposed to be
annuals.
The red dulses belong to the foliaceous and gelatinous part of this
order. The Chondrus crispus, or Irish moss, sold as Carrigeen, is very
common on rocky coasts in the northern seas. It is from three to eight
inches high, and exceedingly varied in form. The frond is thickish,
firm, and elastic, with a stratum of cellular tissue under the skin,
which is probably much developed, as the plant becomes horny when dried.
It is reproduced by tetraspores, in large oval groups scattered all over
the surface, often prominent on one side only, and, in some rare
instances, spores in prominent oval conceptacles are immersed in the
lesser frond divisions. Besides these are warts composed of radiating
threads, possibly antheridia, but not made out.
[Illustration: Fig. 26.—A, Rhabdonia Coulteri, portion of nucleus. B,
Sphærococcus coronopifolius; portion of nucleus and single spore. C,
Wrangelia penicillata, spore threads. D, Cruoria pellita, tetraspores.]
The Rhodymeniaceæ are sometimes filiform, but for the most part they are
compressed flat cellular fronds, spreading widely from a short delicate
stem. They are usually of a blood red, but Rhodymenia palmata, or common
Scotch dulse, is of a dark purple. The tetraspores are variously
disposed, and simple or compound globular conceptacles containing nuclei
are either attached externally to the filiform fronds, or partly
immersed in those that are foliaceous. The spores are produced in the
joints of moniliform threads within the nuclei, which are sometimes
divided into two chambers by threads running from wall to wall.
Rhabdonia (fig. 26 A) belongs to this group.
The Wrangeliaceæ are filiform, many species consisting of a central
thread coated more or less with smaller ones, sometimes so disposed as
to form a most elegant lacework. Each joint of the stem, branches, and
branchlets is beset with whorls of short slender forked and jointed
ramuli. They have clusters of spores in stalked capsules. The spore
threads of Wrangelia penicillata (fig. 26 C) are surrounded by a whorl
of ramuli composed of radiating pyriform spores arising from the
endochrome of terminal cells.
The Squamariæ resemble lichens in spreading themselves in a red crust
over stones and rocks. They have roots below, and warts, on their upper
surface, in which there are tufts of moniliform spore-bearing threads.
The tetraspores of Cruoria pellita are shown in fig. 26 D; its
repeatedly forked filaments taper upwards, and the tetraspores are
formed in the swollen centre cell of the filaments. The Peyssonnelia
grows on shells and other marine objects, and extends from the
Mediterranean to Ireland, and the east coast of North America.
The Polyides rotundus (fig. 23 A), representing the only genus of the
Spongiocarpeæ, has a dark purple solid gristly cylindrical stem,
repeatedly and regularly forked, all being of the same thickness. The
tips of the last forkings, which are small and equal, give the top of
the plant a rounded form. The microscope shows that the stem and
branches are composed of a central column of interlaced threads and
radiating cells; it shows, moreover, that hyaline nuclei containing a
cluster of conical spores whose broad bases radiate in all directions
from a centre, as in fig. 23 _a_, are scattered among the articulated
threads of oblong irregular spongy warts which clasp or embrace the stem
and branches. The tetraspores are buried in the ends of the last forks.
This plant is so like the Furcellaria fastigiata (fig. 23 B), that it
affords a remarkable instance of similarity of form and total diversity
of fructification, not only in the spores and their arrangements, but in
the form of the tetraspores; for in the Polyides they are formed by two
sections, one vertical and the other horizontal, while in the
Furcellaria the endochrome is divided by three annular sections, as in
fig. 23 E.
The Gelidium corneum, common in Britain and almost everywhere,
representing the group Gelidiaceæ, is opaque, firm, and of a dark
purple. The axis with its alternate and repeated branches lying all in
one plane, is composed of confervoid threads. This plant is
distinguished by having its spore-cases or nuclei divided into two
chambers by a fibro-cellular substance; the spores are either attached
to this or to a network of threads; these bodies and the tetraspores are
lodged in the tips of the branchlets. It is one of the most variable of
all Algæ.
The Sphærococcoideæ comprise some of the most common and beautiful Algæ,
remarkable for their brilliant rose and purple tints. This section
consists of those red Algæ which have their nuclei lodged in an external
subglobose conceptacle, the spores being formed at the tips of jointed
threads rising from a substance at the base of the nucleus. A portion of
a nucleus of Sphærococcus coronopifolius, and a single spore magnified,
is shown at fig. 26 _b_. The tetraspores are variously disposed. The
fronds in this family are either gristly or membranaceous, and totally
different from those which follow. They often assume a leafy aspect from
the regularity of the nerves, which sometimes perform the functions of a
stem when the membraneous border has decayed, and then they give rise in
turn to new fronds. That happens in some species of Nitophyllum: a very
short stem rises from a minute disc, and spreads widely into a flat
ribless expansion, more or less deeply slit into broad rounded
divisions. Wavy nerves from the top of the stem spread through the
fronds, which are left bare in winter, and give rise to new fronds in
spring. The leaves of the Delesseria sanguinea, from two to eight inches
long and from one to six inches broad, are of the richest colour and
most delicate structure, with evenly curled edges, and a firm solid
stem, with prominent midrib and nerves. In winter globose stalked spore
conceptacles are borne on the skeleton midribs of the summer’s leaves
from which the margin has decayed, which thus become the stems of the
next year’s plant. In this plant tetraspores in small special stalked
leaflets fringe the skeleton midribs; in the Nitophyllums they are
either scattered in dots over the frond, confined to the centre, or in
lines round the margin. As regards the internal structure of this order,
nothing can be more various, but they never acquire a truly articulate
form. The genera and species of this group are widely distributed; they
have many representatives in the Mediterranean.[41] The genus
Sphærococcus is confined to Europe, while numerous genera are
exclusively tenants of the southern hemisphere. The Gracilaria
lichenoides, the Ceylon moss, is celebrated for its gelatinous
qualities; and the Gracilaria compressa on our own shores is excellent
as a pickle or preserve, and very ornamental. One of the most beautiful
Algæ known is the Grinnelia americana, which abounds on the eastern
coast of North America; fig. 24 _b_, is a vertical section of its
conceptacle, showing the rudimentary placenta and spore threads. It
differs singularly from the Delesseria sanguinea, of which it is an
exact analogue, in the capsules being scattered over the surface of the
frond instead of being situated on the midrib.[42] The Delesseria
sanguinea is now known as the Wormskioldia sanguinea.
The Corallines are florid Algæ, which absorb such a quantity of
carbonate of lime from the surrounding water, that they become rigid,
hard, and often stony. They are purple or pink when fresh, white and
sometimes brittle when dry, and are propagated by strings of spore
threads rising from the base of the nuclei which are enclosed in
conceptacles or spore cases, open at the top. Some are articulate,
composed of closely compacted threads, as the Corallina officinalis, a
pretty little branched and bushy plant, most luxuriant in deep water,
and particularly abundant in the rocky pools. Its urn-shaped spore sacs
are attached to the tips or sides of the branches; fig. 24 _c_ is a
vertical section of one of them magnified, and _d_ is a membrane of the
same, more highly magnified, with impressions of the external cells. The
joints of the articulate corallines, which are flexible and vary much in
length, are either free from carbonate of lime, or ornamented with
calcareous plates; it is through these open spaces that the plant is
believed to obtain nourishment. The forms of the corallines are varied
beyond description; many are mere amorphous crusts on stones and sea
weeds, increasing from the centre outwards as in the lichens, others are
lobed and branched like real corals. Corallines ascend to very high
latitudes, but abound most in warm and tropical seas: either free, or
coating pebbles at vast depths, they form the last zone of vegetable
life.
The Laurenciaceæ have fronds which are soft and thread-like, or solid,
fleshy, and inarticulate; both are repeatedly branched. The colour of
these plants is purple or a dullish red, but they are extremely
sensitive to the influence of light and air, changing through every
shade of orange, yellow, or green, according to the exposure, and like
many other florid Algæ they lose their colour in fresh water. They are
amongst our commonest sea weeds. The Laurencia pinnatifida is the pepper
dulse of Scotland, and is also native on the eastern and western coasts
of North America. Species have been found at the Cape of Good Hope,
Australia, and New Zealand. The fructification in this section is quite
peculiar. They have tetraspores lodged in the branchlets; and egg-shaped
conceptacles with a terminal pore, enclosing nuclei with pear-shaped
spores radiating from a fibro-cellular mass at their base. The
antheridia, which differ in the different species, attain a greater
degree of complication than in other tribes. In Laurencia tenuissima
they form curious lateral twisted plates of a greyish tint, bordered
with large cells. The plate is occupied by the productive cells of a
much smaller size, evidently springing from a cellular branched axis. In
Laurencia pinnatifida instead of a plate there is a somewhat hollow
cup-shaped disc, formed of dart-like vertical groups of pale cells
surmounted by two or three larger oily-looking sacs filled with yellow
pigment. These bodies are sometimes forked, and appear to shoot out from
the mass. L. dasyphylla presents a third modification, the antheridium
being a sac, and the dart-like groups of cells being ejected from the
minute terminal orifice. The moving particles produced in the cells of
these three forms, differ a little in shape, and as they do not
germinate they are believed to be spermatozoids, though no cilia have
been found on them.
The Rhodomelaceæ, the last and highest family of florid Algæ, are, as
the name implies, of a rich red brown colour. None of the other
Rhodosperms can vie with them in peculiarity or variety of structure.
The fronds may be areolate or reticulate, filiform or variously leafy,
articulate or inarticulate.
[Illustration: Fig. 27. Dictyurus purpurascens.]
Some genera, as for example Dasya, have slender, often elegantly
branched threads, while such genera as Amansia and Odonthalia have
instead a flat and pinnatifid frond. The latter, which has a very
conspicuous cellular reticulation, is a genus of high latitudes, but is
common on some parts of the Scotch and North American coasts. The
British seas are rich in many genera of this order, and analogous forms
occur in the southern hemisphere, where there are at least twenty-three
genera. Many are remarkable for their singularity of structure: the
Claudea for example, which is one of the most elegant of the Algæ, has a
cancellated frond and is the ornament of warm seas; the Amansia and
Leveillea which are distinguished by the beautiful reticulation of their
fronds caused by large hexagonal cells; and the Dictyurus, in which the
net forms a spiral web round the principal stem. Fig. 27 shows a portion
of the network of Dictyurus purpurascens magnified. All the genera of
this order possess free areolate hollow conceptacles perforated above,
and containing nuclei, from the base of which short tufts of threads
arise, each bearing a large obovate spore at its apex. The tetraspores
are arranged in series either within the frond, or in distinct pod-like
receptacles called stichidia. Fig. 28 shows the Polyzonia cuneifolia
with its tetraspores arranged in rows in their pod-like stichidia,
together with the areolated conceptacle and spores, all highly
magnified. The antheridia differ in form in the different genera. In the
Dasya they assume that of pods full of cells, in which the motile
particles are generated; in the Rytiphlæa tinctoria the antheridia
resemble those of the Dasya except in being elliptical, and in the
Rytiphlæa pinastroides they are cellular bodies, without any investing
membranes, clothed with delicate hairs.
The form of the Rhodosperms, as well as the limits of the species, like
those of other Algæ, are affected by many circumstances known and
unknown, such as the depth, temperature, saltness, and currents in the
water. The Gelidium corneum varies to such an extent that its forms may
not only be considered as distinct species, but even as belonging to
different genera. The Delesseria alata is sometimes destitute of its
margin, and then its midribs alone being left, it has the form of the
Delesseria angustissima. Several species of the florid Algæ, which in
their natural state have the tips of their fronds even and straight,
occasionally produce hooked and clasping tips.
[Illustration: Fig. 28. Polyzonia cuneifolia.]
Brackish water is often a cause of change. The Irish moss, Chondrus
crispus, when exposed to the fresh water of an estuary acquires great
breadth and thickness, while at low water mark it is thin and has narrow
forked branches, and there are many intermediate forms. The fruit rarely
varies with these changes; its disposition and intimate structure, as
well as that of the frond, are the points of prime importance for the
determination of genera and species in the Algæ.
The MELANOSPERMEÆ, or Melanosperms, are olive-green Algæ, sometimes
inclining to brown. They have fewer species than the Rhodosperms, but
the individuals exceed in abundance and in magnitude all the other Algæ.
These large Melanospermous Algæ, which form marine forests in both
hemispheres, are excessively strong and tough on the exterior but of a
looser texture within, so that the cells of their tissue are of
different sizes and forms, according to the degree of pressure. The
stems and branches are more dense than the leaves. This highest order,
however, has small and delicate Algæ united to the largest by many
intermediate forms. The Melanosperms are either monœcious or diœcious,
and bear their olive-green spores in cases, that is cysts, variously
disposed on the plants. Many have two kinds of zoospores differing in
nothing but size; they are produced in different organs; in some species
both are fertile, in others only one, and, in these cases, the other is
therefore supposed to be a fertilizing body, but however that may be,
there are certainly antherozoids in this group of Algæ, especially in
the order Fucaceæ.
The Ectocarpeæ have many representatives on our coasts, all of which are
tufts of articulated threads from one to eighteen inches long, branched
or simple. They are generally soft, some so flaccid that they cling
together, but sometimes they are firm and stiff. The cysts which are
attached to these threads have various forms; they are spherical,
siliquose (that is, like long pods), or of other shapes, according to
the species; but whatever form they may assume, they are filled with a
dense endochrome. Besides these they have active granules contained in
other distinct organs. M. Thuret has decided beyond a doubt that the
latter are small zoospores, and it is presumed that the endochrome in
the cysts is resolved into zoospores, but of a different order, as in
the Ulvas. These two organs are for the most part situated on different
individuals; in Ectocarpus pusillus (fig. 29 _b_) they are on the same.
The different forms of fruit carpels are represented magnified in fig.
29.
[Illustration: Fig. 29. Fruit of Ectocarpus:—_a_, E. sphærosporus; _b_,
E. pusillus; _c_, E. fenestratus; _d_, E. fasciculatus.]
The Ectocarpeæ contain little or no gelatine, whereas the genera of the
group Chordariæ have soft gelatinous fronds of many forms, either
incrustations, convex lumps, or tubers, like the Leathesia so common on
our coasts; small plants as the Mesogloias, which have soft slippery
filiform stems beset with myriads of moniliferous worm-like branches; or
lastly the Chorda filum, a simple unbranched slimy cylindrical cord,
varying from a quarter of an inch to the thickness of a pencil, and from
one to twenty or even forty feet in length in deep water. The cord is
tubular, divided into chambers by transverse partitions, formed of
interlaced vertical and horizontal articulated threads. It tapers at
each extremity, and the exterior, which is brown, is clothed with
pellucid hairs. Vertical spores are immersed throughout the whole
surface of the cord, and Dr. Harvey says that, mixed with these, there
are numerous narrow, elliptical, transversely striated cells, which
according to M. Thuret produce zoospores. Each plant rises solitary from
its own little disc, but as the Chorda filum is a social plant, vast
assemblies of it cover extensive areas of sand and mud, and form dense
thickets in our northern seas. There are bands of it in the North Sea 15
to 20 miles long, and more than 600 feet wide; there is a submarine
forest of it in Skapta Bay, Orkney; and in passing through the sounds of
the western islands, as between Kerrera and the mainland, there are
others. The long cords always lean in the direction of the tide, and
must oscillate between two zones of rest, one at the turn of the flood,
and another at the turn of the ebb. When dried the people use them for
fishing lines. In the Chordaria divaricata both kinds of spore cysts are
external, and give rise to zoospores.
In the preceding divisions of the Melanosperms the fronds consist of
articulated threads; in the succeeding divisions the fronds are
inarticulate. The latter comprise four very remarkable groups, of which
the Dictyoteæ are distinguished by a leathery or membranous frond,
sometimes cylindrical, but mostly flat, the surface of which is
reticulated and sprinkled with groups or little patches of naked spores
or cysts. The endochrome in the cysts is sometimes quadripartite, or
even divided into eight parts. In one of the genera only, anything like
antheridia have been found. The zoospores produced from the
quadripartite endochrome are large, of a dark colour, and have two
lateral cilia, while the bodies in the filiform much divided antheridia
seated variously in the tufted threads are far more minute and pale, but
with similar cilia. This order obtains its maximum of development in the
tropical and subtropical regions; several species are found in the
Mediterranean, while a few occur on our coasts, and on those of North
America.[43]
The genus Dictyota begins the zonarioid group, whose structure is very
curious. Every band (lacinia) of the frond terminates in a single cell,
by the constant division of which at the lower side, the other cells of
the frond are formed, the terminal cell of the frond being thus
continually pushed onwards. Hence it results that the longitudinal lines
of superficial cells converge, thus affording a ready method of
ascertaining the genus in default of fructification. When a new centre
of growth is to be made, that is, when the frond is to become forked,
the terminal cell divides longitudinally and then each half-cell grows
according to its own law. Fig. 30 shows the tip of the frond of the
Dictyota dichotoma magnified; the cells on its surface are square, and
the interior of each has a spiral structure.
[Illustration: Fig. 30. Dictyota dichotoma:—_a_, tip of young frond;
_b_, cell showing spiral structure; _c_, threads from marginal cells;
_d_, sorus of spores; _e_, _f_, terminal cells dividing into new centres
of growth.]
The Padina Pavonia, or Peacock’s-tail laver of our southern coast, and
those of North America and the Mediterranean, is sometimes included in
the genus Zonaria. The species is remarkable for its wedge-shaped
fronds, which are olive green shaded with rust colour, and, when in
fruit, they are striped across with dark concentric zones, which are
merely lines of spores immersed in the frond and seen through its
transparent superficial membrane. Each zone is ornamented with a fringe
of orange-coloured hairs. Parallel to, or rather concentric with, the
spores, is a row of articulated threads, which bear so strong a
resemblance to the antheridia of the Cutleria that a similarity of
function is suspected by Mr. Berkeley. Species of Zonaria, Padina, and
Haliseris, which is the most highly developed of the Dictyoteæ, are most
abundant in tropical and low latitudes.
The Cutleria multifida is a small plant not exceeding eight inches in
length, of an olive green varied with rusty tints. The frond is a flat
ribless expansion many times variously slit in the upper part. It is
beautifully marked by prominent dot-like tufts of fructification
scattered over both sides of the frond, and grows on rocks and shells in
from four to fifteen fathoms water.[44]
The great Laminariæ form the principal part of those vast submarine
forests which encircle the globe in the arctic and antarctic oceans.
None of these gigantic Algæ are to be met with in low latitudes, but
there are several smaller species. The Laminaria debilis of the
Mediterranean is not more than five inches high, and we have some
ribbon-shaped species also of small size. Besides, many small
individuals of the large species grow on our coasts at low water mark or
below it; but the largest individuals are only found at depths suited to
their size, so that the great Laminaria, or tangle forests, extend from
low water mark to a depth of fifteen fathoms.
The fronds of these Algæ are for the most part leathery and of a
fibro-cellular consistence. The Laminaria bulbosa is the largest of our
sea weeds. Mr. Berkeley says that individuals are sometimes found which
are a sufficient load for a man to carry. A flat stem, often more than a
foot long, rises with a twist from a round hollow bulb a foot in
diameter, throwing out numerous stout fibrous roots below; the stem is
bordered by a thin wavy membrane, whence these plants are commonly
called sea furbelows. At the top of the stalk there is a broad leafy
expansion cut into straps or segments, twelve or more feet long, and
from one to two feet wide.
The Laminaria digitata, commonly called the great tangle, oar weed, or
sea girdle, has a fibrous root, a stem six or more feet long, with a
wide expansion at its top cut into very long narrow segments. The fronds
of some Laminariæ are deciduous; the stem increases in size year by
year, a new frond springing from the apex and replacing the old one,
which at last separates from the point of junction with the new frond,
to which it is attached till the latter has attained its natural form
and dimensions.
The Laminaria saccharina, called the devil’s apron on our northern
coasts, is of a greenish olive when young, brownish when old. It has a
fibrous root, a stem several feet long, ending in a flat ribless
ribbon-like expansion, always very much longer than the stem, and
terminating in a point. The margin of the frond is even, but wavy or
puckered.
‘The fruit of these three great Laminariæ is imbedded here and there in
the surface of the frond, thickening it and forming cloudy patches.’[45]
It consists of thick club-shaped perpendicular cells in which the
endochrome is ultimately divided into four parts. This is certainly the
case in the Laminaria bulbosa, and also in the Alaria Pylaii, a species
of which latter genus, the Alaria esculenta of our own coasts, is a much
esteemed British dulse.
Abundance of colossal Algæ are found in the North Pacific, about the
Kurile and Aleutian Islands, and along the deeply indented and
channel-furrowed northwestern coast of America. The Nereocystis Lutkeana
forms dense forests in Norfolk Bay, and all about Sitka. Its stem
resembles whipcord, and is sometimes 300 feet long. It is exceedingly
slender at the top, where it terminates in an enormous air-bladder six
or seven feet long, and about four feet and a half in diameter at its
widest part, the lower extremity passing into the stem. This huge
air-vessel, which is the usual seat of the sea otter, is crowned with a
tuft of twin leaves mostly rising on five stalks. These leaves, which
are membranous and lanceolate when young, and from one to two feet long
and two inches broad at the centre, are only marked with a few faint
nerves, but they ultimately split lengthwise, cover a large space, and
attain a length of twenty-seven or thirty feet, or even more. The growth
of the Nereocystis must be enormously rapid, since it is an annual, and
must therefore develop its whole gigantic proportions in one summer.[46]
Boats cannot pass through the floating masses of this plant, whose stem
is used for fishing lines, and whose cylindrical air-vessel serves as a
siphon for pumping water out of boats.
The Thalassiophyllum Clathrus is also an inhabitant of the Russian coast
of North America. It is about six feet high, very bushy and branched,
each branch bearing a broad leaf at its extremity which unfolds
spirally, and by this gradual development produces the stem with its
branches and lateral divisions. A spiral border wound round the stem
indicates the growth of the frond, which presents a large convex bent
lamina without nerves, or a leaf of which one-half is wanting. Numerous
long narrow perforations, arranged in a radiating form, give it the
appearance of a cut fan.
The Macrocystis pyrifera and the Laminaria radiata are the most
remarkable of marine plants, for their gigantic size and the extent of
their range. They are met with on the antarctic coasts two degrees
nearer the pole than any other vegetable, except the Diatomaceæ. The
stem of the Macrocystis is slender, smooth, round, and slimy, rising
from a fibrous root, like other Laminariæ, and bearing at its tip a
lanceolate or oblong lanceolate frond. This frond divides at the base;
the fissures extend upwards so as to form two petioles, each of which
swells into an oblong or pyriform air-vessel. Another fissure is formed
in a similar way a little above, and so on, till a single frond may at
the same time have eight or ten fissures, each of which will ultimately
gain the common apex. The margins of the fissures are at first perfectly
smooth, but they soon become ciliated like the outer edge. The
continuity with the fibrous base is at last broken, and the divisions of
the leaves going on indefinitely, the whole reaches the length of some
hundred feet, forming enormous floating masses which are wafted by the
waves hundreds of miles from their origin. Fructification only takes
place in young plants; consequently in such as are still attached to
their native rocks. Even in that youthful state, Mr. Darwin mentions
that such is the buoyancy of this powerful weed, that there is scarcely
a loose block of stone on the coasts of Cape Horn that is not buoyed up
by it.[47] The Macrocystis is native on the shores of the Atlantic, from
Cape Horn to 43° S. latitude; but on the Pacific coast, according to Dr.
Hooker, it extends to the river San Francisco in California, and perhaps
to Kamschatka. The plant is reproduced by pyriform cells, full of
endochrome, in nearly parallel rows imbedded in the fronds.
The rocky coasts of the Falkland Islands are covered with a vast growth
of the gigantic Macrocystis mixed with forests of the arborescent
Lessonia, which forms large dichotomous trees with a stem from eight to
ten feet high and a foot in diameter. The leaves are two or three feet
long, drooping from the forked branches like weeping willows. In the
Lessonia nigrescens the quadripartite endochrome, ultimately resolved
into spores, is contained in thickened club-shaped cells springing
vertically between the surfaces of the frond.
A transverse section of the stem of many of the larger sea weeds
presents zones, formed period by period, corresponding with the
development of the laminæ, roots, and branches. The stem of the Lessonia
bears a strong analogy to that of dicotyledons in having rings of
growth, though there is a great difference. As increase in Lessonia
takes place by the constant division of a flat leaf, the basilar portion
of which becomes the petiole and ultimately swells into a branch, the
stems have always a more or less elliptical form, and their section
exhibits an elliptical core. This form of the core is not however
peculiar, but exists in other Algæ. It is probable that the Lessoniæ,
although attaining so large a size, are really of rapid growth.[48]
The Ecklonia is essentially a southern genus, though one species ascends
to Spain and the Canaries. The frond is pinnatifid, the segments arising
from the evolution of marginal teeth. The stem of the Ecklonia
buccinalis, which is three or four inches thick and strongly inflated
above, exhibits rings of growth with an orbicular central pith.
The group of the Fucaceæ exhibits the highest structure of all the
olive-green Algæ, and forms a large portion of the sea weeds on our
coasts, but they abound more in individuals than in the number of genera
and species. A few have cylindrical stems and branches swelling out at
intervals into large oblong inflated air-vessels, which gives them
buoyancy in the water. The rest have a flat, ribbon-like stem, and for
the most part dichotomous branches with a decided midrib, but no
air-vessels, because they chiefly grow at half-tide level, and are
exposed twice every twenty-four hours. The most common of our fuci, the
Fucus vesiculosus, or bladder-wrack, has a midrib with air-vessels,
generally in pairs on each side of it, formed by the inflation of the
frond; these vessels, however, are frequently wanting, for it is the
most variable in form and most widely spread of the Fuci. The
fructification of this group is contained in large clavate receptacles
or expansions of an orange or greenish yellow colour situated at the
extremities or borders of the branches.
MM. Thuret and Decaisne discovered, by microscopic investigation, that
the fuci have a truly sexual fructification, consisting of male and
female cells inclosed in these receptacles. In the common Fucus
vesiculosus it was found that the male and female cells are either in
different individuals, or in different conceptacles on the same
individual; whilst in the Fucus platycarpus, both the male and female
cells were found to be contained in a globular cavity enclosed in the
flattened receptacles which grow at the extremities of the branches. The
cavity is lined with jointed hair-like filaments formed of cells, some
of which are so long as to project through a pore on the surface of the
receptacle in a spreading brush (see fig. 31, where the whole is highly
magnified). Towards maturity, the cells of some of these filaments
assume an ovoid form; the white viscous, granular matter in their
interior acquires an orange hue, and is divided into a multitude of
hyaline particles, each having an orange spot and two cilia of unequal
lengths, which enable these spermatozoids to swim with great vivacity in
the water as soon as they are set free by the rupture of the cell in
which they are inclosed. Besides these, dark olive-green female cells,
of a large pyriform shape, are fixed to the walls of the same cavity by
very short stems; their contents spontaneously divide into eight spore
cells, never more; each contains a colourless viscous liquid, which is
mixed with protein and yellow-green matter, and is inclosed in a double
coat. ‘The coats are united at the base, and when the spores are ready
for dispersion, the inner coat bursts through the apex of the outer one,
dragging with it a portion of the latter in the form of a little
peduncle. The immediate covering of the spores at length bursts, and
they are set free.’[49] In Fucus serratus, vesiculatus, and nodosus,
swarms of spermatozoids are produced, but M. Thuret has proved by
experiment that they never come to anything of themselves, and the
unfertilized spores perish.
[Illustration: Fig. 31. Vertical section of receptacle of Fucus
platycarpus.]
When a fertilized spore begins to grow, it assumes a pear shape, and
sends out from its narrow end filaments or footstalks containing solid
yellow grains at their extremities, where a hook or claw is formed by
which it fixes itself to rocks or stones. The spore then divides itself
into four equal cells of a brown colour, and by the continued
subdivision of these into four, the plant increases in size, and assumes
a form corresponding to the genus and species of the spore. Dr.
Carpenter mentions that in the Fucaceæ there is also a multiplication by
zoospores. These bodies are produced within certain of the cells that
form the superficial layer of the frond, and swim about freely for a
time in the water after their emission, until they fix themselves and
begin to grow; but these are merely gemmæ.
All the Fucaceæ are tough leathery plants. This is even characteristic
of the genus Cystoseira, various species of which may be seen on our
coasts at low water mark, or in the tide pools. They are little
shrub-like and somewhat thorny plants, not more than three feet high,
with a cylindrical stem and many branches, near the extremities of which
there are inflated air-vessels, sometimes two or three together; in some
species they are lower down. Long spiny conceptacles are situated at the
tips of the branches, but the endochrome does not divide in the germ
cells as it does in the Fuci, so that each cell produces but one spore.
‘Throughout all latitudes the two divisions of Fucaceæ—Fucoideæ and
Cystoseireæ, form the prevailing marine vegetation to which the name of
sea-weed is commonly applied, and the different genera so arrange
themselves as to present, with a few exceptions, a most harmonious
assemblage.’ ‘None of these approach the tropics; the Fucoideæ abound
towards the poles, and there attain their greatest bulk, diminishing
rapidly towards the equator, and ceasing some degrees from the line
itself; while the immense genus Sargassum finds its maximum in lower
latitudes and under the equator itself. In the opposite cold and frigid
zones the waters are inhabited by certain genera of Fucoideæ, which are
in a great measure representatives of one another.’[50] The huge
D’Urvillæa and the Sarcophycus in the Antarctic Ocean represent the
Himanthalia and Fucus proper in the north, and the Cystoseireæ and
Halidrys of the northern seas are represented by the Blossevillea and
Scytothalia in the southern.
The frond of the Himanthalia lorea is a knob about an inch high,
somewhat like a small mushroom; by degrees the top of the knob sinks in,
and the frond becomes cup-shaped. In the second year of growth it throws
out from its centre strap-shaped receptacles from two to three feet long
and the sixth of an inch wide; they are slimy, forked, and entirely
covered with fruit. The true frond sometimes becomes hollow and swells
into a bladder. This singular plant, which grows on our coasts, extends
from Norway to Spain. In the D’Urvillæa, its representative in the
southern hemisphere, the frond and receptacle are united, for the plant,
which is of large dimensions, has dichotomous fronds ten feet long, and
an inch or more in breadth. Their surface is ornamented with large
cavities like a honeycomb, and the fruit imbedded within them consists
of antheridia and club-shaped germ cells with four spores in each. These
plants form a large portion of the wrack and also of the living Algæ
which surround the Falkland Islands and Cape Horn; and they extend to
Western Chili, where the poorer class make a sweet mucilaginous soup of
them. The Sarcophycus potatorum, the only species of its order, is
nearly allied to the D’Urvillæa by the structure of its fruit, and is so
named from pieces of its frond being used to carry water. Many other
olive-green Algæ are peculiar to the southern hemisphere—among them the
Hormoseira, in which the frond, at first even and filiform, becomes
inflated so as to produce moniliform chains of vesicles, parts of which
are at length rough with the apertures of the conceptacles; this plant
has bladder-like air-vessels formed by swollen parts of the frond, like
many of our Fucaceæ.
Those genera which have distinct organs containing air, as the
Sargassum, of which there are numerous species, are either of low
latitudes or tropical, but are sometimes drifted by currents to the
extra-tropical shores. The Sargassum vulgare, however, grows on the
rocks in the Mediterranean. The whole plant is of a translucent reddish
brown; the stem has alternate branches, bearing lanceolate serrated
leaves with a midrib, and generally dotted with dark pores. The
air-vessels are small translucent round balls about the size of a
currant, borne on flat stalks in the axils of the branches, and the
spores are in conceptacles borne on the branchlets just above the
air-vessel. In one variety of this most variable plant, the Uva di mare,
the main stem ends in a loose bunch of these little air-balls.
The Sargassum bacciferum is often found in the Mediterranean, but only
as a wanderer drifted in from the Atlantic, where masses of it, like
floating meadows, occupy an area west of the Azores equal in extent to
that of France, which has never changed its position since the time of
Columbus, on account of the surrounding currents. Fields of it cover the
seas near the Bahama Islands, and another permanent area of Sargassum of
great extent occurs in the South Pacific. The Sargassum bacciferum is of
a pale translucent olive colour, having branched stems, with lanceolate,
midribbed, and serrated leaves, destitute of pores, and little stalked
air-balls in the axils of the branches. The same individual continually
produces new branches and leaves, and thus multiplies its species, but
it never produces fruit; consequently its habits exactly resemble those
of the Macrocystis, and as that plant becomes detached and floats after
fructification, it is supposed that the Sargassum bacciferum may grow on
rocks at the bottom of the Atlantic, between the parallels of forty
degrees north and south of the equator, and when detached after
fructification that it is uniformly drifted to particular spots which
never vary. ‘Multiplication is so rapid in the floating beds of the
Sargassum and Macrocystis, as to render fruit needless; and even the
common Fucus vesiculosus occurs in the Mediterranean under a peculiar
form consisting entirely of specimens derived from sea borne weed
carried in by the current which sets in towards the Mediterranean from
the Atlantic.’[51]
Kelp, the ashes of sea weeds, is the commercial source of iodine. Algæ
growing in deep water contain most of that substance; consequently the
kelp made at Guernsey, consisting chiefly of the ashes of Laminaria
digitata, is richer in iodine than that made elsewhere.
Marine vegetation varies both horizontally and vertically with the
depth, and it seems to be a general law throughout the ocean, that the
light of the sun and vegetation cease together. It consequently depends
upon the power of the sun, and the transparency of the water; so that
different kinds of sea weeds affect different depths, where the weight
of the water, and the quantity of light and heat, suit them best. One
great marine zone lies between high and low water marks, and varies in
species with the nature of the coasts, but exhibits similar phænomena
throughout the northern hemisphere. In the British seas this zone does
not extend deeper than thirty fathoms, but it is divided into two
distinct provinces, one to the south and another to the north. The
former includes the southern and eastern coasts of England, the southern
and western coasts of Ireland, and both the Channels; while the northern
flora is confined to the Scottish seas, and the adjacent coasts of
England and Ireland. The second British zone begins at low water mark,
and extends below it to a depth of from seven to fifteen fathoms. It
contains the great tangle sea weeds or marine forests mixed with fuci,
and is the abode of a host of animals. A coral-like sea weed is the last
plant of this zone and the lowest in these seas, where it does not
extend below the depth of sixty fathoms; but in the Mediterranean it is
found at seventy or eighty fathoms, and is the lowest plant in that sea.
The same law prevails in the Bay of Biscay, where one set of sea weeds
is never found lower than twenty feet below the surface, another only in
the zone between five and thirty feet, another between fifteen and
thirty-five feet. In these two last zones they are most numerous; at a
greater depth the kinds continue to vary, but their numbers decrease.
The distribution in the Ægean sea was found by Professor E. Forbes to be
perfectly similar, only that the vegetation is different and extends to
a greater depth in the Mediterranean than in more northern seas. He also
observed that sea weeds growing near the surface are more limited in
their distribution than those that grow lower down, and that with regard
to vegetation, depth corresponds with latitude, as height does on land.
Thus the flora at great depths in warm seas is represented by kindred
forms in higher latitudes. There is every reason to believe that the
same laws of distribution prevail not only throughout the ocean, but in
every sea.
SECTION III.
FUNGI.
THE Fungi are enormously numerous. No less than 2,000 of the highest and
most conspicuous of these plants have been figured; many more have been
described; multitudes of those inhabiting the torrid zone are unknown;
and the microscopic and parasitic tribes are innumerable. Though with a
few exceptions entirely formed of cellular tissue, the fungi resemble
animals in respiration and chemical constitution. They contain more
azote than any other of the Cryptogamia, and obtain it chiefly from
their food, which consists of animal and vegetable substances alive,
dead, or decomposed. They inhale oxygen, and exhale carbonic acid gas,
so that they never form true chlorophyll. No plants are more dependent
on heat and moisture; many perform all the functions of life and
reproduction independently of light, preferring dark and shady places to
sunshine.
The Fungi form two principal groups, distinguished by their mode of
fructification. In the higher group, _Sporiferi_, the fungus produces
naked spores either single or compound, by means of which it may be
multiplied. In the lower group, _Sporidiiferi_, the fructification
consists of sporidia, enclosed in a distinct sac. The Sporiferi include
the following orders, Hymenomycetes, Gasteromycetes, Coniomycetes, and
Hyphomycetes. The Sporidiiferi include the Ascomycetes and the
Physomycetes. These various groups we may now proceed to examine.
The most important family of Fungi is, without question, that of the
HYMENOMYCETES, the species of which far excel all others in their
richness of colouring, and beauty of form. In this group the hymenium is
free and mostly exposed. It comprises six orders, of which the Agaricini
hold the first place. The genus Agaricus alone comprises 1,000 distinct
species, which assume as many different forms and colours, with only
slight modifications of substance, and it surpasses in number of species
all the other generic groups known.
The Agaricus campestris, or common mushroom, is a type of that vast
group; it consists of two distinct parts, the nutritive and
reproductive. The nutritive part is the mycelium or mushroom spawn of
gardeners, which resembles a mass of white spider’s threads mixed in
inextricable confusion, and carries on for a time all the functions of
the plant. Mycelia may exist for years without bearing the reproductive
part, but fruit never can be produced without spawn. The mushroom
itself, which springs from the spawn, is the fruit-bearing part, in
which the spores are formed and ripened. It is distinguished by a kind
of hat or bonnet called the pileus, supported by a stem. The pileus is
lined by a number of gill-shaped plates or lamellæ radiating from a
common centre; they are the reproductive organs in which the spores are
produced by free cell formation, a process always preceded by a
concentration of the matter within the parent cell, which is then
divided into as many nuclei as there are to be spores. In the higher
fungi, the number of spores thus formed is definite; in the Agarics they
are in groups of four placed at the extremity of a stem, springing from
the summits of these reproductive gills. Most of the Agarics rise from
the ground without any cover; the pileus or cap may show every variety
from a smooth polished surface to hairs or shaggy scales, but some of
the more highly organized have a general wrapper or volva, which
encloses the whole plant, and bursting at last, it leaves some traces
behind. In others, the pileus is at first clothed with fibres, which
vanish or leave traces on its margin forming a veil or curtain. Some
have a membrane attached to the stem, either connected with the volva,
or spread under the gills when young, and, when more or less persistent,
it is called a ring.
The spawn, which is the earliest product of the spore, has a great
variety of forms. Sometimes it is filamentous, sometimes tubular,
creeping extensively, or concentrated in a felted mass, sparingly
developed, or produced in abundance. Besides, it is developed in a great
variety of situations often difficult to detect. The fructification is
alone evident, under innumerable forms, which are as rapid in their
growth as they are for the most part ephemeral in their duration. Some
species have been known to acquire several square inches of surface in a
single night; the plant, however, is often far advanced before it
appears above ground. Some Agarics grow most readily after thunder
storms and abundant rains; certain species are always single; others
grow in dense aggregations; while in several species there is a tendency
to assume a circular arrangement, and that not merely when the spawn is
perennial, but when the whole existence of the fungus is confined to a
few days or weeks.
A mass of spawn is not always produced by a single spore, but by a
collection of spores, from whence it spreads in every direction, and
forms a common belt. In the Agaricus arvensis, Marasmius Oreades, &c.,
it spreads in a circle and bears fruit; and, as it continues to spread,
the same process takes place at each circumference. In this way are
formed the Fairy Rings so frequently seen in pasture lands. The fairy
rings are sometimes of very ancient date, and attain enormous
dimensions, so as to be distinctly visible on the side of a hill from a
considerable distance. It is believed that they originate from a single
fungus, whose growth renders the soil immediately beneath unfit for its
reproduction. The spawn, however, spreads all around, and in the second
year produces a crop, whose spawn spreads outwards again, for the soil
behind forbids its return in the opposite direction. Thus the circle is
continually increased, and extends indefinitely till some foreign cause
destroys it. The manure arising from the dead fungi of the former years
makes the grass vigorous around, so as to render the circle visible even
when there is no external appearance of the fungus; and the contrast is
often the stronger from that immediately behind it being killed by the
old spawn. This mode of growth is far more common than it is supposed to
be.
The depth to which spawn penetrates and the rapidity of its growth even
in the hardest timber if exposed to damp is quite astonishing. Instances
occur in which the spawn of dry rot not only enters wood, but penetrates
solid structures of brick. It overcomes an immense resistance.
The genera of the Agaricini differ in substance; some are almost
ligneous, others leathery or tough, occasionally they are delicate and
deliquescent, and although most of them are entirely formed of cellular
tissue, the Lactarii and Russulæ form remarkable exceptions in having
laticiferous vessels mixed with their cellular structure. These vessels
exist in all parts of the plants, especially in the gills, where they
give out the liquid on the slightest touch. In the Russulæ it is watery;
but in the Lactarii, in which it is either mild or acrid, according to
the species, it is also of different colours, which sometimes change
their tint upon exposure to the air, probably from ozone. In all fungi
there is a small amount of poisonous matter, and the quantity in any
given species is extremely uncertain, so that the same fungus which may
be eaten with safety in one country, is deleterious in another.
In the dark coal mines at Dresden, luminous Fungi cover the roof and
pillars with the most dazzling phosphorescent light, which increases
with the temperature of the mine. Agaricus Gardneri, a species parasitic
on the Pintado palm in Brazil, is highly luminous; and the Agaricus
olearius in the south of France also possesses that rare quality. The
gills under the pileus shine as brightly as a glow-worm, in the dark
crevices of the olive stems in November and December. M. Tulasne found
that the light was extinguished in vacuo or non-respirable gases, whence
he concludes that it is due to a slow combustion without heat, arising
from a chemical combination of the oxygen of the atmosphere, inhaled by
the fungus, with a substance peculiar to the plant.
In a few Agarics the cells are so connected by veins or lateral
branches, that they assume the character of pores, as in the
Chantarelle, a sweet-scented lemon-coloured fungus, whose gills pass
into mere veins, and its inferior fruit-bearing surface is all but even
and uniform, so that it forms a connection between the Agarics and the
Polyporei, a most extensive order of the higher fungi, essentially
distinguished by having a multitude of pores in the smooth under-surface
of the pileus, instead of gills. The pores are generally small; in some
species they are hexagonal, and so large that they look like a
honeycomb. In all, they are the mouths of cellular tubes, packed closely
together side by side, or more closely connected, sometimes easily
separated, sometimes inseparable. They constitute the fructiferous
surface or hymenium of the fungus, and contain the spores. This
structure gives the pileus a thick heavy appearance, and a vast variety
of characters; besides, the substance itself varies in density and
colour. The stem also may be long or short, sometimes wanting
altogether, when the pileus or cap is attached to the surface on which
the fungus is growing. The growth of individual fungi, whether Polyporei
or Agarics, is centrifugal, that is, they spread from the centre of the
pileus, as in the Polyporus fraxineus, which involves every stick and
blade of grass it meets with as it increases in diameter, and continues
to increase for years, till it is occasionally a yard across.
The fructiferous surface in the higher fungi is essentially turned away
from the light, yet, although in many of the lower Agarics it is
uppermost and exposed, such is the tendency to produce the
fructification on the lower side, especially in the Polyporei, that if
the position of the plant be reversed the hymenium or fructiferous
surface is gradually obliterated and a new one is formed on the other
side.
The Polyporei abound in the tropical forests, but species are found in
all latitudes. The higher fungi are more or less plentiful in forests
everywhere, and every genus of trees seems to have one or more species
of fungus peculiar to itself. The Boleti, a genus of the Polyporei,
which are thick fleshy fungi of various forms, and for the most part
brilliantly coloured, grow under trees in the temperate zones, sometimes
in conspicuous circles. When a slice of the Boletus luridus, cyanescens,
or other species is exposed to the air, the white fleshy part acquires a
blue tint in consequence of the action of ozone upon the acetate of
aniline, which was ascertained by Dr. Phipson to be a constituent of
these fungi. According to M. Dutrochet more heat is evolved by the
Boletus æneus than by any other vegetable except the Arum.
The Polyporei destroy decaying trees and timber, and the Merulius
lacrymans or common house fungus, attacks and induces the decay of
timber previously sound. The cap is large, fleshy, spongy and moist, but
delicate and velvety on the under-side, with wide porous dentate folds.
The plant is yellow with a white woolly margin. It grows in a circle,
and its mycelium attracts moisture from the atmosphere, which falls down
in drops from the pileus. The decay of wood induced by the attacks of
this mischievous fungus, is what is called dry rot.
The four principal sub-orders of the Hymenomycetes, or highest fungi,
have a cap or pileus, and an inferior fructiferous surface characterized
by gills, pores, or tubercles, and are connected by intermediate
species; but the other two sub-orders are quite different. The Clavariei
are club-shaped, upright, branching fungi, with the fructification
surrounding the uppermost extremity of some of the stems. The finest
species grow in the Swiss forests where they form an article of food;
some are edible in Britain, but of smaller size.
The Tremellini consists of plants forming a gelatinous mass of a bright
orange, purple, or dark brown colour, which may be seen on rotten sticks
in hedges, and in enormous masses resembling the convolutions of an
animal’s brain on the stumps of dead trees, or at the base of living
ones. They are mostly plants of temperate climates, but the Exidia
Auricula Judæ, or Jew’s ear, is universal. All fungi have a mycelium,
but in this order it is not apparent. The structure of the fruit, as
determined by the microscopic observations of Mr. Berkeley and of M.
Tulasne, is unusual. The fructiferous part is very extensive, being
uppermost and spread over the surface of the gelatinous mass, so as to
follow all its inequalities. Threads rise from this fructiferous surface
bearing on their extremities globular cells exhibiting a concentration
of coloured matter generally divided into four lobes, and from the upper
surface of these globular bodies, a number of flexuous threads spring,
carrying on their tips cymbiform spores.[52]
In the preceding family (Hymenomycetes) the fruit-bearing surface has
free access to the air, but in the group of the GASTEROMYCETES, which
consists of five or six sub-orders, it has neither access to air or
light till the fruit is ripe, for the fructification is enclosed in a
rind of one or two coats, and springs without a stem from a gelatinous
thready, or cellular mycelium. The most important group, Trichogastres,
includes the puff-balls, which grow on the ground. Of these the
Lycoperdon, found everywhere on pasture grounds and meadows, is a
familiar instance. When young it has a milk-white coat filled with
closely packed cells, some of which bear naked spores set upon spicules.
When mature, the whole of the interior vanishes, leaving nothing but a
mass of threads and fruit; the coat becomes brown, bursts open at the
top, and gives vent to a cloud of microscopic spores like the finest
dust. In general the ball is sessile, or it has merely the rudiment of a
stem. The Lycoperdon giganteum, an exceedingly large species, is a
native of a warm climate. The tops of some of the branched threads of
its hymenium swell into pear-shaped cells surmounted by short spicules
ending in spores; when young it is edible, when dry it is used for
tinder and as a styptic, and when ignited its fumes possess a property
similar to that of chloroform. The Batarrea forms a contrast to the
common puff-balls, being mounted on a stem sometimes a foot and a half
high. It has several coats enclosing a thick gelatinous substance in
which the threads carrying the spores are distinctly spiral and closely
twisted.
The sub-order Hypogæi is subterranean, as the name implies. The exterior
coat of these fungi is inseparable from the internal matter, which is
for the most part fleshy. In some species it is dry, in others it
abounds in milky juice; but in all the fruit is formed in hollows
excavated in the interior mass. In some species these cavities are
traversed by threads, and in many species the spores accumulate in such
multitudes within the cavities, as to make it certain that the spicules,
on which the spores are borne, produce successive crops. They are set
free by the rupture of the rind. A species of the genus Melanogaster,
abundant in the south of England, is edible and sold as the red truffle
of Bath; but it is far inferior to the real truffle.
The order Phalloidei has a club-shaped or globose head, composed in the
interior of large cells mixed with fruit-bearing cavities. This head has
a coat consisting of a jelly inclosed between two heterogeneous strata.
The whole interior of the fungus deliquesces, changes to a mucilage, and
drips out of the exterior coat in drops dark with microscopic spores.
The colour of these fungi is often beautiful, but their smell is most
loathsome, tainting the air to a considerable distance; yet the
gelatinous volva of more than one species—an Ileodictyon—is eaten by the
New Zealanders under the name of thunder dirt, and Phallus Mokusin is an
article of food in China.
Certain species of these fungi have a rudimentary stem in their early
stages, but it becomes full of deep pits or cavities and suddenly
acquires an enormous development when the plant approaches maturity. The
cavities are at first strongly compressed, but as the stem increases,
they acquire a rounder form, till at length their vertical tendency is
so strong that the coat or volva of the fungus is ruptured, which could
only be effected by a very strong force. Moreover, the stem is fixed to
the base by so small a point that the plant could not remain erect were
it not for the tubes of the volva which contract on the stem and act as
a sustaining force. Thus these very revolting fragile plants afford a
very striking instance of mechanical power exerted by vegetable matter.
The Myxogastres are an anomalous group of fungi which often appear as
black or coloured spots on dead leaves and twigs. Sometimes their
mycelium or spawn is large and conspicuous, as that of the Reticularia
maxima, which overruns cucumber beds, choking up the breathing pores,
and killing the plants. Species of these fungi are found upon mineral
and vegetable substances dead and alive, and the same species grows upon
plants of very different affinities, so that they depend upon the
atmosphere for their nourishment, and not on their matrix. Like the
puff-balls they end their lives in myriads of microscopic dust spores,
but they begin it as a gelatinous mass, sometimes sparkling as a gem,
brilliant with the metallic tints of gold, silver, steel, or copper.
The gelatinous or creamy mucilage of which these fungi consist, forms a
mycelium which is either diffuse, or creeps over the matrix on which it
grows in anastomosing filaments like a network, or it is arranged
without any definite order. This spawn gives rise to many bodies having
an envelope of one or more concentric membranes, technically called a
peridium, enclosing a gelatinous fertile substance which, when mature,
becomes a mass of scales or threads mixed with spores; the spores are
mostly attached to short threads singly or in groups, sometimes
surrounded by a firm coat or cyst. These bodies are either sessile on
the mycelium or stalked, and are either free or confluent. In their soft
state the tissues are so delicate that they exhibit no structure, but
just as they are passing from the puffy to the dry dusty state there are
indications of it.
It would be tedious to describe the variety of forms assumed by the
fruit-bearing bodies in the different genera of these fungi, or the
manner in which they are ruptured to give egress to the spores, which
differ in colour according to the species, though they are for the most
part red. The forms of the chaffy scales and threads are equally
diversified: in the species of Trichia, the threads contain one or more
spiral filaments, a form peculiar to the vegetable kingdom.
Botanists are now generally of opinion that the Myxogastres are
vegetables, although the singular Amœba-like motions some of them
exhibit, and the nature of the motile bodies they produce, seemed to
assign them a place in the animal kingdom; indeed, even now little is
known of the reproduction and final life-history of these singular
fungi.
Motions precisely similar to those of the Amœbæ, the lowest class of
animal existences, were observed by MM. Hoffmann and Tulasne, and more
especially by M. de Bary,[53] in the Æthalium septicum. It is a yellow
pulpy mass, produced upon a spawn or mycelium consisting of semi-fluid
gelatinous anastomosing filaments, often widely spread through the moist
tan in hothouses.
M. de Bary describes the filaments of the mycelium as full of a
multitude of small colourless corpuscles mixed with large yellow ones;
moreover the branches of this mucous network are described as
continually changing their form, in a manner closely resembling the
pseudopodia of the animal Amœba. They push out new branches, others are
withdrawn, and the whole mycelium frequently advances with a creeping
motion of translation. The yellow pulpy mass produced by the mycelium is
entirely composed of similarly constituted soft filaments about the
thickness of a bristle closely interlaced. They anastomose in all
directions, in long or short meshes, and their upper free extremities
form groups of projections which bristle the exterior surface of the
pulpy mass.
When that mass is about to form spores, all the asperities on its
surface are withdrawn and replaced by a bright yellow network of
irregularly interlaced filaments, which constitutes an envelope to the
interior spore-forming part, consisting of a white central liquid or
plasma containing an innumerable multitude of colourless granules. Both
of these parts are formed at the expense of the filaments which are
decomposed, the yellow granules are absorbed in the envelope, while the
colourless granules retire with the gelatinous matter towards the centre
to constitute the plasma, which is the fruit-producing part. Transparent
globular cells containing nuclei are generated simultaneously in every
point of the plasma, round each of which a portion of the granular
matter is consolidated; it takes a dark purple colour and constitutes a
spore, myriads of which, fine as dust, are thus generated. The whole of
the plasma is consumed in the spores, except a very small quantity,
which forms the threads found mixed with the spore dust. According to M.
de Bary the complete development of the Æthalium, from the instant it
appears above the tan in a hothouse of high temperature to the maturity
of the spores, is accomplished in about fifteen hours. The rapidity of
growth of this fungus is therefore astonishing. Mr. Berkeley mentions
that a mass of it two feet long, formed of many confluent individuals,
was formed upon a piece of iron that had been red-hot twelve hours
before—a proof among many others of the meteoric nature of these fungi,
the atmosphere affording them a sufficient supply of food.
The motions of the plastic matter contained within the dust-like spores
of the Æthalium septicum exhibit amœban motions of the same character as
its mycelium. For when M. de Bary placed ripe spores of that fungus in
water, their skin burst open and the plastic granular matter was set
free under the form of a coherent globular mass without any exterior
membrane. These globular corpuscles exhibited amœba-like changes of
form; processes were pushed out, and then drawn in, till at last they
assumed an elongated cylindrical shape round at one end, prolonged at
the anterior end into a long cilium with which they turned convulsively
round their axis. At the same time vacuoles were frequently seen to
expand and contract alternately in the round extremity. Ultimately these
bodies lost their middle, and at last were divided into two equal parts,
each of which went through the same changes as the primary globule, and
at length assumed the ciliated active form. Besides these active bodies,
there were others which never acquired cilia. The motile bodies were
also discovered in other species of these fungi by M. Hoffmann; there
can be no doubt that they are either zoospores, or of the same nature as
the eel-shaped motile bodies in some of the Algæ; possibly the bodies
which never acquire cilia may be germ cells. There is still much
obscurity with regard to the Myxogastres, inasmuch as the origin of
their mycelium is unknown, whilst in all other fungi mycelium or spawn
are produced by the germination of the spores. The spiral vessels found
in the threads of the Trichia prove that the members of this singular
family are truly vegetables. Professor Fries places the geographical
maximum or centre in the temperate zone, but different species are found
from New Zealand to high northern latitudes.
The Nidulariacei constitute a beautiful order of this family of fungi.
The plants are exactly like a bird’s nest with eggs, sometimes with a
stem, sometimes without. At first the nest or cup has a cover consisting
of several coats, which either burst open with a stellate or irregular
figure, or by the separation of a little lid; then the hollow of the cup
or nest is exposed, and at the bottom are seen one or more sporangia,
that is spore cases, often immersed in jelly and either free or fixed to
the nest by an elastic string. The spore-coats at first contain a
compact circular mass, but a cavity is afterwards formed in the centre,
and the cells which terminate in the walls of the cavity bear spores on
their tips. When ripe the spore cases are ejected by elastic power. The
force with which the sporangium of the species of Sphærobolus is
ejected, far exceeds _in proportion_ the force with which a shell is
projected from a large mortar; this fungus sometimes grows in damp
hothouses.[54] Many Nidulariæ are widely spread, but they thrive best in
warm climates.
The intimate structure and fructification of the higher fungi are for
the most part microscopic but an innumerable mass of the lower fungi are
themselves invisible to the naked eye, living upon all kinds of
vegetable and animal substances, dead, alive, fresh or putrid. They
vegetate upon decayed linen, flannel, leather, and even on metallic and
poisonous solutions. They yield myriads of minute spores wafted by every
breeze. They float in the air we breathe, seeking a nidus in anything
that will supply them with suitable food. There is scarcely a spot on
the earth where these minute spores may not exist, and being insoluble,
they wait where they fall for the growth or decay of the plant or animal
which suits them. As parasites they are most destructive, producing
disintegration, disease, and even death, both in vegetables and animals.
It is the mycelium or spawn which does the mischief, by supplying the
fungus with food at the expense of the victim. The spores of the
Botrytis Bassiana find a nidus in the breathing pores which open into
the trachea of the silkworm; they develop their mycelium in the
air-tubes, which are soon filled up; it then extends into the fatty
matter under the skin, which nourishes the worm during its dormant
state, and, as soon as that matter is exhausted, the victim dies.
In autumn, the common fly, though quite dead, may be seen adhering to
many parts of a room, especially to the window glass, as if it were
alive. In this state it is always surrounded by a halo about an inch in
diameter of whitish dust, consisting of the spores of the Empusa Muscæ,
or fly fungus. The body of the fly is much distended, the rings of its
abdomen are separated by the growth of the mycelium from within, and all
the contents of the body having been consumed by the parasite, nothing
remains but a hollow shell with a thin felt-like layer of the interlaced
mycelia of innumerable fungi, for the fly fungus increases with
wonderful rapidity within the insect. Mr. Berkeley believes the fly
fungus to be merely a condition or phase of one of those anomalous
moulds which grow on dead fish, making them conspicuous as they float on
the surface of the water, by the foggy halo which surrounds them.
Different kinds of parasitic fungi may exist at the same time. Dr. Leidy
found a variety in the stomach of the Passalus cornutus, a beetle that
lives upon decayed wood. Fungi do not attack the carnivorous beetles.
Man is not exempt from these parasites. Fourteen different species of
fungi were discovered by Mr. Hogg in as many cutaneous diseases. There
cannot be the smallest doubt of cutaneous disease being induced by
inoculation with fungi; merely rubbing certain species on the skin is
sufficient. Fungi cause baldness by fixing themselves on the roots of
the hair, and destroying the internal structure of the bulb. Our eyes
are not exempt from attacks of these parasites, for in performing an
operation upon a diseased eye Dr. Hannover found several species of
fungi in it; one of them was globular, and strongly refracted the light.
There is a fungus consisting of from four to sixty-four cells united in
square groups, which infests the stomachs of men and animals, even in a
healthy state; but although fungi produce certain cutaneous diseases,
there is no proof as yet that fever, cholera, or any other epidemic, is
owing to the spores of the fungi which we inhale from the atmosphere.
The family of CONIOMYCETES consists of six groups, two of which are
parasites on living vegetables, the other four growing on those which
are dead, decaying, or dying. They are microscopic plants, and their
mycelium is filamentous, or vesicular, often obsolete; short threads
rising from it bear on their tips either septate spores, or spores like
fine dust, inclosed in oval or bottle-shaped cases, called perithecia,
or in cells united in a cell, like a necklace of beads. We are chiefly
indebted to M. Tulasne and his brother for the obscure and extraordinary
life-history of these fungi.
The parasites on living plants form the two vast groups of Epiphytes and
Entophytes. The Epiphytes exhibit their fructification on the surface of
the plant, while their mycelium penetrates the moist texture of its
interior, which feeds them. All parts are liable to be attacked by these
fungi; they may insinuate their mycelium into the leaves, stem, flowers,
stamens, anthers, and the very heart of the seeds. The mycelium is
generally annual, but sometimes it is perennial, and leaves a crop of
fungi year after year; it disintegrates the tissues of the plant on
which it feeds, and distorts or kills it. Occasionally, the chlorophyll
in the leaves is oxidized, and becomes yellow by the oxygen which the
parasite absorbs.
The Entophytes, which constitute the second group of parasites on living
plants, form microscopic congregations in the interior of the leaves and
tender shoots, the only indication of their existence being a white,
red, or orange coloured spot, which usually becomes black or brown when
the fungus attains maturity. It appears that the same individual of
these entophytes may assume two or more different forms during the
course of its life, and bear two or more totally dissimilar types of
fructification. Of these, the sub-orders Pucciniæ and Uredines furnish
many examples.
[Illustration: Fig. 32. Pucciniæi:—_a_, Aregma speciosum; _b_,
Xenodochus paradoxus; _c_, Puccinia Amorphæ; _d_, Triphragmium dubens;
_e_, Young spores of an unknown Puccinia; _f_, Puccinia lateripes.]
Fig. 32 represents various species of Pucciniæi, which consist of a
thread ending in club-shaped or elongated cells called asci, containing
a definite or indefinite number of septate conidia or spore dust cells.
Each order of plants, as the Rosaceæ, has its own form of these
entophytes. In the tissue of a rose leaf, immediately beneath a bright
golden coloured surface spot, M. Tulasne found two distinct forms of
fungi, living together in a small cavity. The forms were exactly those
of a Puccinia and Uredo. The Puccinia consisted of a short colourless
stem, ending in a club-shaped cell, containing two conidia or spore
cells. These fungi were crowded together in multitudes in a small space
so as to form a solid rounded mass, with their broad tops immediately
under the skin of the upper-side of the leaf, but sometimes they were
arranged in concentric circles. The Uredines, on the contrary, had
colourless branching stems, like threads, bearing on their tops pointed
spore sacs. In some species these Uredines are scattered through the
mass of Puccinia, in others disposed in a circle round it, or in the
centre of the concentric ring where the Puccinia takes that form. It was
long believed that these two forms of fungi living together in the same
cavity were totally different plants, but as in various instances M.
Tulasne perceived that the Uredo had sprung up, shed its spores, and
vanished before its companion had ripened its fruit, he concluded that
the two different forms are merely two states of the same plant, that
the larger spores of the Uredo thus early matured, immediately germinate
and produce the Puccinia, whose fine dust-like spores are merely the
secondary fruit of the Uredo. These minute spores issue through a pore
in the conidia or dust cell and a puncture in the upper skin of the leaf
into the air, whence they are wafted in myriads by the winds; and, if
not too late in the season, they enter into the pores of the leaves and
tender parts of the same or other plants that may suit them, and within
these they form a mycelium, and produce a young Uredo. Even if the
autumnal leaves fall in a moist place before the spores have germinated,
the entophyte will grow on the approach of spring, and ultimately send
its dust spores to enter into young leaves, and grow with their growth.
Although there cannot be a doubt of the existence of the Uredines as a
numerous natural family, M. Tulasne considers the species of certain
genera to be only secondary forms of certain genera of Pucciniæ. Many of
these minute fungi have a third and even a fourth order of fruit; the
principle being carried to a maximum in the order Cæomacei. These
entophytes have a delicate mycelium, which gives rise to short or
obsolete fertile threads, terminated by single spores or chains of
spores. These spores when they germinate produce a second order of
spores; these occasionally produce a third order, and so on successively
even to a fourth or a fifth order. It is always the last and smallest
spores which reproduce the plant. The object of the successive orders
seems to be to diminish the size of the spores and to increase their
number, that they may more easily enter the stomates of the plants they
live upon, and be more easily and widely dispersed by the winds.
The Uredo candida, or Cystopus candidus, which takes its name from the
white spot it forms on the leaves of the cabbage and other vegetables,
is found to produce both female or germ cells and spermatozoids. Long
before the white spot is formed on the leaf, the presence of the
abundant spawn may be perceived by swellings and deformities in the
victim plant. Its filaments, which creep exclusively in the
intercellular canals of the cellular tissue, are tubular, of unequal
diameter, and exceedingly branched, and are always formed of cellulose,
either thick-walled and gelatinous, or thin-walled and membranous. From
this mycelium, little threads hang down, ending in globular vesicles
containing a nearly homogeneous colourless matter, and ultimately an
aqueous liquid; they are supposed to fix the mycelium to the cells of
the victim. According to the examination of Mr. Berkeley, M. Tulasne,
and others, the branches of the mycelium accumulate in a hollow
immediately under the white spot in the skin of the leaf of the plant
attacked. From these branches spring bundles of club-shaped tubes,
directed perpendicularly towards the upper skin of the leaf, and forming
a tuft or little cushion of variable extent. The summit of each of these
club-shaped tubes is formed into a conidium, or spore dust cell, which
separates itself from that below it by taking a globular form. In the
upper end of the remainder of the tubes, new spore dust cells are
formed, and so on indefinitely. These conidia remain attached to one
another in a string by slender constrictions which become thinner, and
at last give way from above downwards, and they escape in succession
through a crack in the skin of the leaf. The quantity of spores that are
generated by the dense mass of these club-shaped tubes must be enormous.
The Cystopus has female reproductive cells, which had escaped notice
from being hid in the plastic matter which nourishes them. They appear
before the spore dust bearing cells, and are formed by terminal or
interstitial swellings in the tubes of the mycelium, which become large
oval cells, separated ultimately by a closure from the rest of the tube
that bears them; they are filled with a granular liquid mixed with large
granules of a coloured fatty matter. The tips of some branches of the
mycelium swell into oval or club-shaped cells containing spermatozoids,
which fertilize the female cells; then the matter within the latter
assumes a globular form, gets a coat of cellulose, and becomes the true
fruit of the Cystopus.
As early as the year 1807, M. B. Prévost had seen that the sporangia, or
spore cells of the entophytes produced zoospores, and recently M. de
Bary has seen them produced, during the germination of the spores,
collected within a sporangium of the Cystopus. When they came into the
water they had two cilia, one of which was short and went first, the
other was long and trailed after the zoospore. Neither M. de Bary nor M.
Tulasne have ever seen zoospores in the fungus itself, but if the drops
of rain or dew round the white spot on the leaf of a plant be examined,
empty sporangia are generally found, and spores in different states of
development.
The Puccinia Fabæ, an entophyte on the common bean, has but one spore in
its cylindrical case, and is considered identical with the Uromyces
appendiculatus. Besides male organs like those of the Æcidium, M. de
Bary found that the bean entophyte has four kinds of reproductive
organs, of which one alone reproduces the original form, while the
others present a well-marked alternation of generations. The Puccinia
forms a prothallus on which conidia, or secondary spore dust cells,
arise; these secondary spores form a mycelium, on which an Æcidium
appears, whose orange-coloured fruit gives rise to a Uredo, and the dust
spores of the Uredo enter the leaves of beans or peas, and grow into a
Puccinia. All the species of Æcidium are similar to one another, and M.
Tulasne is of opinion that they do not constitute a distinct genus, but
that, like many of the Uredines, they are merely a secondary form of
some other fungus, and inhabit the same cavity, as in the case of
Æcidium cyparissiæ and Uromyces scutellatus, Æcidium leucospermum and
Puccinia Anemones, and others.
[Illustration: Fig. 33. Puccinia Graminis.]
The order Pucciniæi comprises epiphytes, as well as entophytes. The
mildew on wheat is caused by the Puccinia Graminis (fig. 33), which
attacks the stem of the plant, and appears on its exterior in a circular
cluster of pear-shaped septate spore cases. These spore cases spring
from a filamental mycelium, whose threads interweave themselves among
the soft tissue of the stem of the wheat, and the fertile threads make
their way through the stomates to the surface. Professor Henslow has
proved that the rust which appears on the leaves and chaff scales of
wheat is owing to the Uredo linearis, a secondary form of the Puccinia
Graminis, and that rust is only an earlier form of mildew; so that the
Puccinia Graminis is a dimorphous and epiphytic fungus. It may be a
question whether the Uredo segetum, which destroys the blossom of wheat,
and reduces the ear to the sooty mass of powder called smut, may not be
the form of some other fungus. The epiphytes of the order Pucciniæi
often appear on the exterior of plants in tufts of brown, yellow,
orange-coloured, or white sporangia.
Fungi are extensively propagated by fragments of their spawn, and the
threads of the mycelia are sometimes diminished in thickness in order
that they may more easily penetrate into the stomates of the plant they
invade. This was discovered by Mr. Berkeley while investigating the
germination of the spores of bunt, a fœtid rust which attacks wheat and
other grasses. It is perfectly analogous to the diminution of the size
of the dust spores in the successive orders of fructification. It is
thought probable that in many of the parasitic fungi, new spores are
formed at the tips of the fructiferous threads of the mycelium as fast
as the ripe spores fall off, whence that enormous mass of minute spores
which a single individual is capable of producing.
Among the multitudes of known parasitic fungi, there is not one that
does not form a mycelium more or less distinct. They do not arise from a
disease in the plant they attack, though they ultimately cause disease
and often death. Each parasite has its own mode of penetrating into the
tissues, and its own manner of vegetating; they attack certain plants,
and avoid others though nearly allied.
Dust spores, single or septate, oozing out of a dark or coloured fungous
mass is characteristic of the group Melanconiei, which is more
remarkable in regard to the mode of fructification than any of the
preceding Coniomycetes, for instead of successively assuming the form
and fructification of two genera, or two orders, the plants successively
assume the form and fructification of two distinct families.
The family of the HYPHOMYCETES takes its name from its filamentous
character. The mycelium gives rise to white, dark brown, or bright
coloured threads, simple or compound, bearing naked spores on their
extremities. Of these there are five sub-orders and many genera.
The sub-order Isariacei has four genera found on the pupæ of moths, on
dead spiders, dead fungi, and dead plants respectively. The group is
characterized by compound threads ending in pulverescent spores. Most of
the caterpillars of the Bombyx Rubi, or bramble moth, fall victims to a
species of Isaria, which has several distinctly different periods and
modes of fructification, and at last assumes the form of a very
beautiful fungus belonging to a different family.
Near Paris, in the month of October, when the caterpillars of the
bramble moth seek for shelter from the cold, in the earth, or under long
grass and withered leaves, M. Tulasne and his brother found that most of
them were surrounded by tufts of a whitish down, which increased so
rapidly that it killed the caterpillars and covered the whole of their
body except the bristly hairs, and assumed characters similar to the
muscardine fungus that kills the silkworm. This down is a mycelium
composed of extremely fine branched filaments felted together, the
upright fertile branches of which bear whorls of branchlets each
terminated by chaplets of from ten to fifteen equal and spherical cells
filled with dust spores. These most minute spores germinated, and put
out filiform creeping germs which quickly emitted many branches ending
in long chaplets of fertile dust-bearing cells.
Points here and there on the felted envelope of the caterpillars became
of an orange colour, took the form of a mycelium, and produced little
orange coloured club-shaped cells which shed abundance of reproductive
dust spores from a ring of white hairs on their summit. Each caterpillar
had from ten to fifteen of these coloured clubs on its sides, which lost
their brightness when they grew old, and had shed their dust spores.
These fungi possessed all the characters of the Isaria crassa, or Isaria
farinosa of Fries.
Later in the season other caterpillars on which this club-shaped Isaria
had not been produced, but which were swollen and white with the felted
spawn of the parasite, gave out orange red club-shaped vessels of a
larger size and deeper tint than those of the nascent Isariacei. They
had no terminal ring of hairs, but some of them had a red spore
dust-bearing felt at their base. Ultimately they assumed all the
characters of the Sphæria militaris of Ehrenberg or Cordyceps militaris
of Fries, which is a bright scarlet fungus half an inch high with a
fleshy upright stem ending in a cup-shaped head containing long
cylindrical sacs called asci, in which the spore cells are so numerous
as to resemble strings of beads.[55] This fungus, therefore, begins as a
member of the family Hyphomycetes, and ends as a member of the family
Ascomycetes.
The order Stilbacei are little globose fungi with or without a stalk,
covered with semi-gelatinous spores. They are united in cushion-like
masses, on decayed wood and dead twigs. The little scarlet masses on
dead currant branches so often seen in gardens, are examples.
The order Dematiei are the black moulds found on damp paper, old damp
linen, dead wood and plants. Their spawn is seldom much developed, the
fertile threads are erect, rigid, dark brown approaching to black,
sometimes of an olive green. The spores on their tops are either simple,
in whorls, or collected into heads, which are large, septate, and even
spiral.
The Mucedines are beautiful microscopic objects both as to form and
colour; they are very numerous both in genera and species and are well
known as red, blue, or green moulds. These fungi spring from many points
of a generally abundant mycelium, in erect coloured threads, bearing on
their tips simple naked spores, spores collected into little tufts, or
spores strung together like beads forming threads either branched or
simple. In this order of fungi there are, moreover, instances of
dualism, the second order of fruit being that of the family of the
Ascomycetes.
The Botrytis, or Peronospora infestans, which causes the murrain in
potatoes, shows how destructive the Mucedines can be. Like other
entophytes, its spores enter the stomates in the leaves of the potato,
and fill the cavities of the leaves with spawn, the ramifications of
which are said to be very beautiful. This creeping spawn then insinuates
itself into the stem and tuber, and from thence it finds its way to the
exterior of the plant, or to some internal cavity, where it fructifies,
bearing large globose sessile bodies yielding fruit of the second order,
and spores on the tips of its fertile branches. The spawn of the
Botrytis spreads rapidly in a circle, and soon destroys the texture of
the leaves and stem, but although it attacks the tuber or potato
generally so called, it does not penetrate deeply. The destruction of
the potato is aided and completed by the Fusisporium Solani, a
microscopic fungus, which takes various forms according to its age and
changing conditions, the last of which seems to be partly gelatinous; it
sometimes hardens the tissues of the potato, but sometimes causes rapid
and loathsome decay.
The thread-like fibres of the spawn of the Peronospora permeate even the
branches and wood of trees. Wasps are frequently seen to frequent hollow
trees, probably in search of the mycelia of some of these parasitic
fungi, which is identical in structure with the material of which their
nests are built. Signore Panceri, professor of comparative anatomy in
the university of Naples, has discovered seven species of Mucedines in
the albumen of hens’ eggs.
Chemical changes in preserved animal and vegetable substances afford
suitable food for the Penicillia mould, if indeed they are not the
immediate cause of these changes. The threads rising from the mycelium
of these moulds terminate in bundles of branchlets carrying at their
summits strings of spores, like necklaces of small beads collected into
bunches like tassels, white, yellowish, blue or red according to their
age or kind. Figure 34 represents various species of Mucedines, in which
_c_ is the Penicillium armeniacum, and _f_ is a spore of
Helminthosporium Hoffmanni; all are magnified. Different species of the
Penicillia form the blue and brick-red moulds on cheese, and the
greenish and grey moulds on jam and preserved fruit. They appear as dry
rot, as orange coloured spots on long kept potatoes, as mildew on cloth,
silk, sugar, meat, and even on weather-beaten window glass. They can
exist in metallic and poisonous solutions by decomposing the chemical
combination, rejecting the metal or poison, and living on whatever
nutriment may be found in the remainder. Like the larger fungi, these
minute plants are sometimes poisonous; the fatal effects occasionally
produced by sausages and spoilt meat are supposed to be owing to
poisonous moulds.
The Mucedines conform to the law prevailing in other low organizations
of having their species widely distributed. The Penicillium glaucum is
found in all countries, especially in the vicinity of man; it inevitably
appears in all saccharine substances, and, according to M. Fries, it is
met with alike in the alps of Lapland and in the oasis of Jupiter Ammon
in the Lybian desert, an example which has no parallel in the
geographical distribution of the higher plants.[56] Ferment, that is to
say, the yeast plant, is a peculiar condition of certain fungi,
including the present species, and is capable of unlimited propagation.
[Illustration: Fig. 34. Mucedines:—_a_, Aspergillus glaucus; _b_,
Aspergillus dubius; _c_, Penicillium armeniacum; _d_, Sepedonium
mycophilum; _e_, Helminthosporium nodosum; _f_, Helminthosporium
Hoffmanni, spore; _g_, Zygodesmus fuscus.]
The real spore-bearing filaments of the Penicillium glaucum are only
developed in air, for when the spores of that fungus are kept submerged
in a liquid favourable for their growth, as in a saccharine solution, or
the juice of the grape, they form an aquatic mycelium on the filaments
of which cells are produced, and carbonic acid gas is given out. These
cells increase by budding or division into chains of ferment, easily
separated into single cells. The Penicillium glaucum is thus developed
in all kinds of liquids, and in almost all kinds of conditions; even the
peculiar knotty filaments observed in its submerged mycelium are not
constant in different liquids. M. Hoffmann has observed with certainty
the passage of Penicillium glaucum into Penicillium candidum, into a
sulphur-coloured Penicillium, and, lastly, into the Coremium glaucum, so
that this fungus is polymorphous, although the conditions under which
the changes take place are unknown.[57]
It appears that substances and liquids do not ferment spontaneously, for
upon examining with a microscope the dust obtained by scraping the
exterior of gooseberries, plums, vine leaves, potatoes, &c., M. Hoffmann
found the short chains of the Torula, the necklace-like ferments of the
Mucedines, and the chaplets of others. Some had already begun to
germinate, and were developed readily when put into water. They had no
doubt been carried by the wind from the dry refuse of fermented
substances which are thrown away.
[Illustration: Fig. 35. Torula Cerevisiæ, showing successive stages of
cell multiplication.]
The yeast of beer was at one time considered to consist entirely of the
cells and chains of the Torula Cerevisiæ (fig. 35). This, which is one
of the Coniomycetes of the order Torulacei, is in its early stage a
colourless transparent globe capable of endless increase by budding.
When in a liquid favourable to its growth, as the wort of malt, buds in
the form of young cells spring from the walls of the globes; these soon
become perfect and acquire buds also, so that in a few hours the parent
cells develop themselves into rows of four or five globes, which remain
in contact while the plant is growing, but separate if anything checks
the fermentation. The full development of the plant only takes place
when the fermentation is allowed to continue for some time, and then it
is capable of producing a variety of forms, which show that it has other
modes of reproduction besides budding.[58] In fact, when the
fermentation is active in the upper parts of the liquid it appears in
chaplets of from four to twelve articulations, or sometimes it ramifies
into little branches.
More recent observations show that yeast is a peculiar state of the
mycelium of various fungi, of which, as already stated, a large
proportion is believed to consist of Penicillium glaucum, though it is
known that other kinds of mould also enter into its composition. These
plants grow naturally in a state of exposure to the atmosphere, but they
have the property of also developing themselves when submerged; and as
in this unnatural condition, which they bear when developed as yeast,
they cannot produce their proper fruit, they propagate themselves by
means of shoots from the altered mycelium. It has been observed that
mechanical injury destroys the vitality of German yeast, which consists
of yeast globules in a dried state. Thus a fall on the ground, or a
bruise caused by a blow or by careless handling, will kill the plant,
and such dead yeast becomes dark-coloured and glutinous, and soon
acquires an offensive smell.
The form of Penicillium glaucum which produces acetic fermentation,
known as the vinegar plant, has a filamental submerged instead of a
vesicular mycelium.
Ferments may be formed in the wort of beer, in the solutions of grape
and cane sugar, in the juice of gooseberries, currants, &c., by means of
the submerged spores of the Uredines segetum and Rosæ, of Ascophora
elegans, Mucor Mucedo, Periconia hyalina and others. There are active
exchanges continually going on between the contents of the globules of
ferments and the exterior liquid, and therefore a continual chemical
action.[59]
M. Pasteur’s experiments on the nourishment of the Mucedines concur with
the observations of others in showing that these plants are the origin
of all fermentation properly so called. When he put a mere trace of the
beer yeast fungus into pure water holding in solution the three
crystallizable substances, sugar candy, an ammoniacal salt, and some
phosphates, the globules of yeast were seen to multiply, deriving their
nitrogen from the ammoniacal salt, their carbon from the sugar, and
their mineral material from the phosphates; at the same time the sugar
fermented. The same results were obtained from lactic yeast. M. Pasteur
now sowed the spores of the Penicillium, or of some other mucedinous
fungus in pure distilled water holding in solution the same ingredients,
except that an acid salt of ammonia was employed to prevent the
development of infusoria, which would soon have stopped the progress of
the microscopic plant by absorbing the oxygen without which fungi cannot
live. The result was the same as in the preceding case. There is
consequently a complete analogy between the ferments, the mucedines, and
plants of more complex structure. If in these experiments any one of the
principles in the solution be omitted, the vegetation is arrested. The
quantities of these substances in the air, the water, or in the spores
themselves, are not sufficient to make up for the suppression of any one
of them. For example, the carbonic acid in the air or water does not
make up for the omission of the sugar. The mucedines and fungi generally
obtain their carbon from their food and from rain water, for rain water
holds in solution nitrates and ammoniacal salts together with salts of
potash and lime, and M. Barral has ascertained the existence of
phosphates also. M. Barral found that the nitrates and ammonia disappear
under the influence of cryptogamic plants.[60]
In the genus Oidium, belonging to the Mucedines, the short threads have
a string of spores, like a necklace of beads, on their tops. Species of
this genus are found on damp paper and honeycombs; also on decayed
oranges, and other spoilt fruits.
The family of the ASCOMYCETES produces spore-bearing cells, called
sporidia, enclosed in long cylindrical sacs or asci, in the definite
numbers two, four, eight, sixteen, thirty-two, &c. The great
characteristic of this large group, consisting of six orders, is the
fleshy soft texture and the more or less complete exposure of the
fructiferous surface. In one division the asci are persistent; this
includes the Elvellacei, the Tuberacei, the Phacidiacei, and the
Sphæriacei. In the other division the asci are often evanescent, and
this includes the Perisporiacei and the Onygenei.
The genera Peziza and Helvella are the principal fungi belonging to the
order Elvellacei. The Pezizæ are a very numerous race, and consist of
brilliantly coloured little cups, with or without stems, and more or
less concave. They grow in the cooler part of the temperate zone, many
show themselves in spring, and some of the largest and most beautiful
appear during the melting of the snow.[61] A few are subterranean, and
128 species inhabit Great Britain, many growing on the ground, others on
stumps of trees, dead sticks and timber, on living plants, damp walls,
&c. They are singularly beautiful, including ‘many of the most elegant
fungi, from the little white and red Peziza elegans, which is sprinkled
over almost every fallen twig of the larch and other conifers; the pale
toothed cups of the P. coronata, which abounds on the dead stems of
herbaceous plants; the scarlet P. scutellata with its edge fringed with
tawny hairs, and the graceful mouse-grey P. macropus, to the gorgeous P.
coccinea, which attracts attention from its elegant form and bright
colours; the more irregular, but not less brilliant P. aurantia, and the
font-shaped P. acetabulum, which might form an elegant pattern for an
architect or silversmith. Fifty others might be mentioned of equal
pretensions to grace of form and brilliancy of colouring.’[62] Some of
the genus are peculiar in their fructification, for in several plants of
this group, besides asci containing eight sporidia, M. Tulasne met with
cells full of eel-shaped particles like those in the Algæ, and although
without motion he considered them to be analogous to the pollen of
flowering plants. In the Peziza aurantia, however, the particles were
staff-shaped and motile.
The genus Helvella may be regarded as Peziza with the cup inverted;
consequently, it assumes the character of a pileus or hat, like a common
mushroom, though often very different in shape, and, instead of
spore-bearing gills, it has asci containing eight sporidia sunk in its
fleshy texture. The pileus is ovate or mitre-shaped in some species and
the margin free, in others it is more or less attached to the stem. When
the pileus and stem are perfectly soldered together, we get the
club-shaped species of the group.
Some obscure forms of this group, forming the genus Ascomycetes, cause
the leaves of the peach, walnut, and pear to blister. They consist of
little more than asci, accompanied by short necklace-shaped thread.
The Morchella esculenta, which is the morel, is so plentiful in some
parts of England, that it is used for making katsup; while the Cyttaria,
which is indigenous in the southern hemisphere, is the staple food of
the Fuegians during many months of the year; its subgelatinous
consistence indicates a nutritious principle. This species has the
peculiarity of growing upon living branches, after the manner of the
jelly-like fungus of the juniper.
M. Tulasne has discovered in Peziza, and in the genus Bulgaria and
others, certain minute bodies, which he considered to be of the nature
of the eel-shaped particles or antherozoids in the Algæ. Besides, he has
shown that several species of Peziza have a second form of fruit. Fries
had long before pointed out the identity of Fusarium tremelloides and
the orange coloured Peziza common on nettle stems. Many of the larger
Pezizæ and Helvellæ eject their sporidia with great elastic force. This
is particularly remarkable in the Peziza vesiculosa, common in hot-beds,
when the sun is shining; the least agitation raises a visible cloud of
sporidia like vapour. The motions of the sporidia in the genus
Vibrissea, which grows on twigs partly immersed in water, is very
peculiar. They are exceedingly long and slender, and, when partly
ejected, they wave about in the sunshine till they are expelled.
The fungi of the order Tuberacei are nearly all subterranean, and their
fruit-bearing surface, as in the truffle, is internal. The asci are
either irregularly deposited in cavities, or in the denser tubers they
are sprinkled through a dark substance which is mottled with a paler
tissue. The truffle, which is the most important and best known of the
order, has a dark corrugated exterior, and the asci are represented by
large pyriform sacs containing sporidia covered with a reticulated or
spinose coat; but these spines are only the angles of continuous cells,
and are beautiful microscopic objects. Truffles prefer calcareous soil,
and a temperate climate. In England, they are found in Rutlandshire, and
numerous species grow in Northamptonshire, but they are smaller than the
continental truffles, which increase in abundance and size towards the
south, and have their maximum in Italy, where they grow on the roots of
trees and vines, and are hunted by dogs, or traced by the presence of a
peculiar fly, and dug up for sale. Sometimes the dogs dig them up, to
the annoyance of the proprietors of vineyards, from the mischief they do
to the roots of the vines.
The Sphæriacei, another order of Ascomycetes, are enormously numerous.
There are 1,000 well known species of this order, and probably twice as
many undetermined; for there is scarcely a twig or dry branch in the
forests, hedges, or gardens on which they may not be found. The rose
tree, the oak, and other plants harbour more than one species at a time.
The genus Cordyceps and many so-called species of Sphæria are only the
ultimate development of fungi of other families. The Cordyceps purpurea,
discovered by M. Tulasne on the ergot of wheat, has a short, upright,
slender stem, with a minute pale purple globose fruit-bearing head. In
this ergotized state the white substance of the grain is converted into
a firm mass without any appearance of meal, and having very powerful
properties. When sown, it is found to produce the Cordyceps. Mr. Currey
found the same plant on the ergot of the common reed, and there are
several other species of Cordyceps, all of which are only the second
form of ergot. To these may be added the Cordyceps militaris of
Ehrenberg, already shown (p. 283) to be the ultimate development of the
Isaria, which attacks the caterpillars of the bramble moth; and the
Cordyceps Robertsii, which grows like a bunch of rushes from the head of
the Hepialus virescens of New Zealand; whilst a kind of wasp in the West
Indies, which continues to fly about after it is attacked, is at last
killed by branching Cordyceps, which project from its head like a pair
of antlers. But the largest of all these parasites grows on an enormous
larva found on the banks of the river Murrumbidgee in Australia. It
appears that species of Sphæriæ are parasitic on insects of very
different affinities in China, America, and Europe. It may be presumed
that, like the Cordyceps militaris, they are the ultimate development of
fungi belonging to other families.
[Illustration: Fig. 36. Sphæriacei:—Sphæria Desmazierii; _a_, ascus;
_b_, sporidia; _c_, mycelium with conidia. Mucedines:—_d_, Botrytis
curta.]
In the genus Sphæria the fungus springs at once from its mycelium, and
consists of a perithecium or external case, to the internal walls of
which the asci are fixed. Each ascus contains eight sporidia or spore
cells, and when the fruit is ripe the asci are emitted through a pore or
slit in the perithecium or external coat. Fig. 36 represents the
fructification of Sphæria Desmazierii. Sphæria bombarda is like an
assemblage of minute black beads lying flat and crowded together upon
decayed wood; each bead is an oblong vesicle pierced at the apex for the
emission of the microscopic sporidia, or spore cells. The Sphæria
aquila, found upon decayed wood, has its fruit-bearing vessels seated
upon thickly matted fine threads. In some species they are in tufts;
others have bottle-shaped perithecia sunk into the stems of the
berberry, laburnum, and decayed oak palings; and lastly the candle snuff
Sphæria may frequently be seen like patches of soot at the bottom of
stakes or gate posts. More than two hundred well ascertained species of
the genus Sphæria are native in Great Britain alone.
The genus Nectria, which forms a connecting link between the genera
Peziza and Sphæria, has several interesting species, as the Nectria
Peziza, which grows in a congeries of most minute cups forming a bright
orange-coloured patch on decayed stumps of trees.
In the order Perisporacei the perithecia, or external cases, are free
and become dehiscent at last, but when young these fungi consist of
cellular jointed filaments like necklaces, rising upright from their
mycelia, and bearing reproductive bodies. In this state they constitute
the mildew on the vine, rose tree, turnip, hop, pea, &c. They are true
parasites, appropriating the juices, and filling up the breathing pores
of the leaves, so as to cause disease and often death. The vine mildew,
which has been called Oïdium Tuckeri, but which is now supposed to be an
imperfect state of Erysiphe, never advances beyond this state,
consequently it never has more than one form of fruit. Mr. Berkeley has
discovered that, on the contrary, the hop and pea mildews, which belong
to the genus Erysiphe, have five different modes of reproduction. The
destructive power of these fungi is strongly illustrated by the
extraordinary energy of their mycelia in draining the vital juices of
the plants on which they live in order to form such various kinds of
fruit; and the quantity of fruit produced is so enormous, that if the
whole were to germinate no genus of plants for which they have an
affinity could escape annihilation. Other species of Erysiphe have at
least three different modes of reproduction. The perithecia of some of
these fungi are beautiful objects for the microscope.
The Physomycetes, which form the sixth and last order of the great
fungus family, have bladder-like fertile cells scattered on threads, the
number of sporidia within the cells being indefinite. The Antennariei
are dark coloured felt-like fungi which run over the leaves of living
trees, and have fruit on black threads, which in some species, when
magnified, resemble the antennæ of certain beetles. The species of this
order are not common in Britain, and they are supposed to be only a
condition of some other fungi. They are certainly spore-bearing plants,
yet the fruit-bearing cells of the Antennaria Robinsonii sometimes
contain a ready formed miniature of the parent plant waiting to be set
free,—a singular analogy between these microscopic fungi and flowering
plants.
The order Mucorini, or moulds, has threads springing from the spawn,
bearing on their extremities large vascular sacs containing asci with
spore cells. The genus Ascophora contains several remarkable species, as
Ascophora elegans, which has two kinds of fruit, and attacks bread while
yet hot from the oven: however, the spores were probably in the dough,
for it has been ascertained that the spores of some of the lower fungi
retain their vitality after being exposed to the temperature of boiling
water. The Mucors are probably found on decayed and decaying matter all
over the world; they grow on fat, on greasy walls, and on decaying fruit
and vegetables.
The extreme minuteness of the reproductive bodies of the microscopic
fungi, many of which are not more than the 20,000th part of an inch in
diameter, and their extraordinary and varied forms even in the same
plant, have made these fungi one of the most difficult studies in the
whole science of botany. There is still some obscurity with regard to
those minute motile bodies supposed to be male particles, and their
analogues, which have not been seen, or have rarely been seen to
germinate. These bodies have been observed in comparatively few genera,
and nothing more than mere molecular motion has been observed in them.
One of the most unaccountable circumstances in the history of the lower
fungi is their sudden appearance in immense numbers, and the rapid
extension of disastrous and destructive epidemics caused by them among
plants and animals, as the potato murrain and the vine disease, which,
though widely spread through Europe and Madeira, leaves the North
American vines unscathed, whether grown at home or abroad. The black
mildews at one time raged so much in the Azores and Ceylon as to
threaten the complete annihilation of the orange and coffee plantations.
Their ravages have been scarcely less among the olive trees in some
parts of Europe; and the Lanosa nivalis, which grows in the melting snow
in spring, is supposed to be the cause in many instances of the death of
the germs of the sprouting rye. The destructive course of most of these
has abated, but the silkworm disease still continues. The fungi require
warmth and a moderate degree of moisture for their development, but the
unwonted multitudes in which the parasites occasionally appear, possibly
indicate some meteoric influences of which we are ignorant.
SECTION IV.
LICHENS.
LICHENS are essentially air plants, being nourished, like the Algæ, by
the medium in which they grow. They vary from a pulverulent or dry
papillose crust, to a leathery or horny expansion, and even acquire an
erect stem. They are independent of the matrix to which they are
attached. Hence they spread their coloured frond, or thallus, in
circular or indefinite patches on old walls, the tiles of houses,
stones, and rocks. They appear in large expansions of red, golden
yellow, grey or white, on barren heaths, under plantations, and on the
stems of aged trees; while others of them often hang from the branches
like long shaggy grey hair, and many form forests of miniature bushes on
the northern plains. The lichen is the last trace of vegetation on the
tops of the mountains, and on the arctic deserts. Some lichens are
patient of severe cold, yet in general they prefer heat and moderate
warmth, and they love bright light so much, that they are usually
barren, or else yield little fruit under shade. Though differing greatly
from fungi in slowness of growth, length of life, and the power of
forming chlorophyll, they resemble them in having a mycelium in their
youth, and in their ascigerous fructification. A perfect lichen without
an ascus would be an anomaly, for the asci contain the true fruit,
associated with vertical threads or elongated cells called paraphyses,
which sometimes bear secondary spores on their summit. The asci with
their paraphyses inclosed in vessels of various shapes, called
perithecia, are aggregated in discs or shields, which form projections
on the surface of the plant. Some of these discs are closed, and give
egress to the spores through a fracture or pore on their surface; others
are open cups of various forms, either with stalks or sessile on the
frond, and through these the spores have egress. Hence the whole order
of lichens is naturally divided into two groups, according as their
discs are open (Gymnocarpei), or closed (Angiocarpei), the first being
incomparably the most important. Fig. 37 shows open, and sections of
closed cups, or perithecia.
[Illustration: Fig. 37. Lichens:—_a_, Trypethelium Sprengelii, pustules,
with sporidia; _b_, Verrucaria variolosa, section of perithecium with
sporidium; _c_, Endocarpon lacteum, thallus with section and fruit; _d_,
Stegobolus Berkeleianus, portion of plant with ascus, and sporidia.]
The highest type of horizontal lichens, of which fig. 37 _b_ is a
perpendicular section, has a firm, spreading, superficial crust or
surface, formed of oblong coloured cells, or of coloured filaments
closely aggregated, and which covers two distinct layers of cellular
tissue. In the layer immediately below the surface, the cells are
globular and of a paler colour; the second layer, or marrow, which is
the origin of fructification, consists of lax, detached, branching rows
of elongated cells and gelatinous matter. These branches of cells spring
up from a fourth layer, which is the base of the plant. It is of a
strong, firm, and tough nature, composed of interlaced filaments and is
often ribbed on the under-side. White fibres fix the base of the plant
to the surface on which it is spread; they are the remains of a mycelium
or matted mass of fibres, from whence lichens spring, which vanishes
when the plant is full grown.
[Illustration: Fig. 38. _a_, Sporopodium Leprieurii, ascus; _b_,
Coccocarpia smaragdina, section; _c_, Lecanora affinis, section.]
Globular bodies of vegetable green called gonidia, like those at the
base of fig. 39 _a_, are arranged in regular parallel rows, and placed
between the surface and the base of the lichen, as in fig. 38 _b_ and
_c_; they are said to proceed from the medullary layer, though Mr.
Berkeley has seen them springing from the threads of the mycelium of
Parmelia parietina. The gonidia find their way to the air through rents
in the surface of the plant, and are washed off by the rains, after
losing a little of their green colour. When they germinate they only
produce a _facsimile_ of the mother plant, as buds do in the highest
classes. But according to the microscopic observations of M. Tulasne,
the true fruit differs little if at all from the asci-bearing fungi.
Through the open discs of the higher lichens, sporidia are discharged
from perpendicular septate asci which, with their paraphyses, are
imbedded in the substance below. The asci are formed by the elongation
of some of the cells of that layer into cylindrical septate vessels,
generally containing from four to eight sporidia, the ultimate result of
as many free cells. In whatever part of a lichen the perithecia may be
placed, the asci and paraphyses invariably originate in the medullary
layer.
[Illustration: Fig. 39. _a_, Paulia perforata, gonidia, paraphysis, and
asci; _b_, Calicium tympanellum, perithecium and sporidia; _c_, Graphis
Leprevostei, with excipulum, asci, and sporidium.]
The thallus or frond of many of these lichens is irregularly covered
with thick convex scales, in each of which are concealed white or grey
bodies, which become flask-shaped, with pores in their extremities. When
mature, they exactly resemble the bottle-shaped perithecia in the genus
Sphæria among fungi. They are lined with branching filaments or other
supports, ending in minute ovoid particles, which escape in myriads from
the flask-shaped vessels long before the spores appear. They are motile,
though not provided with cilia, and are supposed to be fertilizing
particles like pollen, since they do not germinate as spores do. In this
vast family of open-shielded lichens, which comprises every form and
habit of the plant, the preceding type, which is the highest, undergoes
many modifications; but the essential character remains the same, and
the group is so natural, that the species run into one another so as to
leave no very striking distinction.
Commencing with the GYMNOCARPEI, the Parmeliacei first claim notice as
containing the highest types which lichens are capable of assuming, and
as abounding in species. The disc, which is the hymenium, is orbicular
or kidney-shaped, and surrounded by the frond, without any proper
excipulum or cortical covering. A large portion of our most widely
diffused lichens, whether growing on the ground or attached to rocks and
trees, belong to this tribe; they form patches two or three feet in
diameter, which are often of marvellous antiquity, and they grow so
slowly that even small patches are of great age. This tribe consists of
three distinct groups: in two of these the plants are horizontal and
sometimes foliaceous; in the third they are vertical, often branched,
and occasionally pendulous.
The latter group of this large division of lichens comprises the genera
allied to Usnea, which are erect, centripetal lichens, that is to say,
their body or thallus is an upright and generally cylindrical stem, in
the centre of which the layer of marrow, the origin of fructification,
is condensed, and the open discs, or hymenia, are in connection with it,
whether they be situated at the ends of the branches, or on the surface
of the upright stems. The Usneæ have three forms, the pendulous, the
inflated and erect, and the branched or shrubby. The Evernia jubata is
an example of the first, and remarkable for its long, pendulous,
cylindrical branches hanging down like bunches of bluish grey hair. It
is often attached to the branches of aged larch trees, and is
exceedingly picturesque. With the exception of Evernia vulpina and
flavicans, which are brightly coloured, all the rest of the genus have
dull tints. The genus Ramalina has the dull tints of the Evernia, and
some species approach very nearly in form to that genus. The Ramalina
polymorphum and R. scopulorum abound in dyes, while the R. calicaris is
rich in gluten.
Lichens have lost much of their importance since the discovery of the
coal tar colours; nevertheless they afford dyes still in use. In many
species the dying principle is colourless like aniline; but it yields
the most brilliant colours by means of alkalies. The Parmelia parietina,
common on walls and the roofs of houses, gives the chrysophonic acid, a
colourless liquid which becomes bright yellow when treated with an
alkali; and in like manner the vulpinic acid, given by Evernia vulpina,
gives a brown dye. Notwithstanding the quiet greyish green tints of the
Rocella fuciformis and R. tinctoria, no lichens are richer in the purple
substance known as orchil, from which, by means of soda or potash, the
valuable blue substance litmus is manufactured, so important as a test
for acidity. Many of the small moss-like lichens yield orchil, but none
in such abundance as the R. fuciformis, which grows in Madeira, Angola,
Madagascar, and South America. Oxalic and usnic acids are produced by
lichens; indeed the usnic acid combined with green and yellow resins
seems to be more or less a constituent of various lichens. It is evident
that the colour of the dyes is altogether independent of the colour of
the lichen from whence they are obtained.
These aërial plants have a marvellous power of decomposing the light,
and adorning themselves in brilliant hues. In our own country, where the
colouring is often cold from the excess of verdure, the lichen affords a
happy relief by giving a little warmth to the landscape. Even at Rome,
where nature is so gorgeously coloured, the ruins owe much of their
picturesque beauty to the red, white, and golden lichens with which they
are clothed.
The Usneæ are perhaps the most beautiful of the lichens, the colours
being sometimes brilliant, the forms elegant, and when the broad discs
are amply ciliated, the appearance is very striking. The same species
are widely diffused, but the colours are brighter in exotic specimens.
Usnea melaxantha and Usnea Taylori are splendid productions.[63] The
Usneæ are cosmopolitan in genera and species. The genus Cetraria, of
which the Iceland moss is a well known species, forms a connection
between the vertical and horizontal lichens; its thallus is neither
cylindrical nor quite erect, though it becomes more so towards maturity.
The Cetraria tristis has only that degree of inclination which arises
from its crowded mode of growth, and springs like a sea-weed from a
little peltate disc.
The most typical species of lichens occur in the second group of
Euparmeliaceæ, or Parmeliacei proper, in which the disc is at first
closed, and surrounded after expansion by a border arising from the
thallus or frond. The thallus is always horizontal, and expands from the
centre towards the circumference. The genus Sticta, belonging to this
group, is often highly foliaceous, and is not excelled by any horizontal
lichen in brightness of colouring or elegance of form. Even in our own
country the Sticta pulmonacea spreads over a wide area, and is
remarkable for its pitted frond. In this genus the under-side of the
plant is covered with a delicate velvety down, amidst which are
scattered round white spots, which penetrate to the medullary strata.
The erratic lichens are among the peculiarities of the genus Parmelia.
The Parmelia saxatilis, common on stones and boulders of the primary and
metamorphic formation, curls up into a ball, only fixed to its matrix by
a slender thread, which soon gives way, and the ball being dry and as
light as a balloon, is driven bounding by the winds, over the sheep
walks and downs of England. The globular Lecanora esculenta sometimes
suddenly covers large tracts in Armenia, Persia, and Tartary, where the
plants are eaten by the cattle and by the nomade tribes. This species,
with Lecanora affinis, is largely used by the inhabitants of the
countries east of the Levant, and in some parts of Africa is mixed with
wheat in grinding. These species are found, scattered over the ground
without any attachment, in the form of rugged truffle-like objects of
the size of walnuts. Sometimes they are piled together in strata a few
inches thick, by the whirlwinds, after traversing the air for many
miles, which gives rise to the histories of the miraculous descent of
food. During a scarcity, a shower of these lichens fell at Erzeroum, and
there are other well-authenticated instances recorded.[64]
The Peltigeri, or the third group of Parmeliacei, are so named from the
target-like discs on their surface, covered by a veil, which afterwards
disappears. The species frequently spread their beautifully foliaceous
fronds upon the ground, and as the fruit is marginal, it gives the
thallus a digitate appearance. They are often spotted by a little red
fungus. The genus Solorina has the fruit scattered over the frond; in
the Solorina saccata it is at first superficial, but a number of fibres
are formed on the under-side of the discs, which penetrate the soil, and
draw them down below the general surface—a very peculiar structure, in
strong contrast with that of the Solorina crocea, which has veins on the
under-surface but no fibres, and as no rootlets are sent out from the
base of the disc, it remains superficial. M. Tulasne discovered
fertilizing particles in several species of this group.
The order of the Lecidinei contains numerous species of the most varied
habits. There is a distinct and regular series upwards in the genus
Lecidea, the species of which are always crustaceous, and often form
merely a thin, close, adherent fibrous stratum on the hardest flint or
quartz. Some of them disintegrate the hard rocks on which they spread,
possibly by the action of some acid which they contain. From this low
type, the genus rises to the exalted forms with erect branching stems
clothed with foliaceous scales, and brightly coloured. The order is
distinguished by an orbicular disc, contained in a distinct excipulum or
cortical envelope, which is open from the earliest age; but it is
frequently obliterated afterwards by the development of that part of the
medullary stratum where the fruit is formed, the disc in consequence
becoming convex and capitate. This transformation takes place in the
highest forms of the genus Lecidea. The genus Bæomyces has rose or
chestnut coloured convex discs, supported on a stem called a podetium.
The genus Cladonia has a foliaceous thallus producing free scale-like
fronds, from the midst of which spring cylindrical or cup-shaped
podetia, which are sprinkled with leaves. The margins of the cups or
tips of the branches bear an abundant crop of convex, irregular brown,
or deep red discs, often as brightly coloured as sealing wax.
The species of Lecidea bear severe cold, and are wonderfully long lived.
Mr. Berkeley mentions that there are patches of the Lecidea geographica
which probably date from almost fabulous periods.
The fronds of the order Collemacei are gelatinous; the medullary stratum
seems to occupy the whole, though in two species of the genus there is a
distinct epidermic cellular coat. There are moniliform strings of minute
gonidia in the gelatinous matter, and others that are single, arising
from the division of each gonidium and its supporting thread into two:
both kinds differ from the gonidia in other lichens. The fruit-bearing
discs on the surface are open. Members of this order occur in Europe,
Africa, and Australia.
The order Coccocarpei is mainly distinguished by having orbicular discs
entirely deprived of the cortical envelope called an excipulum, or, if
it does exist, it is confounded with the thin membranaceous thallus. The
discs spring at once from the medullary stratum, and contain asci and
sporidia, similar to those of minute fungi (Sphæriæ). Some species of
the genus Coccocarpia only differ from Lecidea in the total absence of
an excidium. The order is chiefly parasitic, and in some cases the whole
plant is little more than a mass of fructification, parasitic upon and
continuous with the substance of other lichens, at whose expense they
live, thus forming an exception to the general habit of lichens, which
are fed by the atmosphere alone. M. Tulasne has discovered that the
genera Abrothallus and Scutula, though consisting almost entirely of
fruit, produce secondary spores—they are the only lichens in which they
occur; while Phacopsis and Celidium bear spermatogonia, which, analogous
to antheridia, contain minute fertilizing particles. The parasitic
genera occur in most parts of Europe and North America.
The Pyxinei are horizontal foliaceous lichens, for the most part fixed
by the centre. They have orbicular discs, and form one of the most
singular groups, both with regard to the superficial fruit, and the
curious convolutions of the perithecia. The fruit-cup, or excipulum, is
at first closed, and in the genus Gyrophora the disc produces a number
of partial fructiferous discs on the original fruit-bearing surface. As
usual the cells from whence the asci spring belong to the medullary
stratum; the border on the contrary is cortical. The thallus is always
foliaceous, and more or less peltate, but it becomes dry and brittle
when exposed to drought. Several of the species have tufts of strong
rootlets by which the frond is fixed to rocks, and some have deep pits,
with corresponding swellings on the upper-surface, from whence they have
their popular name of Tripe de Roche. These lichens, together with some
species of Gyrophora, afforded a miserable sustenance to Sir George Back
and his companions during their journey along the frozen regions of
Arctic America in quest of the north-west passage. It is evident that
plants which derive their sustenance solely from the medium in which
they live, whether air or water, can never become a permanent and
wholesome food for man, though erratic lichens and laver may be eaten
from necessity, certainly not from choice. In the Gyrophora the
perithecia are convolute, in the Umbilicaria they are not; but both of
these genera, which have their seat in Arctic Europe and America,
possess fertilizing particles like most of their class.
In the order Graphidei the disc is linear, simple, or branched, with or
without an excipulum, which is carbonaceous. Many of these lichens are
crustaceous, and in most of the genera the perithecia are much
elongated, pointed at both ends. When they are parallel, or placed at
different angles to each other, they form groups like Japanese or
Chinese characters, whence their name. The genera are determined by the
position of these perithecia, which are much varied. M. Tulasne has
found linear male particles immersed in the crust of some of these
lichens. They have their principal seat in tropical America, though a
few species occur in the frigid and temperate zones of both hemispheres.
The order Glyphidei has no true excipulum; the coloured discs are at
first immersed in the medullary stratum of a crustaceous thallus, the
crust then rises into distinct expansions, in the centre of which the
coloured discs are set like gems in a mosaic. ‘There is in fact no true
border to the disc, the perithecium being reduced to a thick conical
base, from which proceed immediately the asci and paraphyses; each
individual hymenium being surrounded by the intervening medullary matter
injected, as it were, into the interstices.’ The whole surface of
Chiodecton monostichum is productive, and in that genus M. Tulasne found
vessels in the form of little scattered perithecia, containing filiform
curved fertilizing particles. The species of this order are almost
wholly tropical, though the Chiodecton myrticola has been found in
Ireland.
The order Caliciei consists of horizontal lichens, with generally an
ill-developed crust; the discs, which are at first covered by a veil,
are contained in a stalked, or more rarely sessile, excipulum, looking
like little flat-headed pins stuck into the crust; the veil at length
vanishes, and exposes a pulverulent mass of spores, which adhere so
loosely in the Calicium inquinans, that they soil the finger if touched;
in other cases they come out of their ascus like little necklaces. The
species of these lichens are almost entirely confined to Europe and
North America.
In the second division of Lichenaceæ, the ANGIOCARPEI, the discs are
enclosed in an excipulum, which projects from the surface of the plant,
and ultimately discharges the spores from a rupture or pore in its
surface. Besides the fructification consisting of perithecia containing
paraphyses and asci with their enclosed sporidia, there is a nucleus in
each disc, either of firm or deliquescent matter.
Some of the lichens of this group are parasitic, others are aquatic. The
order Limboriei, like parasitic fungi, begin their existence under the
thick skin of the leaves of tropical plants, spread their crustaceous
thallus over their surface, and destroy their beauty, by stopping up
their pores, and preventing the admission of light to their tissues. The
excipulum and perithecia are black, and the latter burst in an irregular
fissure, and are in most cases covered by a beautifully sculptured
crust.
The crustaceous fronds of the Verrucariei are often so thin as to be
inseparable from the substance over which they spread. The excipula are
closed, the walls of the perithecia are often black, and in some species
more or less crowded round a columella. In the Verrucaria muralis
fertilizing particles have been discovered. The plants are widely
distributed, and at least one species spreads its crust over the smooth
stones in running streams.
In the group Endocarpei, the perithecia are immersed in the substance of
the plant, which has, for the most part, a foliaceous horizontal crust,
and a gelatinous nucleus. Some species grow on stones, perpetually or
periodically submerged, or, if not under water, continually wet with its
spray. The Lichina, a genus of the group Lichinei, lives on marine
rocks, and is often dripping with salt-water, and often suddenly dried
up.
The Sphærophorei, or sphere-bearing lichens, have upright stems bearing
globular fruit at the extremity of their numerous branches. At first,
the fruit is only indicated by a swelling, but in time the outer bark
bursts, and exposes the contents of the perithecium, which consist of
asci and paraphyses seated on a central columella. The sporidia are
beautiful objects under the microscope on account of their spherical
form, which is rare among lichens, and their more or less deep blue
tint—a colour by no means peculiar to the sporidia of this order of
plants, for in other orders they are bright scarlet, olive, golden
yellow, or brown.
SECTION V.
CHARACEÆ.
[Illustration: Fig. 40. Nitella flexilis.]
THE Characeæ are submerged annual water plants, growing in stagnant
pools and ditches rather than in running streams. It is a small order
containing but three genera, but the numerous species are dispersed all
over the world, especially in temperate climates. The genera found in
this country are Nitella and Chara. The Nitella flexilis (fig. 40) may
be taken as a representative of the order. Its stem or axis is formed of
very long cylindrical transparent tubes, joined by their flat ends, and
surrounded at each junction by a whorl of long tubes which are forked or
trifid at their extremities. In some species the branches are jointed,
and have whorls precisely like those on the main axis. On the internal
surface of the tubes, which are sometimes several inches in length,
there are four longitudinal bands parallel to the axis of the tubes,
which are occasionally twisted: two of these bands are broad and covered
with oval green particles; while the other two are narrow, transparent,
and colourless. Each tube is filled with a limpid semifluid liquid, in
which pale green particles and jelly-like globules of a starchy nature
float; and when these particles are watched with a microscope, they show
that, in every tube of the plant, a continual current of that liquid,
with its particles, ascends one of the green bands and descends by the
other, even when the stem and branches are twisted; but they never flow
in the colourless bands, though there is nothing to hinder them.
According to the observations of microscopists there is probably a
gyration of an azotized viscid fluid in all plants originating in, and
maintained by, vital contractility of structure, but in none is it so
evident as in the Characeæ. Its rate is increased by heat and diminished
by cold, like the circulation of the blood in animals, because the
activity of the vital energy bears a precise relation to the quantity of
heat received. The gyration is instantly arrested by a shock of
electricity.
The reproductive organs of the Characeæ are of two kinds, both growing
in the axils of the branchlets, namely, dark-red globules, which are
antheridia, and nucules or pistillidia, which contain germ cells.
Sometimes they are found in different individuals, but in most of the
Nitellas they are in the same individual, the globules being placed
closely below the nucules, as in fig. 41, A, B. The envelope of the
nearly spherical globules is formed of eight spherico-triangular valves.
From the middle of the interior surface of these valves, a perpendicular
orange-coloured column extends to the centre of the globule, where its
summit is crowned with a mass of confervoid filaments, which are formed
of a linear succession of minute cells; while from the base of the
column, bands of orange-coloured spherules imbedded in gelatine radiate
along the interior surface of the valve to its margin as shown at C, in
the same figure. After successive changes in the matter within the
confervoid filaments, (fig. 42, D-G), the microscope shows that ‘in
every one of the cells there is formed a spiral thread of two or three
coils, which, at first motionless, after a time begins to move and
revolve within the cell; at last the cell wall gives way, and the spiral
thread makes its way out, partially lengthens itself, and moves actively
through the water in a tolerably determinate direction, by the lashing
action of two long and very delicate filaments with which it is
furnished’[65] (fig. 42 H).
[Illustration: Fig. 41. Antheridia of Chara fragilis:—A, antheridium
developed at base of nucule; B, do., the nucule enlarged, and the
antheridium laid open by the separation of its valves; C, one of the
valves, with its group of antheridial filaments.]
The nucule is an ovoid sac with five long cells spirally twisted round
it, the sac being full of a viscous fluid containing globules of starch
and oil. This nucule falls off when fertilized by the spirally-coiled
ciliated bodies, and then germinates.
The Characeæ may also be artificially reproduced by cuttings; while
under favourable circumstances they are reproduced by nodular bodies
rich in starch grains, which arise from the creeping root, and also by
clusters of cells called bulbils filled with starch grains, which arise
from a modification of the nodes.
The Charas, properly so called, are monœcious or diœcious, more or less
opaque and brittle, their many-jointed tubular stems bearing whorls of
long slender awl-shaped branchlets. The fruit, accompanied by a cluster
of short bracts or ramuli, is placed at intervals in the hollow side of
the branchlets, one at each joint. In the bristly Chara the awl-shaped
branchlets are simple, pointed at the extremity, and composed of about
seven joints, with a whorl of from four to seven short bracts, and
having fruit at each articulation. Many of the species of Characeæ are
thickly incrusted with lime, and the whole order is exceedingly fetid.
[Illustration: Fig. 42. Antheridia of Chara fragilis:—D, E, F,
successive stages of formation of spermatozoids in the linear cells of
the antheridial filaments; G, escape of mature spermatozoids, which are
shown detached at H.]
SECTION VI.
HEPATICÆ, OR LIVERWORTS.
THE Hepaticæ are the small herbaceous plants, which constitute the three
distinct natural orders called respectively Ricciaceæ, Marchantiaceæ,
and Jungermanniaceæ. They are distinguished primarily by the first
having the sporangia valveless, without elaters; by the second having
dependent valvate or irregularly bursting sporangia; and by the third
having the sporangia valvate and erect. Both the latter, moreover, have
the spores mixed with elaters.
The Ricciaceæ, popularly called Crystalworts, which form the lowest
grade of Hepaticæ, are inconspicuous plants, growing in mud, or floating
on water. They have spreading, horizontal fronds, of a delicate cellular
structure and of indefinite form. Their fructification consists of
valveless spherical sporangia, or spore sacs, imbedded in the frond;
cells are formed within these sporangia, each of which cells is divided
into four parts, which become spores, and, when ripe, the surface of the
sporangia is fractured to give them egress. In some species there are
many air passages in the cavities where the sporangia are produced. The
under-surface of the frond is often beset with scales. The genus Riella
differs from all the other genera in having an upright branchless stem
with a distinct wing or limb forming a continuous spiral round it. In
the male plant, the edge of the frond bears the antheridia containing
spermatozoids; in the female plant the sporangia spring from the stem;
they have a separate envelope, and the spores are echinate.
[Illustration: Fig. 43. Marchantia polymorpha:—_a_, gemmiparous
conceptacles; _b_, lobed receptacles bearing pistillidia.]
The Marchantiaceæ rank as higher forms of Hepaticæ. In the Marchantia
polymorpha (figs. 43, 44), which may be taken as an illustration of the
group, the structure of the horizontal frond is complicated, for,
besides the colourless transparent skin, there are three distinct
layers, the uppermost of which consists of cells filled with green
matter, the lowermost (or base of the frond) being formed of close-set
cells full of very solid matter, while between the two there is a cavity
filled with air and loosely branching filaments, which spring from the
base, and consist of green cells fixed end to end, as in fig. 44 B. The
surface of the plant appears to be smooth and shining, but when
magnified it is found to be marked with numerous narrow elevated green
bands, crossing one another diagonally so as to mark out the surface
into a number of small lozenge-shaped divisions (fig. 44 A). These
elevated bands are merely the tops of very solid walls which descend
perpendicularly to the base of the plant, consequently they divide the
internal air chamber into a number of lozenge-shaped compartments, each
of which communicates with the external atmosphere by means of chimneys
opening in the centre of each compartment on the surface of the plant.
These chimneys are so constructed of four or five superposed rings of
cells (see fig. 44 B), that by the expansion and contraction of the
cells of the lowermost ring, more or less air can be admitted into the
cavity below. This is a very beautiful instance of the contractile vital
energy acting for the production of motion, its object being to supply
air, so essential to the health of all plants. White filaments from the
base at once fix the Marchantia to the earth, and supply it with food.
This is the youthful state of the Marchantia polymorpha, but after a
time green points appear from under little reddish scales on the
surface, and these are developed into stalks an inch or less in height,
which terminate differently, some in lobed shields, others in spoked
whorls, like a carriage wheel without the rim (fig. 43). The lobed
shields are rather concave and covered with little elevations, in each
of which there is a flask-shaped cavity with a long neck opening on the
surface of the shield. In all of these hollows there is a mass of cells
full of an amorphous substance, which is changed into spermatozoids,
having the form of delicate spiral filaments, thicker at one end, and
furnished with two cilia, with which they revolve in a spiral within
their cell. At last they emerge from it, and come through the neck of
the flask-shaped hollow to the surface of the shield.
[Illustration: Fig. 44. Marchantia polymorpha:—A, portion of frond seen
from above, showing lozenge-shaped divisions (_a_), with central stomata
(_b_); B, vertical section, showing the layers of tissue, and one of the
stomata (_g_).]
[Illustration: Fig. 45. Marchantia polymorpha:—archegonia.]
[Illustration: Fig. 46. Elater and spores of Marchantia.]
In the companion female receptacles at an early age, certain objects
called archegonia (fig. 45) are found to be concealed between membranes
which connect the spokes of the whorl at their origin. These archegonia
are shaped like flasks with long necks, and each has a germ cell in its
interior, into which a canal leads down from the extremity of the neck.
When this embryo or germ cell is fertilized by the spermatozoids,
instead of producing a new plant resembling its parent, the embryo cell
developes itself into a sporangium containing spores, which are isolated
cells enclosed in firm yellow envelopes and elaters, or ovoidal cells,
each containing a double spiral fibre coiled up in its interior. This
fibre is so elastic, that when the surrounding pressure is withdrawn by
the bursting of the sporangium at maturity, the spires suddenly extend
themselves with such force as to tear open the cell membrane, and jerk
forth the spores which may be adhering to their coils, and thus to
assist in their dispersion. The spores, when they germinate, develope
themselves into little collections of cells, which gradually assume the
form of a flattened frond.[66] It is only when exposed to light and air
that the Marchantias have regular fructification; in shady places they
are reproduced by buds placed in open conceptacles, which are formed out
of green globules that appear in different parts of the frond, and after
a time split open at the summit and expand into singularly graceful cups
or baskets, whose edges are sharply and regularly indented so as to form
a glistening fringe of teeth, while each tooth is adorned with a narrow
fringe. When mature, the basket contains a number of little green round
or oval discs raised on footstalks, and composed of two or more rows of
cells. As soon as these objects, called gonidia, are ripe, they are
detached from their stalks, and being washed out of their basket by the
rain, they quickly grow on the moist earth around; sometimes they
germinate before they leave their nest, and form irregular lobes on the
parent plant. The Marchantia polymorpha, so admirably constructed,
occurs in all temperate climates, and can bear considerable heat
provided it has abundant moisture.
The Marchantiaceæ are divided into three groups, containing fifteen
genera, which are distinguished from each other by the character of
their fructification. They are minutely described in Mr. Berkeley’s
‘Cryptogamic Botany;’ and are widely dispersed both in temperate and
tropical countries, most of the genera being represented in Europe.
The Jungermanniaceæ, or Scale Mosses, in their lowest forms bear the
same resemblance to Lichens that many of the other Liverworts do. These
lichenoid forms are lobed, leaf-like masses, sometimes ribbed and
sometimes not. There are forty genera of this group of plants,
distributed amongst fifteen tribes, and exhibiting great variety of form
and structure; but in external aspect they are so closely connected,
that a graduated series may be traced from the flat-lobed frond to the
higher forms of erect-stemmed foliaceous plants, approaching in size and
structure to some of the smaller mosses.
The higher groups have a distinct upright stem with symmetrical leaves,
which leaves, however, in the lower genera, are merely fleshy fronds
without veins, of numerous and sometimes grotesque forms, while in the
higher, they are generally oval, and disposed in spiral imbricated rows
on each side of the stem and branches, every leaf overlapping part of
that which is adjacent to it like a row of tiles. They are disposed
after two distinctly different plans. In one, the leaves are arranged in
a spiral, which turns from left to right; consequently, the anterior
border of each inferior leaf is covered by the posterior border of that
immediately above it. This constitutes the succubous group of frondose
Jungermannieæ, which embraces five tribes. In the second case, the
spiral of the leaves turns from right to left, and the anterior border
of each inferior leaf covers the posterior border of that immediately
above it. This constitutes the incubous group embracing three tribes of
the frondose Jungermannieæ. Besides the imbricated spiral leaves, there
are small leaves alternating with them, and applied directly on the stem
between them.
The male fruit of the Scale Mosses consists of antheridia, which form
ovoid masses of cells variously disposed; in each of these cells, there
is a filiform spermatozoid coiled in a circle in active motion, which is
maintained after leaving the cell. The female fruit consists of
archegonia, containing embryo cells, which ultimately develope into
sporangia; but the plants arrive at their full development before the
archegonia appear. The same plant may produce more than one crop of
these objects, which are occasionally abortive. The stalk of the
sporangium is first surrounded by the sac of the archegonium, then by an
involucre or circle of bracts, placed in a rosette of leaves. The
sporangia contain cells which ultimately become spores mixed with long
cells containing elaters, that is, elastic spiral filaments. When the
fruit is ripe, the sporangia split into four equal parts, which form a
cross on the top of their stems, and the elaters spring out of their
cells, and disperse the spores.
The plants of this order are also reproduced by gonidia, but in such
numerous genera and species the arrangement of the reproductive bodies
and their minute details are exceedingly varied. They are chiefly
distinguished by the insertion of the fruit, and the form of the
different organs which surround it. They are pretty little plants,
occasionally of a bright green, but oftener inclined to red, purple, and
chocolate colour; a few are fragrant, but they are of no known utility.
They are found in shady woods and moist situations, throughout all
regions of the globe, but are most abundant in damp, tropical forests.
In all the families of the leafy Cryptogamia as well as in the Hepaticæ,
antheridia exist; they differ much in form and structure, but they
collectively agree in developing in the interior of delicately walled
cells, an amorphous substance, coloured yellow by iodine, in place of
which, at the epoch of maturation, spermatozoids appear, thick at one
end and running to a very fine point at the other, and displaying
several spiral convolutions. When rolled up like a watch-spring the
motion is more or less rotary, but if it be coiled in the form of a
corkscrew, the movement is at the same time advancing. The thin end of
the filament always goes first both within the cell and after it comes
out.[67] The whole structure of the Hepaticæ is full of objects of
interest to the microscopic observer.
SECTION VII.
MUSCI, OR MOSSES.
MOSSES approach the higher classes of vegetable life in having roots,
and a more or less upright stem or axis of growth. Like all other
plants, they are chiefly formed of cellular tissue, yet, in the stems,
there is an indication of a separation between the bark and pith by the
intervention of a circle of elongated cells approaching to woody fibre,
which passing into the branches and leaves form a kind of midrib, either
extending to the extremity of the leaf or not. The delicate little
leaves, which are arranged with great regularity, consist of a single or
double layer of cells united by their flattened sides, and rarely
exhibiting any epiderm or skin. Green is the prevailing colour in this
order of beautiful little plants, but, when exposed to the sun and much
moisture, they sometimes become red.
The urn-like vessels (fig. 47 B), containing the spores, are either
terminal or lateral on the stem of the moss, and rise on a slender stalk
from a rosette of narrow leaves possessing a skin perforated by stomata
or breathing pores of simple structure. This is the ultimate result of
fructification; for at a very early period, while the rosettes of narrow
leaves are merely buds, they contain antheridia and archegonia, the
latter or female organs being either inclosed in the same bud, or in
different buds placed on the same or on different plants; but however
that may be, the buds are invariably placed at the base of the leaves,
close to the axis of the plant.
When a male bud is developed, the antheridia are found to be globular,
ovoid, or elongated hyaline bodies, set in a cluster of hairs and
paraphyses, or sterile cellular filaments. Fig. 48 represents the
antheridia of Polytrichum commune, with the paraphyses and hairs, from
the microscopic observations of Dr. Carpenter. The central antheridium
is discharging its spermatozoids; the one on the right is empty, that on
the left immature.
[Illustration: Fig. 47. A, plant of Funaria hygrometrica:—_f_, leaves;
_u_, urns or capsules; S, seta or foot-stalk; _o_, operculum; _c_,
calyptra. B, sporangia of Encalypta vulgaris:—_u_, urns; _o_, opercula;
_c_, calyptra; _p_, peristome; _s_, seta.]
The antheridia are filled with a mass of mucilage containing a multitude
of cells, in each of which there is a spiral filament furnished with
cilia, as in fig. 49. As soon as the filaments are mature, the cells
open; the mobile filaments, or spermatozoids, are free, and come out
through a pore in the antheridia, in multitudes like pollen out of an
anther.
While the antheridia are in process of formation, the female buds expand
and exhibit flask-shaped archegonia, similar to that in fig. 45, seated
in a rosette of leaves. When this archegonium is fertilized by the
spermatozoids, its internal germ cell is developed by cell division into
a conical body elevated upon a stalk; and this at length tears across
the walls of the flask-shaped archegonium by a circular fissure,
carrying the higher part upwards as a calyptra or hood (fig. 47 _c_)
upon its summit, and leaving the lower part to form a kind of collar
round the base of the stalk.
[Illustration: Fig. 48. Polytrichum commune:—group of antheridia mixed
with hairs and sterile filaments (paraphyses), the central one
discharging its contents.]
[Illustration: Fig. 49. Polytrichum commune:—B, cellular contents of an
antheridium previously to the development of the spermatozoids; C, the
same, showing the first appearance of the spermatozoids; D, the same,
mature and discharging the spermatozoids.]
The urn-shaped organ or sporangium has a double wall, and in its centre
a fusiform or deeply winged columella. Between these two there is a
tissue of a most delicate texture, divided into spherical cells, each of
which usually produces four unripe spores, which get an exterior coat
when fertilised, and then it is that the stalk lengthens, and the
flask-shaped archegonium is torn across.
The urns are closed by a lid or operculum of a flat, convex, or pointed
form, which falls off when the spores are ripe to give them egress. The
mouth of the urn in most of the mosses is surrounded by a deciduous
annulus of two or three rows of elastic cells, which are supposed to aid
in scattering the spores, for the mosses have no elaters. Although the
separation of the lid may at once expose the spores, they may be covered
with a membrane entire or toothed at the circumference, or there may be
one or two rows of teeth surrounding the mouth of the urn like a fringe;
these teeth form the peristome, and their number is always a multiple of
4, varying from 4 to 64 (see fig. 47 _p_). Sometimes they are divided
half way down, sometimes they are prolonged into straight twisted hairs.
The teeth arise from the thickening of the walls of two contiguous
cells; when there are two rows of teeth, the outer row frequently arises
from the layer of cells which line the outer wall of the urn, the inner
row from the outermost layer of the spore sac; sometimes, when the
peristome is double, three strata of cells are requisite to form the
teeth. The urns of the Encalypta vulgaris are represented at fig. 47 B;
one is covered with the hood or calyptra, while from the other the lid
has fallen off, showing the mouth of the urn with its toothed peristome.
When a spore begins to grow, its outer coat is ruptured, its innermost
coats protrude and form a projecting extremity, which becomes developed
into a confervoid pro-embryo, analogous to the mycelium of fungi, but is
distinguished by the chlorophyll contained in its cells. Many spores may
concur or not in its formation; but the pro-embryo of each spore is
capable of transforming one or more of the cells seated upon its various
ramifications immediately into buds, which grow up into leafy stems, as
the Funaria hygrometrica, so that here we have the peculiar condition of
one spore giving rise to the development of a number of plants.[68] In
process of growth each of these plants would produce antheridia and
archegonia, and, by the process described, a full-grown Funaria
hygrometrica with its urns and hoods, as represented in fig. 47 A, would
be the result.
If the moss be annual or biennial it dies after bearing fruit; if
perennial, two or more successive crops of archegonia are formed. The
mode of fructification, therefore, resembles that of flowering plants,
with this difference, that the fructification of the latter produces a
young plant from each embryonic cell, while in mosses the fructification
of one embryonic cell produces a sporangium or urn containing spores,
that is, a multitude of reproductive bodies, which have no trace of
cotyledons or axis.[69]
Mosses are also propagated by gemmæ or buds. They are produced in many
situations, sometimes on distinct organs, sometimes on the tips of the
leaves, or on rootlets which grow on various parts of the plants, and
which in some of the mosses form a dense woolly or silky mass of a
bright yellow or brown colour varying to purple. On the fibres of this
mass, green cells appear, which are developed into reproductive buds.
Almost every cell on the surface of a moss is capable of forming, by
continued division, a cellular nodule, which falls off and gives rise to
a germ which grows into a new plant. These nodules are generally
situated at the extremity of the leaves, or on the leaves themselves,
while pro-embryo fibres spring from the leaf cells of many mosses. M.
von Mohl observes that since the cells in the different parts of mosses
are capable of being developed into a bud or embryonic confervoid
structure producing a bud, it follows that in these plants,
notwithstanding their rather complex structure, the subordination of the
individual cell to the purposes of the whole plant is still but small;
and even here individual life readily acquires the preponderance. This
facility of reproduction possessed by the various individual parts of
the plants, accounts for the extensive tracts over which mosses spread
themselves; moreover, some are diœcious, and as the spores might not
always be fertilized, the gemmæ ensure the continued existence of the
species.
Mosses are divided into five principal groups, differing exceedingly in
importance, and in comparative numbers. They are chiefly distinguished
by the position of their fruit. The Pleurocarpi have their fruit
lateral, whether on the stem or branches. They comprise thirteen tribes;
many of them are found in the southern hemisphere, but a considerable
number, especially of the Hypnei, Drepanophyllei, and Hookeriei, are
European. The Cladocarpi are characterised by having their urns seated
on the tops of very short lateral branches, and by their double axis of
growth. The Acrocarpi are distinguished by their main stems ending in
fruit. They comprise twenty-seven tribes, and embrace genera and species
having a wide geographical range. In the Syncladei the branches of the
plant are fasciculate; this group comprises the Sphagnums. The
Schistocarpi are remarkable for their fruit splitting into valves, and
consist of the tribe Andræaceæ. The various tribes depend on the
structure of the urns and leaves, as well as on the natural habits of
the plants. In this numerous class of plants only a few remarkable for
peculiarity of structure can be mentioned.
The species of the acrocarpous tribe Phascei are exceedingly numerous,
and contain the simplest of all mosses; they grow on newly turned-up
soil, and are chiefly annual. Their leaves generally have nerves, and
are bordered by large cells. The urns, which are either sessile, or upon
a short stalk, have not a trace of peristome, and sometimes have a
columella, sometimes not. The spores are large compared with those of
other mosses.
The tribe Dicranei contains numerous species, some of which are the
commonest of mosses in Europe. They are easily known by their single
peristome of sixteen teeth divided half-way down. The leaves are
extremely crisp and convolute, and the hood is spoon-shaped. The
Leucobryum is remarkable for pallid leaves; it has three layers of
cells, a narrow layer of green cells embedded in the centre of the leaf
with a broader layer of colourless cells on each side, whose cell walls
are perforated with large round openings, as in fig. 50 _b_. The mosses
of this order live on sandstone rocks, shady banks, and trunks of trees.
Fig. 50 shows the microscopic structure of the leaves of various mosses.
In the tribe Grimmiei we have frequently a sessile urn, with a single
peristome, and a mitre-shaped hood. The leaves, which are dark green,
have minute hexagonal perforated cells on their upper surface, and a
white nerve projecting from their extremity.
In the tribe Polytrichei the mouth of the urn is mostly closed by a flat
membrane and a hood rough with silky hairs. The leaves are sheathing at
their base and spreading at their tips; except in a few cases they are
rigid; and the nerve often exhibits lamelliform folds. The Polytrichum
dendroides contains scalariform ducts, and starch granules.
The Bryei are of variable size, but a number of European species are
among the finest of mosses, on account of their large leaves and
beautiful double-toothed peristome, the great distinction of the group.
The leaves are margined, toothed, and composed of a loose reticulation
of large rhomboidal cells; in the genus Timmia they clasp the stem at
the base and spread widely at the tip. There are thirty-three British
species of the genus Bryum, many of which with their abundant urns are
extremely ornamental.
[Illustration: Fig. 50. Microscopic structure of leaves of mosses:—_a_,
Octoblepharum albidum; _b_, Leucobryum glaucum; _c_, Sphagnum
latifolium; _d_, Hypopterygium Smithianum; _e_, Eucamptodon
perichætialis; _f_, Andræa subulata; _g_, Campylopus lamellinervis.]
The tribe Splachnei contain many of the most singular and beautiful of
the whole class of mosses. They have large-celled diaphanous leaves and
a straight urn, with the spores radiating from the columella. The urn
has a swelling at its base, often of greater dimensions than the urn
itself. In the Splachnum vasculosum it is purple and very large, but
nothing in comparison of that organ in S. luteum and rubrum, which are
the pride of hyperborean Europe and America. The enormous size of the
swelling, the variety of colouring, the singularity and elegance of
form, and in some cases the unusual dimensions, make the species objects
of great interest. The common S. ampullaceum, when growing in abundance
on the shallow peaty banks of some mountain stream where cattle come to
drink, is scarcely exceeded in beauty by any cryptogam. Species of this
order are abundant in the two hemispheres, but the same species rarely
occur in both. Their habits, too, are different, for while those in the
north only grow on manure, those in the south grow on the trunks of
fallen trees. Three genera occur in Great Britain. Gemmæ are found in
the axils of the leaves in most species of this group.
The tribe of Schistostegei consists of but a single most elegant
species. It inhabits shady caverns, which are sometimes lighted by a
golden gleam from the refractions of the confervoid shoots of its
mycelium-like pro-embryo, which is perennial, and produces a new crop
year after year. The urn is subglobose without a peristome, and when
young the spore cells radiate from the columella as in the Splachna. The
leaves show various intermediate stages between a vertical and
horizontal insertion, and are sometimes perfectly free, while at other
times they are united. This moss is scarce, and confined to the northern
hemisphere.
There are various genera of aquatic mosses in the different tribes, most
of them floating plants. Of these are the pleurocarpous Fontinalei,
which inhabit the northern hemisphere; their urn, with its double
peristome of sixteen teeth, forms a beautiful microscopic object, on
account of the latticed work in the inner row, and the cross bars on the
outer teeth, which are united at their tips by two and two. The common
species have sharp-angled triangular stems, and keeled leaves which
clasp the stem at their base, and are sometimes cleft along the keel.
The syncladeous Sphagnei are aquatic bog mosses of a pale yellowish
green colour. They form but one genus, Sphagnum, which consists of some
eight or ten species and several varieties. The species of Sphagnum, or
common bog moss, hold an important place in the economy of nature. They
are floating mosses, entirely destitute of roots. The stalk of the full
grown plant, like that of land mosses, is constructed of three kinds of
cells; one forms the exterior or cortical layer, another forms the
central pith or axial system, while the third, which is coloured and
somewhat ligneous, comes between the other two. The leaves, which have
their origin in the cortical layer while it is yet soft, consist of two
kinds of cells, one kind being large, elongated, and colourless, and
containing spiral fibres loosely coiled in their interior. The
membranous walls of these cells have large round apertures by which
their cavities have free communication; for certain animalcules, which
sometimes live in the cells, have been seen to pass from one to the
other. Between these colourless cells are some thick-walled, narrow,
elongated green cells, which give the leaf firmness and colour.
According to Mr. Wilson, the fascicles, or bundles of branches, are
disposed round the stem in imbricated spirals, so that, for every
complete spiral formed by five of these fascicles, there are eight
spirals formed of twenty leaves, four leaves being inserted between each
pair of fascicles. The fructification in all these floating mosses is
immersed in the leaves of the stem, the antheridia being globose, and
the spermatozoids having spiral motions, both within the cells of the
antheridia and when they come out. The urns, which are the product of
fructification, and are borne at the top of what appears to be a long
footstalk, but is in reality a pedunculate vaginula, are globular, and
their lids have been observed to be driven off when the spores are ripe,
with such force as to give a distinctly audible report.
The common bog moss grows so rapidly that, rootless as it is, it soon
covers a pool with its matted bundles of branches, and as in a few years
it has no room to spread, the lower stems and branches decay, sink to
the bottom, and begin to form a peat moss, while the upper parts grow
on, so that new stems and branches are perpetually produced. Multitudes
of spores no doubt germinate, and, in this way, the pool is filled up,
and a peat moss is at length formed.
The Sphagnum moss has such a power of absorbing moisture from the
atmosphere, that it forms and maintains the peat mosses and quagmires in
the mountains which feed the streams at their feet. Mosses in general
are almost as much indebted for moisture to the absorbing nature of
their leaves as to their roots; but the loose, large celled, and
perforated leaves of the Sphagnum suck up water like a sponge; even
during the heat of summer, a quantity of water may be squeezed out of a
handful of them. In fact the plant is a perfect hydraulic machine, for a
small stem of it put into a glass of water with its drooping terminal
branch hanging over the edge, acts like a syphon, and soon empties the
glass, pouring the water out through its bending top. Though the peat
from Sphagnum is often too spongy for fuel at present, yet that little
moss now growing on our mountains will yield aniline, magenta, paraffin,
and other illuminating gases to remote generations, although not in such
quantities as the richer vegetation of the coal measures, the products
of a warmer period.
Like all cryptogams the mosses are exceedingly variable and difficult to
distinguish. Not only does the same species show great differences in
size, but even in other respects the characters vary on account of
climate, soil, and exposure.
Beyond that of becoming converted into peat, the uses of the moss family
are not of any great importance. Brooms, mats, and other domestic
articles are sometimes formed from Polytrichum, and in Lapland Sphagnum
not only sometimes enters into the composition of bread, but is used in
place of clothing for new-born babes.
SECTION VIII.
FILICES, OR FERNS.
OF all the spore-bearing families, the Ferns are the most universally
known. They may easily be recognized by the coiling inwards of their
young leaves in spring previous to expansion, and by the arrangement of
the fruit on their undersides when expanded. The Ferns are exceedingly
numerous both in genera and species, and vary from low herbaceous plants
of an inch high, to trees with upright trunks forty or fifty feet or
more in height, bearing a graceful coronet of leaves at their extremity.
The tree ferns come to the utmost perfection in the warm, moist islands
of the tropical oceans. Their boreal limit is about the thirty-seventh
parallel of north latitude, but, on account of the vast extent of ocean
in the southern hemisphere, they reach the fortieth or fiftieth parallel
of south latitude, the more copious evaporation, and the consequently
moister air and soil, being specially congenial to the fern tribe. For
that reason a most luxuriant fern vegetation prevails in Juan Fernandez,
Western Chili, and New Zealand. In the latter there are one hundred and
twenty species, some of which are subarborescent, while others form tree
ferns of considerable altitude. Shade is not absolutely requisite to
ferns, for many of them grow luxuriantly when exposed to the sun,
provided the soil be damp.
The range of the non-arborescent ferns is very extensive. According to
Dr. J. D. Hooker, twenty-one species have been found in Fuegia and the
Falkland Islands, and one grows in matted tufts in Kerguelen’s Land. In
the north the Cytopteris fragilis has been found in the seventieth
parallel of latitude at Minto Inlet, and both it and Polystichum
Lonchitis have been gathered at Disco, on the west side of Greenland,
and Aspidium fragrans on the eastern side.
On account of the extremes of heat and cold in North America there are
only fifty species in all that enormous extent of country; while in
Britain we are indebted to the humidity of our climate for a rich
vegetation of thirty-six species, which adorn our woodlands, our valleys
and mountains in wild profusion. One-half of our ferns are also native
in the Himalaya mountains, where multitudes of British plants are
indigenous. The fern floras of Great Britain and New Zealand are the
richest in species of their respective latitudes, and have several
species in common. The ferns of Tasmania are, with few exceptions,
identical with those of New Zealand; and the occurrence of the rather
common Australian and New Zealand Gymnogramma rutæfolia in the Pyrenees,
and no where else in the whole world, so far as is known, is a
remarkable fact in the distribution of plants.[70]
Whatever the size of a fern may be, its spores (fig. 51 A) are
microscopic. They are produced within the sporangium by cell division,
and are therefore free and variously shaped. They consist of a grumous
mass enclosed in a double coat; and when the spore begins to grow, it
sends out from the cell wall of its inner coat a white tubular
projection or root fibre (fig. 51 B), which passes through the cell wall
of its outer coat. This root sucks up liquid till it expands the inner
coat sufficiently to burst open the outer one, and then it begins to
increase by the subdivision of its cells, till the primary green leaf or
marchantioid prothallus (fig. 51 D) is formed. This prothallus lies flat
on the ground, and is furnished on the under-side with fibrous roots to
fix it, and supply it with food.
[Illustration: Fig. 51. Development of spores of Pteris serrulata:—A,
spore; B, C, early stages of development; D, the prothallus with radical
fibres (_a_, _b_) and antheridia (_h_, _h_).]
Two sets of organs are subsequently developed on the under-side of this
prothallus; one of these is a stalked cell called an antheridium, and is
situated near the roots; the other is an archegonium containing a germ
cell, which is sunk in the cellular tissue. In each of the antheridia,
which are numerous, a cell is formed, which becomes filled with a mass
of mucilage mixed with a number of free cells, containing a flat
ribbon-shaped filament, or spermatozoid, coiled in a spiral manner,
which, as soon as set free by the rupture of the cell, revolves rapidly
by means of several long cilia placed close to the large end. Fig. 52
shows the globular antheridium and the spermatozoids of Pteris
serrulata. The archegonia are fewer in number, and contain the germ
cell, represented in fig. 53, as viewed from above, and sidewise; it is
placed at a little distance from the antheridia, and after being
fertilized by the active filaments or spermatozoids, and matured, it
contains the primordial cell of the young fern, which soon sends forth
leaves rolled up and curled inwards previous to expansion, and a root,
which being sufficient to feed the plant, the flat pro-embryo, which was
the first stage of development, perishes. Thus in the family of ferns
there are two distinct periods of growth, and one only of fertilization.
In flowering plants, fertilization and its products are the final result
of vegetation, and the maturation of the fructification is frequently
followed by death. In the ferns, on the contrary, fertilization precedes
the development of the plant, which, if perennial, continues to bear
fertilized fruit year after year.
[Illustration: Fig. 52. Antheridium and Spermatozoids of Pteris
serrulata:—A, projection of one of the cells of the prothallus showing
the antheridial cell (_b_); B, antheridium fully developed, containing
sperm-cells, each enclosing a spermatozoid; C, one of the spermatozoids
magnified, showing the cilia.]
[Illustration: Fig. 53. Archegonium of Pteris serrulata:—A, as seen from
above; B, side view, showing at A the cavity containing the germ cell;
at B, the walls of the archegonium made up of four layers of cells; and
at C, the spermatozoids within the cavity.]
It is evident that there is an essential difference between the
archegonium of the Marchantia, and that of the ferns. The archegonium of
the Marchantia merely produces a sporangium, while that of the ferns
produces a new plant, and is therefore, in some sense, analogous to the
seed of one of the higher vascular plants, though there is no affinity
between the two.
The roots of ferns, like those of forest trees, are delicate fibres,
which descend either from a woody stem called a rhizome, from _rhiza_, a
root, or from a caudex, so named from the Latin, _caudex_, a trunk. Thus
the stems of ferns are of two distinct types. The rhizome generally
grows horizontally, and creeps along the surface of the soil or rock, or
tree trunk to which it is affixed, but it is sometimes subterranean. In
this, the growing point is in advance of the fronds, which appear at
intervals along the exposed side or sides, and after they have reached
maturity drop off, leaving a clean scar or cicatrix. The caudex varies
from the size of a small wire-like thread to the size of a tree trunk.
It is sometimes elongated, as in the superfine wiry thread-like stems of
Hymenophyllum, the scandent ivy-like rooting stems of Stenochlæna, and
the subterranean, horizontal, widely-creeping stems of Pteris aquilina,
all of which develop single fronds at intervals, but fronds which adhere
permanently to the stem, and which do not fall off, leaving a scar, as
do the fronds borne by the rhizome. The more frequent form of caudex,
however, is that of a short stocky stem producing fronds on all sides
from its apex: very frequently this is a globose or oblong mass growing
erect, yet scarcely reaching above the surface of the soil; but
sometimes it is more lengthened, showing a more or less elevated stem,
or turning sideways, and taking a decumbent position, the young fronds,
however, always rising from its apex. It is on the latter plan, by a
continuous erect and upward growth, that the trunks of tree ferns, which
rise sometimes sixty or eighty feet high, are formed. The common Lady
Fern, Athyrium Filix-fœmina, furnishes an example of the ordinary
herbaceous caudex; as also does the Male Fern, Lastrea Filix-mas. In the
Hart’s-tongue, or Scolopendrium, the caudex is generally compact, and
increases by the formation of new crowns or centres of growth around the
older one, till the whole becomes an almost spherical mass of
considerable size.
The stipes or leaf-stalk of a fern is often of considerable length, and
in its upper part, called the rachis, which bears the leafy portion, is
commonly more or less branched, as well as furrowed on its upper
surface. The fronds, or parts analogous to leaves, are sometimes simple,
as in the Hart’s-tongue, the rachis of which has a symmetrical limb or
wing on each side, so as to form a long, even-margined, narrow,
tongue-shaped frond. If the limb on each side of the rachis be deeply
divided, as in the Polypodium vulgare, the frond is said to be
wing-cleft, or pinnatifid, and the divisions are called segments or
lobes. In most ferns, however, the frond is once, twice, or three times
winged or pinnate, and in such cases the first divisions are called
pinnæ, and the secondary or subsequent ones when present, pinnulæ or
pinnules. The leafy portion, whether simple, pinnatifid, or pinnate, is
always traversed by veins arranged on some definite plan, a most
important circumstance, since the sori or fructification of the ferns is
always produced in connection with the veins.
The fertile fronds, in certain groups, differ in form from the sterile,
generally by the greater or less contraction of their parts. In most
ferns the full-grown fronds are flat, that is, with all their parts
lying in one plane; but, during their vernation, that is, when they
first rise from the stem, they are circinate or curled inwards, like a
crosier.
When a fern acquires a considerable stem, as in Tree Ferns, it consists
of a central or medullary part, consisting of cellular tissue, and an
external or cortical portion, formed of the consolidated bases of the
fronds, in which may be seen, on cutting a section of a trunk, an
irregular zone, formed of fibro-vascular bundles, scalariform ducts, and
woody fibre. Prolongations from this zone pass into the leaf-stalks, and
thence into the midrib of the leaf, whence they spread into its lateral
branches, and ultimately appear in the leafy parts in the form of veins.
It is the arrangement of these bundles of coloured woody tissue in the
cellular tissue of the leaf-stalks of the herbaceous ferns, which give
rise to those peculiar figures on their transverse sections, such as a
star, the letter T, and the heraldic spread eagle, from which latter the
common Brake Fern (Pteris) takes its botanical specific name of
aquilina. Fig. 54 shows an oblique section of the footstalk of a fern
leaf with its bundles of scalariform ducts, as determined by Dr.
Carpenter’s microscopic observations.
[Illustration: Fig. 54. Section of footstalk of Fern frond showing
scalariform ducts.]
The fructification of the ferns is arranged with the most perfect
symmetry, usually on the under surface of the leaf, but sometimes at the
margin, and assuming a great variety of forms and positions. It consists
of sori, that is, groups of nearly globular spore cases or sporangia,
sometimes sessile on the frond, sometimes with a footstalk. They are
always situated on a vein or its branches, or at the extremity of the
veins on the margin of the frond. In fact the sporangia originate in the
cellular tissue immediately in contact with a vein, beneath the
epidermis or skin of the leaf, which is forced up as the sporangia
increase in size, in the form of a whitish membrane, which constitutes
the indusium, or protecting cover of the sori. While the fruit is
advancing to maturity, the indusium separates partly or wholly from the
surrounding skin or epiderm, and subsequently either shrivels or falls
off altogether. In some few species the opening is in the centre of the
indusium, and then it surrounds the sori like a cup; in other ferns, the
skin from both surfaces of the leaf extends beyond the margin, includes
the sori between it, and fulfils the office of an indusium.
[Illustration: Fig. 55. Pinnule of Polypodium bearing sori.]
The sori, as already noted, take a variety of forms, and are variously
situated. Some are round and dot-like, some are oblong and straight,
some are hippocrepiform or horse-shoe-shaped, while some are continuous
in a line-like band. The indusium, when present, takes more or less
exactly the form of the sorus. These peculiarities are so well marked
that they are taken advantage of in the discrimination of genera.
The sporangia or spore-cases are for the most part of globular form, and
are nearly or quite surrounded by a strong elastic ring, which in some
cases is continued so as to form a stalk. When the spores are ripe, this
ring, by its elastic force, tears open the sporangia, and gives free
egress to their contents. The ring assumes various forms. In one large
group it passes vertically up the back of the sporangium, and is
continued to a point called the stoma, where the horizontal bursting
takes place. This form is seen in fig. 56, _a_, _b_. In other groups it
is, though vertical, somewhat oblique, as in fig. 56 _c_. Sometimes,
though more rarely, it is transverse and complete; in which case the
rupture is vertical, as in fig. 56 _d_. In a few cases it is apical,
fig. 56 _e_; and in a few others it is obsolete, fig. 56 _f_. These are
the true Ferns. In one or two small groups, sometimes called
Pseudo-Ferns, the ring is altogether wanting.
[Illustration: Fig. 56. Sporangia of Polypodiaceous Ferns:—_a_, _b_,
Polypodiaceæ; _c_, Cyatheineæ; _d_, Gleichenineæ; _e_, Schizæineæ; _f_,
Osmundineæ.]
The systematic arrangement of the Ferns is chiefly founded on
peculiarities of the sori and sporangia, characters which are quite
intelligible by the aid of a good magnifying lens, these spore-cases
being very pretty opaque objects under the microscope. Thus some of the
primary divisions are founded on the presence or absence of the ring or
annulus on the spore-case. Another series of divisions are founded
mainly on the nature of the ring in those cases when it is present; and,
for the rest, the form and position of the sori come in as
discriminating characters. In this way the main groups are marked out,
but, in the case of the genera, still further recourse is had to the
sori and its covering, and by some modern authors supplementary
characters derived from the venation are brought into use. The following
is the arrangement adopted by Mr. Moore,[71] and which agrees in its
general features with that of most modern observers.
The two leading groups are the _Annulate_ and the _Exannulate_ Ferns,
the first being much the larger division, and consisting of the order
Polypodiaceæ, which comprises the True Ferns, while the second includes
the two orders Marattiaceæ and Ophioglossaceæ, which are some times
called Pseudo-Ferns.
The POLYPODIACEÆ, distinguished by the ring or annulus, which more or
less completely girts the sporangia, offer so much variety of
structure that it becomes necessary to subdivide them; and for this
purpose characters derived from the form, number, or position of the
sporangia, or the structure or development of the ring, are chiefly
relied on. This gives several groups, _e.g._ Polypodineæ, the most
comprehensive of all, including some ten or twelve minor groups, in
which the sporangia are almost equally convex, and have a vertical and
nearly complete ring, and in which the dehiscence is transverse at a
part called the stoma, where the striæ of the ring are elongated, and
apparently weaker; Cyatheineæ, in which the spore-cases are sessile or
nearly so, seated on an elevated receptacle, with the nearly complete
ring more or less obliquely vertical, that is, vertical below, curving
laterally towards the top, and the dehiscence transverse; Matonineæ, a
single species only, in which the sporangia are sessile, bursting
horizontally, not vertically, the ring being broad, suboblique, and
nearly complete, and the dorsal sori oligocarpous, covered by
umbonato-hemispherical indusia, affixed by a central stalk;
Gleichenineæ, with the ring complete, transverse, either truly or
obliquely horizontal, the sporangia globose-pyriform, forming
oligocarpous sori, _i.e._ sori consisting of but few spore-cases (two
or four to ten or twelve), situated at the back of the frond, sessile
or nearly so, and bursting vertically, the fronds, moreover, being
rigid and opaque, and usually dichotomously-branched; Trichomanineæ,
with the ring resembling that of the Gleichenineæ, but the sporangia
lenticular, numerous, clustered on an exserted receptacle, which is a
prolongation of the vein beyond the ordinary margin of the frond, so
that the sori become extrorse marginal, or projected outwards as well
as opening outwardly, while the fronds are pellucid-membranaceous;
Schizæineæ, with the ring horizontal or transverse, situated quite at
the apex of the oval sporangia, which is, in consequence, said to be
radiate-striate at the apex; Ceratopteridineæ, one or two aquatic
species, the sporangia sometimes furnished with a very rudimentary
ring, reduced, as in Osmundineæ, to a few parallel striæ, sometimes
furnished with a very broad and more lengthened ring; and Osmundineæ,
with the spore-cases two-valved, bursting vertically at the apex, the
ring very rudimentary, reduced to a few parallel vertical striæ on one
side near the apex. In all but the last of these groups, the sporangia
are not valvate, and consequently, when they open for the liberation
of the spores, they burst partially or irregularly; but in the
Osmundineæ they split at the top in two equal divisions.[72]
A large portion of the Polypodineæ are either tropical or subtropical.
The genus Polypodium itself is one of the most extensive and diversified
genera of the group. It is chiefly distributed over the tropical regions
of the western hemisphere, but four species are indigenous in Britain,
and of these the Polypodium vulgare, or Common Polypody, is abundant
about the trunks of moss-grown trees, on banks, rocks, and old thatched
roofs. The young fronds appear in May, and rise from five or six to
twelve or eighteen inches in height. They are lanceolate, deeply
pinnatifid, with obtuse, linear, lanceolate, indistinctly serrated
segments. The genus is distinguished by its naked globular sori, which
in this species are regularly disposed in a line on each side of the mid
vein, half way between it and the margin of the leaf. When young, they
are of a yellow or bright orange colour, which changes to brown when
they are ripe. The rhizome, which branches in all directions, is at
first clothed with a skin densely covered with yellowish brown
membranaceous lanceolate scales, which at length fall or become
obliterated, leaving the surface nearly smooth. The Polypodium vulgare
has many varieties, several of which are well marked, especially that
called cambricum, which is twice pinnatifid. The plant is common in
temperate climates.
[Illustration: Fig. 57. Pinnule of Lastrea Filix-mas with sori.]
The Aspidieæ form an extensive and widely distributed group, embracing
several of our common British species. One of these is the genus
Lastrea, which has, for the most part, lanceolate fronds, bipinnate or
tripinnate in division, with linear lanceolate, and usually pinnatifid
pinnæ. The sori are nearly circular, seated upon the back of the veins,
and covered by a reniform indusium, which is attached by its sinus. In
Lastrea Filix-mas (fig. 57), one of our commonest ferns, the fronds
spring up in a rather spreading mass, from the extremity of a long scaly
caudex, and often present a vase-like tuft hollow in the centre. The
rachis, leafy through a third or a fourth of its length, is more or less
clothed with thin membranaceous scales, of a pale often brownish-golden
hue, a peculiarity common to the other Lastreæ. Lastrea Thelypteris, the
Marsh Fern, however, has them not, and differs also from most of its
congeners in having its sori submarginal. Its rhizome has a widely
creeping habit of growth. The Lastrea æmula has globular glands, sessile
over the whole under-surface of the fronds, which secrete a perfume like
new-made hay. From a similar cause, Lastrea rigida was botanically known
as Polypodium fragrans. This coumarine odour is possessed by several
other ferns, notably by Cheilanthes odora.
The Oleandra, which is a foreign genus, has simple fronds articulated
near to the rhizome, and its sori are placed on the tips of the parallel
veinlets. The Oleandra neriiformis, which grows on open spots, has an
erect rhizome, and assumes quite a shrubby habit, rising to from four to
six feet, and bearing at intervals whorls of fronds.
[Illustration: Fig. 58. Sorus and Indusium of Polystichum or Aspidium.]
In the genus Polystichum, the fronds spring in tufts from a short
slow-growing caudex. They are rigid, linear or lanceolate, and either
pinnate, bipinnate, or tripinnate. The upper terminations of the fronds
are sharp and spinous, while the pinnæ are auriculate at their base
above, and oblique below. Circular sori are seated on the anterior
branches of the parallel veins, and these are covered by circular
indusia, opening all round and remaining attached by a short central
stalk. The free venation and this peculiar form of the sorus and
indusium are the distinguishing characters of the genus. Fig. 58 shows a
sorus and indusium of Polystichum or Aspidium, which differ only in
venation. One of the spinous, serrated pinnæ of the Holly fern,
Polystichum Lonchitis, is represented in fig. 59; it is auriculate at
its base above and oblique below, with its sori disposed in regular
series on each side the mid-vein; these often become confluent at
maturity. This beautiful Alpine fern, which is of a deep glossy green,
has linear lanceolate, and simply pinnate fronds. The pinnæ are short,
arranged alternately and obliquely on the rachis, and extend nearly to
its base, which is rather densely clothed with reddish-brown chaffy
scales. The other British ferns of this genus are bipinnate. P.
aculeatum is rigid, but P. angulare is lax, and drooping. One of its
varieties, named proliferum, is abundantly viviparous, producing small
bulbils about the bases of the lower pinnæ and pinnules, which readily
reproduce the plant.
[Illustration: Fig. 59. Pinna of Polystichum Lonchitis.]
All the preceding genera, from Lastrea inclusive, belong to the
Aspidieæ, an enormous tribe, abounding in species, and almost
conterminous with the old genus Aspidium.
The Nephrolepis tuberosa and other species bear sub-translucent tubers
on the rhizome. They are subterranean, ovoid, an inch and a half long,
and filled with a nearly translucent mucus. The tubers have a circle of
vascular bundles forming a sort of balloon, proceeding from a common
base below, and converging to the apex.[73]
Most of the species are tropical, but Lastrea and Polystichum include
several European species, some of which are extremely variable.
The Onoclea is a remarkable genus from the pinnæ being contracted into
berry-like globes. Onoclea sensibilis is a handsome free-growing
American species and appears to have been named sensibilis, from the
particularly rapid withering of the fronds after being gathered.
The genus Cystopteris, which has species in both hemispheres, is the
type of the Cystopterideæ, a small group which approaches Aspidieæ
through Nephrolepis and Acrophorus. The British species of Cystopteris
are small, fragile ferns, growing on walls and rocks in Alpine and
Subalpine districts. Their fronds are for the most part erect,
lanceolate or deltoid, and bipinnate or tripinnate. They are generically
distinguished by having each round sorus covered by a hood-shaped
indusium, which is hollow and attached by its base, and opens towards
the apex of the segment, its free margin being elongated and fringed; in
this respect it bears some resemblance to Woodsia, in which, however,
the indusium projects from beneath the spore-cases equally on all sides,
becoming incurved in a cup-like form. The Cystopteris fragilis, or
Brittle Bladder fern, flourishes in most shady mountainous and rocky
districts, but is also found in the lowlands. It has a decumbent caudex,
which extends slowly, branching and forming new crowns around the old
one, often to the number of several during the summer and autumn. The
fronds rise in tufts from these crowns in April, rapidly attain their
maturity, and die away in succession as their place is supplied by
others, till the frost comes and destroys them all. The tufts vary in
height from two or three inches to a foot or more. The fronds differ
much in form and division even in the same crown, but both they and the
pinnæ are usually lanceolate; the pinnules are ovoid or oblong, always
deeply pinnatifid, and having the segments sharply toothed or serrated.
The Dicksonia group, Dicksonieæ, contains some of the finest tree ferns.
The Dicksonia antarctica sometimes attains a uniform girth of twelve
feet throughout its height of forty feet. In the stem of this fern, the
vascular bundles are symmetrically disposed round the axis so as to form
a closed cylinder. The New Zealanders slice the fibrous coating of the
trunk, and use it for constructing their houses. D. squarrosa reaches
the farthest south of the tree ferns. The D. lanata sometimes forms a
distinct stem but not always, for tree ferns vary much with regard to
the dimensions and elevation of their stem. The caudex of Cibotium
Barometz is covered with long tawny hair; each has its own peculiarity.
The species of Dicksonieæ belong principally to the tropics and southern
isles. The ferns of this group have globose sori, which are submarginal,
seated at the tip of a vein or veinlet. The indusium is lateral,
persistent, and bivalved; the lower valve is formed by the true
indusium, the upper by the altered tooth of the frond folded back. In
Dennstædtia the indusium is cup-shaped, and curiously deflexed; while in
Deparia, another section of the group, the cup-shaped indusium
containing the sorus is extrorse marginal, as shown in fig. 60.
[Illustration: Fig. 60. Sorus and cup-shaped indusium of Deparia
prolifera.]
The group Peranemeæ, or Woodsieæ, is represented in the British Flora by
two species of Woodsia, which are amongst the rarest and most curious of
our ferns, and inhabit the crevices in our highest mountain tops. Their
tufts of fronds are not more than two or three inches high, sometimes
less, and the fronds themselves are lanceolate, and pinnate; in Woodsia
ilvensis they have oblong and usually opposite deeply lobed pinnæ, whose
under-surface is clothed more or less with jointed hairs and long
attenuated scales. The sori in both species are circular, and situated
at the extremity of the lateral veins. When young they are covered with
an indusium, which opens at the centre, and forms a cup round the sori;
afterwards it is divided into numerous jointed and usually incurved
threads.
The Davallieæ are mostly tropical and subtropical ferns. Their sori are
submarginal, placed upon the tip of a vein or veinlet, and enclosed
within a tubular indusium, which is either short and approaching to
cup-shaped, or more or less elongated. In this group the stem assumes
the form of a rhizome, often creeping extensively, and the fronds are
articulated, falling off with a clean scar when they perish. Davallia
canariensis is cultivated in most conservatories, and is known as the
Hare’s-foot fern, from the stoutish scaly rhizomes resembling the foot
of a hare.
Somewhat allied to these are the Lindsæeæ, including Lindsæa and its
allies; these have linear marginal sori, for the most part continuous,
but sometimes regularly interrupted, in which state they approach some
of the Davallieæ, Microlepia to wit. The indusium opens towards the
margin of the frond, and the sori more or less connect the tips of the
veins and veinlets.
[Illustration: Fig. 61. Scolopendrium vulgare.]
The group of Asplenieæ is a very comprehensive one, including, besides
the extensive typical genus Asplenium, the cognate subdivisions of
Diplazieæ and Scolopendrieæ. The latter is represented by Scolopendrium
vulgare, the Hart’s-tongue fern, in which the sori are linear, and
situated in pairs on two parallel veins, so closely approximate that,
though really double, they seem to form but one straight line; they are
not covered with one indusium splitting down the middle, but each has
its own indusium opening in opposite directions. On the under-surface of
the long strap-shaped entire fronds of the common Hart’s-tongue, which
is a type of the genus, the double sori with their opposite indusia form
parallel equidistant straight lines, diverging on each side from the
mid-vein of the frond, as in fig. 61. The fronds are cordate at the
base; sometimes they are forked, and occasionally crisped and wavy, and
the leaf-stem is shaggy with narrow membranaceous scales. The caudex is
very compact and deep-rooted; it does not much elongate, but increases
slowly by the formation of new crowns round the older, attaining
considerable bulk. From this the fronds rise in circular tufts to the
height of a foot or more, and attain from two to three feet, in
favourable shady localities. At first the tufts are straight, but
ultimately they radiate and bend outwards. This plant is exceedingly
variable; hundreds of varieties have been found or raised in Great
Britain. Though twin sori are characteristic of the Scolopendrieæ, they
diverge somewhat in Camptosorus, a North American genus, sometimes
called the Walking fern, from its habit of throwing forward a bud on a
thread-like prolongation of the point of the frond, which becomes
established as a new growing centre, and thus carries the plant onwards.
The group of ferns called Diplazieæ, another section of the Asplenieæ,
typified by the genus Diplazium, is altogether tropical. The sori are
bilateral or double, but placed back to back on the vein, exactly the
opposite of Scolopendrieæ; so that the indusia, instead of opening face
to face, open in opposite directions. The species are rather numerous,
and embrace much variety of form and character.
To the Asplenieæ proper belongs one of our commonest species, the
Athyrium Filix-fœmina, or Lady Fern, which, when growing in moist shady
places, is one of the most elegant of British ferns. Its bipinnate frond
varies from a broad almost ovate outline to a linear lanceolate form.
The numerous fronds often spring from the caudex in a vase-like
arrangement, to the height of four or five feet, those in the centre
nearly erect, but the outer ones, drooping around in all directions,
forming a tuft of lovely feathery foliage. The sori have commonly a
semilunar outline, as in fig. 62; but they are occasionally so much
curved as to acquire a horse-shoe form, and more rarely they are linear,
and they become at length reflected by the growth of the sporangia. In
some varieties they are distinct, in others so close as to become
eventually confluent and completely cover the surface. The Filix-fœmina
is distinguished by the linear junction of the indusium with the frond.
It is one of the most variable of all the known ferns, and is remarkably
prolific, so that new forms may be raised from the spores. There are a
very large number found in this country.
[Illustration: Fig. 62. Athyrium Filix-fœmina.]
All the Asplenieæ have linear sori attached to the side of the vein, so
that they are said to be lateral. The indusium opens on its inward side,
and is sometimes ciliated. There are many tropical ferns of this group,
but the genus Asplenium, which has representatives in all parts of the
world, yields eight or nine species in Great Britain.
[Illustration: Fig. 63. Asplenium Ruta-muraria.]
The Asplenium Ruta-muraria, or Wall Rue, is an indigenous mountain plant
growing in tufts six or eight inches high in the clefts of the rocks,
but has diminished much in size in migrating to the plains, where it is
found on old brick walls and outhouses. The fronds are deltoid and
bipinnate, the pinnules are wedge-shaped and notched, or toothed, on
their upper margin; their colour is deep green, but in exposed
situations they are always covered with a glaucous secretion. The veins
diverge from the footstalk at the base of the pinnules, branch out
above, and extend to the teeth or serratures, as in fig. 63. The sori,
which are produced on the inner side of the veins, are linear,
elongated, and eventually become confluent, covering the whole
under-surface of each fertile pinnule. The indusium, which is only
traceable in the earlier condition of the fructification, is white, and
the free inner margin is somewhat jagged.
Changes as remarkable take place in the sori of the Asplenium
lanceolatum. At first, when the sori are covered by their white
indusium, they are oblong, but they become circular as they enlarge, and
eventually often confluent, so as to form a line round the whole
under-margin of the sharply toothed pinnule. This fern, which is
indigenous in the Atlantic islands, and is found in the Channel Islands,
as well as in the maritime counties of England and Wales, grows in the
crevices of rocks and old walls, or clothes the sides of wells and
deserted mines. It belongs to the section which has a mid-vein.
The genus Ceterach apparently belongs to that section of ferns which
have a vertical ring, and no indusium; but its affinities ally it
closely with Asplenium. Its want of an indusium is supplied, perhaps
occasioned, by the abundance of chaffy scales which cover the back of
the fronds. The lateral veins are alternate and irregularly branched,
with the branches anastomosing towards the margin. The sori are oblong
or linear, attached to the upper side of the anterior branches except
the last, which is on the opposite side of the lower branch. The British
species, Ceterach officinarum, or Scaly Spleen-wort (fig. 64), has
deeply pinnatifid, lanceolate fronds, with oblong, obtuse, alternate
segments, and linear sori. The whole of the under-surface is densely
clothed with brown, pointed, imbricated scales, finely serrated at their
margins, the outermost of which extend beyond the edges of the segments.
They completely cover the sori, and at first hide them entirely from
view. On the unexpanded fronds, the scales are white and silvery. ‘This
plant has an obscure indusium, only perceptible at an early stage of
fructification, and subsequently as a nearly erect membrane, attached to
the back of the vein. It is rendered unnecessary by the arrangement of
the scales, which are disposed in regular series along each side of the
veins and veinlets, pointing outwards, and concealing the sori with
their broad bases, which completely overlap them in their immature
condition.’
[Illustration: Fig. 64. Ceterach officinarum.]
It is well known that the seeds of tropical plants have been brought to
the western coasts of Scotland and Ireland by the great Atlantic
currents; it might therefore be expected, that the spores of Cryptogams
brought to our shores by the same means, and finding a congenial
climate, should become naturalized, as appears to have been the case in
ancient times, with regard to the Ceterach and Asplenium marinum, from
their migrations, as traced by Mr. Johnson. ‘The countries bordering on
the basin of the Mediterranean and the islands and eastern shores of the
North Atlantic appear to have been the original stations of this
remarkable fern. In the British islands its distribution is too partial
to admit of its being regarded as strictly indigenous, though probably
naturalized here at a period little subsequent to the arrival of
Asplenium marinum. It occurs here on limestone-rocks, but more
frequently on old walls and ruins, rooted deeply in the decaying mortar,
and often accompanying Asplenium Ruta-muraria and Asplenium Trichomanes.
Like other natural importations from the south, it is found most
abundantly on the western maritime counties that receive the more direct
flow of the tide, and has progressed slowly towards the northern and
central parts of the kingdom. In Scotland it has not yet traversed
beyond Perth, and is still regarded as a rare species; while in Ireland
its copious distribution seems to indicate an earlier arrival.’
The group Vittarieæ, consisting mainly of Vittaria and Tæniopsis, is
essentially equatorial or sub-equatorial. The plants have narrow
ribbon-like fronds, with naked sori immersed in the very margin of the
frond, in a more or less sunken furrow, there being no indusium.
The Blechnum Spicant, which may be taken as a type of the Lomarieæ, is a
very ornamental fern, with its dark green, linear-lanceolate fronds, of
two forms; the fertile ones erect, pectinate, pinnate, with distinct
narrow linear and acute pinnæ, while the barren ones are smooth,
spreading, and pinnatifid, with broad linear, blunt, approximate lobes.
The rachis is generally smooth, and of a dark purple hue, its leafless
portion being shaggy with membranaceous scales. The barren fronds lie on
the ground in the winter, while the fertile ones are erect, bear fruit
from May to October, and wither when they have shed their spores. In the
fertile fronds the lateral veins are alternate, and extend obliquely
upwards, about half way towards the margin of the lobe, when, by a
sudden turn, each runs parallel to the mid-vein, and anastomoses with
the one above it, thus forming an apparently longitudinal vein on which
the sorus is placed, so as to form a line on each side of the mid-vein;
this is covered with a continuous indusium like a hem, which opens on
the interior side (fig. 65). In the barren fronds the veins do not
anastomose at the margins of the lobe. This species is common in almost
every part of Great Britain, and extends on the Continent from Swedish
Lapland to the borders of the Mediterranean.
The species of Lomaria are marked by having the fertile fronds
contracted, so that the sorus is quite marginal; but there are many
species of Blechnum in which the elongated sorus, placed parallel to the
midrib, is quite distant from the margin, much more so than in Blechnum
Spicant.
[Illustration: Fig. 65. Blechnum Spicant.]
[Illustration: Fig. 66. Pteris aquilina.]
The genus Pteris is the type of another group, the Pterideæ, in which
the sori form a continuous marginal line covered by the attenuated edge
of the frond folded over it, and forming an indusium. This is the
structure in most of the species of Pteris, a large family abounding in
the tropics, and very widely distributed in almost every part of the
globe. In the Pteris aquilina, or Bracken (fig. 66), however, in which
the lateral veins of the leaf are divided two or three times before they
reach the margin, and the extremities of the branches anastomose and
form a vein at the exterior or extreme margin of the leaf, the sporangia
are produced on the upper-surface of the marginal vein, and are enclosed
by an extension of the skin from both surfaces of the leaf; so that the
fructification, which is folded back on the under-surface of the leaf,
is in the earliest stage of its development enclosed between two thin
membranes, both of which have their margins ciliated with jointed hairs,
while, under the microscope, their cellular structure will be found to
differ in accordance with that of the upper and under epidermis, from
which they are individually extensions. This fern is so universal in
Britain, as to exclude in some places almost every other kind of
vegetation, and is so well known, that it may appear superfluous to
mention that the fronds are deltoid, with an elongated, stem-like
petiole two or three times divided, the primary pinnæ opposite, the
ultimate segments oblong, obtuse, and confluent. There are exotic forms
of Pteris in every part of the world, which are mere varieties of the
Pteris aquilina, so that the Bracken may be said to rank as the most
universally distributed of all vegetable productions, extending from
West to East, over both continents and islands, in a zone reaching from
Northern Europe and Siberia to New Zealand, where it is represented by,
and perhaps identical with, the well known Pteris esculenta.
The Allosorus crispus, Parsley fern, or Curled Rock Bracken, which may
be taken as a type of its genus, is by some authors referred to the same
group of Pterideæ, though others regard it as one of the Polypodieæ. It
is a pretty little fern, growing in tufts in sheltered crevices of
mountain rocks, from Lapland to the Mediterranean. The fronds are
deltoid, twice or three times pinnate, and of two kinds. The ultimate
divisions of the barren fronds are wedge-shaped, cut, and toothed; those
of the fertile fronds are linear, oblong, and entire. Although the sori
are at first circular, and situated near the extremities of the lateral
veins, they become confluent in maturity, and being covered by the
reflexed margin of the pinnules of the contracted but scarcely altered
fertile frond, instead of by an indusium, they bear some resemblance in
fructification to Pteris aquilina.
The Adiantieæ form an exceedingly numerous and mostly tropical group of
ferns, in which the rhizome is creeping or globose. Their sori are
linear or oblong, straight or curved, and growing on the under-side of
the edges of the leaf, which are folded over with them, and thus serve
as an indusium. Adiantum has a free venation, but Hewardia, a related
genus, has a reticulated venation. The Adiantieæ have linear, oblong or
lunate sori, seated on the margin of the leaf. Of the seventy species of
Adiantum, only A. pedatum and A. Capillus-Veneris, the first American,
the latter British, are inhabitants of a cold climate. The A.
Capillus-Veneris, which is universally distributed in warm latitudes, is
believed to have migrated in ancient times to the mild and damp
south-western counties of England and Ireland.
The Adiantum Capillus-Veneris, or Maiden’s Hair, has a slender, black,
scaly, creeping, and branching rhizome, from the extremities of which
spring lax tufts of fronds growing from a few inches to a foot high. The
stems with their alternate branches and branchlets are slender,
hair-like, and of a blackish purple tint. The capillary branchlets bear
at their extremities thin, bright, but glaucous green, wedge-shaped
leaves, serrated at their edges. The fibro-vascular bundles which
traverse the stems and branches, on reaching the leaves, spread into a
palmate venation, which terminates at the margin in bifurcations; and
upon these, in the fertile leaves, roundish sori are placed, and covered
by the transverse oblong folds of the edge of the leaf, as in fig. 67.
These delicately graceful ferns flourish almost exclusively in damp
shady crevices of rocks under the spray of cascades, and still more
luxuriantly in deep tropical forests, where the air is loaded with warm
vapour.
[Illustration: Fig. 67. Adiantum Capillus-Veneris.]
The Acrosticheæ are among the most remarkable of ferns in having the
whole under-surface of the frond indiscriminately covered with naked
sori, and in a few species the upper-surface also. They are situated on
the veins and veinlets, from whence they extend into the interstices of
the fertile fronds, which are sometimes much contracted. The species are
almost all tropical or subtropical. A New Zealand species of Lomariopsis
climbs high trees by means of its stout rooting caudex, and has
different leaves on different parts of the plant.
The Cyatheineæ are chiefly arborescent ferns, and, with the exception of
some fossil species, contain the noblest representatives of the
cryptogamic flora. Their fructification consists of dot-like sori, in
which the usually compressed oblique sporangia are placed on an elevated
receptacle, which forms a raised point on the surface of the fronds when
the sporangia are removed. Of these the Alsophileæ are without true
indusia, while the Cyatheæ have a spherical indusium, bursting above or
below, and forming a cup round the sorus. The pulpy substance of the
stem of Cyathea medullaris was a common article of food with the New
Zealanders.
The Gleichenineæ have a creeping or climbing rhizome, and globose or
trigonal sporangia, few in number, and disposed in a radiating manner,
so that the narrow end is internal. They are often seated in a little
cavity, and are highly deciduous. The fronds are generally forked or
trifid, the middle division being sometimes supplied with a little
bulb-like body. They are mostly tropical or subtropical plants, but some
species grow in Chili, New Zealand, and Japan.
The Trichomanineæ are pretty well distinguished from other ferns by
their pellucid membranaceous texture. As regards their technical
characters, they have an oblique and complete ring to the sporangia,
which are of a lenticular form, and are collected about a more or less
elongated receptacle, which is free within either an urn-shaped cup or a
two-valved indusium, projecting from the edge of the frond. They are
chiefly inhabitants of the moist tropical forests, or extra-tropical
regions of damp and mild temperature. Great Britain only owns one
species of the genus Trichomanes, and two of the genus Hymenophyllum,
while species of these two genera constitute one-fifth of the fern
vegetation of our antipode islands of New Zealand. It is more than
doubtful even that the Trichomanes radicans, or Bristle fern, is
indigenous, as it is found in no other part of Great Britain except the
Irish counties of Cork and Kerry, growing on dripping rocks and
waterfalls, or depending from the walls and roofs of caverns; and as it
is also found in the West Indies and the North Atlantic islands its
transit may be accounted for, as in other cases.
[Illustration: Fig. 68. Trichomanes radicans.]
The fronds of the Bristle fern, Trichomanes radicans, are somewhat
deltoid, or of a more elongated form, flimsy, and beautifully
reticulated when viewed with a microscope. They rise at intervals from a
creeping rhizome, bristly, with narrow articulate scales, which often
covers the most precipitous rocks on which it grows with a dark-coloured
network. The rachis is branched and rebranched three or four times, and
the whole being distinctly winged on both sides in the plane of the
frond, the successive branches and branchlets running through the leafy
part become the veins of those parts, so that a segment of the frond is
merely a winged vein, the wings on the branches being however broader
than those on the rachis. The veins, which divide alternately, are hard,
woody, and wire-like, and, when barren, terminate before reaching the
ends of the segments; but, when fertile, they extend beyond the segment,
the tissue of which separates and distends in the form of a more or less
elongated cup around the prolonged vein. At the base of that vein the
sporangia form a small globular cluster; and, as they advance towards
maturity, the vein extends in the form of a bristle, far beyond the
mouth of the cup (fig. 68). The cup is winged in the more luxuriant form
of the plant, in consequence of the double layer of tissue composing the
segment to which it belongs not separating through its whole breadth.
The cup-like cylinder being slightly compressed in the plane of the
frond is indicative of its origin.
[Illustration: Fig. 69. Hymenophyllum tunbridgense.]
A slight difference in fructification separates the Hymenophyllum
tunbridgense, or Film fern (fig. 69), from the Trichomanes, for both
have a creeping rhizome, from which single fronds spring at short
intervals. The leafy parts of the fronds are merely winged veins, and
the wings consist of two layers of cellular tissue finely reticulated.
The Hymenophyllum is distinguished from the Trichomanes by a two-valved
cup spinously serrated, which completely conceals the fruit-bearing
vein, while, in the Trichomanes, the cup is more cylindrical,
smooth-edged, with the fertile vein projecting far beyond it (fig. 68).
The beautiful little Film fern, with its semi-transparent fronds not
more than two or three inches high, grows on shady wet rocks, among the
moss on the branches and roots of old trees, and on the ground near
lakes and rivulets. It is hardy enough to live in the Highlands of
Scotland; but the H. unilaterale, a far less beautiful plant, has its
limit in Unst, the most northern of the Shetland Islands. The
fructification in the latter plant, though the same in position, is
stalked instead of being sessile. The involucre is rounded and ovate,
instead of being cup-shaped, with much swollen convex valves, meeting by
their edges but not compressed towards the apex.
The Schizæineæ are many of them climbing ferns, and for the most part
tropical. Lygodium articulatum climbs trees in New Zealand to the height
of fifty or one hundred feet, and its tough wiry stems are used for
cordage. In one group of this order, represented by Lygodium, the
sporangia are disposed at the back of the frond in imbricated marginal
spikes, formed by a transformation of the ultimate pinnæ. In the other
division, represented by Schizæa, the fruitful spikes are really on the
under-side of the frond, but the frond being reversed they seem to be on
the upper.
The Ceratopteridineæ, sometimes called Parkeriaceæ, are tropical aquatic
ferns, whose sterile fronds, which are membranaceous, with a thick
vascular footstalk, float on the surface of the water, and by the time
these are nearly decayed the fertile fronds are perfected. The latter
are more erect, repeatedly divided and forked, the divisions being
linear. There is only a single genus, Ceratopteris, which has continuous
sori occupying the longitudinal veins at the edge of the frond, and
covered by the indusioid margin. The sporangia have a very broad
incomplete ring, and connect the ringless ferns with the group which
possess them. The spores are triangular, and marked with three sets of
concentric ridges.
The order Osmundineæ contains the Osmunda regalis, considered to be the
finest of all European ferns, which is common throughout Great Britain
in wet spongy soils, and appears to be the only species of the group
that is European. The fronds grow in tufts from a thick woody caudex,
which branches and extends widely by the formation of lateral crowns,
but, if impeded, it elongates and rises in an erect position to the
height of two or even three feet above the soil. The barren fronds of
the large luxuriant tufts are highly developed, and are from six to nine
or even eleven feet high; the fertile fronds are shorter and fewer in
number. The bipinnate character prevails throughout; the primary
divisions are opposite, the secondary alternate, and the pinnules are
oblong and opposite. The sculptured sporangia have a very rudimentary
ring, and, in the fruit-bearing fronds, four or five of the lower pairs
of pinnules have the leafy character, while the remainder develop
clusters of sporangia in place of pinnules. The sporangia on the
fruitful branches are at first pale green, but gradually become reddish
brown, hence the name of Flowering fern. The Leptopteris section of the
genus Todea, almost peculiar to New Zealand, has beautiful transparent
fronds, with naked sporangia placed upon the veins, forming very much
scattered sori.
The MARATTIACEÆ may be partly known by their huge globose rhizome
projecting above the ground, and rough with the processes from which the
leaf-stalks have fallen. This sends out a few large fibrous roots, and
consists of cellular tissue abounding in starch, with small bundles of
fibro-vascular tissue regularly distributed through it. The stipes have
a pair of stipule-like organs at their base; and the sori are either
oblong fronds of a double row of sporangia, which in some cases is
concrete, or they are circular with the sporangia annularly concrete, or
they are connate throughout the fertile portions. These peculiarities
respectively distinguish the smaller groups of Marattineæ, Kaulfussineæ,
and Danæineæ, while the group Marattiaceæ itself is distinguished from
Ophioglossaceæ by having its sori dorsal, that is, set on the back, or
under-surface, of flat leafy fronds. The leaves of Angiopteris evecta
are used as a perfume in the Sandwich Islands, and its rhizome serves
for food, as that of the Marattia salicina does in New Zealand.
The OPHIOGLOSSACEÆ, or Adder’s Tongue ferns, are few in number, and
present comparatively little difference in structure. They are known
among the exannulate series, from the Marattiaceæ, by having their
fertile fronds contracted, bearing their sporangia at the margin, so
that being entirely occupied by sporangia, the fertile fronds appear as
if they were an inflorescence distinct from the foliaceous organs, as
indeed they are analogically. In this group there is the further
difference that the fronds are not circinate, but straight, in their
æstivation.
These Adder’s Tongue ferns are dwarf herbaceous plants, differing
greatly in structure from the true ferns, though there is some
similarity to them in the fructification. They are divided into four
genera, of which Ophioglossum and Botrychium are best known. The base of
the stem is thick and bulbiform, and sends off spreading succulent
roots. The species of the genus Ophioglossum are chiefly plants of a
warm climate, but Ophioglossum vulgatum, or common Adder’s Tongue, is
distributed in almost every part of the globe. In England it is
exceedingly abundant in meadows and pastures, and varies in height from
a few inches to a foot, in moist soil. It has one barren ovate and one
fertile linear frond. The barren frond, which is of a yellowish-green
colour, invests the stem of the fertile frond like a spathe; its form is
ovate, varying to ovate lanceolate, and more or less obtuse, with a
complicated network of anastomosing veins. Bivalved sporangia form two
parallel series on the margins of the club-like terminations of the
fertile frond. Soon after the fruit is shed the fronds die, but one or
two buds are previously formed at the base of the plant, which remain
dormant till the following spring.[74]
The genus Botrychium has few species, about half of which grow in North
America. Botrychium Lunaria, or Moonwort, is the only species indigenous
in Great Britain. This plant, which is from five to six inches high, has
a rather long succulent stem, invested at the base by the dark-coloured
membranous remains of withered leaves. It has one barren frond, the base
of whose stem is a sheath, through which the tall, fertile frond rises.
The barren frond has from three or four to seven pairs of opposite
smooth pinnæ, of a lunate shape and glaucous green colour, overlapping
each other; they are smooth, crenated on the margin, and occasionally
lobed, which gives them a fan shape. The fertile frond is longer than
the barren one, and ends in a panicle or thyrse. The sporangia, which
are large and bivalved, are disposed in two regular series upon the
divisions of the panicle, and directed towards the upper or inner face
of the frond. This fructification bears a strong resemblance to that of
Osmunda regalis, the Flowering fern.
The stem of this plant has been assumed to be solid and branched, which
is by no means the case; for, upon dissection, it is found to be hollow,
and at its base the fronds of the following year may be detected more or
less perfectly formed, and the rudimentary bud of the succeeding year
within the latter; the position of the barren and fertile fronds being
reversed in the successive developments.
SECTION IX.
EQUISETACEÆ, OR HORSETAILS.
THE Equiseta are leafless, herbaceous plants, annually renewed from a
creeping rhizome, and growing in marshy land, in pools and ditches, on
the banks of rivulets, and in rivers, from Lapland and Siberia to within
the tropics. There is but one genus, and few species. The largest of the
ten or eleven species, which are indigenous in Great Britain, is not
more than five or six feet high, but they are of greater size in warm
climates. The Horsetails begin their lives precisely like the Ferns;
for, when a spore begins to germinate, it forms a marchantioid leaf or
prothallus lying flat on the ground, upon which are produced antheridia,
full of cells, in each of which there is a spermatozoid with numerous
cilia. Archegonia are also formed on the prothallus. These, after
fertilization, give rise to the perfect plant, which throws out a
rhizome, whence new shoots are produced. ‘The structure of the rhizome
is very different from that of ferns. In an early stage it consists of a
central column of cellular tissue, sending off about eight radiating
plates, which connect it with an external cylinder of the same tissue,
and opposite to each of which there is, in the central column, a
vascular bundle consisting of annular vessels passing into spiral. At a
later period, tissue grows from the walls into the cavities, in such
wise that they are more or less perfectly obliterated.’[75] From the
rhizome, which often extends to a great length, the stems of the
Equisetums rise in the form of rough, rigid, hollow cylinders, striated
longitudinally, and articulated at intervals by separable joints. Each
articulation is invested at its base by a toothed membranaceous sheath,
from beneath which, in the greater number of the species, whorls of
branches spring, jointed like the stem, and similar to it even to the
number of teeth in the sheaths and striæ on the surface, but unbranched.
[Illustration: Fig. 70. Equisetum giganteum:—_a_, fragment of stem with
branches; _b_, cone or spike of fructification; _c_, one of the scales
of the cone; _d_, spore with its elastic filaments.]
The fructification is occasionally on separate stems, which make their
appearance before the barren ones, and are in general unbranched and
succulent, bearing a cone on their apex. The surface of the cone is at
first smooth or indistinctly reticulated, but it eventually splits into
numerous octagonal, brown, shield-like discs, spirally arranged, which,
separated from the stem, are found to have a stalk, and to bear on the
under-side four or eight pendent sac-like bodies of a whitish hue. These
are the sporangia, which open on the inside by a slit for the discharge
of the spores. The inner coat of the cells of the sporangium is composed
of beautifully spiral tissue. The spores arise by cell division, each
being covered by a separate membrane, which ultimately forms a pair of
elastic fibres attached by their middle, closely coiled round the spore,
as it is formed within the cell; but, when the spore is liberated, they
extend; the least moisture, however, even the breath, makes them
contract.
The Equisetaceæ differ from the Ferns in having spiral instead of
scalariform vessels in their structure; but they agree in having only
one period of fertilization, after which they produce a crop of spores
year after year, and these, when sown, give rise to the primary leaf
with its antheridia and archegonia—a perpetual cycle.
The whole substance of the stems of the Equisetums is so completely
penetrated with silex, that a silicious skeleton remains after the
herbaceous part is destroyed, and in some species as much as thirteen
per cent. of the whole plant, and fifty per cent. of its ashes are found
to be pure silex. On account of the crystallization of the silex, the
Horsetails form some of the most beautiful of microscopic objects. If a
fragment of the cuticle is magnified and viewed by polarized light, the
colours are seen to be intense and vivid, and the arrangement of the
silicious particles so elaborate and symmetrical as to resemble
necklaces of diamonds and coloured gems. The rows of crystals run in
lines parallel to the axis of the stem, the greater number being so
disposed, but the rest are grouped so as to form ovals joined together
by a chain of particles forming a sort of curvilinear quadrangle, these
rows of oval combinations being arranged in pairs. The effect is
brilliant when they are seen in polarized light. According to Sir David
Brewster every particle has a regular axis of double refraction; but
Professor Bailey, of the United States of America, states that the
effect under polarized light is not produced by the silicious particles,
but by the organized tissues, since, after these have been destroyed,
the silex shows no double refraction.
The vascular tissue of the Equisetums shows them to be of a higher class
than the Ferns. The recent forms vary much in size, but even the
Equisetum giganteum of Brazil, which is eighteen feet high and
three-quarters of an inch in diameter, is incomparably less than the
Calamites and other fossil Equiseta, which appear in the coal measures
and new red sandstone. Recent species are found in Iceland and the high
northern latitudes of America, in the tropics, and in most parts of the
world, except Australia and New Zealand.
SECTION X.
MARSILEACEÆ, OR RHIZOSPERMÆ.
[Illustration: Fig. 71. Pilularia minuta:—_a_, mature plant, natural
size; _b_, receptacle, slightly magnified.]
THE Marsileaceæ, a natural tribe of small perennial aquatic herbs, have
a filiform creeping rhizome with alternate erect leaves, curled in
vernation like those of the ferns. The sporangia are enclosed in oval or
spherical leathery capsules, or receptacles, which contain two
dissimilar forms of reproductive organs—sporangia and antheridia, and
are sessile, or nearly so, on the rhizome at the base of the leaves,
whence the general name of Rhizospermæ. The Pilularia globulifera, or
Pillwort, the only British species of its genus, may be taken as a type.
Its rhizome creeps over sand or mud, at the margins of lakes and pools,
where it is always submerged, or in sandy or gravelly places, which are
only occasionally overflowed. At regular intervals the rhizome sends off
a tuft of roots and a tuft of leaves opposite to them; the leaves are
smooth, erect, and very slender, in deep water almost hair like, varying
from four to five inches in height. The solitary globular receptacles,
brown, hairy, and about the size of a small peppercorn, spring from the
axils of the leaves, supported on so short a stem that they appear to be
sessile. Fig. 71 shows an Algerian species of its natural size. The
receptacles are divided by cross partitions into two or more cells, and
separate at maturity into four equal valves. Each cell has a sort of
placenta to which the sporangium and antheridia are attached; and its
upper half is lined with three minute, sessile, obovate, yellowish
bodies, which are the antheridia; the other half is occupied by a
larger, roundish, or oblong sessile sporangium, containing one spore,
which has a firm outer coat, tapering to a point, and leaving a cavity
at the top of the nucleus. According to Hofmeister, this cavity becomes
filled with cellular tissue, constituting a conical prothallus confluent
with the nucleus. A single archegonium is formed in the centre, the
orifice of which corresponds with the apex of the prothallus. The
antheridia contain numerous granules, from which long spiral delicate
spermatozoids are ultimately developed.[76] The embryos of the young
plants push forth their radicle in one direction to fix them to the
soil, and a frond or leaf in the opposite direction. Thus these minute
plants, approaching so nearly to a monocotyledon, exhibit a high
organization, only inferior to that of the Lycopods.
The group contains the genera Marsilea, which has leaves made up of
cuneiform lobes, and resembling those of some leguminous plant; and
Salvinia and Azolla, both of which consist of small floating plants,
which mostly occur in tropical or subtropical countries.
SECTION XI.
LYCOPODIACEÆ, OR CLUB MOSSES.
THE Club Mosses are mostly perennial plants, with slender creeping
stems, often several feet or yards in length, occasionally erect, and
clothed by small, sessile, closely set, often imbricated leaves without
veins. They have in some instances a habit resembling that of Conifers.
The stems consist of a mass of thick-walled, often dotted cells,
enclosing one or many bunches of scalariform tissue, which sends off
branches to every leaf and bud. The scalariform tissue is accompanied by
fine, elongated, and sometimes rather coarser cells, which are
occasionally reticulated. The stem approaches to that of ferns, but the
bundles of vascular tissue are confined to the centre. The branches of
the stems are often bifurcate, and terminate in one or a pair of
cone-like spikes, which are either sessile or stalked. The sporangia are
sessile in the axils of the imbricated leaves or bracts that cover the
cones. Two kinds grow on the same plant, one of them bivalved,
containing a powdery substance, whose particles a high magnifying power
shows to be globular spermatozoids: these do not germinate; and the
other three-valved, enclosing comparatively large, nearly spherical
granules, marked with three prominent ridges, radiating from one
extremity. These certainly germinate, forming by cell-division a
prothallus of hexagonal cellular tissue, adherent to and confluent with
the spores, as in Marsileaceæ, or penetrating their cavity, but without
the protrusion of threads, as in ferns or mosses. On this prothallus,
archegonia are soon produced, the embryo being formed from a cell at its
base; this sends down roots on one side, and a minute stem with the two
primary leaves of the young plant at the other. The plant thus bears a
close resemblance to a young plant of the dicotyledons, and exhibits,
with regard to fructification, the highest organization of which the
Cryptogams are capable, although in their tissues the Lycopods are
inferior to the Equisetaceæ and Marsileaceæ.
There are four genera of this order, very widely dispersed, most
abundant and larger in the hot moist parts of India and the Indian
islands, but large tracts are covered with them within the arctic
circles and in temperate latitudes. There are 200 species of the genus
Lycopodium, or Wolfs-claw, and of these, six only are indigenous in
Britain; they grow in very exposed situations, as in the case of L.
clavatum, which lives on upland heaths and pastures, and which has a
procumbent stem, creeping for many feet or yards, sending out branches
in all directions, with a pair of cones at their extremities, and strong
roots at intervals to fix it to the ground. Lycopodium inundatum forms
large patches on the marshes in the south of England. New Zealand has
many more species of this genus than Britain, and some of the noblest
specimens. The dried spores of the L. clavatum are so inflammable, that
they have been used on the stage to produce the effect of lightning.
Some Club Mosses yield a blue dye, a colour which is now obtained of a
better quality from coal tar. Others possess cathartic properties, but
although they have been used as medicine, they are very dangerous from
the violence of their effects.
The genus Selaginella has a moss-like habit. The stem is generally
creeping, and flat-looking from being clothed with distichous leaves;
but sometimes it is tall and erect, having feathery branches clad with
leaves. The sporangia are sacs, with two or three valves containing
large spores and sub-globose antheridia, containing orange-coloured or
minute scarlet bodies, ultimately developed into spermatozoids. The
species are very numerous and greatly varied, and mostly tropical.
From a comparison of the structure of the stems of the fossil plants,
Sigillaria and Lepidodendron, Dr. Hooker concludes that they are highly
developed Lycopods, approaching closely in structure to the highest
class of plants. The stems of the former differ chiefly in size from
those of the Lycopodium, and the cones of the Lepidostrobus differ in
their greater development and in the thickness of their scales from
those of the Conifers. The spores of the Lepidostrobus ornatus too are
sphericotetrahedral, like those of the Lycopodium. But inferior as the
existing Lycopods are to their fossil ancestors, their analogy to the
Conifers gives them a more exalted position in the vegetable world than
their tall and graceful allies, ‘the tree ferns.’
The gradual change of structure from the lowest to the highest
cryptogamic form is accompanied by a singular variety in the mode of
reproduction, and a degree of vital energy scarcely to be expected, at
least in beings of such low organization as the yeast plant, which
produces gemmæ in vast and rapid profusion, each gem or bud being only a
facsimile of the parent cell. In like manner the mother plant is
reproduced by the germination of the green zoospores, the green globular
cells in the fronds of the lichens, the motile gonidia, and the discs in
the baskets of the Marchantia, especially those of the Marchantia
polymorpha, which have such vigorous vegetation that they form stomates
on whichever side is turned to the light, and roots on the other. The
result of all these is an individual perfectly similar to its parent,
like that produced by a bud and cutting of a tree, or the axillary fruit
buds of the Begonia. The leaves of that flowering plant, as well as
those of the Achimenes and Gloxinia, possess the property of reproducing
the parent plant, for, when laid on moist earth and slit in different
places, a young plant rises from the upper side of the fracture, and
roots shoot down from the under. Although this manner of growth
resembles the germination of the embryo of an archegonium, it is widely
different, for the embryonic cell is fertilized by the spermatozoids, so
there is a certain analogy but not the smallest affinity. The highest
vegetable classes can reproduce the mother plant in many ways, but they
have nothing akin to the alternation of generations exhibited by many of
the lower tribes, nor yet to conjugation like the Desmidiaceæ and
Diatoms. Possibly the spores resulting from these two modes of
reproduction, as well as the resting spores, may produce new species;
certainly those resulting from fructification do occasionally yield new
varieties.
Many spores produce the plant directly, others indirectly, as most of
the fungi; but if the definition of a perfect plant be that which bears
the fructification, the mycelium of a mushroom constitutes the plant,
for the mushroom itself is only a kind of sporangium or spore-case.
Nevertheless, the spores of the Puccinia and other microscopic fungi,
which are the cause of the rust and mildew in wheat, give rise to a kind
of prothallus, ‘a slight fore-shadowing’ of the prothallus of the
Marchantia and Mosses, which only produce sporangia, and those of the
Ferns, Horsetails, and Club Mosses, whose archegonia contain the embryo
of the plant itself. The fructification of the Lycopods is the highest
of which the Cryptogamia are capable, and brings them into a singular
analogy with the flowering class. For in the ovule, or seed-vessel of
the flower-bearing race, a large cell is formed, containing mucilaginous
matter, which, soon after fructification, is converted into a mass of
cellular tissue, which gives rise to the cell containing the embryo,
just as the prothallus of a Lycopod gives rise to the archegonium
containing the embryo of the plant. The linear leaves of existing
Lycopods and the cones of their fossil allies, are similar to the leaves
and cones of the Coniferæ: there is no affinity, but an analogy existing
between these two groups sufficient to make them form links between the
two great divisions of the vegetable kingdom. The Lycopodiaceæ probably
may be regarded as the highest of the Cryptogamia, and Coniferæ as the
lowest of the flower-bearing class.
SECTION XII.
GENERAL STRUCTURE OF FLOWERING PLANTS.
IN some of the Cryptogamic families fertilization takes place before the
plant itself is developed. In the two highest classes, those containing
the great groups of Flowering Plants, on the contrary, it is the
ultimate result of the inflorescence, which consists of calyx, corolla,
stamens, and pistils, all which are the later expansions of the cellular
tissues and groups of vessels which have in earlier stages of
development formed the leaves. They contain the same materials, and in
fact they are leaves modified in form, structure, and function.
Although the almost innumerable diversities in the form of leaves must
produce corresponding diversities in the inflorescence, yet the general
characteristics are the same or similar in both of the great botanical
classes. The structure of the calyx and corolla which form the floral
envelope of the fructification, is similar to that of leaves. The calyx
consists, in its early stage, of several parts called sepals, which have
all the characters of leaves; subsequently they are sometimes united by
their edges, so as to form a tube, or are otherwise modified. The
corolla only differs from a leaf by greater delicacy of organization; it
has fewer stomates or breathing pores, and the veins have less woody
fibre and sometimes many spiral vessels; the veins in the petals of
chickweed and some other plants are entirely composed of spiral vessels.
The calyx and corolla protect the fructifying organs consisting of
stamens with their anthers, and the pistil or pistils.
The stamens are formed of very fine filaments, and the anthers, when
young and still enclosed in the unexpanded flower, are full of a liquid
which is afterwards changed into a delicate homogeneous, cellular
tissue. Then the internal part of that tissue becomes divided into two
kinds of cells, one of which goes to form the walls of two lateral and
parallel chambers or loculi, separated by a part more or less continuous
with the filament. The other kind of cells are developed into pollen
grains within the chambers. They gradually form a cylindrical assemblage
of mother cells. Within each of these, four cells are ultimately formed,
each containing a single pollen cell. In all plants, except the aquatic,
the pollen cells are soon clothed by the deposition of one or more
layers of cells, which form the outer membrane, on which are impressed
figures or markings peculiar to each species of plant, such as slits,
points, papillæ, sharply defined circles, pores, &c. The colour of the
external layer is generally yellow, rarely green, blue, or red; the
thread is usually white, except in the fuchsia and some others. When the
pollen is ripe the chambers dehisce to emit it. The opening is usually a
slit between the valves which close the chambers. The length of the slit
is variable, and the form of the pollen grains is equally so; they are
commonly ellipsoidal, and attenuated at the extremities; but in all
cases they are beautiful microscopic objects.
The contents of the pollen grains consist of a liquid containing
mucilaginous granules, which is in circulation. The granules increase in
number towards the period of maturation, mingled with oil globules, and
frequently starch. The circulation of the fluid ceases long before
maturation in all cases, except the Zostera; but the granules of the
contents exhibit an active molecular motion often within the pollen
cell, and always after expulsion even in pollen taken from dried
specimens. After the liquid ceases to circulate, it becomes concentrated
and contracted; and when the pollen is only enclosed in one delicate
membrane, it simply bursts the vesicle; but, as most pollen grains have
a double coat, they burst open at the slits, or pores, on their surface,
and through these the internal membrane, in the form of a tube, is
protruded, and emits its contents; but it may be projected to a
considerable distance without bursting.
The pistil in a very young state is a greenish concave body scarcely to
be distinguished from a nascent leaf; it becomes more concave; and
finally the borders meet and unite, so that a hollow body is formed,
which ultimately becomes a perfect pistil consisting of three parts, the
apex or stigma, the style, and the ovary, in which the ovules, or unripe
seeds, are formed and fertilized, the latter afterwards becoming the
fruit.
The ovary is composed of an outward skin containing cellular tissue and
vascular bundles of spiral vessels, which, running upwards, converge
towards the style; they vary in number, and are sometimes ramified. A
single ovule, which is an unripe seed, may be produced within the ovary,
or the ovary may be divided into two or more compartments, in each of
which an ovule may be formed and attached to the ovary by a mass of
cellular tissue, and not unfrequently by a single thread.
The style, when examined with a microscope, is a hollow tube, or canal,
extending from the cavity of the ovary to the stigma. It consists of
cellular tissue with vascular bundles near the circumference, which pass
upwards in straight lines, and end below the summit. In certain cases
the canal is open; in others it is obstructed by lax cellular tissue
having many gaps. The stigma is an expansion of that lax tissue at the
point where the canal opens externally. At the time of fructification,
the canal of the style is occupied by soft humid elongated cells mixed
with a viscous fluid, which exudes upon the surface of the stigma,
rendering it moist and glutinous. When the pollen grains adhere to that
humid substance, the tubular extension of their inner lining, with its
contents, passes down the style and fertilizes the unripe seeds.
After fructification the anthers, stigma, and conducting tissues wither,
yet, in many fruits, additions are made to the ovary and its contents by
the remains of some of the other parts of the inflorescence. In the
apple, which is a simple fruit, the skin of the calyx forms the skin of
the apple, and the flesh or edible part is developed out of the ovary
and the remainder of the calyx, while the inner layer of the ovary forms
the horny cells containing the ripe seeds. In the strawberry, which is a
compound fruit, the pulp is the enlarged fleshy receptacle bearing the
simple fruit on its surface. In the orange, the membranous partitions of
its segments are the linings of the ovary, and the pulp is formed of lax
large-celled cellular tissue developed within it.[77] All fruits have
the mark of the style; it is very evident in the apple and orange.
Simple fruits are formed by a single flower; multiple fruits, like the
ananas, or pine-apple, and fir cone, are formed of masses of
inflorescence in a state of adhesion.
Spiral vessels are frequently found passing from the fleshy part of
fruits into the seeds; they are very numerous in the seeds of the
Collomia grandiflora and others, coiled up and compressed by the outer
skin; but they start to their full length as soon as released. The coats
of the seeds of various plants, when seen through a microscope, are
beautifully marked. That of the Bignonia is surrounded by a fringe of
elongated spiral vessels, the seed of the poppy has a reticulated
surface, and many have wings like those of the ash tree, or down like
the thistle, that they may be dispersed by the wind. The tendency of
roots to strike downwards is so great, that seeds right themselves
whichever way they may fall.
SECTION XIII.
MONOCOTYLEDONOUS, OR ENDOGENOUS PLANTS.
THE structure, growth, and reproduction of the flower-bearing vegetation
offer objects of the highest interest, though very different from those
furnished by the flowerless class. The plants whose seeds have but one
lobe form a transition from the lowest to the highest class of
vegetables, and include those which furnish the principal articles of
food to man and animals. They are all flower-bearing, and consist of
numerous families of land and water plants. The most important are the
palms, which serve for food in tropical countries, the grasses, which
are cosmopolite and social, covering extensive tracts of country with
rich verdure, and including the Cerealia, which have been cultivated
from such remote antiquity, that the grasses from whence they were
derived are unknown. This monocotyledonous class includes, besides the
palms and cereals already mentioned, the sugar-cane, the bamboo and
other canes, together with reeds, rushes, screw pines, most garden
bulbs, the singular and beautiful race of Orchids, and a multitude of
other ornamental and useful plants.
The seeds of this class consist of a thin skin covering a mass of white
or green matter, which, when ripe, constitutes the starch and flour in
the Cerealia. Within that matter lies a fleshy lobe with the infant
plant imbedded in one corner. The same general structure may be traced
throughout the class, though the lobe with its embryo in the surrounding
matter may be covered with a hard coat, as in the cocoa-nut, or inclosed
in a shell covered with a rich fruit, as in the date. The infant plant,
as it lies in its seed lobe, has a stem with a leaf bud or plumule at
one end, and an embryo root at the other; and, as soon as the seed
begins to germinate, it is nourished by the conversion of the starch of
the surrounding matter into sugar. At first the whole young plant is of
cellular tissue, but, as soon as the seed leaf appears above the soil,
it decomposes the carbonic acid of the atmosphere, absorbs the oxygen,
and consolidates the carbon. Then cords of fibro-vascular tissue are
formed within it, which converge at its base, and unite with the radicle
of the young plant to form the solid centre of the underground stem,
from whence the real roots descend, like slender white cords, which
supply the plant with food. The seed lobe having set the current of the
cell sap in motion disappears, and the aërial or above-ground stem,
which now consists of pith, sap, wood, and skin or bark, grows rapidly
in length to its first node, or thickened part, where the first leaf or
leaves appear, for the first leaves generally appear at the top of the
stem in this class, and are annually succeeded by others rising above
them from the top of the elongated stem, as in the palms and grasses.
The stem or axis of a palm is a cylinder with a graceful plume of leaves
at its summit, and a cone of white roots at its base. A section of the
stem perpendicular to its axis exhibits a mass of cellular tissue with a
number of dark-coloured spots irregularly scattered throughout the whole
with the exception of an envelope of dense cellular tissue, which forms
a kind of bark. The dark spots are the sections of bundles consisting of
two or three very wide and large vascular ducts enclosed in extremely
fine woody fibre and spiral vessels closely pressed together. These
bundles after having formed the ribs and footstalks of the leaves, enter
the upper part of the stem, approach towards its centre, bend down for a
short distance, then turn towards the exterior, interlace with the
bundles of the previous year, which have followed a similar course, and
make a circle of rough marks on the surface of the stem. The stem then
increases in length, and a new plume of leaves crowns its top; the same
process is repeated; and, as this goes on indefinitely, the lower part
of the stem is rough, and sometimes even rugged, as that of the Palmyra
palm. In consequence of this manner of growth, a perpendicular section
of a stem shows a series of curves intersecting each other and
originating in points gradually ascending as the palm grows in height.
These curves proceed from every point of the circumference of the stem,
present their convexities to the centre, and bend round, and enter the
leaves, which are long, lanceolate, and often pinnate with veins running
through them longitudinally. In some palms the greater part of the
centre of the trunk is altogether cellular tissue, but each species has
an arrangement of its own. Some palms, as the date palm, are diœcious,
one plant bearing male, the other female flowers. In these cases, the
pollen is carried from one tree to the other by the wind or by insects,
and, as much is lost in the transit, there is always more produced than
is required to fertilize the female flowers.
The Graminaceæ are very numerous, but the general structure is virtually
the same, whether it be a simple herbaceous grass, a sugar cane, or a
bamboo. The stem of the grasses is jointed, and furnished with long
lanceolate leaves, springing alternately to the right and left of each
successive joint, and the parts of the stem between the joints are
embraced and nearly surrounded, as by a sheath, with the expanded bases
of the leaves; for, in this class of plants, one growth always springs
from the interior of that which precedes it. In youth the stems of the
grasses are solid, and the bundles of woody tissue which surround the
pith are parallel to one another and to the axis of the stem; but, at
the joints, they are turned aside and are condensed into a node or
joint, where they enter into a new arrangement, and pass into the
lanceolate leaf, to form its longitudinal ribs. The compression of the
fibro-vascular tissue is so great as to form a septum across the
interior of the stem as the node. This is the case in all the grasses;
for the pith, which is of moist green cellular tissue when they are
young, soon disappears, so that a full grown grass is a hollow stem
articulated at intervals by solid joints, from whence the long narrow
leaves spring alternately, and as the bases of the leaves are continuous
with the stem, they do not fall off when they wither.
In the majority of cases, the inflorescence of the Graminaceæ consists
of a pistil and three anthers; but in the Anthoxanthum odoratum, or
sweet-scented vernal grass, and some others, there are but two anthers,
while some few species have more than three. In general, the flowers are
hermaphrodite; but in certain genera they are monœcious, as in Zea and
Zizania; and in others they are polygamous, as in Andropogon and
Sorghum. The grasses are, for the most part, low and herbaceous; but the
Arundo Donax of Southern Europe, and the sugar cane and bamboo of the
tropics are lofty strong-growing plants, the latter reaching as much as
50 or 60 feet in height. The stems and leaves of the grass family are
strengthened by silex; many of them are so entirely coated with it, that
their leaves are as sharp as the edge of a knife. It gives hardness to
the beards of wheat and barley, and is often found in concrete masses,
called tabasheer, in the joints of bamboos, which in the Indian jungles
have been set on fire by the friction of their silicious coats during a
gale of wind. As no solid matter can enter the roots of a plant, the
silex must be absorbed in a state of solution; and, as it coats the
surface, which is full of pores, the liquid is removed by perspiration,
and the silex is consolidated. The whole of this family abounds in
sugar, and its farinaceous products are too well known to require any
notice.
The grasses seem to have been the means of revealing the earliest dawn
of plant life, for the Hon. Sidney Osborne discovered that the
colourless protoplasm, or organizable liquid extracted from the roots of
young wheat, produced spontaneously and simultaneously double ovate
vesicles, or cells, such as are found in the roots themselves; and in
that liquid, though hermetically sealed in glass tubes, the formation of
these vesicles or cells was as active after six months as in the liquid
freshly taken from the young plant. Mr. Osborne believed these vesicles
to be the earliest organisms of plant life, and that this is the direct
and prevailing mode of production of the embryo. This accords with the
observations of Messrs. Wenham and Devey.
Many other very remarkable plants belonging to the Monocotyledons might
be mentioned, as the Dracæna Draco, or Dragon tree of Teneriffe, one of
the most ancient trees existing; the Pandanus, or screw pine, with its
aërial roots, indigenous in the islands of Oceania; and the Zostera, or
sea wrack, the only flower-bearing plant except one that inhabits the
ocean; their flowers are minute and bisexual, are rarely produced, but
they cover large areas with long grassy leaves. Bulbous plants, and the
Orchidaceæ, the most splendid ornaments of our gardens and hothouses,
are members of this class. Of the former, the snowdrop, crocus,
colchicum, arum, hyacinth, narcissus, tulip, and lily form a group of
singular beauty.
A bulb is merely a subterranean stem remaining permanently in the
condition of a bud. It consists of a disc, or conical plate, which is
the point of growth whence the flower-bearing stem and the leaves
spring. These are surrounded by leafy scales of a fleshy character
overlapping one another. The Liliaceæ and the Amaryllidaceæ contain many
of the most remarkable and beautiful bulbous plants known in gardens. To
the former order belong the hyacinth, tulip, and onion, in which the
growing point is surrounded by a series of fleshy tunics, each of which
encloses its predecessor; as also the lily itself, in the bulb of which
the growing point is enclosed in fleshy scales imbricated in rows one
above the other. One or two circles of roots descend from the
circumference of the disc in the form of slender, soft, white cords or
threads, with a spongy termination of cellular tissue to imbibe water
and other liquids for the nourishment of the plant. These bulbs are
reproduced by buds, or offsets, developed in the axils of the fleshy
scales, which fall off either the first or second year of growth.
Offsets are also produced by the arum, crocus, and meadow saffron or
colchicum, whose bulbs form a solid mass, and are called corms. These
offsets merely reproduce a _facsimile_ of the parent. In these and all
flowering plants fructification must take place before a new form or
variety can be expected.
The Orchids surpass every plant in the vegetable world, for the variety
of means employed by nature to continue the race. The blossoms are, for
the most part, at once both male and female, though some plants are
diœcious, but, except in rare instances, as in that of the Bee Orchis,
not a blossom can be fertilized without the aid of insects. The flowers
of the Orchideæ are constructed upon a fixed plan, which can be traced
through the innumerable variety of beautiful, singular, and often
grotesque forms which they assume. A corresponding variety is exhibited
in the admirable contrivances and adaptations which enable insects to
detach the pollen from one blossom, and carry it to fertilize another
blossom.
[Illustration: Fig. 72. Orchis mascula:—A, side view of flower, with a
portion cut away; B, section through one side of rostellum, with
included disc and caudicle of pollinium; _a_, anther; _r_, rostellum;
_d_, disc.]
The British Orchideæ belong mainly to three natural groups: the Ophreæ,
which comprise the Orchis, Ophrys, and other common Orchids; the
Neotteæ, comprising Epipactis, Neottia, Spiranthes, &c.; and the
Malaxeæ, represented by Malaxis. Orchids have white, fibrous roots, and
many of them a pair of fleshy tubercles. From thence a straight stem
springs up, ending in a spike of blossoms, each of which is attached to
the stem by a twisted stalk, containing the ovarium, as represented in
fig. 72. Ribbed lanceolate leaves rise from the roots, and some are
attached sparingly to alternate sides of the stem. The calyx is formed
of three pointed sepals, one perpendicular, the others horizontal.
Instead of being green they are usually coloured. Sometimes the whole
plant is green. One petal (fig. 72) is much larger than the others,
occasionally assuming the most extraordinary forms. It is called the
labellum, or lower lip; it secretes nectar, a sweet juice, to attract
insects, and is often produced into a long, hollow, spur-like nectary,
as in fig. 72 _n_. On that side of the spur which is opposite to the
labellum, the organs of reproduction are so placed, that an insect
alighting upon the labellum cannot insert its proboscis into the nectary
tube to eat the honey, without touching them.
[Illustration: Fig. 73. Orchis mascula:—Front view of flower, with
sepals and petals removed; _a_, anther; _r_, rostellum; _s_, stigma;
_l_, labellum.]
Mr. Darwin, in his admirable work on the ‘Fertilization of Orchids,’
assumes the Orchis mascula as a type for explaining the mechanism of the
reproductive organs of the Orchidaceæ generally. Fig. 73 represents a
front view of the flower with all the sepals and petals cut off, except
the labellum, or lip; and fig. 72 is a side view of the same, with the
near half of the labellum cut away, as well as the upper portion of the
near side of the nectary, or spur.
In all common Orchids there is only one stamen, which is confluent with
the three pistils, to form what is called the column. The edges of the
labellum are attached to the sides of the column, leaving a space known
as the chamber or mouth of the nectary, and thus the mouth has the
column on one side, and the labellum on the other. Although the three
pistils are united into one, the three stigmas are not; for the stigma
of the upper pistil is transformed into a pouch-shaped viscous mass
(_r_, fig. 73), called the rostellum, bearing no resemblance whatever to
a stigma, while the other two stigmas form a bi-lobed confluent mass or
stigma below it, through which the pollen grains fertilize the ovary,
which latter forms the apparently twisted stalk, by which the blossom is
attached to the stem. In fact, the rostellum (_r_) projects into the
mouth of the nectary, and overhangs the confluent stigmas (_s s_). The
lowest and narrowest part of a hood-shaped anther (fig. 73 _a a_,) is
attached to the back of the rostellum, and consists of two rather widely
separated oblong cells, which open longitudinally in front. Each cell
contains a pollen-mass, or pollinium. Fig. 74 represents one with its
club-shaped mass of pollen grains, its stalk, and viscid disc. These
objects are seen in situ in fig. 73. A pollinium consists of a number of
wedge-shaped packets of pollen grains held together by exceedingly
elastic fine threads, as in fig. 75; the packets unite into the
club-shaped head, and the threads form the stem; the viscid discs are
formed by the rostellum.
[Illustration: Fig. 74. Pollinium of Orchis mascula:—_p_, pollinium;
_c_, caudicle; _d_, disc.]
[Illustration: Fig. 75. Pollen grains of Orchis mascula.]
At an early period of growth the rostellum consists of a mass of
polygonal cells full of brownish matter, which soon resolve themselves
into two balls of an extremely viscid semifluid substance void of
structure. These two discs are flat on the top, and rounded below. They
lie quite free within the rostellum, except at the back, where each
viscid disc firmly adheres to a small portion of the exterior membrane
of the rostellum. The ends of the two stalks of the pollinia, or
granular masses, are strongly attached to these little discs, or balls.
[Illustration: Fig. 76. Pollinia of Orchis mascula, showing front view
of the discs and caudicles within the rostellum:—_r_, rostellum; _c_,
caudicle; _d_, disc.]
At first the membrane forming the exterior surface of the rostellum is
continuous; but as soon as the flower opens, the slightest touch causes
it to rupture in such a manner as to set free from it, and from one
another, the little discs, while at the same time the anther cells
themselves split in a longitudinal direction from top to bottom. In this
state none of the organs are changed; they are merely set free, and
ready for change, yet maintaining their normal positions. The labellum,
which is the largest petal of the flower, lies on one side of the
nectary tube, and the rostellum projects into it on the other; hence, if
an insect alights on the labellum, and pushes its head into the tube, in
order to reach the honey in the nectary with its proboscis, it cannot
fail to touch and depress the rostellum (see fig. 76), so that one or
both of the viscid discs carrying the pollinia will stick to it, and as
the anther cells are open in front, when the insect withdraws its head,
one or both of the pollinia are drawn out of their cells, and stick
upright on it like horns. In that position they never could fertilize
any blossom which the insect might afterwards visit; but Mr. Darwin has
shown that their position is changed by a contrivance that is not
surpassed in beauty in the whole vegetable world. While a pollinium is
upright on the head of the insect, the little viscid disc which supports
it contracts on being exposed to the air, so as to cause the pollinium
to sweep through ninety degrees towards the apex of the insect’s
proboscis, and this is accomplished in about thirty seconds, the time an
insect would take to fly to another flower. The pollinium in this
position would exactly strike the surface of the stigma when the insect
inserts its proboscis into the nectary of a flower.
It was long ago noticed by Robert Brown, that the stigma is very viscid,
but not so viscid as when touched to pull the whole pollinium off the
insect’s head, yet sufficiently viscid to break the elastic threads by
which the packets of pollen grains are tied together, and leave some of
them on the stigma. Hence a pollinium attached to an insect can be
applied to many stigmas, and fertilize them all. Mr. Darwin mentions
having seen the pollinium of Orchis pyramidalis adhering to the
proboscis of a moth with the stalks alone left, all the packets of
pollen having been left glued to the stigma of the flowers successively
visited. It appears that insects, for the most part, only remove one
pollinium at a time, and that the rostellum returns to its normal
position to prevent the viscid matter of the discs of the remaining
pollinia from being exposed to the air.
[Illustration: Fig. 77. Orchis pyramidalis:—Front view of flower with
upper sepal and petals removed; _a_, anther; _s_, stigma; _r_,
rostellum; _l_, labellum.]
The Orchis pyramidalis is considered by Mr. Darwin to be the most highly
organized species of the British Orchids he has examined. It has sharp
leaves, and a close pyramid of white or rose-coloured blossoms. The
upper sepal and the two upper petals form a hood, protecting the anther
and stigmatic surfaces from the weather. In fig. 77, a front view of a
blossom, these are cut off as well as in fig. 78 _a_, which represents a
side view of the same blossom deprived of half the labellum, and the
upper part of the nectary, or spur. The labellum is long, with three
equal entire lobes, and is produced into an awl-shaped spur, or nectary.
It has a small ridge on each side of its narrow base to guide the
proboscides of insects to the mouth of the nectary, which, besides being
small, is partially closed by the pouch-shaped rostellum, which is
placed very low in this species, and is flanked on each side by a
distinct stigmatic rounded surface (fig. 77 _s s_). The rostellum (_r_)
is hollowed out on its under-side in the middle, and is filled with a
fluid. Instead of two little viscid discs or balls, as in the Orchis
mascula, there is but one which is saddle-shaped (figs. 78, 79),
carrying on its nearly flat top or seat, the two stalks of the pollinia,
of which the two truncated ends firmly adhere to its upper-surface.
Before the membrane of the rostellum ruptures, the saddle-shaped disc
forms part of its continuous surface. The upper membrane of the disc is
rather thick; it is lined with a layer of highly adhesive matter formed
within the rostellum.
[Illustration: Fig. 78. Orchis pyramidalis:—Side view of flower, with a
portion cut away; _a_, anther; _r_, rostellum; _l_, labellum; _n_,
nectary.]
[Illustration: Fig. 79. Disc of Orchis pyramidalis seen from above, with
one pollinium, flattened by force.]
[Illustration: Fig. 80. Pollinia of Orchis pyramidalis, attached to
saddle-shaped disc.]
[Illustration: Fig. 81. Pollinia of Orchis pyramidalis with the disc
contracted.]
[Illustration: Fig. 82. Pollinia of Orchis pyramidalis:—_a_, as
withdrawn by the insertion of a needle; _b_, as after the second
contraction.]
When the flower opens, the saddle-shaped disc is set free by the rupture
of the membrane of the rostellum. Then the rostellum, which projects
into the minute round orifice of the nectary, is easily depressed by the
proboscis of a moth, and as the now naked and sticky under-surface of
the saddle-shaped disc is uncovered, it adheres to the proboscis, and is
withdrawn with it into the air, carrying the two pollinia (fig. 80) on
its exterior surface. Almost instantly the saddle is exposed to the air,
a rapid movement takes place. The two flaps curl inwards (fig. 81), and
embrace the proboscis, and the pollinia, at first parallel, become
divergent. A second movement now takes place, which causes the divergent
pollinia (fig. 81), which are at right angles to the proboscis, to sweep
through ninety degrees towards the tip of the proboscis, so as to become
depressed, and lie on each side of it. Hence, when a moth pushes its
proboscis between the guiding ridges of the labellum into the nectary of
another flower, the two thick ends of the pollinia will exactly strike
against the two stigmas. These stigmas are so viscid that they hold and
rupture the elastic threads which bind the packets of pollen grains
together, and some dark green grains are seen even with the naked eye
remaining on the two white stigmatic surfaces. In the Orchids, as in all
other flowering plants, tubes sent out by the pollen grains penetrate
through the stigmas, and fertilize the ovules in the twisted ovary. The
double movement of the pollinia, while on the moth’s proboscis, is owing
to the rapidity with which the viscid matter contracts and dries. Both
butterflies and moths frequent this Orchis. Mr. Darwin enumerates
twenty-three species of these insects which he had seen with the
pollinia of the Orchis pyramidalis attached to their proboscides. Many
had two or three pairs; the proboscis of the Acherontia had seven, and
that of the Caradium had no less than eleven of these saddles attached
at regular distances from top to bottom of its proboscis. Few of the
species of Orchis are visited by bees.
On account of some resemblance in form, the Ophreæ are named after
insects. The Fly Ophrys differs in no material respect from the other
Orchids. The stem or caudicle of the pollinium, instead of being
straight, as in the Orchis mascula, is doubly and almost rectangularly
bent. The upper membrane of the disc to which the stalk of the pollinium
is fixed, being the summit of the rostellum, is exposed to the air, and
becomes dry when the flower opens, consequently the disc, though viscid
enough on its under-side to stick to an insect’s head, is incapable of
shrinking, and causing that depression of the pollinium, characteristic
of all the species of Orchis. The labellum has no spur, but at its base,
just below the stigma, there is a deep depression representing the
nectary; and as the pollinia, which cannot be shaken out of their cells,
or pouches, are certainly, though rarely, extracted, Mr. Darwin
conceives that small insects crawl along the labellum to its base,
strike against one of the pouches, extract a pollinium, and fly with it
sticking on their head to another blossom, and that while bending their
heads into the hollow at the base of the labellum, the pollinium, owing
to its doubly bent stalk, strikes the sticky stigmatic surface, and
leaves pollen grains on it. There can be no doubt that this plant is
visited occasionally by insects, as it cannot be fructified without
them; but it is scentless, and as no nectar has as yet been found in it,
their motive for visiting it is unknown.
[Illustration: Fig. 83. Epipactis palustris:—Side views of flower, with
lower sepals cut away: A, with lip in natural position; B, with lip
depressed as by an insect.]
The fructification of the Ophrys apifera, or Bee Ophrys, is independent
of insects, for the stalks of the pollinia are extremely long, thin, and
flexible; and although their viscid discs still remain in their pouches,
as soon as the flower expands, and the anther cells or pouches open, the
heavy thick ends of the pollinia fall out of their cells, and hang
freely down in the air exactly opposite to the stigmatic surface. A
breath of air is sufficient to make them vibrate, strike the stigma with
their pollen mass, and leave pollen grains on its sticky surface.
[Illustration: Fig. 84. Epipactis palustris:—C, side view of flower,
with sepals and petals and half the labellum removed: D, front view of
column: _a_, anther; _r_, rostellum; _s_, stigma; _l_, labellum.]
The great tribe of British Neotteæ is characterized by a free anther
standing like a hood behind the stigma; the pollen grains are tied
together with threads, and attached to a viscous cap lying on the top of
the rostellum. The Epipactis palustris is a type of this group of
orchids. Its spike is short, and the pink blossoms stand out
horizontally from the stem on long ribbed footstalks, which contain the
ovaries. Fig. 83 A is a side view of the flower in its natural position,
with the lower sepals alone removed. The labellum, or lowest petal, is
interrupted in the middle by a kind of flexible hinge: the basal part is
a cup-shaped trough, at times abounding in nectar; the extreme part is a
wavy leaf (fig. 83 B). The entrance to the nectary cup is nearly closed
by the hood and the large anther; but, on account of the elasticity of
the hinge, the weight of an insect is sufficient to give access to the
nectar; but no sooner is the labellum relieved of the weight, than it
springs up into its natural position, and the insect creeps backwards
and comes out at the top of the flower with the viscous cap clasped
round its proboscis, and the pollen grains attached to it ready to
fertilize another blossom. In fig. 84, C represents a section of the
Epipactis, and D is a front view of the column.
[Illustration: Fig. 85. Listera ovata:—Side view of flower, with sepals
and petals cut away: _a_, anther; _col_, summit of column; _p_, pollen;
_r_, rostellum; _s_, stigma; _l_, labellum; _n_, nectar-secreting
furrow.]
Of all the British Orchids, the Listera ovata, or Twayblade (fig. 85),
has the most curious structure. It grows in woods and pastures, has a
creeping root, oval leaves, a downy stem, and yellowish green flowers.
Fig. 85 represents a lateral view of a blossom, with all the sepals and
petals removed, except the labellum. In this plant the rostellum (_r_)
is large, thin, convex in front, concave behind, and arches over (_s_)
the stigmatic surface. When the flower is full blown, the anther cells
(_a_) are already open, and the naked and friable pollen grains united
by a few threads, which form the pointed tips of the pollinia, rest upon
the concave back of the rostellum. The labellum, which is contracted at
the base, is exceedingly long, hanging down like a narrow ribbon. It is
divided half-way up, and furrowed along the middle, from the bifurcation
close up to the base of the stigmatic surface (_s_). The borders of the
furrow are globular, and secrete much nectar. The rostellum is
internally divided into a series of longitudinal cells, or chambers,
which contain and expel viscid matter with violence on the slightest
touch, and the viscid matter sets hard in two or three seconds, and soon
assumes a purplish brown tint. So exquisitely sensitive is the
rostellum, that a touch from the thinnest human hair suffices to cause
the explosion. As the pointed tops of the loose pollinia lie on the
crest of the rostellum, they are always caught by the exploded drop.
This never fails. So rapid is the explosion, and so viscid the fluid,
that it is difficult to touch the rostellum with a needle quick enough
not to catch the pollinia already attached to the partially hardened
drop, and consequently the slightest touch of any small insect which
enters the flower, suffices to explode the rostellum, and the pollinia
which attach themselves to its proboscis are carried by it to the next
flower to adhere to the viscid stigma and fertilize it. Mr. Darwin has
seen two Hymenopterous insects retreat from one of these plants with
bright yellow pollinia on their heads, and Mr. C. K. Sprengel saw an
insect of that kind leave pollen upon a stigma. The action and structure
of Neottia Nidus-avis is almost identically the same as that of Listera
ovata.
The Malaxis paludosa, or Bog Malaxis, the smallest of British Orchids,
is a rare plant, and differs from all of them in having its labellum
turned upwards instead of downwards. Its lower margin clasps the column,
making the entrance into the flower tubular. In this orchis the upper
sepal and two upper petals are reflexed, to allow insects freely to
visit the flower. In many orchids the labellum is properly directed
upwards, but assumes its usual position as the lower lip by the twisting
of the ovarium, or pedicel, of the flower. In the Malaxis paludosa,
however, the twisting has been carried to such an excess, that the
flower occupies the same position it would have held if the ovarium had
not been twisted at all, and which the stalk ultimately assumes when
ripe by the process of gradual untwisting. This little plant belongs to
a genus distinguished by having a movable and deciduous anther, and is
one of the British orchids that have allied forms among the exotic
genera. All Dr. Lindley’s vast tribes of Epidendreæ, and the still more
numerous and splendid Vandeæ, have not a single British representative.
The structure of the exotic orchids is often very complicated, and they
possess many properties unknown in the British genera. Their labellum,
or lower lip, is so varied, and sometimes so singular, that it baffles
description; besides, it often possesses peculiar motions, sometimes
from structure, at other times from irritation. In the Bolbophyllum
Rhizophoræ, the labellum is attached to the column by a very narrow thin
strap, elastic as india rubber, which oscillates in a singular manner;
while that of the B. barbigerum has a beard of fine hairs in almost
constant agitation. The irritability of the labellum in many of the
allied forms of the orchids is one of their remarkable properties; the
slightest touch sets them into motion. The Australian genus Caleana
possesses it in the highest degree; for when an insect settles on its
labellum, it suddenly shuts up against the column, and encloses it, as
it were, in a box.
Some of the exotic orchids have a pseudo-diœcious character. Thus some
species of Catasetum have two long horns, or antennæ, attached to the
rostellum, which stand over the labellum, and the pedicels of the
anthers are fastened down in a curved position; so, when an insect
alights upon the labellum and touches the antennæ, the excitement is
conveyed by them to the rostellum, the attached edge of the disc of the
anthers is ruptured, and they straighten themselves with such force,
that not only do they drag the balls of pollen and anther cells from
their places of attachment, but the whole pollinium is jerked forward
over and beyond the tips of the horns, to the distance of two or three
feet. The insect, disturbed by so sharp a blow, or after having eaten
its fill, flies with the pollen adhering to it to fertilize the female
plant, which differs from the male in having no antennæ. Thus the agency
of insects is as requisite to fertilize these semi-diœcious as
hermaphrodite orchids.
The Vanilla, which is cultivated for its aromatic pods in Tahiti,
Bourbon, and the East Indies, does not bear fruit without artificial
aid, which shows that the American insect, which fertilizes it in its
own native home, is not indigenous in the places mentioned. It appears
that many exotic orchids require a less elevated temperature than has
hitherto been supposed.
The form and position of the nectary are exceedingly varied. In certain
species, both of the native and tropical orchids, they are always dry;
but Mr. Darwin has discovered that, in these cases, the walls of the
nectaries are thick and formed of two coats, and that a liquid is
contained between them, to which the insect penetrates by piercing the
inner wall. The exotic orchids are, for the most part, larger, and
require larger insects to fertilize them than our small ones, whose
organs are generally microscopic. A curious instance, both of this and
of the extraordinary form of the nectary, is found in the Angræcum
sesquipedale, a Madagascar orchid, with large six-rayed flowers, like
stars formed of snow-white wax. It has a green whip-like nectary,
sometimes as much as a foot long, and, from the structure of the plant,
it appears that the pollinia never could be withdrawn, until a large
moth, with a wonderfully long proboscis, attempts to drain the last drop
of nectar from the bottom of the nectary.
Notwithstanding the vast diversity in the form of the orchids, they are
homologous in their general structure. Mr. Robert Brown was the first to
observe, that an orchid flower consists of fifteen organs, arranged
alternately, three within three, in five whorls. This accords perfectly
with a system of spiral vessels developed at an early age in all
orchids. Mr. Brown and three of the greatest living botanists[78] have
each traced the spirals from six bundles surrounding the ovary in the
footstalk to the different organs of the flower, and have found that
they consist of fifteen bundles corresponding to the fifteen organs of
the flower; namely, three sepals, three petals, six anthers in two
whorls (three of which are rudimentary), and three pistils, with their
stigmas: these are arranged in alternate whorls, and undergo many
modifications. The pistils and anthers are confluent, and form the
column; the uppermost stigma becomes the rostellum; the three inner
anthers are rudimentary, one forming the front of the column, and the
other two forming the membranous sides of the hood which protects the
pollen; and, lastly, the two lower anthers are united to the sides of
the lowest petal, and form the labellum, which accounts for its great
size, frequent tripartite form, and peculiar manner of attachment.
This system has been wonderfully modified to produce the varied groups
of extra-tropical and brilliant forms of exotic orchids, yet it can be
traced in all.
SECTION XIV.
DICOTYLEDONOUS, OR EXOGENOUS PLANTS.
IN Endogenous plants the seeds have but one lobe, and the growth is
invariably from the interior. In the Exogenous class, on the contrary,
the seeds have two lobes, and the increase in growth is external: hence
the botanical distinction of Exogenous plants. Although the distinctive
character of the highest class of vegetables is to have seeds with two
lobes, yet the structure and position of the seeds are extremely
diversified. Many have horny coats, such as the pips of apples and
oranges; or hard ones, as nuts, and the stones of plums and cherries.
They are sometimes on the outside of the fruit, as in strawberries, but
oftener within it, as in the melon, the pear, and a variety of others.
These succulent substances nourish the young seeds for a time, but, when
they are matured, the light and heat which ripened the fruits now
combine to accelerate their decay and decomposition, in order to set the
seeds free.
Whatever be the size or form of the seeds, whether large or microscopic,
they invariably contain two seed lobes or primary leaves, consisting of
cellular tissue, between which the miniature plant, with its radicle,
stem, and terminal bud lies concealed. At the end of the first year
there is little difference in the structure of a young woody plant,
whether from a one or two lobed seed; the distinction begins the second
year. In herbaceous plants, the stem, which is in general annual, is of
loose cellular tissue, with separate bundles of fibro-vascular tissue
running from the roots upwards, and passing at last into the leaves,
where they form the ribs or veins. Many of the higher forms of plants
have fleshy underground stems, as instances of which may be named the
Corm, as seen in the crocus and colchicum, the Tuber, as in the potato,
the Rhizome, as in the fleshy rootstock of Iris florentina, which yields
the violet powder of the shops, and indeed most bulbs may be considered
as modifications of stems, though they are more strictly analogous to
buds. The edible parts of the carrot, turnip, parsnip, and radish are
not stems, but highly developed succulent roots, the unusual development
of which is a direct result of cultivation.
A young tree, at the end of the first year of its growth, has
subterranean roots, with their branches and fibrils, and an aërial stem,
often more or less branched, formed of bark, wood, and pith, with a few
leaves at its extremity, all exceedingly tender. Every succeeding year a
new cylinder of woody fibre and vascular tissue is formed between the
wood and the bark, both in the stem and branches. It follows from this
manner of growth, that the stem of a tree, consisting of bark, wood, and
pith, is formed of a series of cylinders or extremely elongated
concentric cones closely united, so that a transverse section exhibits a
series of concentric circles or zones from the surface of the bark to
the central pith. The structure of the branches is similar, but the
number of zones depends upon the age. Since all the tissues that have
been described are combined to form the organs of nutrition and
reproduction in a full-grown tree, it affords the best general
illustration of the organization of the highest class of vegetables.
Every part of a tree or plant, except the top of the stigma and
extremity of the roots, is covered by an extremely delicate film of
cellulose, closely pressed down upon one, two, or three layers of
transparent colourless cellular tissue compressed into a tubular form,
which constitutes the cuticle. These flattened cells, which are firmly
united to one another by their edges, differ in shape in almost every
tribe of plants. In the monocotyledons they are elongated in the
direction of the parallel ribs of the leaves; in the highest class they
differ little from circular discs, but they have large sinuosities in
their edges, which make their junction very irregular. The upper cells
of the cuticle are lined with a waxy substance, which protects the plant
from damp; and in many cases it contains more or less silex.
This general covering, or cuticle, is perforated by numerous pores,
especially on the under-side of the leaves, and the green tender parts
of the branches; they are the organs of respiration. These pores, or
stomata, are usually formed of two crescent-shaped cells, joined
together by their points or horns, so as to leave an open space like a
mouth, through which the plant breathes. These, however, are only the
guardians: the opening between them leads into a cavity full of air,
which is the antechamber to an interior cavity. The valves of the
stomata open and shut according to the humidity or dryness of the
atmosphere. All plants of both classes, woody or herbaceous, have
stomates, except water plants, fungi, fuci, and others of the lowest
class. They are generally very abundant on the under-surface of leaves.
They are sometimes in vast numbers on both sides of the leaves, and are
essential to the life of the plant. According to MM. Payen and Liepner,
silex and azote, together with calcareous and alkaline salts, are
invariably found in the cells forming the skin of the roots, stems,
leaves, fruits, hairs, and spines. The strong cohesion of the skin,
together with the presence of these substances, becomes a defence
against the wearing effect of the weather, without diminishing the
transparency of the tissues.
The bark is divided into three regions, or zones. The external coat,
lying immediately under the skin, is formed of one or many layers of
cubical or oblong cells, elongated horizontally. They are transparent
and colourless at first, but become brown and opaque with the colouring
matter of cork as they grow older. On that account it is called the
suberous zone, and sometimes acquires great thickness, as in the Quercus
Suber, or cork tree.
The green cellular envelope comes next to the corky layer, and consists
of prismatic cells and laticiferous tubes, which form an irregular
wide-meshed network, elongated in the direction of the axis of the tree,
and sometimes constituting the chief thickness of the bark. This zone,
as well as the succeeding, increases imperceptibly by new layers added
to its interior, while the exterior coats of the bark perish annually.
In some trees they are annually cast off in plates and large flakes, as
in the Oriental plane, whose stem and branches look as if they had been
peeled in autumn.
The liber, which is the third and innermost zone of the bark, generally
consists of several layers of cellular tissue, traversed longitudinally
by bundles of woody fibre and laticiferous tubes.
The generating layer of cambium, in which all the phenomena of growth
takes place, is a semi-fluid mucilaginous substance, which comes between
the liber and the wood. It is most abundant in the spring, and is the
origin of all horizontal growth. This mucilage is really made up of a
vast multitude of cells, with cell-walls as delicate as those of a soap
bubble, which gradually undergo transformation into woody fibre,
laticiferous ducts, spirals, &c., thence called the cambium zone. The
whole of this matter spontaneously divides into two parts: one forms a
new layer of liber on the interior of all those which precede it, and
the other a new ring of young sap-wood, exterior to all its
predecessors. A portion of the cambium, in its unchanged or liquid
state, always remains between the wood and the bark, which are never in
absolute contact.
As a new cylinder of wood enclosing all its predecessors is annually
formed, the section of a stem perpendicular to its axis exhibits a ring
of woody fibre, alternating with a ring of spotted and rayed vascular
tubes, which constitute the silver grain of the wood. The rings are more
and more crowded, and narrower towards the centre, and at last become
impervious to the sap, which only rises through the younger part of the
sap wood. In fact, a large portion of the solid fibres of most plants
have ceased to take any active share in the performance of vital
functions, and, like the solid heart of an oak, retain their integrity
simply because they are not exposed to influences which would cause
their decomposition. A vegetable tissue exposed to ordinary chemical
action, can only remain entire so long as it is performing vital
functions. The arrangement of the woody fibre and ducts in the different
orders and genera is much varied. The breadth of the rings of wood shows
the effect of good and bad seasons; and in extra-tropical latitudes,
where there is alternately a period of growth and repose, their number
frequently indicates the number of years’ growth; so that the age of a
tree may be approximately, if not exactly, learned from a critical
examination of a section of its stem.
The innermost cylinder of wood is lined by the medullary canal or tube
containing the pith. It is a delicate membrane, entirely formed of
hollow spiral tubes. The pith, which fills the canal, is of greenish
cellular tissue when young, full of sap, and occasionally, though
rarely, mixed with vascular and spiral tissue. It passes uninterruptedly
to the end of every branch, leaf bud, and flower. Perpendicular plates,
called medullary rays, radiate from the medullary sheath and end at the
bark, dividing the whole mass of the wood into triangular or
wedge-shaped sections. They are thin plates of cellular tissue,
stretched horizontally between the central pith and the bark. In each
family of trees and shrubs they have a different arrangement, but in all
they keep up a horizontal communication between the centre and the
circumference, though they do not all extend throughout the whole length
of the stem; some do, others do not. Thus the cellular tissue forms a
horizontal system, while the fibro-vascular ducts constitute a
perpendicular system of tissues. In some trees the pith is scarcely
perceptible, and in others it diminishes or vanishes with age, as in the
oak. In the alder and other plants it dries up, breaks into pieces, and
the canal is filled with air.
In the stem and branches of the Coniferæ, there is scarcely any mixture
of vessels amongst the woody fibre, the vascular system generally
consisting exclusively of glandular woody tissue, except in the
medullary sheath, where spiral vessels are found in small numbers.
The subterranean growth, or descending axis of trees consists of large
branches, sometimes tending downwards, but more frequently spreading in
extensive ramifications, not far from the surface of the earth. Their
growth and structure are similar to those of the stem, but the cylinders
of wood are less apparent; they have medullary rays, but no pith; they
merely connect the active roots with the stem, and fix the plant firmly
in the ground, for they have few or no pores, and contribute little to
the nourishment of the plant, except by conveying liquids from the
fibrous roots to the upper growth. The active feeding roots spring from
them in the form of bunches of white fibres, like cords or threads,
which sink straight down into the ground. These real roots are of
cellular tissue enclosed in vascular tubes and spiral vessels, which
terminate at a little distance from the extremity, leaving a point of
loose spongy cellular tissue, called the spongiole, which absorbs from
the ground the liquids that nourish the plant. These root fibrils are
temporary organs; they die on the older parts of the subterranean
branches, and are succeeded by others on the new.
The various tissues which form the stem of a tree form, in the same
manner, though in diminished numbers, the complicated ramifications of
the branches and the leaf-stalks, and terminate in the leaves
themselves. Under the transparent film which forms the skin on the
upper-surface of a leaf, there is a layer of soft thin-walled
cylindrical or prismatic cells, closely pressed together, and full of
green vegetable matter, or chlorophyll. Several layers of thick-walled
cells follow, each more loosely aggregated than that which precedes it,
and fuller of void spaces, till in the last green layer on the
under-side of the leaf the cells are globular, with numerous large
irregular void spaces, united in a reticulated system filled with air,
and in direct communication with the atmosphere by means of the
innumerable stomata, which are to be found in the under-surface of the
leaves of all land plants of the higher classes, and which are their
organs of respiration.
The form of the leaf is determined by the arrangement of the vascular
bundles, which are in communication with those in the interior of the
stem, and branch out in various directions through the green layers:
these branches unite again, and form the skeleton of the leaf, which is
often a delicate maze of the finest lacework of nerves. The vascular
system is double, consisting of an ascending and descending portion. The
ascending portion, which is continuous with the medullary sheath,
becomes continuous at the apex of each nerve of the leaf with the
descending portion, which is beneath and in contact with it throughout
its ramifications. This descending portion at the base of the
leaf-stalk, or petiole, becomes continuous with the bundles of the
liber. In the upper part of the nerves of the leaf there are spotted
vascular ducts, in the lower part there are laticiferous vessels. Those
on the upper side carry the rising sap to the green matter, where it is
elaborated and matured, and then it passes into the vessels on the
under-side of the nerves or veins, which carry it down the liber.
Buds are generally formed of scales closely imbricated round the young
leaves, which are variously folded and firmly packed; they contain the
rudiments of the whole plant, and as in a large tree they are renewed
every year, the sources of life are all but infinite.
The spines with which many plants are armed are of two kinds; one is
permanent, being an excrescence from the wood, as in the blackthorn; the
other proceeds from the bark, and may be stripped off, as in the rose;
both contain silex, and are covered by the skin common to the whole
plant.
Few plants of any kind are without hairs, which are chiefly found on the
young shoots, and on the under surface of the leaves. They are either
formed of a transparent elongated hollow cell, or consist of a number of
transparent colourless superimposed cells, sometimes jointed, but more
frequently rectilinear. When they sting, as in the nettle, they are set
upon a kind of bulb composed of cells which secrete the acrid colourless
liquid which causes the irritation, and when slightly pressed send it
through the hair, the point of which breaks off as it enters the skin of
the hand.
The hairs are so transparent that the gyration of the azotized liquid,
called protoplasm they contain, has been distinctly traced, a motion so
universal in some part of the structure of plants that according to the
observations of Mr. Wenham, the difficulty is to find a plant, aquatic
or terrestrial, in which it does not take place at some period of its
growth. The gyration in any given cell preserves a uniform direction; in
different cells the direction is different. It will persist in a
detached part of a plant for several days or even several weeks. It is
arrested by cold, and recommences its gyration when the temperature is
raised.
It has been mentioned that in the primordial cell the solid coloured
particles often form a nucleus in the centre of the viscid liquid called
protoplasm, which is continually diminished by the increase of the
watery vegetable sap. At length the protoplasm in the hairs is reduced
to mere threads extending from the cell wall to the nucleus, so that the
latter looks like a spider in the middle of its web. These threads are
really streams of the viscid protoplasm flowing through the more liquid
cell sap from the nucleus to the cell wall, where they turn and flow
back again in another thread. When there are several currents in the
same cell, the nucleus, which is the common point of departure and
return, is the centre of the vital activity of the cell, though it does
not always maintain a central position; in the cells of the leaves of
the Vallisneria spiralis the nucleus even follows the protoplasm, which
flows in a broad stream up one side of the cell and down the other, as
in the Chara. In most plants the gyration is transitory, for the nucleus
which always exists in young cells is dissolved as the cell advances in
age, and the protoplasm is so much diminished in quantity, that its
motion is imperceptible. There are exceptions, however, as in the hairs
of the nettles and some other plants, where it is persistent.
The motion is in general very slow. The thinness and minuteness of the
currents may be imagined, since they and the cells containing them are
microscopic objects, and the solid particles carried by the liquid,
which afford the means of tracing its course, are not more than between
the three and the five thousandth part of an inch in diameter. M.
Schleiden ascribes the motion to changes in the form of the cells
produced by an internal vital action, while Professor Karsten believes,
from observations he made on the rotation of liquids in the hairs of the
common nettle, that it is a phenomenon of diffusion, depending upon the
chemical changes taking place in the cells of the hairs independent of
any contractibility, not referable to them.
The whole of the tissues that exist in a well-grown tree are not to be
met with in each of the numerous woody and herbaceous plants of the
first class; some may be wanting, and those that do exist may be, and
generally are, much modified both in form and size. All the trees in the
temperate zone, and most of those in the tropics, belong to the class of
Exogens; but the annual rings of wood are less distinct in the latter,
the periods of repose and activity depending upon the dry and wet
seasons not being so decided as our winter and summer. The leaves of
tropical plants have a thicker skin than in colder climates, to defend
them from an ardent sun. The structure of herbaceous plants in all
countries is lax and juicy, they have abundance of pith, large medullary
rays, and zones of fibro-vascular tubes, which separate the pith from
the bark. In fact, each herbaceous and ligneous family has a structure
and properties peculiar to itself; but although there is almost an
infinite diversity of form and character, the general type of the class
may be traced in all.
Vegetable matter consists of carbon, hydrogen, oxygen, and nitrogen, yet
no plant can combine these simple elements into organic substances; they
imbibe them by their roots and leaves under the form of carbonic acid,
water and ammonia: these they have the power of decomposing, and
recombining their simple elements into new compounds. Carbon forms the
hard part of plants, and enters extensively into their most delicate
structure; but it is never found free. Combined with hydrogen and oxygen
it not only constitutes the cell wall cellulose, which may be regarded
as the skeleton of the vegetable world, but hundreds of compounds
differing decidedly in their properties, yet consisting only of these
three elements united with one another in different quantities and
proportions. Proteine, a compound of all the four simple elements, is a
mucilaginous substance, which lines the primordial cell, is homogeneous
at first, and afterwards more or less granulated. It is present wherever
the vital energy is in activity.
Although these four primary elements form the basis of vegetation,
plants require other substances which they absorb from the ground in a
state of solution, such as silex, or rather silicious salts having a
base of potash or soda, the carbonates, sulphates, and phosphates of
lime, the phosphate of manganese, and the oxides of manganese and iron,
with various other metals and substances in a state of combination and
solution. A few are universal constituents, as the earths and alkalies;
in general each race of plants only absorbs such as are peculiar to
itself. Soda abounds in the Algæ and is found in the Liliaceæ,
Cruciferæ, and other plants that are indigenous on the sea-coast, or in
brackish marshes. Potash exists in land plants, and cannot be replaced
by soda, for however rich the soil may be in soda, they do not thrive in
it. The ashes of land plants consequently contain the metal potassium,
while most of the Algæ yield sodium; they also yield chlorine, iodine,
and bromine in a state of combination. It is proved by spectrum
analysis, that every plant, with the exception of the very lowest,
contains a variety of metals in infinitesimal quantities, as lithium,
rhodium, and others; but they are not essential to the welfare of the
plants. Iron is the most frequent constituent in very small quantities;
there are also occasional deposits of soda, lime, and a little
manganese. All the various substances which enter the vegetable system,
are combined in definite proportions into an infinite variety of organic
compounds in different plants, and in different parts of the same plant,
for the decomposed matter is carried by the ascending sap to every part
even of the highest trees. Throughout the whole process the law of the
division of labour prevails; to each part of a plant, and to each group
of cells, its own duty is allotted.
The vegetable sap, consisting of water, carbonic acid, ammonia, and
other substances, which enter the spongy extremities of the roots in a
liquid state, rises in the form of a crude fluid through the whole loose
texture of herbaceous plants, through both the wood and pith of trees
under two years’ growth, and in older trees and shrubs it rises through
the sap-wood of the stem into the branches, and thence into the leaves,
the limit of ascent in all plants, so that in spring, all the cells are
full of sap. The vascular ducts are capillary tubes, and the cellular
tissue is an assemblage of closed cells or sacs, whose wall or
cell-membrane is permeable by liquids; hence the imbibition of the roots
and the rise of sap in the plant are essentially due to capillary
attraction acting contrary to gravitation. The ascension of the liquid
is inversely as the diameter of the capillary tubes and cells in the
stem and branches; the quantity raised is the same at all heights, and
the velocity of ascent is inversely as the height.[79] As soon as the
leaves are expanded, they evaporate a quantity of water through their
stomata during the day, so that in a tree or any plant, an enormous
extent of evaporating surface aids in raising the sap by creating a
vacuum in all the upper cells and vessels, by which the force of suction
and the rapidity of ascent are increased. It appears that the water
evaporated by the leaves is in exact proportion to that taken up by the
roots to supply its place; but as soon as the young branches are formed,
the buds for the following year produced, and when the leaves are full
of the chlorophyll which they have consolidated during the summer, the
evaporation is less, the sap ceases to rise, the spirals and vascular
ducts in the medullary canal and sap-wood are left dry, and fill with
air, which they convey to every part of the plant except the bark, to
assist in assimilation, that is, in the formation of organic compounds.
During the whole of this process the leaves and other green parts, which
are the organs of vegetable respiration, are most active. They absorb
carbonic acid gas from the atmosphere by day, and exhale oxygen. For by
the direct action of solar light the carbonic acid gas and ammonia in
the crude sap are decomposed, part of the oxygen is set free and
exhaled, and the rest, with part of the remaining elements, combine to
form chlorophyll, which is a compound of starch and a little nitrogen.
The oxygen inhaled by plants during the night, combined with other
elements in the sap, forms oxidized vegetable compounds.
M. Kosmann, of Strasburg, observed that both the leaves of plants and
their corollas give out a ponderable quantity of ozonized oxygen, much
more than that which exists in the air, and that the quantity is less in
the night.
All parts of plants that are _not green_ exhale carbonic acid gas, and
inhale oxygen, like animals, night and day; if prevented from inhaling
oxygen they lose their vital power, are soon suffocated, and the plant
dies. The expiration of oxygen by the leaves is connected with the
nourishment of a plant, the inspiration of that gas is connected with
its life.
When the sap is completely organized by respiration, evaporation, and
the chemico-vital agency of light, it descends chiefly through the
cambium, lying between the liber and the wood. From this layer the sap
distributes to each organ capable of increase, the requisite nutritious
liquids, deposits various organic compounds, and annually renews the
cambium. Part of the sap in its descent runs into the wood through the
horizontal medullary rays, in the cells of which it deposits starch. The
descent of the sap is no doubt due to gravitation.
The latex is a general name for those white or coloured juices peculiar
to some plants. It is separated in the leaves from the descending sap,
which is always colourless, and consists of a clear liquid, thickened
and coloured by white, yellow, reddish-brown, or green globules floating
in it; it does not turn blue under the action of iodine, therefore it
does not contain starch. These proper juices differ as much in quality
as in colour; some contain fatty matters, others substances of a totally
different nature, as caoutchouc; a few are bland and nutritious, many
acrid and poisonous; some contain alkaloids, others have none. These
juices are by no means essential to the life of the plant, for sometimes
they are wanting in their most essential parts, and they are found in
certain species and not in others most nearly allied. Certain it is,
that tropical lactescent plants which do not produce their proper juices
when brought to a cold climate, still produce their milk vessels.
These vessels follow the ramifications of the veins of the leaves in the
highest class, and also in some of the monocotyledons. In the stem the
milk vessels belong especially to the layers of the bark, where they
take the form of long reticulated perpendicular ducts, through which the
proper juices descend towards the roots.
Each plant has its own system of milk vessels, and M. Lestiboudois has
found that the coloured liquids have a rapid motion; the movements are
very complicated, not from point to point, but in such a manner that the
granules are carried by the liquid into all the ramifications of a
complicated network.
The septa, or divisions between the primordial cells, exert a powerful
influence upon the substances contained in the sap as it permeates
through them, no doubt acting as a dialysing membrane, which separates
the gelatinous from the crystalloid matter. The latex is probably
separated from the sap by the septa in the cells of the leaves, and sent
into the vessels peculiar to it, and then, while the sap is descending
and passing through the cambium, it is likely to be dialysed by the
septa between the cells of that layer, arresting the protein, and other
gelatinous substances, and allowing sugar, starch, and other crystalloid
matter to pass freely, and form deposits of organic compounds for the
following year. For perennial plants in extra-tropical countries remain
in a dormant state during the winter; their cells are then full of
organic compounds under the form of protein, as well as sugar, gum, &c.,
but especially starch, which is converted into sugar or dextrine, when
spring awakens the plants to renewed life and activity.
The composition of inorganic matter is very simple; there are
comparatively few radicals, and the substances are compounded of few
equivalent atoms, at most eight or ten, sometimes only two or three.
Carbonic oxide is formed of one atom of carbon and one of oxygen;
carbonic acid is formed of one atom of carbon and two of oxygen; and
acetylene, M. Bertholet’s base of synthetic compounds, contains two
atoms of carbon and two of hydrogen, chemically united; but no organic
compound contains less than three equivalent atoms, generally a great
many more. For example, citric acid, which is lemon juice, contains 12
atoms of carbon, 5 of hydrogen, and 11 of oxygen; while strychnine
contains 44 atoms of carbon, 23 of hydrogen, 4 of oxygen, and 2 of
nitrogen. Experiment has proved that the powers which maintain stability
among the numerous and complex constituents of organic substances
decrease in energy as the number of the equivalent atoms augments; hence
such compounds are in less stable equilibrium than those of inorganic
bodies, and are more liable to be disturbed and changed into new and
more stable forms.
As the chemical functions are not the same in all the cells, situated as
they are in different parts of a plant, they elaborate different
substances from the same materials. Besides, new substances are
introduced with the growth of the plant, to be acted upon by the light
and heat of the different seasons, so that numerous compounds may be
formed out of a given number of the primary elements. For example, the
ultimate elements of wheaten flour, or a grain of ripe wheat, are
carbon, the three elementary gases, sulphur, phosphorus, calcium,
magnesium, and silex; but during the germination and growth of the
plant, its flowering, forming the seed, and ripening the grain, certain
portions of these elements chemically combine in definite proportions to
form cellulose, starch, sugar, gum, gluten, fibrin, albumen, casein, and
fat, all of which are found in wheaten flour.
However much plants may differ in their organic products, they all agree
in producing protein, which takes an active part in the formation of
cells; and all produce neutral hydrates of carbon, such as cellulose,
starch, sugar, gum, &c., which consist of carbon, combined with hydrogen
and oxygen in the exact proportion that forms water. Many of them have
precisely the same quantity of carbon, and only differ in the quantity
of the aquatic element, as for example, lignin, starch, and cane-sugar,
which consist of 12 parts of carbon, in a state of combination with 8,
10, and 11 parts of water respectively; indeed the affinity between many
of these neutral hydrates is of a most intimate character. Some of their
varieties are isomeric, that is to say, they contain the same
ingredients in the same proportions, and yet they differ essentially in
regard to their properties.
Next to cellulose, starch is the most universal and distinctive of
vegetable productions, being a constituent of all plants, except the
fungi. It abounds in the grains and other seeds, and supplies the young
plant with food till it can feed itself. In both of the flowering
classes it occurs in small colourless transparent grains, either
floating in the sap, attached to the walls of the cells, or accumulated
within them. Starch globules of very small size are imbedded, either
singly or in groups, in the granules of chlorophyll, or leaf green; the
manner in which the green coating takes place is unknown. Starch is an
organic substance, varying from grains of inappreciable minuteness to
such as are visible to the naked eye, and of such a variety of forms
that it can be ascertained with tolerable certainty by what plant a
grain of starch has been produced. The small grains are generally
globular, but whatever the form may be, each consists of a series of
superimposed layers of different densities, which exhibit coloured rings
and a black cross in polarized light.
Starch is an early and transient product of young plants, which is
destined to be changed into nutritious substances at a later period, but
being insoluble in cold water it is unfit to travel with the sap.
However a ferment called diastase produced during the incipient
germination of the grains and seeds, in the tubers of potatoes, &c.,
being in a state of change, imparts that state to the starch, and
converts it into a sweet soluble matter known as dextrine or starch-gum
which is capable of being carried throughout the plant with the sap, and
which is itself ultimately changed into sugar. Dextrine is an ingredient
in the primordial cell. Starch, dextrine, and cellulose are isomeric:
consisting of the same elements with different characters.
The woody part of trees and shrubs, the fibres of hemp, flax, of the
Agave, and many other plants, are formed of cellulose, the purest form
of that substance being bleached flax and linen. During the progress of
vegetation, the cells of the ligneous tissue of trees, also those of
woody and fibrous plants, which are transparent and colourless when
young, become internally coated or filled with sclerogen, the colouring
matter of wood, a substance of various hues. In extra-tropical countries
it is generally some shade of brown, sometimes dark, sometimes so pale
as to be almost white with a yellowish or reddish tinge; and
occasionally it is beautifully marked as in the wood of the olive. In
tropical countries the colours are more vivid and varied, deeper and
even black, as in ebony. This colouring matter has the same quantity of
oxygen as cellulose, but it contains hydrogen and more carbon, hence
wood is combustible in proportion to the quantity of sclerogen it
contains. In beech it forms half of the wood, in oak two thirds, and in
ebony nine tenths, so it is the most highly combustible of the three.
The additional carbon is obtained by increased respiration, the hydrogen
by decomposition of water in the sap.
Sugar is almost as universal a constituent of the higher classes of
plants as cellulose and starch, for besides the saccharine juice of
innumerable plants, starch, the acids of unripe plants, and even the
acrid juice of the fig and other plants, is turned into sugar as the
plant advances to maturity, and the fruits ripen. Manna and other
saccharine exudations from the leaves or stems of trees, as the lime
tree, are probably intercepted by the dialysing septa of the cells, and
exude to the exterior through the pores of the skin. The sweet juice
found in the nectaries of flowers is formed in other parts of the plant,
and rarely flows to the flower before it is full blown; the quantity is
at its maximum during the emission of the pollen, and ceases when the
fruit is formed. In diœcious plants and that singular and beautiful race
the Orchideæ, it is evidently intended to attract insects for their aid
in fertilization.
Vegetable oils, resins, and wax, consisting of the same simple elements
as the hydrates, form a large class of inflammable organic substances in
which hydrogen predominates. Olive oil is a rare instance of a fixed oil
being obtained from a fruit; some laurels have that property also, but
the fixed oils are chiefly found in seeds, as the walnut, hazel nut, and
the almond, in which the principle of oil is in its greatest purity. It
is particularly abundant in hemp seed, and in a great variety of plants
the starch in the seed is changed into oil to nourish the embryo, till
the seed lobes are above the ground, and the true lobes appear.
Resins, gums, and wax, being colloid substances, are dialysed and
ejected from the system either through the fissures in the bark, or by
pores in the leaves. The resins exude through the bark from canals that
run between the cells of the plant, in solution, and are consolidated by
the oxygen on coming into the air. The herbaceous zone in the bark of
the fir and pine family furnish an abundant supply of resins and
balsams; the camphor tribe and the Amyrids are rich in them, as
frankincense, myrrh, balm of Mecca, and the Olibanum, supposed to be the
frankincense of scripture.
Wax is a frequent vegetable production, especially in the torrid zone,
where many of the wax-bearing plants supply the natives with light. An
exudation through the pores of many plants coats their surfaces with
resin or wax. Young buds are often covered with resin to protect them
from cold and wet during the winter and early spring, as those of the
horse-chestnut and balsam poplar. It is wax that gives the bloom to the
plum, cherry, and grape, and the rain drops lie on the waxy surface of
the cabbage leaf, like balls of diamond, from the total reflection of
light at their point of contact. Wax protects plants from damp in a
rainy climate, and prevents too strong perspiration from the fleshy
leaves of the aloe, cactus, and other inhabitants of the parched and hot
regions in the tropics.
The vegetable substances hitherto under consideration are neutral, but
the remarkable compounds albumen, fibrin, and casein, already mentioned
as constituents of wheaten flour, not only contain carbon and hydrogen
with a little oxygen, but azote and small quantities of sulphur and
phosphorus. Each of these three organic compounds is the same, whether
derived from animal or vegetable matter. Thus albumen is chemically the
same, whether obtained from wheat and other grains, from arrowroot,
dahlia roots, the serum of blood, or the white of an egg. As it
constitutes the film or thin coating of the primordial cell, and
combines with dextrine in its internal viscid lining, it not only forms
an ingredient in all vegetable organisms, but plays an important part in
the growth of the whole vegetable world. Fibrin is chemically the same
in the juice of plants and in blood, in which it exists as a liquid
during the life of the animal, and as a fibre after death. It forms the
basis of the muscular system in animals, and that extracted from the
juice of plants coagulates spontaneously like blood. Casein is
chemically identical, whether derived from the curd of milk, or from
peas and beans. Azote is a very important principle in these substances
as well as in the gelatinous substance gluten. It forms an essential
part of the animal structure, and is either highly nutritious or
deleterious in the vegetable, being at once one of the most valuable,
contradictory and powerful agents in nature.
Chemists have formed by synthesis compounds identical with all the fixed
and essential oils, for confectioners can now give the flavour of the
pear, orange, quince, pine apple and other fruits by means of artificial
chemical compounds. All the saccharine substances have not yet been
artificially obtained, nor the albuminous substances, albumen, fibrin,
and casein.
It cannot be a matter of surprise, when chemists form organic substances
out of inorganic elements, that they should succeed in transforming
compounds produced by living plants into new compounds, as that of
changing the vegetable acids into alcohols, which is now done. But some
of the acids themselves are synthetically formed out of inorganic
elements; as for example the oxalic, the most common of all the
vegetable acids, which is found most abundantly in the Oxalis or wood
sorrel, and is a frequent constituent of the highest and lowest plants.
The formic acid, which is the acrid stinging principle in ants, is also
synthetically formed; it is found in the juice of the stinging nettle
and in decaying pine leaves, and contains hydrogen like all the other
vegetable acids. These acids result from an augmentation of oxygen
during nocturnal respiration, which penetrates deeply into the vegetable
structure.
Octahedral, prismatic, and stellar microscopic crystals formed by the
chemical combination of the natural acids with bases imbibed by the
roots, are deposited in the cells under the skin, and in all parts of
plants. However, they appear most frequently as bundles of needle-shaped
crystals of carbonate of lime, lying side by side in the hollow of a
cell. They are known as raphides, from _raphis_ a needle, and may be
easily seen under the skin of the medicinal squill. Large single
crystals of oxalate of lime, octahedral or prismatic, are found in the
cells under the skin of the onion and other plants; and stellar crystals
of the same substance abound so much in the common rhubarb that the best
specimens of the dry medicinal root contain as much as thirty-five per
cent. of them; while certain aged plants of the cactus tribe have their
tissues so loaded with them as to become quite brittle. The calcareous
base in some instances is combined with tartaric, citric, or malic acid.
The crystals of some raphides are 1/40th of an inch long, others are not
more than the hundredth; they are brought into view by polarized
light.[80] Spherical raphides between the 1/2000th and 1/4000th of an
inch in diameter have been discovered scattered profusely through the
tissues of the leaves, and those parts of plants which are modifications
of the leaves; they may be seen under the skin of Pelargoniums and other
plants, and it is supposed that few if any orders of plants are without
them.[81]
Although azote forms 788 thousandth parts of the atmosphere, none, or at
least no appreciable quantity of it, is absorbed by the vegetable world;
that great principle of nourishment is entirely supplied by ammonia and
nitric acid, imbibed by the roots, and decomposed by the chemico-vital
power. Here it shows its capricious character by combining with other
simple elements in the bark, to produce the most precious medicines in
some plants, and in others the most deadly poisons, while no vegetable
substance is perfectly nutritious without it.
The milk sap, when exposed to the air, coagulates into a tenacious
viscid solid. The white juice is generally acrid, or narcotic, or both,
and for the most part extremely poisonous, though exhibiting strong
contrasts even in nearly allied species. In the order Euphorbiaceæ or
Spurgeworts, comprising nearly 1,500 species, a large proportion are
hurtful; but there is a gradation from mere stimulants to the most
formidable poisons. This order furnishes the Ethiope and the native
Brazilian with poison for their arrows. It contains the Manchineel, and
Excœcaria Agallocha, the most poisonous of plants; even the smoke from
the burning branches of the Excœcaria affects the eyes with insufferable
pain. The white juice of the Fig, one of the Morad order, is violently
poisonous; in many, as in the common fig, it is acrid and irritating.
The Antiaris toxicaria, the celebrated Upas-tree of Java, which is of
the Artocarpeæ or Bread-fruit order, owes its virulence to its milky
juice, which contains strychnia, the most fatal of drugs. Dangerous and
acrid as these orders are, the Bread-fruit, abounding in starch,
supplies the inhabitants of the East Indian islands with excellent food;
the milky juice of the Cow-trees, chiefly of the Bread-fruit and Fig
orders, furnishes a wholesome beverage to the South Americans; and the
Manihot or Cassava, a poisonous spurgewort when raw, yields when roasted
nutritious food to whole nations, the heat driving off the dangerous
principle. Caoutchouc, a most harmless substance, is the solid produce
of many of the most acrid and virulent juices of plants belonging to the
preceding orders; the poison is probably left in the liquid. The
chemico-vital power is strikingly illustrated by the number of safe and
excellent fruits produced by trees full of the most deleterious juices,
whether milky or not. Some of the finest fruits in the Indian
Archipelago are products of eminently dangerous species of the
Sapindaceæ or Soapworts. The acrid juices of the leaves and branches,
are so much diluted with water in the fruits, that they become
innocuous, or they may be changed into sugar, as in the common fig.
Nothing can surpass the virulence of the juice of the Upas-tree, yet its
nuts are eaten with impunity, and the pulpy contents of the fruit of the
Strychnos nux vomica is food for birds. The leaves and berries of the
potato are so strongly narcotic, that an extract from them is
intermediate in power between that from deadly nightshade and hemlock,
yet the potato itself, like the cassava, is rendered wholesome by being
boiled or roasted.
The alkaloids are alkaline substances formed in the bark and milky
juices of plants, always combined with an acid during the life of the
plant. The chemical structure of this class of substances is very much
alike, and chemists have succeeded in forming many of them
synthetically; they all contain azote, and have a great affinity for
acids. The bark of the different species of Cinchoneæ, especially the
Cinchona cordifolia and C. Condaminea, yield three alkaloids—namely,
cinchonine, quinine, and cusconine—they are all formed of carbon,
hydrogen, and azote in the same proportions; but the first has one atom
of oxygen in addition, the second has two atoms in addition, and the
third has three; so that in these alkaloids the carbon, hydrogen, and
azote combine to form an organic radical, which is oxidized in three
different degrees. Six of the alkaloids have been obtained from opium,
which is the solid portion of the milk juice of the poppy; of these,
morphine seems to be the narcotic principle; and the orange-coloured
milk sap of the Chelidonium, a very poisonous and acrid plant of the
poppy order, has furnished chelidonine. The Colchicum order, containing
the meadow saffron or autumnal crocus, and Veratrum album or white
hellebore, as well as many other plants, yield alkaloids, all of which
are medicinal or poisonous, according to the dose.
There is scarcely a people, however savage, that has not discovered some
exciting narcotic. Opium is almost universally smoked or eaten among
Eastern nations; and bhang, a strong narcotic, obtained from the leaves
of Indian hemp, is in equally universal use among the Brazilian savages
and Hottentots, but especially among the Malays, who are excited to
madness when they smoke it too freely. The same intoxicating effect is
produced by a strong liquor prepared from the Datura sanguinea, a
species of stramonium; and its congener tobacco, now all but a necessary
of life among civilized mankind, was smoked by the natives of the
American continent, before the arrival of the Europeans, as a relief
from hunger.
Coffee has been long in use on account of its stimulating principle
caffeine, which is now discovered to be the same with theine, the
latter, however, being less exciting, unless the tea plant grows in a
very hot climate. In countries where nature furnishes few narcotic
principles, wine, beer, and spirits supply their place, especially in
the far north, where animal heat is rapidly carried off by the cold, and
carbon must be furnished to satisfy the all-devouring oxygen which we
draw in at every breath.
Caffeine, the highly azotized principle of coffee, obtained from tea
leaves and coffee beans, is one of the substances known as neutral
crystallisable principles. Similar substances are found in asparagus,
pepper, almonds, the bark on the roots of the apple, pear, plum, and
cherry trees, as well as in the bark of the willow. The two last are
especially analogous, and contain no azote, as the others do.
The colouring matter of flowers is a fluid contained in cells, situated
immediately under the skin, which itself is perfectly transparent and
colourless. The whiteness of the white Camellia, rose, lily, and other
flowers, is supposed to be owing to the total reflection of light from
the cells immediately below the skin, which are either full of air, or
of a colourless liquid. The predominating colours are yellow, red, and
blue, with the various intermediate tints. Sometimes these colours are
converted one into another in the petal after fertilization, at which
period the colours are brightest. The chemical nature of these liquids,
the cause of their variety, and their definite arrangement in one and
the same petal, do not seem as yet to be ascertained.
The parts of plants that are not green inhale oxygen from the
atmosphere, and exhale carbonic acid gas exactly like animals. During
the chemical combinations of the oxygen with the carbon derived from the
nutriment to form the carbonic acid gas, heat is necessarily evolved,
especially in the flower, the point of maximum heat varying with its
expansion. The blossoms of the Aroideæ, or Arums, are remarkable for the
evolution of heat. According to Saussure, a blossom of the common Arum
maculatum consumes five times its volume of oxygen in twenty-four hours
previous to its evolution of fruit, so it is not wonderful that the
chemical combination of such a quantity of oxygen should produce a
strong development of specific temperature. By M. Dutrochet’s
observations, the heat evolved by the Arum maculatum has a maximum in
the day and a minimum in the night, and he found that it exceeded the
heat of the surrounding air by between 25° and 27°. The heat of the
Colocasia odorata, another Arad, was determined by several observers to
be even 50° above the warmth of the air. The heat evolved by germinating
seeds when in a heap is not from fermentation; it is owing to their
consumption of oxygen and expiration of carbonic acid gas. The
temperature of all vegetating parts of plants, the roots, leaves, young
juicy shoots, &c., is far superior to that of their flowers. It arises
from the nutritive process, and has a maximum at noon, and a minimum at
midnight, like that of the flower. The growth of plants is most vigorous
at noon; consequently there is then a greater evolution of heat.
Water in small quantities is secreted night and morning from the points
of the leaves of many plants, probably to relieve them from a
superabundance of liquid, which evaporation is insufficient to carry
off. The arums are remarkable for the quantity they eject. It falls in
drops from the points of the leaves. About half a pint is given out
every night by the enormous leaves of the Caladium distillatorium, a
species of Arad. In that plant, and in the Colocasia, the water flows in
canals along each rib into a general duct, which runs along the border
of the leaf, and terminates in an orifice upon the surface.
Since electricity is developed by chemical action in unorganized matter,
it may be inferred that it is also developed within the vegetable cell
where so many organic compounds are formed; but it is probably given off
from the points of the leaves or by evaporation from their surfaces.
Professor Fleming ascertained by actual experiment, that the sap of a
leaf, and its surface, are in different electric states; he also found
that the surface of the spongioles of the roots of plants and the
ascending sap have opposite electricities. Both of the preceding cases
the Professor ascribes, in part at least, to organic changes which take
place during vegetation. Slight currents of electricity were obtained
from the petioles of flowers, but fruits and tubers give powerful
electrical currents due to the reaction of different vegetable juices
upon one another. The tuberose is said to emit scintillations and dart
small sparks of light in a hot electric evening, and gardeners have long
been aware that mushroom spawn is most prolific in stormy weather.
The irritability of the tissues of plants which renders them liable to
be acted upon by external causes, has occupied the attention of many
celebrated botanists. From experiments by Professor Ferdinand Cohn and
his pupil M. Krabsch upon the irritability of the stamens in the florets
on the discs of composite flowers, more especially the Centaureas, they
have come to the conclusion that susceptibility to the excitement of
light, as well as to that of mechanical and probably electrical impulse,
is possessed by all young vigorous tissues, and upon comparing the
phenomena of these with those of animal irritability, they further
conclude that the faculty of responding to external irritation by
internal movements and change of form, belongs to cells, and holds good
in the vegetable as in the animal kingdom. To be irritable, to change
its normal form as a vessel of excitation, and to revert to the normal
form after a while by its internal elasticity, are characteristics of
the living cell. In plants these properties are met with only when the
vital processes are in full activity, and therefore are particularly
noticed during the period of flowering, when the processes are at the
maximum. And it may be remarked that the stamens, in which irritability
is most frequently noticed, are the only organs in which an elevation of
temperature measurable by the thermometer occurs, although a certain
degree of heat is generated in all plant cells by the chemical process
going on within them. It is to be supposed that irritable properties
belong to all parts of plants, but that they exist in an intensified
degree, and for a certain epoch, in those parts where their results
arrest attention, as in the stamens of the Centaurea, berberry, cactus,
Cistus, nettle, &c., and in the anthers of the Stylideæ, the leaves of
Dionæa muscipula, and many others, all of which are more or less
affected by the external action of mechanical force and electricity; for
it is scarcely possible that plants should not be under the influence of
atmospheric electricity, since every shower of rain forms a perfect
conductor between the clouds and the earth. The motion does not always
immediately follow the excitement; plants often require to be rudely
shaken before the movement begins. M. Hofmeister has observed that all
young shoots and leaves become curved by mechanical shaking.
Light is the most universal and important exciting cause in the
vegetable world. The mouths of the stomata are opened by the influence
of light. The leaves, young shoots, and tendrils turn to the light; it
regulates the sleep of plants, as well as the diurnal motions of the
daisy and sunflower. The opening of blossoms and of folded leaves which
had been closed in sleep during the night, shows the susceptibility of
their tissues to the influence of light, an influence beautifully
exhibited by the orange-coloured Eschscholtzia, which shuts its golden
blossoms under every passing cloud.
All M. Cohn’s experiments prove that in the Mimosa pudica, which is
highly sensible to the action of light, heat, electricity, and touch,
‘the propagation of the external excitement, proceeds in the same mode
as in animals, and there is little doubt that the vascular tissue (which
contain spiral vessels) constitute the special bundles adapted for the
purpose, and that the phenomena of contractibility depend upon a
muscular tissue.’[82]
From Professor Franklin’s experiments it appears that ‘the motions
resulting from external causes are owing to vital contractibility, and
that they are governed by the same laws which regulate similar action in
the animal kingdom. Their energies vary with the vigour of the plant;
they are exhausted by over exercise, and require rest; and like animals
they are lulled and put to sleep by chloroform and narcotics.’
END OF THE FIRST VOLUME.
LONDON: PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
Footnote 1:
Professor Faraday.
Footnote 2:
Professor Helmholtz.
Footnote 3:
The address of the president, Sir William Armstrong, C.B., to the
British Association at Newcastle-on-Tyne, 26th August, 1863.
Footnote 4:
J. Tyndall, Esq., on Force.
Footnote 5:
The Law of Exchange was independently proved by Messrs. Tyndall,
Kirchhoff, Angström, and Balfour Stewart.
Footnote 6:
Tyndall on Heat.
Footnote 7:
Analogous to transparent media which receive their colour by stopping
or absorbing some of the colours of white light and transmitting
others.
Footnote 8:
‘Connection of the Physical Sciences.’
Footnote 9:
‘Connection of the Physical Sciences.’
Footnote 10:
They are called vacuum tubes, and are filled while open by putting one
end in communication with the vessel in which the gas is generated,
and the other end in communication with an air pump. As soon as the
atmospheric air is pumped out, the gas rushes in and fills the tube,
the communication with the vessel containing the gas is cut off by
fusing up that end of the tube, and as soon as the gas is sufficiently
rarefied the other end is fused up also. An electrical discharge that
will not pass through one inch of air, will pass through thirty or
forty inches in a vacuum tube.
Footnote 11:
‘Connection of the Physical Sciences.’
Footnote 12:
Lectures of much interest by Dr. William Odling in the _Chemical News_
of 1862.
Footnote 13:
M. H. Kopp.
Footnote 14:
Lectures by Dr. Crace Calvert on improvement and progress of calico
printing and dyeing since 1851.
Footnote 15:
This prediction, made in his Treatise on Light published in 1826, has
been completely fulfilled by the discovery of four new metals by
spectrum analysis.
Footnote 16:
Phil. Mag. vol. iv. 1834, p. 114.
Footnote 17:
‘On the Spectrum of the Electric Spark in Compound Gases,’ by Mr. J.
M. Seguin. Comptes Rendus.
Footnote 18:
The light of the electric lamp is produced by an apparatus which
successively makes and breaks an electric current, whereby the
terminal charcoal points become red-hot.
Footnote 19:
‘On the Means of Increasing the Intensity of Metallic Spectra.’ By Mr.
W. Crookes.
Footnote 20:
_The Chemical News and Journal of Physical Science_ for July 4, 1863.
Footnote 21:
Dr. W. A. Miller has shown that these invisible highly refrangible
rays exist in the vapours of all metals, and has obtained photographs
of their spectra (see Phil. Trans. 1862, p. 876), which correspond to
the spectra of fluorescence.
Footnote 22:
In γ Cassiopeiæ, Mr. Huggins has detected a second _bright_ line in
the red part of the spectrum. He has also found that these two bright
lines agree in position with the two brightest lines of the spectrum
of hydrogen, and may therefore be considered due to luminous hydrogen.
Footnote 23:
Sir John Herschel, who is of the highest authority with regard to the
nebulæ in both hemispheres.
Footnote 24:
Since this observation, Mr. Huggins discovered that two small comets
give an analogous spectrum.
Footnote 25:
From further observations Mr. Huggins is of opinion that the red
colour of this planet is not due to its atmosphere, but is peculiar to
certain parts of its surface.
Footnote 26:
‘Quarterly Journal of Science,’ April 1864.
Footnote 27:
Jan. 7, 1865.
Footnote 28:
On the latest discoveries concerning the sun’s surface, by Balfour
Stewart, Esq., in the ‘Chemical News and Journal of Physical Science’
of April 1865.
Footnote 29:
According to Payen. Like starch, it is stained blue by iodine.
Footnote 30:
‘On the Functions of the Nitrogenous Matter of Plants.’ By M. L.
Garreau, ‘Annales des Sciences naturelles,’ t. xiii. 1860.
Footnote 31:
‘Remarks on the Vessels of the Latex, the Vasa Propria, and the
Receptacles of the elaborated Juices of Plants.’ By M. Lestiboudois,
‘Comptes rendus,’ 1863.
Footnote 32:
The author is indebted throughout many parts of this section to the
excellent work of Dr. Carpenter, and to the ‘Cryptogamic Botany’ of
the Rev. M. J. Berkeley, from which also many of the cuts are derived.
Footnote 33:
Mr. Berkeley’s ‘Cryptogamic Botany.’
Footnote 34:
‘Dr. Carpenter’s Microscope.’
Footnote 35:
These motions were discovered, and are described by Dr. Harvey, in his
‘Manual of British Marine Algæ.’
Footnote 36:
‘Cryptogamic Botany.’ By the Rev. M. J. Berkeley.
Footnote 37:
‘Cryptogamic Botany.’ By the Rev. M. J. Berkeley.
Footnote 38:
‘The Microscope.’ By Dr. Carpenter.
Footnote 39:
‘Cryptogamic Botany.’ By the Rev. M. J. Berkeley.
Footnote 40:
‘British Seaweeds.’ By Mrs. Alfred Gatty.
Footnote 41:
‘Flora Italica Crypta.’
Footnote 42:
Mr. Berkeley’s ‘Cryptogamic Botany.’
Footnote 43:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 44:
Mrs. Gatty’s ‘British Sea Weeds.’
Footnote 45:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 46:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 47:
‘Voyage of the Adventurer and Beagle,’ by Mr. Darwin.
Footnote 48:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 49:
In the Fuci each kind of fruit is discharged on the surface of the
receptacle before fructification.
Footnote 50:
Hooker’s ‘Flora Antarctica.’
Footnote 51:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 52:
Berkeley’s ‘Introduction to Cryptogamic Botany.’
Footnote 53:
‘Des Myxomycetes,’ par M. Antoine de Bary; et Mémoires par MM. Tulasne
et Hermann Hoffman, ‘Annales des Sciences Naturalles,’ 4me séries.
Footnote 54:
‘Introduction to Cryptogamic Botany,’ by the Rev. M. J. Berkeley.
Footnote 55:
‘Sur des Isaria et Sphæria Entomogens,’ par MM. L. et H. Tulasne, de
l’Institut, ‘Annales des Sciences Naturelles,’ 4me série, 1857.
Footnote 56:
‘On the Geographical Distribution of Fungi,’ by M. E. P. Fries, of
Upsala, Sweden.
Footnote 57:
Memoir by M. Hermann Hoffmann, upon Fermentation, in the ‘Ann. des
Sciences Naturelles,’ 4me série, 1860.
Footnote 58:
‘The Microscope,’ by Dr. Carpenter.
Footnote 59:
M. Hoffmann.
Footnote 60:
‘Comptes rendus,’ Nov. 12, 1860.
Footnote 61:
‘Geographical Distribution of Fungi,’ by M. Fries.
Footnote 62:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 63:
Berkeley’s ‘Introduction to Cryptogamic Botany.’
Footnote 64:
Berkeley’s ‘Introduction to Cryptogamic Botany.’
Footnote 65:
Dr. Carpenter’s ‘Microscope.’
Footnote 66:
Dr. Carpenter’s ‘Microscope’
Footnote 67:
‘Anatomy and Physiology of the Vegetable Cell,’ by M. Hugo von Mohl.
Footnote 68:
M. von Mohl, on the ‘Vegetable Cell.’
Footnote 69:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 70:
Dr. J. D. Hooker on the ‘Distribution of Ferns,’ in Berkeley’s
‘Cryptogamic Botany.’
Footnote 71:
‘Index Filicum,’ by Thomas Moore, F.L.S.
Footnote 72:
Moore, in ‘Treasury of Botany.’
Footnote 73:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 74:
Charles Johnson, Esq.
Footnote 75:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 76:
Berkeley’s ‘Cryptogamic Botany.’
Footnote 77:
Inflorescence and fructification are fully explained, and applied to a
vast number of plants, in ‘Structural and Physiological Botany,’ by
Arthur Henfrey, Esq.,—a work of great research and merit.
Footnote 78:
Messrs. Hooker, Darwin, and Brongniart.
Footnote 79:
Professor Matteucci.
Footnote 80:
Dr. Carpenter, ‘Microscope.’
Footnote 81:
Annals of Natural History for 1863.
Footnote 82:
Annals and Magazines of Natural History for 1863.
Transcriber's Note
This book uses inconsistent spelling and hyphenation, which were
retained in the ebook version. Some corrections have been made to the
text, including correcting the errata and normalizing punctuation.
Further corrections are noted below:
p. 29 heat indedendent matter -> heat independent matter
p. 166 next the visual center-> next to the visual center
p. 179 in dependently -> independently
p. 251 isr eproduced -> is reproduced
p. 343 ig 56 -> Fig 56
p. 351 the fonds are articuled -> the fonds are articulated
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