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Title: An Elementary Text-book of the Microscope - including a description of the methods of preparing and - mounting objects, etc.
Author: Griffith, John William
Language: English
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                        [Illustration: Plate I.

                      _London, John Van Voorst._]



                                  AN

                         ELEMENTARY TEXT-BOOK

                                  OF

                            THE MICROSCOPE;

               INCLUDING A DESCRIPTION OF THE METHODS OF
                 PREPARING AND MOUNTING OBJECTS, ETC.

                                  BY

                  J. W. GRIFFITH, M.D., F.L.S., ETC.,

     MEMBER OF THE ROYAL COLLEGE OF PHYSICIANS; CONJOINT AUTHOR OF
                   THE MICROGRAPHIC DICTIONARY, ETC.

                     WITH TWELVE COLOURED PLATES,
                        CONTAINING 451 FIGURES.

                            [Illustration]

                                LONDON:

                   JOHN VAN VOORST, PATERNOSTER ROW.

                              MDCCCLXIV.

               [_The right of translation is reserved._]



PREFACE.


The object of this little work is to furnish an elementary course of
instruction in the use of the Microscope, and on its application to the
examination of the structure of plants and animals. Assuming that the
reader has had no previous acquaintance with the Microscope, or with the
study of natural history, I have attempted to render the descriptions of
the objects as simple as possible. At the same time, the technical terms
have been added and explained, in order gradually to render them
familiar to the reader, and thus facilitate the future study of larger
and more detailed works. The objects figured and described comprise the
principal structures and more minute forms of both the vegetable and the
animal kingdom, those having been selected which are common and readily
procurable.

A chapter has been given upon the optical principles on which the
action of the instrument depends (which will assist the reader to
understand the operation of its constituent parts), including a sketch
of the subject of polarized light. The order in which the subjects are
treated is scientific, and particular directions have been given for the
examination of the objects.

The small size of the work has necessitated the exclusion of figurative
descriptions, so that it is adapted rather for a worker than a reader;
at the same time, the matter forms a course, and must be taken as a
whole for the proper comprehension of the subjects. The technical terms
used are referred to in the Index, so as to furnish to some extent a
glossary of terms; and their derivation is given, to facilitate their
recollection. The figures, with very few exceptions, are drawn from
nature, and are coloured that the objects may be more easily recognized.
The magnifying powers under which they have been drawn are denoted by a
small number placed beneath each figure: and the particular attention of
the reader is requested to this point; otherwise the whole subject will
be utterly confused; so much does the appearance of objects vary under
different powers.

Directions are given for preparing and mounting objects, implying that
the reader will collect specimens for himself, which is to be strongly
recommended as the best method of acquiring a practical and useful
acquaintance with the objects. These will serve to furnish permanent
landmarks in the great ocean of structural forms, will probably recall
in after-years pleasant recollections of early excursions in search of
the beauties of nature, and, surely, deepen the conviction of the
existence of their All-wise Creator.

J. W. G.



CONTENTS.


CHAP.                                           PAGE

I. THE MICROSCOPE                                  1

II. THE MOUNTING OF OBJECTS                       10

III. VEGETABLE ELEMENTS AND TISSUES               19

IV. VEGETABLE ORGANS                              31

V. FERNS                                          49

VI. MOSSES                                        54

VII. ALGÆ                                         64

VIII. LICHENS                                     91

IX. FUNGI                                         96

X. ANIMAL ELEMENTS AND TISSUES                   113

XI. ARTICULATA                                   127

XII. RADIATA                                     153

XIII. PROTOZOA                                   155

XIV. OPTICAL PRINCIPLES                          167

PLATE I. [FRONTISPIECE.]

VEGETABLE TISSUES, &c.

Fig.

1. Leaf of Geranium: cells, chlorophyll, and intercellular passages.

2. Cells of Apple.

3. Starch-granules: _a_, of Wheat; _b_, of Arrowroot; _c_, of Potato;
_d_, of Oat; _e_, of Lentil; _f_, of Rice.

4. Cells of Potato, containing Starch.

5. Garden Rhubarb-stalk: _a_, raphides; _b_, reticulated duct; _c_,
spiral vessel; _d_, woody fibre; _e_, annular vessel.

6. Wood-cells from stem of Chrysanthemum.

7. Deal, transverse section: _a_, glandular tissue; _b_, woody fibre.

8. Deal, longitudinal (radial) section of glandular tissue.

9. Deal, longitudinal section of woody fibre.

10. Deal, tangential section.

11. Holly, wood of: _a_, porous cells; _b_, _d_, _e_, wood-cells; _c_,
dotted ducts.

12. Hairs, vegetable: _a_, _b_, of Groundsel; _c_, of London Pride; _d_,
_e_, of Geranium; _f_, of Chrysanthemum.

13. Epidermis of Geranium-leaf.

14. Style of Crocus, with pollen-granule and-tube.

15. Pollen-grain of Crocus, with pollen-tube.

16. Pollen of Primrose.

17. Pollen of Sunflower.

18. Pollen of _Convolvulus major_.

19. Caraway-seed.

20. Needle-point.

21. Sting of Nettle.

22. Hair of Spiderwort.

23. Hair of Spiderwort, single cell.

24. Epidermis of Geranium-petal.

25. Petal of Chickweed.

26. Sepal of Chickweed.

27. Seed of Poppy.

28. Epidermis of _Deutzia_.

29. Seed of Mignonette.

30. Pollen of Chickweed, dry.

31. Pollen of Chickweed, in water.

32. Flower of Chickweed.

33. Epidermis of petal of Chickweed.

34. Hairs of calyx of Chickweed.

35. Hairs of seed of _Collomia_.

36. Stem of Dicotyledon, section of.

37. Stem of Monocotyledon, section of.

38. Seed of Shepherd’s Purse, transverse section.

39. Stamen of Chickweed.

40. Stigma of Chickweed.

41. Ovary of Chickweed.

42. Leaf of Chickweed.

43. Seed of Wallflower, section of.

44. Seed of Wallflower, radicle and cotyledons.

45. Embryo-sac of Chickweed.

46. Embryo-sac of Chickweed.

47. Embryo-sac of Chickweed.

48. Wheat, cotyledon and leaves of, section.

49. Mustard-seed, cotyledons and radicle.

50. Mustard-seed, transverse section.

51. Chickweed, seed of.

52. Seed of _Eccremocarpus scaber_.

53. Grain of Wheat: _a_, cotyledon; _b_, embryo; _c_, radicle; _d_,
albumen.

54. Ovule of Wallflower.

55. Cotyledons of Chickweed.

56. Plum-stone, section of.



                                   A

                               TEXT-BOOK

                                  OF

                            THE MICROSCOPE.



CHAPTER I.

THE MICROSCOPE.


The microscope (from μικρὁς, little, and σκοπέω, to see), so called
because it enables us to see objects which are too small to be seen with
the naked eye, consists of several parts, each of which has its special
use. As the proper management of these is of great importance in the
successful application of the instrument to minute investigations, we
shall commence with the consideration of their names and uses, including
those of the more important pieces of accessory apparatus.

_Microscope._--The foot of the microscope is that part which supports
the instrument upon the table; it is connected above with the stand, of
which it is often considered a part. The stand sometimes consists of a
single rod or pillar; but in the best microscopes it is composed of two
upright plates, between which, at the upper part, the rest of the
microscope swings stiffly upon an axle. Arising from this axle,
indirectly through the medium of parts which require no special mention,
is an arm, to which the body is fixed. The body is moveable up and down
by one or two large milled heads, connected with a grooved rod or
pinion, which works in the teeth of a rack fixed to the back of the
body, or of the arm which supports the body. The large milled heads form
the “coarse movement,” as it is called.

On the top of the arm, or on the front and lower part of the body of the
microscope, is placed the “fine movement,” consisting of a small milled
head, with a fine screw, for moving the body through very small
distances.

Next is the “stage,” or flat plate, upon which the objects to be viewed
are placed. This is often so arranged that, by turning two milled heads,
the object can be moved backwards and forwards, or from side to side; it
is then a “moveable stage.”

The eye-piece slides into the upper end of the body; and the
object-glass screws into its lower end.

Beneath the stage is the mirror, which reflects the light through the
object, the object-glass, and eye-piece to the eye.

_Object-glasses._--The object-glasses are the most valuable parts of the
instrument. There are generally three or more of them; and, by means of
an “adapter,” any object-glass can be made to fit any microscope. Great
care is required in their use, especially to avoid scratching the lower
surface of the glass, which is sometimes accidentally done by pressing
the surface against any hard body, or allowing such a body to fall upon
it. When not in use, the object-glasses should either be put away in the
brass boxes or covered with a small bell-glass, to prevent their
receiving any injury.

The object-glasses possess various magnifying powers, according to the
distance at which they require to be placed from the object for distinct
vision: this is not, however, absolutely correct, yet may serve as a
general expression. Thus we have a 1-inch, ½-inch, ¼-inch object-glass,
&c. The object-glasses, for brevity, are often called powers.

As a beginner may at first have some difficulty in distinguishing a high
from a low power, it may be remarked that the size of the lower glass is
larger the lower the power: but, in the case of the better
object-glasses, the focal distance is engraved on the box in which the
object-glass is packed when put away.

As the coarse movement raises or depresses the body and object-glass
through comparatively large distances, it must be used only with the
lower object-glasses, _i. e._ those of low or little magnifying power,
as the 2-inch, 1-inch, or ½-inch, or to bring the object-glass near the
focal distance with the higher powers; whereas the fine movement serves
to adjust the higher powers, as the ¼-inch, &c.

If the object-glasses should become soiled on the lower face, this
should be wiped very gently with an old silk handkerchief or piece of
very soft wash-leather, previously shaken to displace dust. The same
method will answer to cleanse the upper surface of the eye-piece.

Great care must be taken that a slide which has been warmed in any
experiment be not placed near the object-glass until quite cold.

_Mirror._--The mirror has sometimes one silvered face only, at others
two--one flat, the other concave. The flat surface is used to reflect
the light upon the object when the light is too great with the concave
surface.

Beneath the stage, in most microscopes, is a circular moveable
“diaphragm,” perforated with holes of various sizes, to allow more or
less of the light reflected by the mirror to pass through, as may be
required.

When opake objects are viewed, the mirror should be turned aside, so as
not to reflect any light through the stage.

_Eye-pieces._--With all microscopes, two or more eye-pieces are
supplied. These possess different magnifying powers, and are lettered or
numbered accordingly; the lowest power with the earliest letters of the
alphabet, or with the smallest numbers, thus: A, B, C; or 1, 2, 3, &c.

_Forceps._--These are fine pincers, for holding minute bodies to be
viewed as opake objects. In use, they are inserted by a stem connected
with a joint, in an aperture, generally in the stage; and are moveable
in all directions.

_Live-box._--This is a brass slide, perforated in the middle, to the
aperture in which is soldered a short piece of brass tube, closed at the
top with a circular plate of thin glass. A rather wider and longer piece
of brass tube slides over the former; this is also closed at the top by
a thin glass plate, so as to allow of an object being confined or
compressed between the two glass plates. It is used for examining living
objects in water.

_Knife, &c._--For cutting slices or sections of objects, a very sharp
knife with a thin back will be found useful; or a razor may be used for
the same purpose. And for picking minute objects to pieces, or
dissection, fine needles, cut off short with pliers, the blunt ends
being thrust into hair-pencil sticks, will be requisite.

A pair of fine surgical forceps will also be required, for taking up
minute objects. These should be without teeth, and the spring-action so
weak that the points can be very easily approximated.

_Dipping-tubes._--For removing minute objects from water, two or three
narrow glass tubes, of different lengths, are very useful. These are
called “dipping tubes,” and are used thus:--the tube being held upright
between the second finger and the thumb, the fore finger is placed at
the top of the tube to close it; the tube is then put into the water
until the lower end is close to the object, when, on suddenly removing
the fore finger, the water will rise in the tube, carrying the object
with it. The fore finger is then again applied to the tube, and, as thus
held, the water will not run out. The tube is then held over a
watch-glass, or a slide, upon which the water and object will fall on
removing the fore finger.

A small glass spirit-lamp will be found very useful. The spirit for
burning should be methylated alcohol, or wood-naphtha. As these spirits
are inflammable, great care should be taken to keep the stock-bottle
away from a candle or other flame, when filling the lamp.

_Achromatic condenser._--A very important piece of apparatus, when high
powers are used, is the achromatic condenser; it is not, however,
usually supplied with the cheaper microscopes. It consists of a brass
fitting, placed beneath the stage, into which an object-glass is
screwed, in an inverted position, _i. e._ the small end of the
object-glass being placed uppermost. It serves to condense the light to
a focus upon the object, so as to illuminate it more brightly; and as it
can be elevated or depressed by a milled head and rack-work, the object
can be viewed by either converging or diverging rays.

_Simple microscope._--For examining the larger kinds of objects, and for
dissection, a simple microscope is very useful. This consists of a
stand, a stage, and an arm supporting a simple lens or combination of
lenses, but without the body of the compound microscope (as the ordinary
microscope is distinctively called). For most purposes, common
plano-convex or doubly convex lenses are sufficient to form the
object-glasses of a simple microscope. With the best microscopes, an
“erector,” or tube containing a pair of lenses, fitted within the body,
renders the compound microscope capable of answering most of the
purposes of a simple microscope.

_Polariscope._--An expensive but interesting and useful addition to a
microscope is a polarizing apparatus, or polariscope. This consists of a
Nicol’s prism, or a plate of tourmaline, placed beneath the stage, and
another in the body of the microscope or above the eye-piece; both in
brass fittings. The former is called the polarizer, and the latter the
analyzer.

_Rotating disk._--Another most useful piece of apparatus, for moving
opake objects whilst under the microscope, in all directions, is Smith
and Beck’s “rotating disk.”

_Slides._--The slides upon which objects, especially those to be viewed
as transparent objects, are to be placed, should be made of crown or
plate glass. They are usually 3 inches long, and 1 inch wide; but I
prefer them 2½ inches long, and 1 inch wide, simply because they take up
less room in a cabinet, and because they do not project beyond the stage
on either side. They should not be more than 1/20th of an inch thick,
and as colourless and clear as possible. The edges should be ground or
filed, to prevent their scratching the stage.

_Covers._--The covers are square pieces of very thin glass, less in
breadth than the slides, so as not to reach their margins; and of
various thicknesses, the thicker and stronger being used to cover large
objects for examination under the lower powers, and the thinner serving
to cover very delicate objects requiring the higher powers.

_Side condenser._--For illuminating opake objects, a large plano-convex
or doubly convex “bull’s-eye” lens, or side condenser, is used; this is
fixed to an arm, which slides on a stand, so as to be capable of being
raised or lowered to a suitable height. This is placed between the
source of light and the stage, and at such a distance from the latter
that the light may be brought to a focus upon the object. Sometimes a
“Lieberkuhn” or concave silver reflector is used for this purpose.

These are the most important pieces of apparatus required in examining
microscopic bodies. But the beginner will do well, if he have the
achromatic condenser and the polarizing apparatus, to lay these aside
until he has had considerable practice in examining objects simply with
the mirror and the lower powers.

_General method of observation._--In the ordinary use of the microscope,
the object to be examined is laid upon the middle of a slide, which is
placed upon the stage. The object is then brought under the centre of
the object-glass, the mirror inclined half towards the light and half
towards the object, until the object is seen to be illuminated, when,
upon looking through the eye-piece and adjusting the coarse and fine
movements, the object as it comes into focus will be seen, as it were,
drawn upon a white disk, which is called the “field.”

When the object is wet, it cannot be viewed without the application of a
cover, because the water evaporates and condenses upon the under surface
of the object-glass.

To avoid the danger of injuring the object-glass, or crushing the object
by lowering the body and object-glass too much in adjustment to focus,
the best plan is to lower the body by means of the coarse movement until
the object-glass appears near the object to the eye placed on one side
of the stage, and then to apply the eye to the eye-piece, and turn the
milled head so as to raise the body and object-glass until the object is
brought into focus.

In the examination of an object, it is best to begin with a low power,
so as to obtain a view of the general arrangement of its parts, and
then to apply the higher eye-pieces and powers, so that the more minute
structural details may be observed.

_Illumination._--The illumination or proper management of the light in
using the microscope is of very great importance. The best light,
especially with the low powers, is daylight, particularly that reflected
from white clouds; this is least injurious to the eyes. But as daylight
cannot always be used for microscopic investigations, and as cloud-light
is insufficient with the higher powers, some kind of artificial light
must be supplied. That mostly used is the light of a reading oil lamp or
of a gas-burner, a candle being quite useless, on account of the
flickering of the flame with the slightest draught. A moderator lamp has
the defect of too great height; otherwise this would be better than any
other oil lamp. A powerful and excellent light for the highest powers is
afforded by a short paraffine-oil lamp.

As intently looking at strongly illuminated objects is injurious to the
sight, the amount of light allowed to pass the diaphragm should be no
more than is agreeable, and sufficient to show the object distinctly.

In using the higher powers, the field is much less bright with the use
of the same light than in the case of the lower powers; and difficulty
is often found in obtaining sufficient light for the distinct vision of
the object. The achromatic condenser is of most important service here;
but it is sometimes requisite, even when this is used, to condense the
light upon the mirror by a shallow bull’s-eye; or a large common
metallic reflector may be used for the same purpose.

The most convenient manner of proceeding in regard to the use of the
individual eyes is to apply the left eye to the eye-piece, so that the
right eye may be used in finding the stage-movements, or in moving the
slide, without removing the eye from the eye-piece. If this arrangement
be adopted, the light should be placed towards the left-hand side of
the microscope. But the best way to avoid injuring the sight would be to
use both eyes for viewing the objects in turn, although most microscopic
observers make use of one eye only for this purpose.

The structure of many transparent objects can be best seen when the
mirror is turned more or less obliquely to one side, so as to view them
by oblique light, as it is called: we shall refer to this point
hereafter.

As a rule, objects are best seen by transmitted light, or as transparent
objects, although it is well to examine objects under both kinds of
illumination, _i. e._ by transmitted and reflected light.

If during the use of the microscope, after removing the eye from the
instrument, the impression of the light remains perceptible to the
sight, the light used has been too strong, or its action too long
continued; and the instrument should be at once laid aside for a time.



CHAPTER II.

THE MOUNTING OF OBJECTS.


The mounting or “putting up” of microscopic objects signifies their
preparation in such way that they may be preserved for future reference
and observation.

As a general rule, objects should be mounted in that manner by which
their structure is best and most clearly shown; but in certain instances
the objects are mounted so as to make their structure difficult of
detection, that they may form test-objects of the power and quality of
the microscope.

Some objects require to be mounted in the dry state, while others are
best mounted in liquid; some again as opake, others as transparent
objects: these must be considered separately.

_Dry opake objects_ were formerly mounted by gumming them upon small
coin-shaped pieces or disks of cork, blackened upon the surface with a
mixture of fine lamp-black and thin warm size, laid on with a
hair-pencil. They were kept in a drawer, to the bottom of which a sheet
of cork was glued, the disk being transfixed by the pin, so that the
free or projecting pointed end of the pin could be thrust into the
sheet-cork. This plan may still be adopted in the case of common
objects, as seeds, &c.; but it is objectionable, on account of the
facility with which the bare objects are knocked off or injured by dust.

Hence dry opake objects are usually mounted in such manner as to be
enclosed in a cell, the sides being formed by a ring of glass-tube or
cork, or a square piece of leather, cardboard, or paper, with a hole cut
or punched out of the middle. The glass rings are best; but as they are
expensive, some of the other substances are generally used. The size
and thickness of the material from which the rings are made must
obviously vary according to the size and depth of the object. The rings
are cemented to the middle of ordinary slides; and it is best to keep a
number of them ready prepared. The cementing material must vary
according to the nature of the ring used. If this consists of glass,
Canada-balsam or marine glue is best. In using the former, the ring is
gently heated over the flame of the spirit-lamp, and a thin layer of the
balsam applied to its upper or under surface, by means of an iron wire
with a little balsam on its end; it is next warmed over the spirit-lamp,
so that the surface is entirely and evenly coated. A clean slide is then
slightly heated, the ring laid upon it, and gentle pressure is used to
squeeze out the excess of balsam; and the slide is kept at a gentle
heat, until on cooling the balsam becomes so hard as not to be indented
with the finger-nail. Marine glue is applied in the same way as the
balsam, except that prolonged heat is not required to harden it, for it
becomes hard on cooling. The balsam may also be replaced by black japan
or asphalte.

The pieces of cork, leather, or paper are best fastened to the slides
with solution of shellac or sealing-wax in methylated alcohol, or with
white hard varnish.

When the ring or piece has been firmly fixed to the slide by either of
the above cementing materials, so as to form the sides of the cell, the
bottom is to be covered with a piece of black paper, cut to fit it
exactly, and fastened to the surface of the slide with a little gum, or
of either of the above varnishes. As soon as this is thoroughly dry, the
upper surface of the cell-wall, whether of glass or cork, &c., is thinly
covered with varnish, and a clean thin-glass cover laid upon it, and
very lightly pressed; the object is then permanently preserved.

The main points to be observed are, that the object and varnish are
completely dry, and that the cell is thoroughly closed. If the latter be
not the case, more varnish must be applied to any little openings which
may have been left; and it is better to apply the varnish in very small
quantities at a time, the application being renewed as soon as the
previous layer is quite dry.

_Dry transparent objects_ are usually small and delicate; for, unless
they are so, their structure cannot be well seen. In mounting these, a
square piece of note-paper or tracing-paper, with the centre cut out,
may be fastened to a clean slide with a little paste, gum, or shellac
varnish. When this is thoroughly dry, the object is placed in the vacant
space, a clean dry cover laid on, and the varnish applied by means of a
hair-pencil to the edges in very small quantities. This will run in
between the under surface of the edges of the cover and the upper
surface of the paper, and when dry will cement the two together.

Supposing that the object is so delicate that it cannot be removed from
the surface of a slide, if it will not be injured by heat, a good plan
is to draw a square or circle around the object with a little black
japan, then to heat the slide gradually until the japan is not indented
with the finger-nail when cold. A clean slide is then laid upon the ring
of japan, the whole again gently warmed, until the varnish is softened,
and the cover lightly pressed so as to be in contact all round with the
varnish. The slide must then be rapidly cooled, by being laid upon a
piece of metal, which prevents the varnish from running in so as to
spoil the object.

Many dry transparent objects can be preserved by mounting in Canada
balsam. This is the best process for mounting objects in general; but
only those can be so preserved which are not injured by drying, and
which are not rendered too transparent by the balsam. If the object to
be mounted in balsam be small, it is thoroughly dried, and then a drop
of oil of turpentine added to it upon a slide; the slide is then gently
warmed, which causes the turpentine to evaporate. When this has nearly
all evaporated, a drop of balsam is allowed to fall upon the object from
the end of a wire held at a distance above the flame of a spirit-lamp. A
warmed cover is next laid upon the balsam, and gentle pressure applied
until the cover is sufficiently depressed. The slide is then kept at a
gentle heat until the balsam is quite hard when cold. The superfluous
portions may be removed with the point of a knife, and any residues
cleaned off with turpentine or a little benzole on a cloth.

Another way consists in laying a cover upon the dry object on a slide,
adding a drop of turpentine, and warming the whole over a spirit-lamp
until all air-bubbles are displaced, then continuing the application of
the heat until most of the turpentine has evaporated. A drop or more of
the balsam may next be applied to the edge of the cover, when it will
run in and mix with the turpentine. The whole is then gently heated
until the balsam is hard when cold, more balsam being added if
necessary, to replace the turpentine which has evaporated.

When the objects are large, they should be pressed as flat as possible
without injury between two slides, being retained until dry by enclosure
between the prongs of an American clothes-peg; or the slides may be
fastened at the ends by sealing-wax. When perfectly dry, the object
should be immersed in turpentine, kept in a common gallipot, until all
the air-bubbles have been entirely displaced, and the object appears
very transparent. It is then removed from the turpentine with forceps,
drained, laid upon a slide, and melted balsam dropped upon it until it
is quite covered. A clean dry slide is then laid upon its surface, and
the two slides gently pressed together, the two slides fixed at the ends
by sealing-wax, and the whole allowed to cool and dry. If requisite,
more balsam is added to fill up any vacuities. When the balsam has
become hard, the excess is cleaned away with a knife and turpentine, and
the object is permanently mounted.

If the object should be spoiled by the presence of air-bubbles, the
slides and object should be immersed in turpentine or methylated
alcohol, until the whole of the balsam is dissolved; the remounting may
then be proceeded with as at first. If the slides have been immersed in
the alcohol (which is the quickest method), the object must be soaked in
turpentine before the balsam is reapplied.

If, after an object has been mounted in balsam, on applying heat,
bubbles resembling air-bubbles should be formed, the object must not be
considered as spoiled; for these are merely bubbles of the vapour of
turpentine, and will disappear spontaneously after a little time.

A quick way of mounting in balsam is to drop the melted balsam at once
upon the dried object; but as air-bubbles are very apt to be produced in
this way, the beginner had better previously apply the turpentine.

As balsam is very viscid, and adheres firmly to everything with which it
comes in contact, some care is required in its use. Young microscopists
very generally manage to soil the microscope, tables, chairs, papers,
books, and even their clothes with it. It may be easily cleaned off,
however, with turpentine or benzole.

_Moist objects_ are best preserved, whenever practicable, in glycerine.
There are, however, two important objections to its use: one is, that it
makes objects very transparent; the other is, that it often wrinkles and
distorts them, by withdrawing their watery contents. Hence only those
objects can be preserved in glycerine, which are not too transparent,
and which are sufficiently firm to resist the tendency to collapse.

When the objects are tolerably flat, and sufficiently firm to bear the
pressure of the cover, they may be mounted by adding a small quantity of
glycerine to them lying on a slide; the cover is then applied, and a
little of the cement mentioned below applied warm with a hair-pencil
around the edges of the cover to fasten it to the slide. Care is
required that the glycerine applied be no more than sufficient; for
wherever it has touched the cover or the slide, the cement will not
adhere. Superfluous portions may be sucked up with a piece of clean
moist sponge or a corner of blotting-paper.

When it is required to mount a large number of objects in a short time,
the cement need only be applied to two opposite sides of the cover,
leaving the other two sides open.

When the objects require to be protected from the pressure of the cover,
the sides of a cell must be made with the cement or black japan upon the
slide before the cover is applied, a further quantity being used to
close the cell as usual.

A very strong solution of chloride of calcium may be used for the same
purposes and in the same way as the glycerine. It has the advantage of
not making the object so transparent; but it has the disadvantage of
crystallizing slightly in a dry atmosphere. In most cases, I prefer it
to glycerine.

A large number of interesting objects cannot, however, be preserved in
either glycerine or chloride of calcium, without their value being
impaired by the cause mentioned above. Many kinds of liquid have been
recommended for preserving these, all agreeing mainly in being
inefficient. The objection to them is, that they are evaporable; and
after the object has been mounted for some time, the liquid creeps
between the cement and the slide or cover, at some spot, and evaporates;
and if the cement be not quite hard, the inner and more liquid portion
of it runs into the cell, and spoils the object. A solution, containing
a grain of salt and a grain of alum to the ounce of distilled water, is
as good as any other; or simply distilled water in which a piece of
camphor has been kept. In use, the cell is first formed by making a
circle or outline square on the slide with black japan, and heating this
carefully until it becomes solid when cold. The object is then laid in
the cell, the liquid added, and the cover applied, any excess being
removed with blotting-paper. The cell is to be closed with old black
japan or gold-size, applied round the margins of the cover with a hair
pencil. A second and a third layer of the varnish may be applied upon
the first, when it has become hard outside. Although black japan and
gold-size are generally used for the cement, I prefer that mentioned
below.

When large preparations are mounted in liquid, the cell-walls are either
formed of glass rings, or they are built up with four oblong pieces of
glass, cemented to the slide and to each other with marine glue. A very
good preservative liquid for large specimens is a solution of chloride
of zinc, in the proportion of 20 grains to the ounce of distilled water.
A mixture of spirit of wine and water, in the proportion of 1 part to 2,
or 1 to 4, is often used for the same purpose.

When preparations are mounted, the cement and the adjacent parts of the
slide and cover should be coated with a solution of sealing-wax in
spirit, which hardens the exterior of the cement.

The cement above alluded to is made by melting together 5 parts of
rosin, 2 parts of balsam, 1 part of bees’-wax, and 1 of red ochre. The
cement is best kept in a little metallic cup, and melted over a
spirit-lamp when used. It should be applied while hot, with a hair
pencil; and cools very quickly.

The preservative liquids should be kept in corked bottles, a hair pencil
being fixed into the under part of the cork; or, what is better, in
stoppered bottles, the stopper being prolonged to a point nearly
reaching the bottom of the bottle.

As soon as the preparations are mounted, they should be labelled, the
labels being kept ready gummed. The balsam should be kept in a capped
bottle, such as is used for holding solutions of gum, with an iron wire
for removing portions as required. By keeping, the balsam becomes
thicker; it may be thinned by the addition of oil of turpentine, and the
application of a gentle heat.

The black japan, &c., may be procured at any oil-shop; the glass rings,
cell-sides, covers, &c., from Mr. Norman, 178 City Road, or of the
microscope-makers.

Mounted objects should be kept in shallow drawers, and be laid flat--not
standing on edge.

_Magnifying power._--Before entering upon the consideration of the
objects themselves, a word or two must be said upon the magnifying
powers. In the plates of this work, the serial number of each figure is
expressed by large numerals placed above the objects, while the number
of times the object is magnified is indicated by small numerals placed
beneath. The latter must be understood to express the number of times
the drawing is larger than the object in one dimension. Thus,
considering fig. 13, Plate I. to be an inch in width (for it is really
somewhat less), being magnified 150 times in the direction of the width,
the object itself is about 1/150th of an inch in size; and it is
represented magnified 150 times linear, or 150 diameters, as it is
called.

A knowledge of the number of times the object is magnified is of the
greatest importance in making use of the drawings; for, without it, the
observer will be unable to apply such a magnifying power of the
microscope as will enable him to see the structural appearances figured
in the drawings.

The observer must also be acquainted with the magnifying powers of his
microscope with the various object-glasses and eye-pieces. These are
usually given when the instrument is purchased. Or they may be
determined approximatively thus:--An ivory scale, with 1/100th of an
inch engraved upon it, is placed on the stage, and viewed as an opake
object, both eyes being kept open; and the size of the image of one of
the gradations is measured with compasses, upon the stage as seen with
that eye which is not applied to the eye-piece. The number of 1/100ths
of an inch contained in the measure obtained with the compasses
represents the magnifying power. Thus, supposing the image of the
1/100th of an inch on the scale appears magnified to the length of 1
inch on the stage; the magnifying power is 100 diameters, or 100 times
linear. This proceeding is difficult to any one unaccustomed to the use
of the microscope, yet by practice it becomes very easy. Other methods,
which require the use of the camera lucida, are given in the
Micrographic Dictionary.



CHAPTER III.

VEGETABLE ELEMENTS AND TISSUES.


We may now enter upon the consideration of the microscopic structure of
objects, beginning with those which are derived from the vegetable
kingdom, as they are more easily procured and prepared for examination
than those belonging to the animal kingdom; moreover they are not so
transparent, and hence are more readily distinguished under the
microscope, which is of importance in the case of an unpractised
observer.

_Cells._--The elements of which all plants consist are cells. Cells, in
their simplest condition, are microscopic, rounded, colourless, closed
sacs or vesicles, resembling small bladders (Plate I. fig. 2), and
consist of a thin, transparent, colourless, vegetable skin or membrane
(_a_) called the cell-wall. The cells are well seen in a little of the
pulp of an apple (fig. 2), or in a section of almost any soft part of a
plant. A high power is usually required to show them distinctly, on
account of their minute size. The outline of the cells is seen to be
double, one line indicating the inner, the other the outer, surface of
the cell-wall, the space between the two lines corresponding to the
thickness of the cell-wall.

In the pulp of the apple, the cells are loosely connected, and so retain
their rounded form; but in most parts of plants, the cells become
crowded and squeezed together, from their ordinary or normal expansion
being limited in certain directions, so as mutually to alter each
other’s shapes. The sides then lose their originally rounded form and
outline, becoming more or less straight (Pl. I. figs. 1 & 4),--the
cells at the same time mostly adhering to each other, so as to be
separated with difficulty.

The forms thus produced are various and interesting, and have all
received names by which they are distinguished. They are described in
works on botany in two ways--according to the outline (which is the most
common, as this expresses the appearance usually presented in sections
and on the surfaces of vegetable structures), or according to the entire
or solid form, which it is often a difficult matter to determine.

_Cellular tissue._--Cells aggregated thus form a tissue, which is called
cellular tissue or _paren´chyma_ (παρά, among, and ἕγχυμα, poured
substance), because it fills up the interstices of the other tissues of
plants.

In technical descriptions, the cell-structure is often left out of
consideration; and bodies composed of parenchymatous tissue are
described as being reticulated or netted, because the united sides of
the cell-walls appear as a network covering the surface.

It must be understood that parenchymatous cells are such only as have
the three dimensions of solidity (viz. the length, breadth, and depth)
nearly equal.

_Intercellular passages._--The observer will not have examined many
sections of cellular tissue, without noticing certain irregular black
lines running between the cells, as in a piece of a Geranium-(_Pelargo´nium_-)
leaf (Pl. I. fig. 1). These lines arise from the existence of passages
between the cells, containing air; and they are called intercellular
passages. By gently warming a section containing them in water over a
spirit-lamp, or by moistening the section with a drop of spirit, the
passages will be filled up with the liquid, so as to become transparent.
When the intervals between the cells are larger and broader, they are
called _intercellular spaces_.

So far, cells have been considered simply in regard to their form, as
vesicles, either rounded or altered in shape by mutual pressure. We have
now to notice the matters contained within the cells, or the
cell-contents.

_Cell-contents._--In most cells, especially when young, a minute,
rounded, colourless body may be seen, either in the middle or on one
side, called the _nucleus_; this is very distinct in a cell of the pulp
of an apple (Pl. I. fig. 2 _b_). And within this nucleus is often to be
seen another smaller body, frequently appearing as a mere dot, called
the _nucle´olus_.

The nucleus is imbedded in a soft substance, which fills up the entire
cell (Pl. I. fig. 2 _c_); this is the pro´toplasm (πρῶτς, first, πλἁσμα,
formative substance). As it is very transparent, it is readily
overlooked; but it may usually be shown distinctly by adding a little
glycerine to the edge of the cover with a glass rod, when it contracts
and separates from the cell-walls, as in the lower cell of fig. 2. The
protoplasm in some cells is semisolid and of uniform consistence, while
in others it is liquid in the centre, the outer portion being somewhat
firmer and immediately in contact with the cell-wall. In the latter
case, it forms an inner cell to the cell-wall, and is called the
_primordial utricle_. The terms “protoplasm” and “primordial utricle”
are, however, used by some authors synonymously.

The protoplasm is the essential part of the cell, and it forms or
secretes the cell-wall upon its outer surface in the process of
formation of the cell considered as a whole. It is also of different
chemical composition from the cell-wall, being allied in this respect to
animal matter.

_Chlor´ophyll_ (χλωρὀς, green; φὐλλον, leaf).--On examining a section
of any green part of a plant, as the green substance of a
Geranium-(_Pelargonium_-) leaf, it will be seen that the green colour
does not arise from the whole substance being coloured, as appears to
be the case to the naked eye, but from the presence of little grains or
granules of a green colouring-matter in the protoplasm of the cells.
This green matter is called chlorophyll. If the cells be crushed, the
granules will escape, and can be examined in the separate state.
Chlorophyll is most abundant in those parts of plants which are exposed
to the light.

_Starch._--In many cells of plants, particularly those which have
attained their full growth, other granules, larger than those of
chlorophyll, and colourless, are met with; these are the
_starch-granules_ (Pl. I. fig. 3). They are usually rounded or oblong,
and exhibit on the surface a number of rings, one within the other, or
concentric, as it is called. In the centre of the innermost ring is a
black dot or streak, arising from the presence of a little pit or
furrow, and called the _hilum_.

The starch-grains may be readily seen within cells in a thin section of
a potato (Pl. I. fig. 4); here they are very numerous, and larger than
in most other plants. A separate grain is represented in fig. 3.

The appearance of rings in the separate grains arises from the
starch-granules being composed of numerous concentric coats or layers,
like those of an onion.

A very simple and striking method of determining whether any granule is
composed of starch or not, consists in adding to it, when placed in
water on a slide, a drop of solution of iodine. As soon as this touches
the granule, it assumes a beautiful purple colour, the depth of tint
depending upon the quantity of the iodine-solution; if this be very
considerable, the granule appears almost black. The section of potato
forms a very interesting object when moistened with the iodine-solution,
the starch-granules becoming beautifully coloured, whilst the cell-wall
remains colourless, and the protoplasm becomes yellow.

The form of the starch-granules differs in different plants, so that
the kind of plant from which starch has been derived may be
distinguished by attention to the size, form, and structure of its
starch-granules. Thus, the granules represented in Pl. I. fig. 3, which
it will be noticed are all drawn under the same power, are derived from
different plants,--_a_ being those of wheat-flour, in which the hilum is
obscure, and the rings faint; _b_ is a granule of West Indian arrowroot,
in which the hilum forms a transverse crack; _c_ is a granule of
potato-starch, in which the hilum is a dot, and the rings are very
distinct; _d_ represents the compound granules of the oat, the separate
granules being figured below; _e_ is a granule of lentil-starch, with
its long dark hilum and elegant oval concentric rings; and _f_
represents a compound and separate granule of rice-starch. It will be
noticed that the granules of oat-and rice-starch are angular, as it is
called.

The knowledge of the peculiar forms of the starch-granules is important
in a practical point of view, for it enables us to recognize them when
mixed as an adulteration with other substances, and also to distinguish
the different kinds of starch from each other. Thus table-mustard, as it
is called, is principally composed of the cheaper wheat-or pea-flour,
which is easily recognized by the structure of the starch-grains.
Arrowroot is considerably dearer than potato-starch; hence in trade the
latter is fraudulently sold for the former, the adulteration being
detected with difficulty by the eye, but easily under the microscope.
Again, rice is largely mixed with wheat-flour, as it makes inferior
flour into very white bread; and this may also be readily detected under
the microscope. The reader can now understand how valuable the
microscope is in detecting adulterations, with a knowledge of the
various forms and structures of substances, especially with the aid of a
few chemical tests.

Starch-grains are altered by boiling in water, becoming swollen and
often changed into curious forms, the rings becoming faint or
disappearing. If a piece of boiled potato be examined, the
starch-granules will seem to have vanished from the cells, which are
swollen and covered with an irregular kind of network. The network
consists of parts of the protoplasm situated in the interstices of the
starch-granules, and solidified or coagulated by the heat. On crushing
the cells by pressing upon the cover, the starch-granules will escape,
swollen and partly fused together; but they may easily be recognized as
consisting of starch by the iodine test.

The granules of “tous les mois” starch are particularly well adapted for
showing the concentric rings, the granules being about twice as large as
those of the potato.

Starch-granules are best examined in water; and a small quantity only of
the starch must be placed on the slide, if the structure of the granules
is to be seen clearly. They may be mounted in glycerine, although this
makes them very transparent.

To those who possess a polariscope, starch-granules are particularly
interesting, as they exhibit a black cross, and, with a plate of
selenite laid beneath the slide, a beautiful play of colours.

In addition to the starch and chlorophyll, the cells of plants contain
other matters, as gum, sugar, &c.; but as they are dissolved in the
cell-liquid, they are not visible. In the cells of certain plants,
however, spherical globules, with light centres and black outlines, will
be met with: these consist of oil.

_Raph´ides._--Lastly, occurring in the cells of plants, especially such
as are soft and juicy (succulent), will be found minute, hard,
colourless crystals, called _raphides_ (ῥαφὶς, a needle). These are most
frequently needle-like or acicular (_acus_, a needle), but sometimes
prismatic or rod-like with flat sides; they are also not unfrequently
grouped into little tufts. They may be readily found in a piece of the
stem of garden-rhubarb (Pl. I. fig. 5 _a_), or of the common balsam.

_Porous and spiral cells._--The walls of the cells of cellular tissue
are sometimes covered with little dots (Pl. I. fig. 11 _a_), or
slit-like markings; the cells are then called porous cells. A specimen
of them may be obtained from a section of the pith of the elder
(_Sambúcus nígra_).

Sometimes cells exhibit the appearance of a spiral line marking their
walls, as if a little bell-spring were coiled up in them (Pl. III. fig.
2 _a_). These are called spiral cells, or spiral fibrous cells, and the
tissue formed by them is called fibro-cellular tissue.

We now leave the cells of ordinary cellular tissue, to examine those in
which the dimension of length predominates, so that they form tubular
cells; and first of those required to possess strength and firmness,
combined with flexibility. These qualities are met with in the cells
constituting

_Woody tissue._--Of this there are two forms, called respectively
wood-cells and woody fibres.

The _wood-cells_ are moderately long, more or less tapering and
overlapping at the ends; and the cell-walls are thickened, so as to
possess considerable firmness. These cells are found in the wood of
stems, as in the white woody portion of an ash stick, that of a
lime-tree, the stem of a Chrysanthemum, &c. (Pl. I. fig. 6). They are
closely packed, and the tissue formed by their union is called
_prosen´chyma_ (πρὸς, close, ἔλχυμα, tissue).

In the other kind of woody tissue the cells are very long and slender,
strong, yet flexible, gradually tapering at the ends, where they overlap
each other; and they have thick walls, so that, when divided
transversely, the cavity appears almost filled up (Pl. I. figs. 5 _d_,
9, & 7 _b_). This tissue is called _woody fibre_ or _pleuren´chyma_
(πλευρἀ, rib, ἔγχυμα), from its strength.

The walls of the cells of woody tissue are often covered with dots,
either simple or with an inner dot (Pl. I. fig. 6 _b_, fig. 11 _b_), or
with streaks (Pl. I. fig. 6 _a_) or with a spiral fibre (fig. 11 _b_,
_c_), either alone or with dots also.

This tissue is of great importance in plants, from its strength and
flexibility; it forms a considerable part of the veins of leaves, the
inner bark (_liber_), and of the wood of the stems of trees. It is also
very useful to man: for it constitutes hemp, of which rope and string
are made; flax, of which linen is made; cocoanut fibre; bast, used by
gardeners for tying up plants, which is the inner bark of the lime; and
jute, which is the inner bark of an Indian lime-tree.

In the white woody part of the stems of trees belonging to the fir-order
(Conif´eræ), as a piece of deal or pine, which is mainly composed of
wood-(prosenchymatous) cells, the cells exhibit rows of minute circular
markings (Pl. I. fig. 10). These were formerly supposed to be solid
bodies or glands; hence the tissue is still sometimes called glandular.
Within the outer ring of each marking is an inner central dot, or
sometimes an oblique streak. The side view of the cells (Pl. I. fig. 8
_a_), which is seen in a tangential section, shows that the markings are
minute pits, each being opposite to one of an adjacent cell, and sunk
inwards towards the centre of the cell, the inner dot or streak being a
thinner portion of the cell-wall. This glandular tissue of the Coniferæ
is interesting as forming a test-object for the defining power of the
microscope, which should show the two rings sharply and free from
colour; the section of the wood should be examined as a dry transparent
object.

The difference between the woody fibre and the wood-cells of coniferous
wood may also be seen well in a piece of deal, as cut up for fire-wood.
If the end of a stick of this be examined with the naked eye, parts of
brown rings will be seen traversing the whiter portion of the wood.
These brown rings consist of woody fibre; the white portion of
wood-cells. On making a very thin transverse section, the interior of
the woody fibres is seen to be almost entirely filled up (Pl. I. fig. 7
_b_), while the cavity of the wood-cells is much more open (Pl. I. fig.
7 _a_); the former also contain globules of turpentine.

It must be remarked here that some botanical authors include both forms
of woody tissue under the term prosenchyma. But, as we shall see
hereafter, the form of the prosenchymatous cells being sometimes used as
a character for distinguishing the cells of leaves, to which the term
pleurenchymatous cells would be inapplicable, the above distinction will
be found important.

_Vessels, vascular tissue._--In the next form of tubular cells, these
are broader and softer than the cells of woody tissue, thin-walled, and
the ends pointed; and their walls exhibit spiral or ring-like markings,
or rows of dots (Pl. I. fig. 5 _c_, _e_, _b_), indicating the existence
of one or more spiral fibres or rings. When the vessels contain spiral
fibres, they are called spiral vessels (Pl. I. fig. 5 _c_); when they
contain ring-shaped portions of fibre, they are called annular
(_an´nulus_, a ring) vessels (Pl. I. fig. 5 _e_); and when the spaces
between the fibres are partly filled up, leaving only dots, the deposit
forming a kind of network, we have a reticulated (_réte_, a net) vessel
(Pl. I. fig. 5 _b_). This tissue can easily be obtained from a piece of
cooked rhubarb, the stem of a balsam, or from any soft-stemmed plant.
Vessels very frequently contain air.

_Ducts._--The tubular cells forming ducts (Pl. I. figs. 5 _b_, 11 _c_)
are large, more or less flattened or blunt at the ends (truncated); and
the cell-membrane at first closing the ends is often removed or
absorbed, so that the ducts communicate with each other, to allow of
the free passage of the sap through them. Their walls are invariably
covered with markings, consisting of either simple or bordered dots,
resembling those met with in the preceding forms of tissue. The ducts
are often easily recognizable with the naked eye, in transverse sections
of stems, by the large pores which they form in the wood. These may be
well seen in a section of a piece of cane. The tissue composed of dotted
ducts is called bothren´chyma (βὁθρος, pit); but the term is principally
applied to those ducts in which the dots are simple, _i. e._ have no
inner dot.

The structure of the above forms of tissue may be best understood in
relation to their development. It has been stated that the essential
part of the cell is the protoplasm. As cells grow older, new matter is
deposited by the protoplasm upon the inner surface of the cell-wall,
either to a small extent, evenly and uniformly, as in ordinary
parenchyma, or unevenly, in the form of spiral layers, forming fibres or
bands, leaving bare spaces, where the original cell-wall exists alone.
The matter thus deposited is called secondary deposit, the original
cell-wall being the primary deposit. When the secondary deposit covers
the interior of the cells except at certain slit-like spaces, we have
the appearance figured in Pl. I. fig. 6 _a_. When the deposit forms a
spiral fibre, or a series of rings, we have the spiral or annular vessel
or duct. And when the interspaces between the coils of a close spiral
fibre are filled up except at certain spots, we have the dotted or
reticulated vessel or duct.

In many instances, these deposits are present together: thus, sometimes
the outermost deposit leaves rounded pits or dots, while an inner
portion forms a spiral fibre (Pl. I. fig. 11 _b_); or one layer leaves
simple rounded pits, while the other leaves smaller slits or dots placed
opposite the former (Pl. I. fig. 6 _b_).

In some cells the cavity is almost entirely filled up by secondary
deposit, which leaves minute canals radiating from a small cavity in the
centre to the circumference, as seen in the transverse section of a
plum-stone (Pl. I. fig. 56); here the canals appear as dark lines. In
others, again, the secondary deposit forms several distinct layers,
leaving channels very similar to those of the last; an example is met
with in the gritty tissue of the pulp of a pear.

The obvious use of the pits and channels in the above tissues is to
preserve the permeability of the walls of the elements, which would be
destroyed if the walls were equally thickened all over.

_Cell-formation._--New cells are formed by the division of old or parent
cells. The actual process of division is difficult to observe, as it
requires prolonged observation; but cells are often met with in all
stages of division, of which some instances will be pointed out
hereafter. The cell-division takes place in two ways, either according
to the endogenous (ἔνδον, within, γεννἁω, to produce), or the exogenous
(ἔξω, outside, γεννἁω) method. The manner in which the division takes
place in the former is this:--At first a slight indentation or
constriction of the protoplasm occurs at the line of division; this
deepens until the protoplasm is completely divided. The freshly divided
surfaces then become coated with a new portion of cell-wall, so as to
make two or more new cells, which either remain in contact or separate
from each other. In some cases, the divided portions of protoplasm
become coated all over with new cell-walls.

In the exogenous process, a portion of the protoplasm protrudes from the
surface of the cell, carrying the cell-wall before it, so as to form a
little bud-like body; this is next cut off at its point of junction with
the parent-cell, and coated, as in the first case, with a new cell-wall,
so as to form a new cell.

_Preparation._--In examining the vegetable elements and tissues, very
thin sections must be made with a razor or thin sharp knife; these are
then to be placed in a little water on a slide. As the structures are
all minute, the distinctness with which they are seen will mainly depend
upon the proper thinness of the sections. When sections of dry stems are
to be examined, the black margins of the air-bubbles contained in the
cells often render the structure indistinct; these must therefore be
displaced by first wetting the tissue with methylated alcohol, and then
adding water to it in a watch-glass or on a slide; or the tissue may be
soaked in warm water for some hours: and this is mostly requisite in
preparing thin sections of dry tissues.

Attention must also be paid to the manner in which the section is made,
or the direction in which the portion of the plant is cut. There are
three important directions which must be distinguished, producing
transverse, longitudinal, and tangential sections. If the cuts be made
across the length of a stem, for instance, the section is called
transverse. If the cuts be made in the direction of the length, through
the centre, the section is longitudinal; and if the cuts are made in a
direction parallel to a line running down the centre of the stem, but
nearer its margin, it is a tangential section. It is scarcely necessary
to mention that an oblique section is intermediate between a transverse
and a longitudinal section.



CHAPTER IV.

VEGETABLE ORGANS.


The vegetable elements and tissues which have been described form,
either separately or by their combination in various ways, the organs of
plants. To these we shall now pass, and consider the structure of the
principal organs of the members of the vegetable kingdom.

_Leaves._--Leaves in their simplest form consist of a single sheet or
layer of parenchymatous cells or cellular tissue, an example of which
may be found in almost any moss (Pl. III. fig. 30). The granules of
chlorophyll will often be very distinctly seen in these cells. The first
addition to this form of leaf is a row or two of prosenchymatous cells
running longitudinally down the middle of the leaf, so as to form a
rudimentary vein or nerve. In other and more highly developed leaves,
the layers of cells are numerous, and traversed by bundles of
wood-cells, vessels, and ducts (fibro-vascular tissue), forming the
veins,--the entire surface being covered with a skin or membrane, called
the epidermis.

_Epider´mis_ (ἑπἱ, upon, δἑρμα, skin).--This membrane is composed of one
or more layers of colourless, closely packed cells (Pl. I. figs. 13 &
28), the colour it occasionally exhibits usually arising from some of
the underlying cells of the leaf being seen through it, or remaining
adherent to it when stripped from the leaf. It is easily separated, by
making a cut in a soft leaf, and peeling it off with a fine pair of
forceps, or by soaking a leaf for some time in water and then stripping
it off. It must be remarked that the epidermis covers not only the
leaves, but every part of the plant.

_Hairs._--Arising from the epidermis are the hairs of plants. These are
thread-like or filamentous prolongations of the epidermis beyond the
surface of the leaf (Pl. I. fig. 12), consisting of cells arranged end
to end. They are often branched, sometimes star-shaped (stellate) (fig.
28), and present great varieties in form, as shown in the figures, the
plants from which these were drawn being mentioned in the Description of
the Plates. Sometimes hairs terminate in a little head (Pl. I. figs. 12
_c_, _d_, _e_), the cell or cells composing which secrete a colouring or
a viscid substance; they are then termed glandular. The hairs of plants
are particularly interesting to the microscopic observer, not only on
account of their curious forms, but in connexion with the remarkable
phenomenon of the circulation of the cell-contents, or _rotation_, as it
is called, observable in them. This is difficult to be perceived by any
one unaccustomed to microscopic observation, because the particles by
which the motion of the cell-contents becomes evident are exceedingly
minute; but practice in the use of a high power will overcome this
difficulty. The hairs which exhibit the phenomenon best are those of the
American Spiderwort (_Tradescan´tia Virgin´ica_), which is to be found
in every garden. It may, perhaps be recognized thus:--The plant is about
a foot and a half high; the leaves are sword-shaped and channelled, and
the flowers are purple, in heads, and 1½ inch in diameter. The hairs are
attached to the sides of the stamens, towards the lower part or base.
The stamens should be carefully picked off with forceps, and placed on a
slide in a drop of water; the hairs should then be separated with the
mounted needles, and a cover applied. Under a low power, the hairs are
seen to be beaded or monil´iform (_moníle_, a necklace), and of a fine
purple colour (Pl. I. fig. 22). On applying a high power, as the
¼-inch, the individual cells will come distinctly into view, and the
nucleus will be seen very clearly as a roundish granular mass (Pl. I.
fig. 23 _a_). On carefully examining the cell-contents, delicate lines
will be observed radiating irregularly from the nucleus, some passing to
the top of the cells, while others run towards its base, as in the
figure; and on very close inspection, the portions of protoplasm of
which these lines consist, will be found to move slowly and steadily,
the motion becoming perceptible by means of the minute granules of which
the protoplasm consists. The currents return at the ends of the cell,
there being no passage of the contents of one cell into the cavity of
either of those adjacent. During this examination, it will be noticed
that the surface of the cell-wall is striated with fine wrinkles.

It may be remarked that the hairs should be taken from flowers which
have only just opened; for this curious and inexplicable rotation is
connected with the growth of the cell; and when this has attained
maturity, it no longer occurs. The phenomenon may be observed in many
other hairs of plants, as those of common groundsel (_Senécio vulgáris_)
(Pl. I. fig. 12 _a_, _b_), and in the cells of the leaves of some
water-plants; but I must refer to the article “Rotation” in the
Dictionary for further information.

The most important variety of hair is that derived from the Cotton-plant
(a kind of Mallow), and forming the cotton of commerce. These hairs
spring from the epidermis of the seeds. The cells composing it are very
long and soft, becoming flaccid and easily bent when dry (Pl. IX. fig.
13).

_Stings._--Stinging hairs or stings may be well illustrated by reference
to the common large nettle (_Urtíca dioíca_). In this plant they consist
of a thick-walled cell, bulbous at the base, which is imbedded in the
epidermis (Pl. I. fig. 21), the pointed end being terminated by a very
minute dilatation or knob. The sting contains an acrid liquid, which
escapes when the little knob is broken off in wounding the skin, and
produces the well-known irritation. By the side of the figure of the
sting is represented the point of a fine needle (fig. 20), showing that
the expression “sharp as a needle” has no force when microscopic bodies
are in question.

_Stom´ata_ (στὁμα, mouth).--On viewing a strip of epidermis, the
observer will be sure to notice certain oval or roundish bodies (Pl. I.
fig. 13 _a_), composed of mostly two kidney-shaped cells in apposition
but leaving a chink between them; these are the stomata. They
communicate beneath with the intercellular passages, of which they may
be considered the mouths; and by their agency a direct communication is
established between these passages and the air. The two cells which
guard the orifice are termed the “guard cells.”

Stomata are most numerous on the under surface of leaves; they are
entirely absent in plants growing under water, and in most of the lower
plants. In many of the stomata, viewed in the ordinary way, the air
situated between the guard cells is indicated by the black spot or dot
present; but after a time, or by the application of a gentle heat to the
slide, the air becomes displaced by the water, and their structure
becomes very distinct.

In certain plants, the epidermis is imbued with flint or sil´ica; so
that even when burnt to an ash the stomata are still quite distinct.
Examples of this may be found in the stalk or culm of grasses, as in
straw, the shining epidermis of which is siliceous; or the epidermis of
canes. Among the lower plants, this peculiarity is especially curious in
the species of _Equisétum_, or mares’-tails.

The manner in which the veins of leaves are arranged is worthy of
special attention, as it forms one of the characters by which the two
leading divisions of the Vegetable Kingdom are characterized. Thus in
one of these divisions the veins are branched, so as to form a network
throughout the leaf; the plants with these netted veins, to which belong
our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens;
while in the second division, the veins run parallel to each other,
being little or not at all branched, and not forming a network. The
plants with parallel veins, among which are our grasses, lilies, &c.,
are the Monocotylédons or En´dogens.

_Stems._--In the stems of plants, the tissues are arranged round a
centre; otherwise, in the simpler and lower plants, they agree in
structure with leaves, the centre being occupied by some element of
fibro-vascular tissue, as simple wood-cells, a few vessels or ducts.

In the higher or flowering plants, the stem exists in two distinct
forms, corresponding to the differences above noticed in the arrangement
of the veins of the leaves; these must be considered separately.

In the _Dicotyledons_ or _Exogens_ (Pl. I. fig. 36), the centre of the
stem, in a transverse section, is seen to be occupied by the pith or
_medulla_, which is represented in the figure by the innermost circle.
Immediately outside and around this is a narrow ring, indicating the
section of a sheath to the pith, and called the _medullary sheath_. Next
comes a broad ring of wood of the first year’s growth (fig. 36 _a_),
traversed, from the pith to the bark, by wedge-shaped paler rays, termed
the medullary rays. Outside the first year’s wood is the newer and paler
wood of the second year (_b_); and so on, a new ring of wood being added
outside the preceding layer for each year of growth of the stem.

On the outer side of the wood is the inner bark or _liber_ (fig. 36
_c_); and outside this is the spongy outer bark (_d_), covered by its
epidermis.

These structures are of different composition, as may be best seen in
longitudinal sections. The pith and the medullary rays consist of
cellular tissue, the cells being mostly rounded in the former, and more
closely pressed together and squarish in the latter. The medullary
sheath consists of vascular tissue; and the wood, of wood-cells
traversed longitudinally by bundles of vascular tissue and ducts, the
latter being larger and more distinct towards its outer boundary. The
liber is composed of woody fibre, and the outer bark of cellular tissue.

The new woody matter being deposited outside the old, between the bark
and the previously formed layer, gives origin to the term exogen (ἔξω,
outside, γεννἁω, to produce). These structures may be examined in the
section of a branch of the lime-tree or lilac.

In the _Monocotyledons_ or _Endogens_ (Pl. I. fig. 37), there is no
distinct bark, nor pith, nor medullary rays--the entire stem consisting
of cellular tissue with isolated bundles of fibro-vascular tissue
scattered through it. Moreover the new substance is added to the centre
of the stem, or within the old; hence the term endogen (ἔνδον, within,
γεννἁω). A section of a piece of cane will exhibit this structure.

To examine the structure of stems, sections must be made in various
directions. The relative position of the component parts of a stem are
best seen in a transverse section; but the structure of the tissues is
most evident in longitudinal sections, and under the higher powers. The
annual rings of the Exogens are best observed in transversely sawn-off
pieces of perfectly dry stems, which have been polished with sandpaper,
and varnished with spirit varnish.

_Roots._--The structure of roots is very similar to that of stems; there
is, however, no distinct pith, nor are there stomata on the epidermis;
and the vessels are replaced by ducts. The very fine rootlets or
radicles of water-plants often show the rotation of the protoplasm very
distinctly.

_Flowers._--The various parts of flowers, being each a modified leaf,
present the same general structure as the latter. As the reader may not
be acquainted with the names of these parts or organs in the higher
plants, and as we shall have to compare them with their representatives
in the lower forms of vegetable life, it will be well briefly to
indicate them. A common and beautiful yet despised flower (Pl. I. fig.
32) may serve for illustration; this is chickweed (_Stellária média_),
which can be found everywhere. The outermost circle of flower-leaves,
which forms a kind of cup to the rest of the flower (_a_), is the
_calyx_; the separate leaves being called the _sepals_. The row within
this, in most flowers consisting of brilliantly coloured pieces, forms
the _corolla_ (_b_); the individual pieces being the _petals_. When the
two kinds are equally coloured, or not distinguishable, the whole is
called the perianth, as in a tulip. When the segments of the _perianth_
are dry and chaffy, as in the flowers of grasses, the outermost are said
to constitute the _glumes_, and the innermost the _paleæ_. Within the
ring of petals are certain thread-like organs called _stamens_ (_c_);
and these consist of a _filament_ (fig. 39 _a_), surmounted at the top
or apex by the _anther_ (fig. 39 _b_), which is usually coloured, and
consists of two lobes. The anthers when ripe burst, and discharge a
coloured dust; this is the _pollen_. Lastly, within the stamens is the
central organ of the flower, the _pistil_, and sometimes there are
several of them. The pistil consists of three parts, viz. a swollen
base, the _ovary_ (fig. 41 _b_), surmounted by a column or _style_ (fig.
41 _a_), and which is crowned by a viscid and often hairy summit, the
_stigma_ (fig. 40*). In chickweed there are 3 styles.

It must be remarked that, in the flowers of some plants, stamens alone
are present, while others contain pistils only, although most flowers
contain both organs. When the stamens and pistils occur in separate
flowers on the same plant, the plant is said to be _monœcious_ (μὁνος,
single, οἶκος, family); when all the flowers of distinct plants contain
either stamens only or pistils only, the plant is _diœcious_ (δις,
twice, οἶκος); and when the stamens and pistils occur together in all
the flowers of the same plant, the plant is said to be _hermaphrodite_.
These terms had their origin in the idea that the differences of plants
in respect to these organs were analogous to those of the sexes in
animals. All the parts of a flower have their special uses: thus the
calyx and corolla protect the delicate organs enclosed by them, until
they attain maturity. The petals also, by their brilliant colours,
attract insects which feed upon or collect the honey of the flowers;
these at the same time conveying the pollen which adheres to their
bodies from one flower to the stigma of another. The stamens and pistils
are organs of fructification, it being essential for the fertilization
of the flowers that the pollen should come into contact with the stigma.
We will now consider some interesting points of structure in these
organs.

_Petals._--The petals often form most beautiful microscopic objects, on
account of the curious shape and structure of the cells of their
epidermis, and the splendid tints of the colouring matters contained in
them. As petals are mostly too thick to allow of the cells being
distinctly seen in the entire state, a little cut should be made in them
while gently stretched on the finger, and the epidermis carefully
stripped off with forceps; the strip should then be laid on the slide in
water as usual: in this way the curious patterns of the epidermic cells
will become very distinct. The petals of a red geranium (_Pelargónium_)
may be used to illustrate them (Pl. I. fig. 24). The structure may be
best understood by reference to the epidermis of the leaf of a geranium
(Pl. I. fig. 13), in which the cells present wavy or undulate walls. In
the petal (fig. 24), the walls are inflexed at tolerably regular
distances, so as to give rise to the appearance of a row of teeth lining
the cell. If the strip of petal be folded, so as to exhibit the side
view, it will also be seen that the cells project outwards from the
surface to form a bluntish point or papilla, or the petals are papillose
as it is called; and the surface of the membrane around the papillæ is
finely wrinkled, so as to present the appearance of very delicate
radiating lines or striæ. Intermediate degrees of this inflexion may be
found in various flowers, between the slight condition seen in fig. 13
and the extreme state of fig. 24, as in the snapdragon (_Antirrhínum
május_).

_Anthers._--The cavities of the anthers are lined with fibro-cellular
tissue, the fibres of which aid in discharging the pollen; this may be
seen by dissecting an anther of London pride (_Saxif´raga umbrósa_), or
of a wallflower (_Cheiran´thus cheíri_) in water. It also exists in
chickweed.

_Pollen._--The pollen consists of minute grains called the
_pollen-granules_. They may be viewed either in the dry state as opake
objects, or when immersed in water as transparent objects. As it is
often difficult to moisten them, they may be touched on the slide with a
little spirit, and then a drop of water added. Their forms are very
varied and curious, but they are difficult of observation from their
minute size. They consist of one or more coloured cells, and these cells
are remarkable for their surfaces exhibiting spines, networks, folds,
and markings of various kinds. Thus in the primrose the pollen-granules
are cylindrical, the surface being furrowed (Pl. I. fig. 16); in the
sunflower the granules are spherical, and covered with tubercles
surmounted by spines (fig. 17); in the garden convolvulus the surface of
the spherical granules is covered with an elegant network, in the
meshes of which are also situated spines (fig. 18); and in the granules
of chickweed the surface presents pits, with minute tubercles in the
centre (figs. 30 & 31). The pollen-granules are often considerably
altered by immersion in water; so that, in judging of their structure
when examined in water, the resulting alteration must be taken into
account.

When ripe pollen-granules have been immersed in water for a short time,
one or more minute tubes will be seen protruding from their surface;
these are the _pollen-tubes_, and the granular protoplasm contained in
them is called the _fovil´la_. In the process of fertilization of the
flower, the pollen-granules fall upon the viscid stigma; the
pollen-tubes are then protruded, and, passing down the intercellular
spaces of the style (Pl. I. fig. 14), enter an aperture in the ovule or
young seed, which is thus endowed with the power of growing into a new
plant. The pollen-tubes are often very long, and they do not exist fully
developed in the pollen-granules, but grow down the style, just as the
little rootlet of a seed grows into the soil. The style of a crocus will
serve for dissecting out with mounted needles the long and very slender
pollen-tube (Pl. I. fig. 15).

_O´vary._--The ovary by its growth and enlargement becomes the fruit.
There are many interesting microscopic structures to be found in fruits
and the seeds they contain, a few of which may be noticed here.

On examining the surface of the rind or pericarp (περἰ, around, καρπὀς,
fruit) of an orange, little dots will be seen, paler than the rest of
the surface. These are receptacles of secretion, or glands, containing
the evaporable or volatile oil upon which the fragrance of the orange
depends. They consist of loose cells, surrounding a central cavity, and
are imbedded in the rind.

Other receptacles of secretion, called _vittæ_ (_vitta_, a band), occur
in the wall (pericarp) of the fruit of the Umbelliferæ, or Parsley Order
of plants, and their arrangement forms characters for distinguishing the
genera. They may be well seen in caraway-seeds; for the caraway-plant is
one of the Umbelliferæ. It must be observed that a caraway “seed” is not
really a seed, but consists of half the fruit; for, on careful
examination, one side of it will be found to be flattened, the
flattening resulting from the mutual pressure of the two half-fruits at
that part; moreover the dried style exists at its summit. In the figure
(Pl. I. fig. 19), the flattened part of the seed is next the observer.
The seed has five evident longitudinal ridges, one at each corner or
angle. The vittæ are dark-coloured (fig. 19 _a_), and placed one between
each pair of these ridges; and they consist of long flattened spaces in
the substance of the pericarp, with transverse markings, indicating
internal cross partitions. In botanical works, the presence of five
ridges, with single vittæ in the intervals, is given as a character by
which the half-fruits (carpels) of the caraway are to be distinguished.
But on closely inspecting the flattened surface, another ridge is seen
running down its middle; so that the seed really has six ridges, one of
which is smaller than the rest from the pressure of the other half.
Hence the character of five ridges with single vittæ is incorrect.

The vittæ contain the volatile oil to which the fragrance and pungency
of the fruit is owing, although some of the oil exists also in the cells
of the kernel or albúmen, which forms the white and greater part of the
seed.

The skin of a reddish apple, peeled off in the manner described for
petals, exhibits beautifully the red colouring matter of different tints
in adjacent cells, while the pulp displays the cell-contents, as already
mentioned. The latter may also be easily examined, from their large
size, in most of the softer fruits, as that of the snowberry or the
cucumber.

As the ovary or fruit approaches maturity, the petals and stamens wither
and fall off, the calyx often remaining, and being sometimes adherent to
the ovary, at others free or unattached to it.

_Seeds._--During the ripening of the fruit, the seeds contained within
it are gradually becoming further developed.

The seeds themselves are covered outside by a skin or coat called the
_testa_ (_testa_, a shell). This is remarkable for frequently displaying
various kinds of figured patterns, consisting of raised networks,
ridges, little knobs or tubercles, &c. Examples of these may be found in
the seeds of the poppy (Pl. I. fig. 27), mignonette (fig. 29), and
chickweed (fig. 51).

Some seeds are winged, as it is called, _i. e._ furnished with an
extension of the testa beyond the margin of the seed. This not
unfrequently consists of aggregated fibre-cells, the spiral fibre being
very distinct, as in the seeds of _Eccremocar´pus scáber_ (Pl. I. fig.
52). In the seeds of another curious plant in this respect, viz.
_Collómia grandiflóra_, the fibre-cells are separate, so as to resemble
hairs, and very mucilaginous, and in the dry seed are closely pressed to
its surface. If a portion of the testa of these seeds, which can be
procured at the seed-shops, be cut off, laid on a slide, a cover
applied, and when the object is in focus, a drop of water be added, in a
short time water softens the mucilaginous walls of the cells, the power
of the spiral fibres comes into play, and the cells expand so as to form
a very interesting object; the cells, in their expansion, apparently
writhing like so many minute worms (Pl. I. fig. 35).

The seed itself, which is contained within the testa or seed-coat,
consists essentially of the young plant or embryo. This is composed of
three parts, viz. the _plúmule_ (_plumula_, a little feather), or the
young stem; the _rad´icle_ (_radicula_, a little root), or the young
root; and one or two, rarely more, imperfectly developed or rudimentary
leaves, the _cotyle´dons_ (κοτυληδὠν, a cup).

These structures are closely packed in the seed, and are not easily
recognized at first. By keeping seeds moist for a day or two until they
begin to grow, or _germinate_ as the seed-growth is called, they are
readily detected, and may then be more easily found in the dry seed.

When somewhat advanced in growth, they are familiar to every one,
although they may not be recognized by their names. In table “mustard
and cress,” the whole consists of these organs of the two plants; the
white stalk directed downwards being the radicle, the two green
leaf-like lobes the cotyledons, and between the latter directed upwards
is the very minute plumule, which is more easily seen when the plants
have been allowed to grow larger. This structure of the seed is
important to be known, because the absence or presence and the number of
cotyledons afford characters, corresponding with those already mentioned
in respect to the veins of the leaves and the structure of the stem, for
distinguishing the great divisions of the Vegetable Kingdom. Thus, the
Exogens are Dicotyledons (δις, twice), their seeds having two
cotyledons; while the Endogens are Monocotyledons (μὁνος, single),
having one only; and the Cryptogam´ic plants are Acotyledons (α,
without), their seeds (spores) having none of these organs.

Some seeds consist entirely of the embryo, surrounded by the testa. But
in many others there is also present a usually whitish, firm cellular
substance, called the _albúmen_ (_albumen_, white of egg).

The albumen of seeds often affords good specimens of secondary deposit,
the cells being almost entirely filled with it. An example may be found
in a section of vegetable ivory, of which ornaments are sometimes made;
its structure resembles essentially that of the plum-stone. In other
instances the cells contain secreted matters, as starch, oil, &c.; and
sometimes the cotyledons also contain starch and oil. An example of the
former exists in the albumen of wheat; and of the latter, in the
horse-chestnut, the filbert, and mustard-seed.

The albumen and cotyledons serve to supply the embryo with nutriment
until the roots have grown sufficiently to enable them to absorb it from
the soil; the cotyledons also serve as temporary leaves.

The form and relative position of the radicle and cotyledons serve to
distinguish certain groups of plants. This may be illustrated by the
natural order Cruciferæ, or that containing the mustard, wall-flower,
&c.

Thus, in one group, which may be represented by the wall-flower, the
cotyledons are flat or plane (Pl. I. figs. 43 & 44), the radicle being
applied to their edges. This is best seen in a transverse section (fig.
43). They are then called _accum´bent_ (_accumbo_, to lie against); and
the botanical sign is O=. In the second group, the cotyledons are plane
(Pl. I. fig. 38), with the radicle applied to the back of one of them,
as in the seed of the common shepherd’s purse (_Capsel´la bur´sa
pastóris_) (Pl. VII. fig. 19). They are then termed _in´cumbent_
(_incumbo_, to lie upon), and the sign is O||. While in the third group
the cotyledons are folded in the middle, like the leaves of a book (Pl.
I. figs. 49 & 50), and the radicle is enclosed between them, as in the
white mustard (_Sina´pis alba_). The cotyledons are then called
_condu´plicate_ (_conduplico_, to fold); and their sign is O> >.

The plants above-mentioned are evidently all Dicotyledonous, or their
seeds have two cotyledons; and they contain no albumen.

In the Monocotyledonous division, which may be represented by a grain of
wheat (Pl. I. fig. 53), the single cotyledon forms a minute sheath
(_a_), enclosing the plumule (_b_), the radicle (_c_) being here but
little developed at first, the greater part of the grain consisting of
the albumen (_d_); the grain should be softened in water before
examination. In the germinated grain the cotyledon appears as a pale
sheath, surrounding the convolute green leaves of the plumule; which may
be best seen in a transverse section (Pl. I. fig. 48).

_Fertilization._--A few words must now be said regarding the formation
of seeds, and the action of the pollen-tubes in the process of
fertilization.

In the earliest stages of growth, the young seeds, or _ovules_ as they
are called, appear as little buds, arising from the inner wall of the
ovary; and the part from which they arise is called the _placen´ta_
(_placenta_, a cake). In chickweed (Pl. I. fig. 41 _c_), the placenta
forms a central column; and when the ovules are a little older, they are
found to have separated somewhat from the placenta, but retaining a
connexion by means of a little cord or stalk, termed the _funic´ulus_
(_funis_, a cord). The ovules may be readily found in the ovary or young
pod of a wall-flower, the placentas forming four lines, running
longitudinally down the interior of the pod.

In this early condition the ovule consists of a mass of cellular tissue;
and as new formations are soon added to it, it is termed in this state
the _núcleus_. Around the nucleus are then formed two coats, an outer,
called the prímine, and an inner, termed the secun´dine. These coats or
membranes are open at one end, so as to leave a passage down to the apex
of the nucleus; the opening is called the _forámen_. These structures
are well seen in the ovule of the wall-flower (Pl. I. fig. 54), the
foramen in the figure being indicated by a *; it will be noticed also
that the funiculus runs down one side of the ovule, so as to terminate
at the bottom or base of the nucleus. In ripe seeds, the spot at which
the funiculus has been attached is mostly perceptible in the form of a
scar. The slight prominence of the foramen can also often be
distinguished, as in the seed of chickweed (Pl. I. fig. 51*); in the
ripe seed the foramen is termed the _mícropyle_, and towards it the
radicle of the embryo is always directed.

One of the cells of the nucleus near its apex then enlarges, so as to
form a sac, called the _embryo-sac_. This is excessively thin and
transparent (Pl. I. figs. 45 _b_ & 47); and in it, also at the end next
the foramen, one or more (in the chickweed one) smaller cells are formed
from the cell-contents of the embryo-sac, which are called the
_embryonal vesicles_ (Pl. I. fig. 45 _a_).

Thus far developed, the embryo exists prior to the expansion of the
flower and the discharge of the pollen. The embryo-sac is not figured in
this early condition, the embryonal vesicle being then smaller than that
in fig. 45 _b_, although occupying the same position.

When the pollen has escaped from the anthers and fallen upon the stigma,
the pollen-tubes growing down the intercellular passages of the style,
enter the foramen of the ovule, and so reach the apex of the nucleus, at
which the embryonal vesicle contained in the embryo-sac is situated. The
end of the pollen-tube then adheres to the embryonal vesicle, and such
interchange of cell-contents takes place between them as effects
fertilization.

The process of cell-formation in the fertilized embryonal vesicle then
takes place rapidly, new cells being formed by the division of its
cell-contents (Pl. I. fig. 45 _a_); and it will be noticed that the new
cells are formed at the end of the embryonal vesicle, opposite to that
situated at the apex of the embryo-sac. As the cell-division and
formation proceed further, a mass of new cells is produced (Pl. I. figs.
46 _c_ & 47), forming the rudimentary embryo; and from this, by further
growth, the perfected embryo (fig. 55) results; or, to use a fashionable
technical term, the simply cellular embryonic mass becomes
differentiated into the radicle, cotyledons, and plumule, forming the
embryo. It will be remarked that the position of the embryo in fig. 55
is the reverse of that in figs. 46 & 47, the radicle in the former being
directed downwards, whilst that of the embryo in the figure of the
embryo-sac (fig. 47) is directed upwards.

The embryonal structures are very difficult of detection; but it happens
that in our little chickweed they are more easily dissected out than in
most other plants. For this purpose, the ovules, placed on a slide and
lying in water, should be picked to pieces with the mounted needles,
under the simple microscope. They may be preserved in chloride of
calcium or glycerine.

A clear distinction must be drawn between seeds, which result from the
process of fertilization, and buds, which are formed independently of
this process. Both consist essentially of embryo plants; but while the
former originate from a single cell, the latter are outgrowths of a
parent stem, from which their tissues are derived; and while the former
propagate the species, the latter increase the individual.

The obvious use of seeds is the distribution of the species by the
formation of new individuals.

In the general outline which has been given of the elements, tissues,
and organs of plants, they have been examined principally as existing in
the higher groups, or those of more complex structure; and to enter
further upon a description of these plants would involve the
consideration of variations in the form and arrangement of the organs of
which they are composed. As these can mostly be investigated without the
use of the microscope, we must pass to those in which the entire plant
consists of little more than simple or parenchymatous cells, and in
which the representatives of the flower are so inconspicuous, or are
reduced to so elementary a condition, that the plants included in the
Division have been termed _Cryptogam´ic_ (κρυπτὀς, concealed, γἁμος,
union--figuratively for reproductive organs) or Flowerless Plants. The
reproductive organs of the Cryptogamia are usually termed the
fructification, implying that they produce fruit, but not flowers.

PLATE II. [PAGE 49.]

FERNS AND LICHENS.

Fig.

1. _Chlorococcum vulgare._

2. _Parmelia parietina._

3. _Parmelia parietina_, section of a saucer (apothecium).

4. _Parmelia parietina_, gonidia and asci.

5. _Parmelia parietina_, asci and paraphyses; 5 _a_, spores.

6. _Calicium clavellum._

7. _Calicium clavellum_, stalked apothecia.

8. _Calicium clavellum_, apothecium.

9. _Polypodium vulgare_, frond.

10. _Polypodium vulgare_, lobe of frond.

11. _Polypodium vulgare_, group of capsules (thecæ).

12. _Polypodium vulgare_, capsule and spores.

13. _Polypodium vulgare_, spore germinating.

14. _Polypodium vulgare_, prothallium.

15. _Polypodium vulgare_, portion of prothallium, with archegonia.

16. _Aspidium filix mas_, frond.

17. _Aspidium filix mas_, pinnules with sori.

18. _Aspidium filix mas_, single pinnule.

19. _Scolopendrium vulgare_, frond.

20. _Scolopendrium vulgare_, portion of frond.

21. _Cladonia coccifera._

22. _Cladonia cornuta._

23. _Cladonia pyxidata._

24. _Cladonia rangiferina._

25. _Cladonia rangiferina_, ends of podetium.

26. _Graphis scripta._

27. _Graphis scripta_, lirellæ.

28. _Graphis scripta_, asci and spores.

29. _Graphis scripta_, spore.

30. _Opegrapha betulina._

31. _Opegrapha betulina_, lirellæ.

32. _Opegrapha betulina_, lirella.

33. _Opegrapha betulina_, ascus with spores.

34. Scalariform ducts of Brake (_Pteris_).


[Illustration: Plate II.

W Bagg sculp

_London: John Van Voorst._]



CHAPTER V.

FERNS, OR FIL´ICES.


The general appearance of plants belonging to the class of Ferns is so
well known that it need scarcely be described, especially since the
introduction of the glass plant-cases, by means of which the air can be
kept so damp that ferns are now grown in the very heart of our cities.
Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or
fronds (_frons_, a leaf) as the leaf-like organs of the lower plants are
called, arising in tufts from the stems, give them the elegant
appearance we are called upon to admire, whenever they are met with. The
brownish spots or stripes seen upon the back or under surface of the
fronds, and consisting of the fructification, form also a simple
character by which they may generally be distinguished; although in a
few of them the fructification is placed upon a distinct stalk. The stem
or rhi´zome (ῥἱξωμα, a root) of a fern is mostly situated just beneath
or at the surface of the ground, and is commonly mistaken for the real
root, which is buried in the earth. It is brownish outside, and covered
with scurfy scales or _ramen´ta_ (_ramentum_, a shaving). These scales
are interesting microscopic objects, from the distinctness with which
they exhibit the cellular network.

A section of the rhizome exhibits the fibro-vascular tissue arranged
differently from that in the stems of either Exogens or Endogens. It
forms curiously curved longitudinal plates, a very abundant component of
which is the scalar´iform (_scala_, a ladder) duct (Pl. II. fig. 34).
The walls of the scalariform ducts are angular, and the secondary
deposit is arranged in the form of transverse bars, somewhat resembling
the steps of a ladder; which structure is best seen in a transverse or
slightly oblique section. The fronds are usually cleft nearly down to
the main vein or midrib (fig. 9), or pinnatifíd (_pinna_, a feather,
_findo_, to cleave); sometimes the segments are similarly cleft, so that
the fronds are bipinnatifid. The manner in which the veins usually
branch is also peculiar, each branch separating from the point of
division at an acute angle with the original direction of the vein (fig.
20), so as to be forked. It is also worthy of notice, that the young
frond is rolled up into a flat spiral, or is cir´cinate (_circino_, to
go round), before it opens.

We will now examine one or two species more minutely.

POLYPODIACEÆ. _Polypódium vulga´re_ (Pl. II. fig. 9, one-third of the
natural size), a member of this family, is very common on old trunks of
trees, on banks, &c. The frond is deeply pinnatifid, the segments being
oblong, blunt (obtuse), scalloped (crenate) at the edges (PI. II. fig.
10), and becoming gradually shorter towards the apex of the frond.

On the back of the fronds are the little orange-coloured groups of
capsules (Pl. II. figs. 9 & 10); these are called _sor´i_ (σορὀς, a
heap). The capsules or _thécæ_ (θἡκη, a case), a magnified group of
which is represented in fig. 11, consist each of an aggregation of
cells, fixed to a stalk (fig. 12); and along the back of the capsule is
a close row of thicker cells, forming an elastic ring, the _an´nulus_.
When the seed-like bodies or spores (σπορἁ, a seed) are ripe, the
annulus becomes straightened from its elastic power, and tears the
capsule open, so that the spores are set free and scattered.

_Aspid´ium filix mas_ (Pl. II. fig. 16, reduced to one third or fourth
of the natural size), is the most common British fern. In this the
fronds are pinnate, _i. e._ the segments (pinnæ) corresponding to those
of _Polypodium_ are cleft entirely to the main stalk, the pinnæ (fig.
17) being pinnatifid; and the segments of the pinnæ or the pinnules
(fig. 18) are oblong, obtuse, and saw-edged (serrate). In all botanical
works the fronds are incorrectly said to be bipinnate.

The sori are brown, kidney-shaped (fig. 18), and differ from those of
_Polypodium_ in being covered by a thin membrane (_indúsium_), which is
fixed to the frond at the notch.

_Scolopen´drium vulgáre_ (Pl. II. fig. 19), the Hart’s-tongue Fern, is
common in hedges and on moist banks. Its fronds are simple or undivided,
strap-shaped, heart-shaped at the base, and narrowed to a point at the
apex. The sori (fig. 20) are brown, narrow, longish (linear), and
transverse or slightly oblique. The indusium is cleft down the middle,
so as to form a longitudinal fissure or suture.

_Reproduction._--The spores of the ferns (Pl. II. fig. 12 _a_) resemble
in appearance the seeds of flowering plants on a small scale. They are
usually brown, covered on the surface with little tubercles or other
markings, and when kept on a slide in a moist atmosphere, as over a
saucer of water covered with a bell-glass, they germinate. When this
takes place, one or more short, brownish, hair-like radicles emerge from
one part of the surface (Pl. II. fig. 13), and a process containing
chlorophyll issues from an adjacent spot. As growth proceeds, the latter
by cell-division gives rise to a flattened two-lobed leafy
cotyledon-like body or _prothal´lium_ (πρὀ, before, θαλλὀς, leaf), with
numerous rootlets springing from the base of the lobes. The prothallium
is of a peculiar dull-green colour, different from that of the young
frond which is subsequently formed. This arises from the absence of
stomata and intercellular passages containing air; for the air in these
passages of leaves and petals contributes greatly to the production of
the brightness of their colours. Moreover, the cells of the prothallium
resemble those of the parenchyma of a leaf, the epidermis with its
wavy-margined cells being absent.

When the prothallium has attained its full development, minute scattered
protrusions from its cells occur on the margin or under surface,
resembling short and blunt hairs; and each of these becomes partitioned
off to form a new cell, within which a number of crowded smaller cells
are produced. These organs are called _antherid´ia_ (anther, and εἶδος,
resemblance); and within each of the crowded smaller cells is contained
a very minute, colourless, coiled fibre, furnished with still finer
filaments, called _cil´ia_ (_cilium_, an eyelash); the ciliated fibres
being termed _spermatozo´a_ (σπἑρμα, seed, ξῶον, animal). At a later
period, other organs are found also on the back of the prothallium.
These are larger than the antheridia, and are composed of several cells,
arranged around a central canal which leads to an embryo-cell situated
at its base (Pl. II. fig. 2). These organs are the _archegónia_ (ἁρχἠ,
beginning, λὁνος, offspring). When the antheridia are ripe, they
discharge the spermatozoa, which are enabled to swim about by means of
their cilia in water (rain), and entering the canal, reach the
embryo-cell, which thus becomes fertilized. When fertilized, the
embryo-cells produce the little fronds which afterwards grow into the
mature plants.

Hence the spores of ferns differ strikingly from the seeds of the higher
plants in not containing the embryo radicle and cotyledons already
formed, these being produced during or after germination; also in the
fertilizing organs, viz. the antheridia or representatives of the
anthers, and the archegonia or the representatives of the pistils, being
produced from the cells of the prothallium.

The more minute of these structures are too difficult of observation and
preparation for any one unaccustomed to microscopic manipulation, so
that they have not been figured in detail; the figures given will,
however, serve to guide the observer in their recognition.

_Preservation._--The ferns may be easily preserved in the entire state,
by laying them flat between sheets of coarse unsized paper, and
subjecting them to moderate pressure in a screw-press; the paper should
be changed, or dried before a fire every two or three days, and the
pressure repeated until the specimens become dry and rigid. They may
then be mounted on sheets of paper, being fastened either with thread
passed round the stalk or portions of the frond with a needle, and tied
in a knot behind, or with strips of paper gummed at the ends.

The minute structures may be preserved either in the dry state or in
glycerine.



CHAPTER VI.

MOSSES, OR MUS´CI (MUSCUS, MOSS).


I need scarcely refer to the figures in Pl. III. to enable the reader to
recognize the Mosses; every one knows them at once by their remarkably
uniform general appearance, their miniature-plantlike form, their
crowded little leaves, concealing the slender wiry stems, their growth
in patches, and their curious urn-shaped fruits raised up on slender
bristle-like stalks.

The leaves of the mosses are simple, _i. e._ not cut into segments, and
consist of one or two layers of cells. The thinness of the leaves
enables these cells to be seen very distinctly, the closely united
cell-walls giving the leaves a netted or reticulated appearance (fig.
48), and the grains of chlorophyll being generally few and readily
distinguished. The veins of the leaves, or the nerves as they are
usually called, scarcely deserve the name; for neither they nor even the
stems contain fibro-vascular tissue, but consist simply of elongate
closely packed cells, and often the leaves have no nerves.

The fruit of the mosses consists of a _capsule_, sometimes called a
_sporan´gium_ (σπορἀ, seed, ἄγγος, vessel), usually placed at the end of
a slender stalk, called the _séta_ (_seta_, a bristle); but sometimes
the stalk is absent or extremely short, when the capsule is said to be
ses´sile (_sessilis_, sitting). The young capsule is covered with a thin
extinguisher-like cap or _calyp´tra_ (καλὑπτρα, a cover), which is
carried up as the capsule and its stalk grow, so as to be either
entirely thrown off, or to remain covering a greater or less portion of
the capsule, when this attains maturity.

PLATE III. [PAGE 54.]

MOSSES.


Fig.

1. _Sphagnum acutifolium_, expanded leaf.

2. _Sphagnum acutifolium_, cells of leaf.

3. Spermatozoa of _Polytrichum piliferum_.

4. _Sphagnum acutifolium._

5. _Sphagnum acutifolium_, capsule.

6. _Gymnostomum truncatulum._

7. _Gymnostomum truncatulum_, leaf.

8. _Gymnostomum truncatulum_, capsule and operculum.

9. _Gymnostomum truncatulum_, spore.

10. _Dicranum heteromallum._

11. _Dicranum heteromallum._

12. _Dicranum heteromallum_, leaf.

13. _Dicranum heteromallum_, operculum.

14. _Dicranum heteromallum_, calyptra.

15. _Dicranum heteromallum_, capsule; 15 _a_, peristome.

16. _Tortula muralis._

17. _Tortula muralis_, leaf.

18. _Tortula muralis_, capsule; _a_, tooth of peristome.

19. _Tortula muralis_, operculum.

20. _Tortula muralis_, calyptra.

21. _Tortula muralis_, archegonia.

22. _Polytrichum piliferum._

23. _Polytrichum piliferum_, leaf.

24. _Polytrichum piliferum_, calyptra.

25. _Polytrichum piliferum_, antheridial stems.

26. _Polytrichum piliferum_, single head.

27. _Polytrichum piliferum_, antheridia and paraphyses.

28. _Funaria hygrometrica._

29. _Funaria hygrometrica._

30. _Funaria hygrometrica_, leaf.

31. _Funaria hygrometrica_, capsule; _a_, operculum.

32. _Funaria hygrometrica_, stalk-like body.

33. _Funaria hygrometrica_, young archegone.

34. _Funaria hygrometrica_, more advanced archegone.

35. _Funaria hygrometrica_, section of young capsule.

36. _Funaria hygrometrica_, calyptra.

37. _Funaria hygrometrica_, antheridia.

38. _Funaria hygrometrica_, spores.

39. _Funaria hygrometrica_, annulus.

40. _Funaria hygrometrica_, archegonia.

41. _Funaria hygrometrica_, antheridial head.

42. _Funaria hygrometrica_, peristome.

43. _Hypnum rutabulum._

44. _Hypnum rutabulum_, leaf.

45. _Hypnum rutabulum_, capsule.

46. _Hypnum rutabulum_, spores.

47. _Hypnum rutabulum_, peristome.

48. _Hypnum rutabulum_, cells of leaf.

49. _Bryum capillare._


[Illustration: Plate III.

W Bagg sculp

_London: John Van Voorst._]

The calyptra is either simply mitre-shaped, or _mítriform_ (Pl. III.
fig. 24), or it is half-cleft, or _dimid´iate_ (figs. 14, 36). When the
capsule is ripe, the upper part usually separates at a circular
horizontal line (fig. 8) as a kind of lid, which is called the
_oper´culum_ (_operculum_, a lid), and thus the spores are enabled to
escape. The rim of the capsule, from which the operculum has separated,
forms its mouth, and this often exhibits a fringe of teeth (figs. 15,
18, 31), arranged in one or more rows; sometimes the teeth are replaced
by a membrane, or, again, both teeth and a membrane may be present. This
mouth-fringe is the _per´istome_ (περἰ, around, στὁμα, mouth). In many
mosses, an elastic row or ring of cells is situated between the mouth of
the capsule and its operculum, called the _annulus_ (figs. 18 & 39);
this, when the capsule is ripe, aids in throwing off the operculum.

It is important to become acquainted with the structure and arrangement
of these parts, as they form characters by which the families and genera
of mosses are distinguished.

The capsules of the mosses form very beautiful microscopic objects,
especially those furnished with a toothed peristome.

Most of the mosses produce their fructification in the winter and
spring.

The class of mosses is divided into two Orders, according to whether the
fruit-stalk is terminal, _i. e._ arises from the end of the stem or its
branches, or whether it is lateral, arising from the side of the stem.
Those with the fruit-stalk terminal, or the end fruited (Pl. III. fig.
22), form the _Ac´rocarpi_ (ἄκρα, summit, καρπὀς, fruit); while those
with the fruit-stalks lateral, or the side-fruited mosses (fig. 43),
constitute the _Pleu´rocarpi_ (πλευρἀ, side). The new shoots or young
branches of the stems of mosses are termed _innovations_.

We will now examine a few common mosses more in detail, beginning with
the ACROCARPI.

_Sphag´num acutifólium_ (Pl. III. fig. 4) is found in pools or bogs,
growing at the margins so as to be partially immersed. In this moss, the
upper branches are grouped into a head. The leaves are crowded, and
overlapping or im´bricate (_imbrex_, a tile) on the elongate stems; they
are egg-shaped (ovate) on the main stems (fig. 1), and narrower or
ovate-lanceolate on the branches; they are nerveless, and finely toothed
at the apex. The capsule (fig. 5) is roundish-ovate, without a
peristome, and the operculum is flattened. The grouped arrangement of
the upper branches renders the species of _Sphagnum_ easily recognized.
The structure of the leaves is also very peculiar and characteristic
(fig. 2). The cells of which they consist are of two kinds, one (fig. 2
_a_) being colourless, elongate, pointed, and containing a spiral fibre;
the other consisting of shorter and narrower obtuse cells, containing
chlorophyll, and situated between the former. In many of the former kind
of cells, little round apertures exist on the under surface, and minute
animals may sometimes be found imprisoned in them.

Another species of _Sphagnum_, _S. obtusifolium_, is common, and greatly
resembles the above, but has shorter and thicker stems, and
rounded-ovate, very concave, and obtuse leaves.

_Gymnos´tomum truncat´ulum_ (Pl. III. fig. 6) is a common little moss,
found on banks and in fields and gardens.

In this there is no peristome, although, in the young condition, a
membrane extends more or less over the interior of the mouth of the
capsule. The stem is slender, rigid, and simple, or but little branched.
The calyptra is dimidiate; the operculum is present (fig. 8), and
terminates above in an oblique beak, or it is obliquely rostrate
(_ros´trum_, a beak) as it is called. The leaves are obovate (fig. 7)
or ovate with the broader part remote from the stem, and narrowed at the
apex, where the nerve protrudes or is ex´current (_excurro_, to run
out). The spores (fig. 9) are reddish brown and smooth.

_Dicránum heteromal´lum_ (Pl. III. figs. 10 & 11) is probably the first
moss the reader will meet with on banks and heaths in the early spring;
and it will be sure to be noticed on account of the bright green colour
of the patches and the beautiful orange-brown capsules.

In this moss the capsule is nodding (cer´nuous) (Pl. III. fig. 15), and
has a single peristome, consisting of sixteen equidistant teeth, each
being deeply cut or cleft longitudinally (fig. 15 _a_), so that there
are thirty-two teeth altogether; and these are marked with internal
cross-bars, or transverse ridges. The calyptra is dimidiate (fig. 14);
and the lid is furnished with a long oblique beak (fig. 15 _b_). The
leaves are crowded, strongly nerved (fig. 12), lanceolate at the base,
and very narrow towards the apex, which is toothed; they are, moreover,
curved, and bent towards one side, or sécund.

_Tor´tula murális_ (Pl. III. fig. 16) may be found on the top of almost
every wall and on waste ground.

In this moss the peristome is single (fig. 18), consisting of thirty-two
spirally twisted teeth, arranged in pairs. They are narrow and slender,
and each is composed of two longitudinal portions (fig. 18 _a_), one of
which is pale yellow, the other reddish brown, like the capsule, and
both are fringed and covered with very minute papillæ. The capsule (fig.
18) is oblong, the ring or annulus remaining for some time. The lid is
conical (fig. 19), with a longish somewhat oblique beak, and the
calyptra is dimidiate (fig. 20). The stems are very short; the leaves
(fig. 17) are oblong, obtuse; the nerve strong, and projecting as a
colourless spirally striated bristle. The bristles often give the
patches of the moss a hoary appearance on wall-tops. The margins of the
leaves are folded back or recurved, giving them a peculiar thickened
appearance.

The largest of our mosses are contained in the next genus, viz.
_Polyt´richum_, some of them having the stems from 2 to 4 inches, or
even more, in height; they are common on heaths and in woods.

_Polyt´richum pilif´erum_ (Pl. III. fig. 22) is very common on open dry
heaths. This moss has simple stems, with the leaves crowded on the lower
part of those which are fertile or fruit-bearing. The fruit-stalk is
terminal (acrocarpous); the capsule ovate, 4-sided or quadrangular, with
a knob or struma (_strúma_, a swelling) at the base, the lid having a
short beak. The calyptra (fig. 24) is half-cleft (dimidiate) and very
hairy. The peristome is single, and consists of sixty-four teeth. The
leaves (fig. 23) are lanceolate, nearly upright, the margins folded
inwards or inflexed; and they end abruptly in a saw-edged or serrated
hair-like point.

_Poly´trichum commúne_, which is also very common, is larger than the
last species, and may easily be distinguished by the curved and serrate
leaves, which have no bristle-point.

In the early spring, patches of both these mosses may be found, in which
the stems are terminated by little rosettes (figs. 25 & 26); these will
be referred to presently.

Keeping still to the end-fruited or Acrocarpous mosses, we have next to
mention _Funa´ria hygromet´rica_ (Pl. III. figs. 28, 29), which is
readily distinguished from most other mosses by the pale apple-green
colour which it possesses before the capsule ripens. It is extremely
common on walls and waste ground.

The capsule of this moss (fig. 31) differs from those of the preceding
mosses in the peristome being double (fig. 42), or composed of an outer
and an inner row of teeth. The outer row consists of sixteen oblique
reddish teeth, which are marked with transverse bars or trabe´culæ
(_trabecula_, a little beam), and their points are connected by a
net-like thin plate. The inner row contains also sixteen teeth, arising
from the division of the membrane lining the capsule; these are
yellowish, thin, and placed opposite the outer teeth. The capsule itself
is pear-shaped or pyriform, orange-red when ripe, curved, and with the
mouth oblique. The calyptra (fig. 36) is half-cleft, and expanded as if
blown out below. The lid (fig. 31 _a_) is convex and obtuse; and the
annulus (fig. 39) is large and easily separable. The fruit-stalks are
curved near the top. The leaves (fig. 30) are ovate, concave, entire,
with a nerve reaching the apex, which is acute and prolonged into a
little point, or apic´ulate. The spores (fig. 38) are small and reddish
brown. The specific name (_hygromet´rica_) of this moss expresses its
hygrometric property; for if either the recent and moist moss be dried
or the dry moss wetted, the fruit-stalk gradually twists in opposite
directions in the two cases.

The last of the Acrocarpous mosses which we shall notice, _Bry´um
capil´lare_ (Pl. III. fig. 49), is tolerably common on trunks of trees,
on the ground, and sometimes on walls.

The capsule of this moss has a double peristome or mouth-fringe; the
outer consisting of sixteen reddish-brown, equidistant, transversely
striped teeth; the inner composed of sixteen thin keeled teeth, more or
less split down the middle, and with two or three intermediate cilia.
The capsule is nodding, smooth, oblong, pear-shaped, slightly narrowed
below the mouth; the lid being somewhat convex, and furnished with a
short slender beak. The calyptra is dimidiate. The leaves (fig. 50) are
obovate, the nerve extending beyond the point, rendering them
bristle-pointed. The seeds (fig. 49 _a_) are small and green.

This moss serves to illustrate a great difficulty, which will often
occur to the student, in determining whether a moss is end-fruited or
side-fruited. For in this, as in many other end-fruited mosses, a little
side-shoot or young branch (innovation) grows from the main stem
immediately below the leaves surrounding the base of the fruit-stalk, so
that the fruit-stalk appears to arise from the side of the stem. The
only method of overcoming the difficulty is to examine carefully the
comparative size and thickness of the stem and the shoot, and to
determine which is the weaker and so the newer. The leaves surrounding
the base of the fruit-stalk, which are mostly somewhat different in
structure from the stem-leaves, are called the _perichæ´tial_ (περἰ,
around, χαἱτη, bristle) leaves.

From among the side-fruited or Pleurocarpous mosses we shall select one
only, _Hyp´num rutab´ulum_ (Pl. III. fig. 43), which is common on the
trunks of trees and on banks.

In this moss, the nodding unequal curved capsule (fig. 45) has a double
peristome, resembling that of _Bryum_ (fig. 47). The calyptra is
half-cleft, and the lid conical and shortly beaked. The stem is
reclining or procumbent, and the pale green imbricated leaves (fig. 41)
are ovate and pointed, faintly saw-edged, the nerve becoming indistinct
at about the middle. It will be noticed that the cells of the leaf (fig.
48) have the prosenchymatous form, or are elongate with pointed ends;
and that the fruit-stalk (fig. 45) is rough with little grains.

_Fructification._--The fruit-producing organs of the mosses are of two
kinds, comparable to those of the flowering plants, but with their names
changed, as in the case of the ferns; the representatives of the anther
being called antheridia, and those of the pistil archegonia. The
antheridia may be best examined in _Polytrichum piliferum_ or _commune_,
the patches of stems with red rosette-like heads (figs. 25, 26) being
readily found in the spring on open heaths. The coloured leaves forming
these heads differ in form from those of the stem, being broader and
very sharp-pointed, and have received the distinctive name of
_perigónial_ (περἰ, around, γνος, offspring) leaves. In the centre of
these leaves, which must be separated with mounted needles in a drop of
water, the antheridia (fig. 27), forming oblong cellular green sacs,
will be seen; and intermingled with them will be found some slender pale
filaments, composed of mostly two rows of cells, which are the
_paraph´yses_ (παρἁφυσις, a side growth). If the antheridia are quite
ripe, they swell somewhat in the water, and from the free or unattached
end a very delicate, colourless, cellular mass gradually escapes. If the
antheridia are not quite ripe, the mass must be liberated by dissection.

On carefully examining this mass under a high power, it will be seen to
consist of very delicate rounded cells (fig. 3 _a_), each containing a
coiled filament, revolving more or less rapidly. After a time, these
filaments (fig. 3 _b_) escape, so that they may be examined more
minutely. They are excessively delicate, and are best seen when dried on
the slide. Each consists of a very slender curved filament, with a still
finer filament, or cilium, arising from it on each side. These are the
spermatozóa or spermatozóids (στἑρμα, seed, ξῶον, animal, εἶδος,
resemblance).

In _Funária_ the antheridia (fig. 37) may also be found, by careful
examination, in the little green heads terminating some of the stems
(fig. 41, of the natural size). In this moss, the paraphyses are
inflated at the summit into little knobs, or they are capitate (fig.
37). The pistil-like organs of mosses, or the _archegónia_, from which
the capsule is formed, must be looked for in the winter or early
spring. They occur in the parts of the stems from which the fruit-stalk
subsequently arises, and are surrounded by perichætial leaves, so as to
resemble in general aspect the antheridial heads. They are readily found
in _Tortula_ and _Funaria_, which are always at hand.

The archegonia (Pl. III. fig. 21) differ in form from the antheridia,
being flask-shaped, with a neck and a dilated base. The neck contains a
slender canal, and within the base is a special embryonal cell, from
which the capsule is subsequently formed. The spermatozoa of the
antheridia pass down the canals of the archegonia, and fertilize the
embryonal cells; but one archegonium only comes to maturity in each
head, the others ceasing to grow, and withering, in which condition they
are found at the base of the fruit-stalk when the capsule is fully
formed. The embryonal cell grows by subdivision, so as to form a
stalk-like body, which as it rises extends the archegonium upwards until
it splits across near the base. Thus the archegonium becomes split
horizontally into two parts, the upper and longer of which forms the
calyptra, whilst the lower remains as a very short tube or sheath
(_vagi´nula_) surrounding the base of the fruit-stalk. The cellular
stalk-like body then swells at the summit, the swollen portion gradually
becoming developed into the capsule, by resolving itself into an outer
wall lined inside with a coat forming the outer row of teeth at the top,
and within this a thinner membrane or spore-sac, the cleft upper margin
of which forms the inner teeth; and within this are contained the
spores. The mass of cells within the spore-sac remains, forming a
central column, called the _columella_.

These stages of growth may be readily traced in _Funaria_. In Plate
III., fig. 40 represents two fertilized archegonia of the natural size,
surrounded by the perichætial leaves; fig. 33 is a still more advanced
archegone. In fig. 34 the calyptra has separated from the vaginule, and
contains the stalk-like body, which is represented alone in fig. 32, the
dark summit indicating the commencing formation of the capsule. Fig. 35
represents the young capsule, in which all the parts are more advanced
in growth.

When the seeds of mosses germinate, they produce at first a green
_Conferva_-like filament, which branches at one end, the cells
containing green endochrome, while brownish little roots are given off
from the other end. The young leafy buds or young stems arise from these
confervoid filaments.

_Examination._--In the examination of the mosses, the capsules should be
viewed as opake objects while fixed in the forceps; and to discover the
minute structure of the teeth of the peristome, a capsule should be
wetted with spirit, then immersed in water, slit up with fine scissors,
and spread out with the mounted needles, so as to form a transparent
object. In this way, the curious structure of the teeth becomes very
distinct.

It must be noticed that, in the mosses, the antheridia and the
archegonia usually occur in separate flower-like heads; or the mosses
are either monœcious or diœcious (p. 38).

_Preservation._--The mosses may be dried under pressure, and preserved
entire in the same manner as the ferns or the flowering plants. If
simply dried without pressure, their structure can be readily made out
at any future time, by immersing them in water, or by keeping them for a
few hours in a moist atmosphere. The minute structures of mosses may be
mounted in solution of chloride of calcium, or in glycerine; they keep
extremely well without closing the cells.



CHAPTER VII.

ALGÆ (ALGA, SEA-WEED).


The plants belonging to the Class Algæ grow in water, either in that of
the sea or in fresh water; a few of them, however, being found on damp
earth, damp walls, &c. The marine Algæ are commonly known as sea-weeds;
but the fresh-water Algæ generally receive but little popular notice,
forming, as they do, slimy masses or strata, of a green or brownish,
sometimes red, colour.

Algæ are of simple structure, consisting entirely of cells; in some
these are single, in others, united end to end, to form threads or
filaments, or grouped into a leaf-like expansion, or collected few
together into a little spherical group or a flat plate. They possess
none of the fibres, vessels, or ducts of the higher plants, although
some long and slender cells, existing in the stalks of the fronds of the
larger kinds, bear considerable resemblance to woody fibre. They exhibit
no distinction of stem and leaf, but consist of fronds representing the
stem and leaf combined and undistinguishable. And the term frond must be
understood to signify the separate parts arising from the point of
attachment when they are fixed; and in the case of those which are
unattached or free, the entire plant is called a frond.

The Algæ are divided into three Orders, viz. the Fucoid´eæ or
olive-coloured Algæ, the Florid´eæ or red, and the Confervoid´eæ or
green Algæ.

FUCOID´EÆ, Fucoid Algæ, or Melanospor´eæ (μἑλας, black or dark). The
plants composing this order form our largest sea-weeds, and are found
everywhere in the sea and on the sea-shore. They are of an olive-green
or olive-brown colour, and usually become darker on drying.

PLATE IV. [PAGE 64.]

MARINE ALGÆ.


Fig.

1. _Dasya coccinea_, piece of.

2. _Dasya coccinea_, portion with capsule (ceramidium).

3. _Dasya coccinea_, portion of main filament.

4. _Dasya coccinea_, section of filament.

5. _Melobesia polymorpha._

6. _Melobesia polymorpha_, portion with capsules (ceramidia).

7. _Jania rubens._

8. _Jania rubens._

9. _Lithocystis Allmanni._

10. _Ceramium nodosum._

11. _Ceramium nodosum_, filament.

12. _Ceramium rubrum_, filament.

13. _Ceramium rubrum_, tetraspore.

14. _Ceramium rubrum_, end of filament.

15. _Ceramium rubrum_, capsule (favella).

16. _Fucus vesiculosus_, receptacles of.

17. _Fucus vesiculosus_, capsules (conceptacles).

18. _Fucus serratus_, antheridial conceptacles.

19. Spore of _Fucus vesiculosus_.

20. Antheridia of _Fucus serratus_.

21. _Plocamium coccineum_, sporophyll.

22. _Plocamium coccineum_, with capsule (coccidium).

23. _Plocamium coccineum_, portion of frond.

24. _Plocamium coccineum_, tetraspore from sporophyll.

25. _Polysiphonia fastigiata_, portion of.

26. _Polysiphonia fastigiata_, filament with capsules (ceramidia).

27. _Polysiphonia fastigiata_, portion of filament.

28. _Corallina officinalis._

29. _Corallina officinalis_, portion of filament.

30. _Corallina officinalis_, capsule (ceramidium).

31. _Enteromorpha compressa._

32. _Enteromorpha compressa_, cells of frond.

33. _Hypnea purpurascens_, capsule (coccidium).

34. _Hypnea purpurascens_, spores.

35. _Hypnea purpurascens_, filament.


[Illustration: Plate IV.

W Bagg sculp

_London: John Van Voorst._]

_Fúcus vesiculósus_, with its parallel-sided or linear olive-brown
fronds, is known to every one as the seaweed which is hung up to act as
a weather-glass. The fronds have a central stout vein, or midrib, and
scattered air-bladders, mostly in pairs.

The fructification consists of yellowish oval enlargements of the ends
of the fronds, called the _receptacles_ (fig. 16); but these are
somewhat variable in form, being often angular or truncate. On holding
one of the receptacles to the light, it will appear to contain a number
of little grains imbedded in its substance, slightly projecting above
the surface, and in the centre of each is a minute dot or pore. These
grains are the capsules, or _conceptacles_, and contain the spores. The
substance of the receptacles is composed of a beautiful network of
colourless, jointed, cellular fibres (figs. 17 _a_ and 18 _a_), the
meshes of which are filled with a transparent gelatinous substance; but
immediately around the conceptacles the cells are shorter and more
closely packed. The spores (fig. 19) are arranged in the conceptacles in
a radiate manner; they are brown, and surrounded by a colourless sac,
called the _perispore_ (περἰ, around, σπορἀ, seed); and between them are
numerous slender, colourless, jointed filaments, the _paraph´yses_. The
spores are not, however, truly single spores, for they ultimately divide
into eight segments or sporules, each of which is capable of producing a
new plant.

In the conceptacles of some fronds of _Fucus_ no spores will be found,
the conceptacles (fig. 18) being filled with elegantly branched
colourless filaments (fig. 20), the ends of many of them being distended
into little yellowish sacs; these are the _antherid´ia_. The antheridia
contain large numbers of exceedingly minute spermatozoa, furnished with
two cilia, and very similar to those existing in the antheridia of the
mosses; these, escaping through the pore of the conceptacle, fertilize
the spores.

The figure (20) in the plate was drawn from a conceptacle of _Fucus
serrátus_, another common species, differing from _F. vesiculosus_ in
having the margins of the frond serrate; the antheridia of the two
species do not, however, differ in any important respect. To examine the
conceptacles of _Fucus_ and their contents, the receptacles should be
soaked in water, if not fresh, and thin sections made with a sharp
knife. They form very beautiful objects, and may be preserved in
chloride of calcium or glycerine.

FLORID´EÆ, or Rhodosper´meæ (ῥὁδον, rose, σπἑρμα, seed).--The second
Order of Algæ, forming the Florideæ (_flos_, a flower), comprises the
red sea-weeds; a few of them are purple, or greenish-red; so that by the
colour alone they may be readily distinguished from the Fucoids, and
from nearly all those of the next Order, the Confer´voids. A few of them
are leaflike, or possess flat fronds; but most of them consist of finely
divided or feathery fronds. They are often found upon the sea-shore of a
dirty white colour, the colouring matter having been decomposed or
washed out by rain.

We shall consider a few of the genera and species under the heads of the
families to which they belong.

CORALLINA´CEÆ, the Corallines, or calcareous Algæ.--In this family we
have the beautiful _Corallína officinális_ (Pl. IV. fig. 28), the common
Coralline, which is very abundant on the sea-shore, attached to larger
sea-weeds, shells, and rocks. It is hard and chalky, from the presence
of a large proportion of carbonate of lime in its minute cells. The
fronds are composed of jointed and branched filaments. The
fructification (figs. 29 and 30) consists of ovate cellular capsules, or
_ceramid´ia_ (κερἁμιον, earthen vessel), placed mostly at the ends of
pinnate stalks, and containing a tuft of somewhat club-shaped jointed
spores, springing from the base of the capsules (fig. 30). When ripe,
the spores escape from a pore or hole in the end of the capsules. The
spores are 4-jointed, and hence are called _tet´raspores_ (τἑτρα,
_four_).

To observe these spores, the capsules must be soaked in strong vinegar
for some hours, and then washed with water, to dissolve the calcareous
matter.

_Jánia rúbens_ (Pl. IV. figs. 7 and 8) is another common and very
elegant little coralline, and is of a pale red colour. It differs from
the last in the branches being dichot´omous (δἱχα, in two, τομὀς,
cutting) or forked, instead of pinnate. The capsules, or ceramidia, have
also two short horn-like branchlets, placed one on each side, near the
end.

The genus _Melobésia_ has the frond crustaceous, _i. e._ forming a hard
crust or layer. _M. polymor´pha_ (Pl. IV. figs. 5 and 6) is common on
shells, stones, &c. The capsules (ceramidia) here form little blunt
cones, scattered over the crusts, and containing the tufted tetraspores,
as in _Corallina_.

_Lithocys´tis Allman´ni_ (fig. 9) is very minute, and not uncommon upon
sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous
layer of cells, closely investing the body to which it is attached; its
fructification is unknown.

Leaving the family of crustaceous Florideæ, we shall now pass to those
of softer consistence, although all the marine Algæ contain a
considerable quantity of calcareous matter.

RHODOMELA´CEÆ.--In this family we have the large genus _Polysiphónia_,
in which the frond (Pl. IV. figs. 25 and 26) is filamentous, the
filaments being apparently jointed and longitudinally striated. The
filaments are composed of rings of cells (fig. 27), arranged end to end,
and containing dark endochrome. The ends of the colourless cell-walls
separating the endochromes of the cells of adjacent rings produce the
jointed appearance; while the striated appearance is caused by the dark
cells being elongate and the cell-walls thick, so as to form white
interspaces.

The fructification consists of capsules (ceramidia), attached to the
sides of the branches, containing pear-shaped spores, with tetraspores
imbedded in swollen branches of separate plants.

_Polysiphónia fastigiáta_ (fig. 25, a small piece) is common, attached
to the fronds of _Fucus_. Its filaments are rigid, bristle-like, of the
same breadth throughout, forked, and forming globular brown or yellowish
tufts, from 2 to 4 inches long. The joints are broader than long, each
with 16-18 of the dark cells. In the centre of the branches of this
sea-weed is a row of curious objects (fig. 26 _a_), consisting of a
dark-coloured body surrounded with irregular spiny marginal processes,
and with a colourless short process above and below. These require
further investigation.

_P. nigres´cens_ is also common among masses of seaweeds. Its filaments
are brown, pinnate, the branches awl-shaped, and the joints about as
long as broad.

_Dásya coccin´ea_ (Pl. IV. figs. 1 and 2, representing small portions of
a filament) is a very common filamentous red sea-weed of the same
family. The filaments are 6-8 inches long, and bipinnate,--the larger
ones somewhat resembling those of _Polysiphonia_, in being composed of
parallel longitudinal cells, arranged round the centre, but containing
also smaller intermediate cells; while the smallest branches (fig. 2),
which arise in tufts, consist of a single row of cells, little longer
than broad. The fruit consists of ovate capsules (ceramidia), placed at
the base of the branches, and containing a round mass of spores. There
is also another kind of fructification, occurring on distinct plants;
this is formed of one or two rows of tetraspores, immersed in pod-like
capsules, called _stichid´ia_ (στἱχος, row).

DELESSERIA´CEÆ.--In this family, the typical or most highly developed
genus of which, _Delesséria_, has beautiful leaf-like rose-red fronds,
we shall examine the common _Plocámium coccin´eum_ (Pl. IV. figs. 23 and
22). This is of a fine red colour; the fronds are from 2 to 12 inches
long, and consist of numerous branched and bushy filaments. These are
compressed, with the branchlets arranged in alternate rows on the two
margins of the stem. The end branchlets are acute and pectinate
(_pecten_, a comb), or arranged like the teeth of a comb. The cells of
which the filaments consist are small and angular, giving the surface
the appearance of being elegantly netted under a high power. The fruit
(fig. 22) consists of globular capsules, called _coccid´ia_ (κὁκκος, a
berry), placed in the axils or forks at which two branches separate, and
containing a mass of angular spores. There are also tetraspore-pods
(stichidia), as in _Dasya_; and tetraspores (fig. 24) in little
leaf-like altered branches (fig. 21), called _spor´ophylles_ (σπορἁ,
seed, φὑλλον, leaf), and antheridia are present.

RHODYMENIA´CEÆ.--In this family we have _Hyp´nea purpuras´cens_ (Pl. IV.
fig. 35). The filamentous pale purple frond of this sea-weed is from 6
inches to a foot or more in length, the branches being alternate and
spreading. The fructification consists of capsules or coccidia (fig.
32), immersed in the branches, and containing the spores (fig. 34).
Tetraspores also occur in the cells of the surface of the filaments.

CERAMIA´CEÆ.--This is the last family to be noticed. _Cerámium nodósum_
(Pl. IV. figs. 10 and 11), which belongs to it, is a most delicate and
elegant filamentous sea-weed, commonly found attached to other
sea-weeds. The filaments are hair-like or capillary, irregularly
dichot´omous; they consist of colourless cells, 3 or 4 times as long as
broad, and with thick walls. The junctions of the cells are swollen
(fig. 11), and covered with very minute dark red cells, giving them a
knotty and jointed appearance to the naked eye or under a low power. The
globular capsules, or _favel’læ_ (_favus_, a honeycomb), containing the
numerous spores, are situated at the ends of the branchlets, and the
tetraspores (fig. 11) in twos or threes on the outer margins of them.

In _Cerámium rúbrum_, which is also very common, being found attached to
stones, rocks, and the larger Algæ, the filaments (Pl. IV. fig. 12) are
stouter than in _C. nodosum_, branched so as to form tufts from 2 to 10
inches long, and their ends forked, with the tips hooked inwards (fig.
14). The central cells of the filaments are large and rounded, and their
walls are entirely covered with a layer of very small angular red cells.
The globular capsules (fig. 15), or _favellæ_, are situated on the suter
surface of the branches, stalked, and supported by 3 or 4 short
branchlets. The tetraspores (fig. 13) are imbedded in the branches,
towards the ends. The capsules called favellæ differ from the coccidia
in the walls being simply membranous, while the walls of the coccidia,
like those of the ceramidia, are composed of cells.

The tetraspores are usually imbedded, among the cells of the superficial
layer of the filaments, and are not very easily recognized by an
unpractised eye; it will be observed in the figures that they are
sometimes cleft horizontally, at others obliquely.

CONFERVOID´EÆ.--This Order consists principally of the green freshwater
Algæ, although some of them are yellowish brown, purple, or red, and
some are marine. Their general structure may be best illustrated by
selecting certain common examples from the families composing the order.
The families are 13 in number. The species which are figured in the
plates are found in fresh water, except when otherwise stated.

PLATE V. [PAGE 70.]

FRESHWATER ALGÆ.


Fig.

1. _Conferva floccosa_, single filament.

2. _Lyngbya muralis_, single filament.

3. _Ulothrix mucosa_ (?), filament.

4. _Synedra radians_, prepared frustules.

5. _Synedra radians_, tuft of natural frustules.

6. _Cladophora crispata_, with zoospores (_a_).

7. _Batrachospermum moniliforme_, portion of filament.

8. _Batrachospermum moniliforme_, filament.

9. _Closterium acerosum._

10. _Draparnaldia glomerata._

11. _Spirogyra quinina._

12. _Spirogyra nitida_, filaments conjugating.

13. _Zygnema cruciata._

14. _Coleochæte scutata._

15. _Xanthidia_ in flint.

16. _Micrasterias rotata._

17. _Gomphonema acuminatum._

18. _Gomphonema acuminatum_, prepared frustules.

19. _Ankistrodesmus falcatus._

20. _Pediastrum Boryanum._

21. _Hyalotheca dissiliens._

22. _Pinnularia viridis._

23. _Fragilaria capucina._

24. _Fragilaria capucina_, prepared frustules; _s_* side view of _F.
virescens_.

25. _Scenedesmus quadricauda._

26. _Schizogonium_, probably a form of _Lyngbya_.

27. _Campylodiscus costatus._

28. _Nitzschia minutissima_, front view.

29. _Nitzschia minutissima_, valves.

30. _Epithemia turgida._

31. _Diatoma elongatum_, natural frustules.

32. _Diatoma elongatum_, prepared frustules.

33. _Cocconeis placentula._


[Illustration: Plate V.

_W Bagg sculp_

_London: John Van Voorst._]

CONFERVA´CEÆ.--On removing some of the soft green matter found adhering
to the stems of water-plants in any pool or pond, one of the species of
_Conferva_, _C. flocculosa_, is almost sure to be met with. On close
inspection with the naked eye, the green filaments of which it consists
are just visible, as extremely fine, soft, silky threads; and, under a
high power of the microscope, the filaments are seen to be unbranched,
and composed of a single row of cells (Pl. V. fig. 1), or joints, as
they are called in technical works; these are 2 or 3 times as long as
broad. In some specimens the joints are swollen, so as to present a
rounded outline. In another common species, _C. bombyc´ina_, the
filaments are somewhat more slender, and the joints are from 3 to 5
times as long as broad.

_Cladoph´ora crispáta._--This Confervoid forms large, entangled,
dull-green masses, composed of branched, tufted, somewhat rigid and
coarse filaments. It is often a troublesome overrunner of the
fresh-water vivarium. The filaments are composed of thick-walled cells
(Pl. V. fig. 6), from 4 to 6 times as long as broad, and often
containing minute starch-granules.

The Confervaceæ have two modes of reproduction. The first of these
consists in the division of the endochrome of the joints into a number
of distinct segments, each of which becomes furnished at one end with
two very slender cilia (Pl. V. fig. 6 _a_). After a time, these ciliated
bodies, which are called _zo´ospores_ (ξῶον, animal, σπορἀ, seed) or
_gonid´ia_ (λονἠ, seed, εἶδος, resemblance), escape from the cells
either by their rupture or through a papillary orifice, and swim about
in the water, ultimately losing their cilia and growing into cells
resembling those of the parent plant. In the second method, which
occurs, for instance, in _Conferva bombycina_, certain of the joints
enlarge so as to become rounded or inflated; their endochrome then
becomes coated with a new cell-wall, and so forms a spore, which
subsequently escapes from the cell and germinates.

CHÆTOPHORA´CEÆ.--_Draparnal´dia glomeráta_ forms small green jelly-like
masses, adhering to sticks and stones in water. These consist of
branched filaments (Pl. V. fig. 10), prolonged at the ends into
colourless hair-like points, and composed of single rows of cells, the
green endochrome forming a band across the middle of each cell, the ends
being colourless.

In _Coleochæ´te scutáta_ (Pl. V. fig. 14) the cells are closely united,
so as to form a minute flat green disk. In the natural state, this
beautiful little object adheres to the submerged leaves and stems of
water-plants, and is therefore difficult to be found. But if a few
healthy water-plants be kept for some time in a glass jar, the little
_Coleochæte_, which is about as large as a pin’s head, will often be
found adhering to the side of the glass.

BAT´RACHOSPER´MEÆ.--The members of this family resemble to the naked eye
the little masses of _Draparnaldia_, and they are found in the same
localities. They are of various colours, being green, brown, purple, or
red. They consist, as in _Bat´rachosper´mum monilifor´me_ (Pl. V. fig.
8), of branched filaments, which have a knotty appearance under a low
power. The larger filaments are composed of cells arranged end to end,
the knots consisting of numerous smaller whorled filaments, _i. e._
filaments arising from around them at the same level (fig. 7). The cells
composing the whorled filaments are beaded or moniliform, and are
prolonged into colourless hair-like points. The globules seen among the
branches (fig. 7) consist of groups of spores.

ZYGNEMA´CEÆ.--The members of this family resemble the Confervaceæ in
consisting of simple cellular filaments (Pl. V. figs. 11, 13), but
differ from them in the elegant arrangement of the endochrome: this
forms beautiful spiral bands, as in _Spirog´yra quini´na_ (fig. 11), or
star-shaped masses, as in _Zygne´ma crucia´ta_ (fig. 13). A remarkably
curious phenomenon met with in them is the manner in which the spores
are formed, and which is known as _conjugation_. In this process the
opposite cells of two distinct filaments, lying near together, push out
protrusions of the cell-walls, which meet and open into each other,
forming cross tubes, as in _Spirog´yra nit´ida_ (Pl. V. fig. 12). The
contents of the opposite cells of the filaments then unite, forming
large spores, which remain either in the cells of one of the filaments
or in the cross tubes.

The three species figured are common in clear pools.

DESMIDIA´CEÆ.--The Desmidiaceæ are truly microscopic, few of them being
even perceptible to the naked eye without the very closest examination.
They are very beautiful, on account of their bright green colour and
often elegant forms. Many of them are very common, existing in every
pond or ditch; but they abound most in clear open boggy pools on heaths.
On placing some water containing them in a glass jar and exposing it to
the light, they will often be found adhering to the glass, or forming a
layer on the surface of the muddy sediment.

The Desmidiaceæ consist mostly of single cells (Pl. V. figs. 9, 16); and
these consist of two equal halves or segments, as indicated either by a
paleness of the endochrome or a deep constriction at the line of
junction, which is called the _suture_. The cells are often elegantly
lobed and cut, or spiny; and in many the surface exhibits minute
markings, consisting of little protrusions of the cell-wall outwards, or
inflations, as they are called.

Their reproduction is effected by division and conjugation. In the
process of division the cells gradually separate at the suture, and a
new half-cell is formed upon each old half, which grows until it attains
the size and form of the original half of the parent-cell. The
conjugation is effected by two cells approximating so that their sutures
are near together, the cells then open at the sutures, and the effused
contents become united to form a spore or sporange, from which one or
more individuals are formed. These spores are often elegantly spinous on
the surface.

Among the species selected for illustration is _Clostérium acerósum_
(Pl. V. fig. 9), in which the cells are single, elongate, very slightly
curved or lunate; the endochrome forms long bands, often containing
numerous globules or transparent vesicles. At each end of the cells is a
round transparent vesicle, containing exceedingly minute granules, which
exhibit a trembling kind of motion. Between the cell-wall and the
cell-contents very fine currents may also be detected, forming a
circulation resembling that in the hairs of _Tradescantia_.

In _Micrastérias rotáta_ (Pl. V. fig. 16) the cells, which are single,
are deeply cleft into two segments at the suture, the segments being
again regularly cut into five lobes, which are toothed or dentate.

In _Hyalothéca dissil´iens_ (Pl. V. fig. 21) the cells are united into a
cylindrical filament, and are surrounded by a very delicate gelatinous
sheath. In _Ankistrodes´mus falcátus_ (Pl. V. fig. 19) the cells
resemble those of _Closterium_ in shape, but are aggregated into
faggot-like bundles, and are very much smaller. In the beautiful little
_Pedias´trum boryánum_ (Pl. V. fig. 20) the cells are aggregated into a
disk, the marginal cells being bidentate or having each two points, so
that the whole resembles a star. The species of _Pediastrum_ are
reproduced by the contents of each cell subdividing into numerous
ciliated segments or zoospores, which subsequently escape in a mass from
the cell, ultimately losing their cilia, and reuniting to form a new
individual.

In _Scenedes´mus quadricau´da_ (Pl. V. fig. 25) the oblong cells are
united, side by side, the outermost cells being furnished with a bristle
at each end. The division of these cells takes place obliquely, so that
in the divided groups the cells are situated in two alternate rows.

The spores of many of the Desmidiaceæ are spinous, and they are often
found fossil in flint (Pl. V. fig. 15). To detect them in this
substance, thin slips of flint may be examined under a half-inch power;
or the chips of flint may be cemented to a slide with balsam, and ground
down on a hone.

The Desmidiaceæ must be mounted in the moist state: the smaller ones
will keep well in chloride of calcium; but the larger ones are injured
both by that liquid and by glycerine. The remarks made upon mounting, at
page 15, are especially applicable to these delicate organisms.

DIATOMA´CEÆ, or Siliceous Algæ.--The members of this family are singly
very minute; but when existing in large numbers, as they are often found
at the bottom of ditches and ponds, on the submerged stems of
water-plants, or upon damp ground, they form yellowish-brown evident
masses or strata. They occur both in sea-and in fresh water. They
usually consist, like the Desmidiaceæ, of single cells, which are called
frustules. But they are especially characterized by the cell-walls being
imbued with silica or flint, so that if the frustules be heated to
redness upon the point of a knife or a slip of platinum-foil, which
destroys the organic part of the cells, the coat of silica remains,
exhibiting the perfect form of the original cells or frustules. The form
of the frustules is very different in the various genera and species, as
represented in Pl. V. figs. 22, 23, 27, 30, 31, and Pl. VI. figs. 16,
17, 23; and it will be noticed that, in the figures, two views are given
of each frustule, _f_ indicating the front view, and _s_ the side view.
In all the front views, as in Pl. V. fig. 22, one or more lines will be
observed running longitudinally down the middle of the frustules, and
corresponding to the indications of division existing in the cells of
the Desmidiaceæ. Each half of a frustule is called a _valve_, and the
line at which these valves meet is called the _suture_. That side or
aspect of the frustule in which the suture lies (fig. 22 _f_) is the
_front view_; and the other aspect of the frustule (fig. 22 _s_) is the
_side view_. The frustules are mostly four-sided--the main breadths of
the two opposite valves forming two sides, and the bent margins of the
valves, with the back and front of the hoop, forming the two other
sides; so that the view presented by the side of a frustule is the same
as that of a single valve. The suture is the line at which the division
of the frustules takes place in the formation of new individuals. In
this process the cell-contents divide into two parts, as in ordinary
endogenous cell-formation,--the two new surfaces thus produced becoming
coated with a new portion of cell-wall or valve, so that two frustules
now occupy the place of the original one. At the same time a siliceous
band, encircling the frustules at the line of suture, is formed to fill
up the interval between the edges of the parent valves; this is the
_hoop_ (Pl. V. fig. 22 _f_; Pl. VI. fig. 10 _f_), and beneath it lie the
two newly formed valves. In many cases I believe that each half-frustule
becomes coated with a new entire cell-wall, with its siliceous valves.

The frustules of the Diatomaceæ are constantly undergoing division when
in vigorous growth. After the frustules have divided, the new ones
either separate entirely, as is perhaps most commonly the case; or they
remain united, sometimes completely, so as to constitute a filament (Pl.
V. fig. 23), while at others the frustules cohere only at the angles (Pl
VI. fig. 23), so as to form a zigzag chain.

In some species, the frustules are attached to foreign bodies by means
of a gelatinous cushion (Pl. V. fig. 5; Pl. VI. fig. 7); while in others
they are situated upon a simple or branched gelatinous stalk (Pl. V.
fig. 17) or stipes (_stipes_, a stem).

When the frustules are examined in the living state, the cell-contents
resemble those of ordinary vegetable cells, excepting in regard to the
colour, and exhibit granules and globules, and sometimes a nucleus is
visible. It will also be noticed that many of the free frustules move
slowly across the field of the microscope; but the cause of the motion
is unknown.

When the frustules have been properly prepared, the surface of the
valves exhibits a number of coarser or finer markings, consisting of
dots, lines (striæ), flutings, or networks, &c., arranged with great
regularity and symmetry, often of extreme minuteness, and rendering them
exquisite objects under the microscope. The exhibition of these markings
requires not only that the valves shall be properly prepared and
mounted, but that the object-glasses be of good quality, and that the
management of the light be thoroughly understood; so that to a beginner,
their examination is often a matter of great difficulty; for only the
very coarsest or largest of these markings can be perceived in the
natural frustules.

The appearance of these markings, and even their apparent absence or
presence, frequently depends upon the kind of illumination used: thus,
under one kind of illumination the valves may appear simply white or
coloured, while under another they appear covered with lines, and under
a third with dots. It will often be observed, also, that the colour of
the valves varies according to the illumination and the power used--the
same valve appearing white, yellow, brown, blue, &c.; and the wet or dry
state of the frustules often cause a decided difference in their
appearance as regards colour.

To illustrate the forms and markings of the frustules and valves, we may
select the following species taking first those which occur in fresh
water.

In _Epithémia tur´gida_ (Pl. V. fig. 30), the side-view or valve (_s_)
exhibits transverse or slightly radiating lines, with intermediate rows
of dots--these markings being continued over the margins of the valves
so as to appear also in the front view (fig. 30 _f_), ceasing at the
hoop. The frustules are curved or arcuate (_ar´cus_, a bow) in the side
view, oblong and narrowed at the ends in the front view.

In _Fragilária capucína_ (Pl. V. fig. 23), which is extremely common in
fresh-water pools, &c., the frustules are united side by side into long
filaments, which are often twisted. In the separate and prepared
frustule, the front view (Pl. V. fig. 24 _f_) is rectangular, the valves
(_s_) being narrowly lance-shaped or lanceolate. The valves under
ordinary illumination appear colourless and without markings, but, by
proper management of the light, very fine transverse striæ are seen upon
them, consisting of rows of very minute dots. Fig. 24 _s*_ represents
the valve of _Fragilaria vires´cens_, a nearly allied species.

_Diat´oma elongátum_ (Pl. V. fig. 31) is often found with the above. Its
frustules are coherent at the angles. The front view (fig. 32 _f_) is
rectangular, often slightly narrowed in the middle; and the valves are
narrowly linear, and capitate at the ends; they are also transversely
striated.

In _Synédra splen´dens_ (Pl. V. fig. 5) the frustules radiate from a
soft gelatinous cushion. They are linear in the front view (fig. 4 _f_),
the valves (fig. 4 _s_) being gradually narrowed or attenuated from the
middle to the ends, and exhibit transverse striæ interrupted opposite a
middle longitudinal line.

In _Campylodis´cus costátus_ (Pl. V. fig. 27) the frustules are
disk-shaped and curved, so as somewhat to resemble a saddle. The
markings consist of central dots, with radiating coarse flutings.

_Nitzsch´ia minutis´sima_ (Pl. V. fig. 28) has oblique valves, _i. e._
the front half of the suture is not opposite the back portion; the
valves (fig. 29) are constricted in the middle, and the ends narrowed
and prolonged. The markings consist of a row of oblong dots or puncta
(_punctum_, a point). This species often forms yellowish layers upon
damp paths, &c.

In the next group, the valves have a longitudinal line running down the
middle of the valves, with a little knob or nodule in its centre (Pl. V.
fig. 22 _s_), both consisting of internal thickened portions of the
valves.

In _Cocconéis placen´tula_ (Pl. V. fig. 33) the valves are oval, and the
markings consist of longitudinal rows of minute dots, with a marginal
row of puncta; these markings are invisible under ordinary illumination.

In _Gomphonéma acuminátum_ (Pl. V. fig. 17) the frustules are attached
to a branched stalk (stipes); they are wedge-shaped or cúneate
(_cúneus_, a wedge) and transversely striate (fig. 18), the striæ
consisting of dots.

In _Pinnulária vir´idis_ (Pl. V. fig. 22), which is very often seen
slowly traversing the field of the microscope when a drop of pond-water
is examined, the frustules in the front view are linear, the valves
being elliptic oblong, and transversely striated, the striæ consisting
of furrows.

In _Gyrosig´ma (Pleurosig´ma) attenuátum_ (Pl. VI. fig. 16) the valves
are sigmoid, or somewhat resemble a Greek ς (sigma) in outline, and the
markings consist of rectangularly crossed rows of very fine dots; in the
front view, the frustules are linear-oblong with truncate ends.

_Tabellária flocculósa_ (Pl. VI. fig. 23) has the frustules adherent
only at the angles, as in _Diátoma_. They are rectangular, and in the
front view exhibit a row of longitudinal dark lines interrupted in the
middle; these have been compared to the vittæ of the fruit of the
Umbelliferæ, and have received the same name.

Among the marine species may be mentioned _Melosíra nummuloídes_ (Pl.
VI. fig. 9), in which the frustules are united into a chain or
cylindrical filament. This is very common among sea-weeds, &c.; and it
illustrates well the process of division of the frustules (fig. 10 _f_).
The valves are covered with fine dots, and near each end of the
frustules is a projecting rim encircling it, and appearing as a curved
line extending beyond the margin of the frustule in the front view. In
_Actinocy´clus undulátus_ (Pl. VI. fig. 5) the frustules are separate,
disk-shaped, and the valves are divided into six equal parts by six
rays, each alternate portion of the surface of the valves being situated
on a lower level than those adjacent, so that an alteration in the focus
is required to bring into view the dots on any two adjacent divisions of
the valve. The surface of the valves is covered with easily recognized
dots. The form of the surface is best seen in the front view (fig. 5
_f_) when the frustule is placed on its edge.

_Rhabdonéma arcuátum_ (Pl. VI. fig. 7), which is very commonly found
attached to sea-weeds, resembles _Tabellária_ in the frustules having
the vittæ (Pl. VI. fig. 8 _f_). The frustules form short filaments,
attached by a little gelatinous cushion. The valves have transverse
striæ, interrupted in the middle (fig. 8 _s_).

_Gyrosig´ma (Pleurosig´ma) angulátum_ (Pl. VI. fig. 17) resembles _G.
attenuátum_ in the sigmoid form; but the markings consist of lines
crossing each other obliquely; and these are resolvable into rows of
dots (fig. 17 _a_) under suitable illumination.

In _Coscinodis´cus radiátus_ (Pl. VI. fig. 3) the frustules are
disk-shaped, the valves being elegantly sculptured with easily
recognized cell-like markings or dots, so as to resemble a piece of
vegetable cellular tissue. But in some other species the dots are very
minute, and difficult to be shown satisfactorily. These markings consist
of depressions or pits in the surface of the valves. That this is the
case may easily be seen by examining a fragment of the valve, when the
shadings of the broken ends of the netted thicker portions, which
project like teeth, strongly contrast with the difficultly
distinguishable portions of the thin interspaces. The fossil forms from
the Bermuda deposit are best for the investigation of this structure;
many of these are extremely beautiful microscopic objects, their
markings resembling those on the engine-turned back of a watch.

The detection of the finer markings of the Diatomaceæ, which, according
to my view, consist of depressions like those upon the valves of
_Coscinodiscus_, is a matter of great difficulty to those who are
unaccustomed to the use of the microscope, and who have not a complete
set of apparatus. The main point to be attended to in bringing them into
view, is to use one-sided oblique light, _i. e._ to turn the mirror by
its stem as much as possible to one side, and then to incline it so as
to throw the light upon the object. In this way the valves of the
species of _Gyrosigma_, for instance, appear covered with lines (Pl. VI.
figs. 16 and 17); but the lines are spurious, _i. e._ they are the
optical expression of rows of minute dots (figs. 16 _a_, 17 _a_); and
when oblique light is thrown upon the valves from all sides, by means of
a special achromatic condenser, in which the central rays are excluded,
the dots become distinct, and the markings resemble those on the valves
of _Coscinodiscus_. To show the finer dots clearly, a valve should be
crushed, so as to obtain a fragment as flat as possible; for the surface
of the valves is curved more or less in all the species. The valves of
_G. angulatum_ are generally used to test the quality of the
object-glasses of the microscope, and also for practice in “making out”
the lines and dots; there are, however, many Diatomaceæ more difficult.

As the nature of these markings is a disputed point, the discussion of
which is not adapted for an elementary work, I must refer for further
details to the ‘Micrographic Dictionary;’ it may be remarked, however,
that some observers have regarded them as cells, and others as
elevations or tubercles on the surface of the valves.

The preparation of the valves for showing the markings should be
effected by burning the frustules, or the mass containing them, on a
strip of platinum-foil over a spirit-lamp. The incinerated mass should
then be transferred to a slide, and the valves separated with the
greatest care by a bristle mounted in a hair-pencil stick under a low
power of the microscope.

This is, however, a substitute for the proper method, which is dangerous
in the hands of one unpractised in chemical manipulation. It is
this:--The mass of Diatomaceæ (the water containing it having been
carefully poured off as far as possible) is put into a Florence
oil-flask, and strong nitric acid (aquafortis) gently added, more than
sufficient to cover it. The mixture is then carefully boiled over a
spirit-lamp for some time. When it is cold, distilled water is added,
the whole shaken, and allowed to settle. The watery part is then gently
poured off, more water added, and this poured off after settling, and
the process repeated until a drop of the water evaporated to dryness on
a slide leaves no residue. The Diatomaceæ then form a white sediment at
the bottom of the water, and can be transferred to a slide with a
dipping-tube. The drop is then dried with a gentle heat, and the valves
mounted as dry transparent objects (p. 12).

If the valves have coarse markings, they may be mounted in balsam; but
if the markings are fine, balsam makes them much more difficult of
detection.

Many of the most beautiful Diatomaceæ are found in the fossil state; and
specimens of these are sold already mounted. I would advise those
unacquainted with them to purchase a slide of the “Bermuda” or
“Richmond” earth, which abounds in the species of _Coscinodiscus_; and
of the “San Fiore deposit,” which contains many species of _Epithemia_,
_Navicula_, _Pinnularia_, &c. These may be procured from Mr. Norman, 178
City Road, or from the microscope-makers.

VOLVOCIN´EÆ.--The Volvocineæ are inhabitants of clear fresh-water pools,
on heaths and bogs. They are very minute, of a rounded or plate-like
(tabular) form, of a green colour, and are pretty readily distinguished
from most of the other Algæ by their free motion; for they swim about in
the water like animals, as which they were formerly considered. They
consist usually of groups of thick-walled soft cells, each being
furnished with one or two cilia, by means of which the movement of the
compound bodies is produced.

In the beautiful _Volvox globátor_ (which is not uncommon) the cells
form a hollow sphere (Pl. VI. fig. 18), studded with exceedingly minute
green spots or zoospore-like bodies, representing the green endochrome
of the component cells, and from each of which very fine radiating lines
extend, so as to give the surface a netted appearance; the lines
consisting of delicate processes of the endochrome, which may be
compared with those existing in the cells of the hairs of
_Tradescantia_. In the interior of the parent globes are often seen
several young organisms, usually eight, of a deep green colour; these
escape by the rupture of the parent, so as to form independent beings.
Sometimes they are found of a yellow colour, and furnished with a thick
transparent coat; these are called “resting spores,” as they remain for
some time before undergoing their full development.

The cilia of _Volvox_, of which there are two to each of the component
cells, are difficult to detect; they are best seen when the organism is
dried without a cover, or after moistening them with a little solution
of iodine, which dyes them brown.

_Synúra volvox_ (Pl. VI. fig. 13) is a still more minute member of this
family, and is often found rolling along among _Confervæ_. The greenish
zoospore-like bodies of this Alga have one cilium only, and arise from a
common centre by a narrowing of the base (fig. 14).

In _Gónium pectorále_ (fig. 11 _a_) the green bodies, which are sixteen
in number, and furnished each with two cilia, are grouped into a flat
square plate; and in the very minute _Gonium tranquil´lum_ (fig. 11 _c_)
these bodies are also sixteen in number, and arranged in a tabular form,
but are without cilia.

SIPHONA´CEÆ.--The structure of this family may be illustrated by the
genus _Vauchéria_, of which two or three species are common on damp
ground or in freshwater pools, forming a green layer. At first sight,
the filaments of which the little plants consist appear like those of a
stout _Conferva_; but on close examination they are found to be
branched, and not jointed, consisting of a single cell from end to end
(Pl. VI. fig. 26). The reproduction is effected by the agency of two
kinds of organs, antheridia and capsules (sporangia), situated near each
other (fig. 26 _a_) on the walls of the filaments, of which they are
protrusions or outgrowths--their cavities being separated from that of
the filament by a partition or septum. The antheridia produce
spermatozoa, and the sporangia each a spore, the one fertilizing the
other in the ordinary manner. In addition to this method of
fructification, zoospores are also produced--the ends of the filaments
becoming swollen, the contents cut off by a septum, and forming single
large zoospores covered with cilia, the further development of which
resembles that occurring in the Confervaceæ.

OSCILLATORIA´CEÆ.--The members of this family are commonly found in
stagnant water or on shaded damp ground, especially in the cold seasons
of the year, forming green strata or masses.

PLATE VI. [PAGE 84.]

FRESHWATER ALGÆ.


Fig.

1. _Oscillatoria autumnalis._

2. _Oscillatoria nigra._

3. _Coscinodiscus radiatus._

4. _Nostoc minutissimum._

5. _Actinocyclus undulatus._

6. _Bacterium._

7. _Rhabdonema arcuatum._

8. _Rhabdonema arcuatum_, prepared frustules.

9. _Melosira nummuloides._

10. _Melosira nummuloides_, prepared frustules.

11. _a_, _b_, _Gonium pectorale_: 11 _c_. _Gonium tranquillum._

12. _Spirulina oscillarioides._

13. _Synura volvox._

14. _Synura volvox._

15. _Gyrosigma attenuatum_, front view.

16. _Gyrosigma attenuatum_, side view; 16 _a_, portion of a valve.

17. _Gyrosigma angulatum_; 17 _a_, portion of a valve.

18. _Volvox globator._

19. _Glœocapsa._

20. _Chara vulgaris_, globule.

21. _Chara vulgaris_, portion of filament.

22. _Chara vulgaris_, branch with nucule and globule.

23. _Tabellaria flocculosa._

24. _Tabellaria flocculosa_, prepared frustules.

25. _Palmella cruenta._

26. _Vaucheria Ungeri_ (_sessilis_).

27. _Vaucheria Ungeri_, capsule.


[Illustration: Plate VI.

_W Bagg sculp_

_London: John Van Voorst._]

_Oscillatoria autumnális_ (Pl. VI. fig. 1) occurs everywhere upon damp
shaded banks of ditches, especially when newly made, forming a
greenish-black closely adherent stratum. Under the microscope it is seen
to consist of innumerable palish-green filaments; these are jointed or
transversely striated, some being straight, others curved, the ends
often exhibiting a writhing or worm-like movement. The appearance of
these fibres is peculiar, seeming as if they were solid throughout, and
so differing from that of the Confervaceæ, in which the cell-walls are
readily distinguishable from the cell-contents. The fibres easily break
across at the joints; and the last few segments are often narrowed and
rounded, so as to form a blunt point. When they have been left in water,
they exhibit colourless tubular sheaths surrounding and extending beyond
them. These sheaths consist of the consolidated outer portions of the
cell-walls; for when the cells undergo transverse division, and expand
by growth in the direction of the length of the filament, the original
septa or inner walls are broken through, and their remains may often be
seen on the inner surface of the sheath, appearing as little teeth.

_Oscillatoria nigra_ (Pl. VI. fig. 2) is another very similar species,
forming blackish-green masses, and is common in ditches. It has longer
filaments than the last, with narrowed and slightly curved ends; and the
endochrome is distinctly granular.

In two other genera of this family, _Vib´rio_ and _Spirulína_, the
filaments are spiral. _Vib´rio spiril´lum_ is excessively minute,
colourless, and found in decomposing vegetable mixtures. The short
filaments move rapidly through the water, with a corkscrew-like motion.
In _Spirulína oscillarioídes_ (Pl. VI. fig. 12), which is more rarely
found in clear pond-waters among _Confervæ_, the filaments are greenish,
and form a beautiful simple spiral, resembling that of a very slender
spiral vessel.

_Lyng´bya murális_ (Pl. V. fig. 2) is very common on damp walls, gravel
walks, &c. It forms a bright grass-green layer, consisting of somewhat
rigid curled filaments. The endochrome is usually broader than long; and
the cells of the filaments are often found empty, the endochrome having
escaped in the form of gonidia.

Pl. VI. fig. 6 represents a species of _Bactérium_ which is not uncommon
in decomposing vegetable liquids; the filaments are short, curved,
pointed at the ends, and have four joints.

Fig. 26 represents a _Schizogónium_, found upon damp paths. The
filaments resemble those of _Lyngbya_, but are united in pairs.

Fig. 3 represents a filament of a _U´lothrix_, which is common in
freshwater pools, showing the curious manner in which the endochrome is
arranged in the cells, forming bands partially lining the cell-walls.

NOSTOCHA´CEÆ.--Two species of the typical genus _Nos´toc_ will serve to
represent this family. _Nos´toc commúne_ is found on damp ground or in
ponds, and forms to the naked eye firmish, olive-green, skin-like,
plaited masses, an inch or more in diameter. Under the microscope it is
seen to consist of numerous beaded fibres, imbedded in worm-like
gelatinous sheaths; these are curved and interwoven to form the compound
mass. In the middle of many of the filaments is an enlarged colourless
cell, called the vesicular cell, which is related to the reproduction,
but in a manner not yet determined.

_Nostoc minutis´simum_ (Pl. VI. fig. 4) forms solid gelatinous
bluish-green masses, varying in size from a pin’s head to a pea; it is
found upon unhealthy water-plants kept in glass vessels. The component
filaments are very slender, wavy, and the sheaths often have a brownish
tinge.

ULVACEÆ.--These Algæ are mostly marine--some, however, being found in
brackish or fresh water, or on damp ground, thatch, moss, &c. They are
generally of considerable size, forming flat or tubular fronds, often
several inches long, a few being filamentous. They consist of one or
more sheets or layers of cells, containing mostly green endochrome. This
at first fills the cells, but subsequently becomes converted into single
spores, or subdivided into numerous ciliated zoospores.

_Ul´va latis´sima_ is very common on the sea-coast, being found attached
to stones, shells, &c. It forms a broad, flat, green, rounded or oblong,
thin frond, wavy and crumpled at the margins, and from 6 to 18 inches in
length. The minute cells form two layers, adherent to each other. The
zoospores formed are numerous in each cell.

_Enteromor´pha compres´sa_ (Pl. IV. fig. 31) is also common in the sea
and in brackish ditches; it is often found floating. The frond is green,
tubular, flattened or compressed, and branched, the branches being
usually simple and narrowed at the base. The frond consists of two
layers of minute cells, separated by a space rendering it hollow. The
zoospores are numerous in the cells (fig. 32).

PALMELLA´CEÆ.--These Algæ are found in fresh or salt water, or on damp
earth, wood, &c. They are green or red, forming round or irregular
masses or strata. They consist of loosely connected cells, imbedded in a
gelatinous mass or matrix, thus forming a frond.

_Chlorococ´cum vulgáre_ (Pl. II. fig. 1) is very common upon the bark of
elm-trees, palings, &c., forming a green granular crust. It consists of
minute rounded or oval cells, mostly undergoing division into twos,
fours, or eights. These cells are attached to the sides or ends of very
fine colourless filaments. It is most probable that this organism, which
is usually placed among the Algæ, consists of the gonidia of a Lichen.

_Chlorococcum muror´um_ forms a somewhat similar but soft and thin green
layer, upon damp walls or other porous bodies. It consists of very
minute oval green cells, with thick walls, and imbedded in the ends of
prolongations of a gelatinous matrix.

_Palmel´la cruen´ta_ (Pl. VI. fig. 25) forms a portwine-red layer at the
bottom of damp walls or on the ground. It is composed of pale red cells,
imbedded in no definite order in a colourless gelatinous matrix. The
cells are filled with red granules, and are often found undergoing
division.

Pl. VI. fig. 19 represents a species of _Glœocap´sa_, in which the
cell-envelopes do not soften and unite to form a gelatinous matrix, as
in _Palmella_ and other members of the family, but are persistent. This
species occurs in fresh water containing Confervæ.

CHARA´CEÆ.--This family consists of the single genus _Chára_, the
systematic position of which is not agreed upon by authors; as however
its structure will be better understood after what has been gone over,
it may be conveniently considered here.

There are several species of _Chara_, the one illustrated, _Chara
vulgáris_ (Pl. VI. fig. 21), being commonly found in ditches and pools.
It consists of long main stems, often a foot in length, which are
branched, and surrounded at tolerably regular intervals by whorls of
branchlets. In some species, the stems and branches consist simply of
elongated cells, arranged end to end; while in others, of which _Chara
vulgaris_ is one, the central cells are surrounded by a number of
narrower spirally arranged cells, forming an outer coating.

The Charæ have long formed interesting microscopic objects, on account
of the circulation of the protoplasm being visible in the cells, as in
the hairs of _Tradescantia_. This is best seen in those species in which
the outer layer of cells is absent from the stems, and which were
formerly arranged in a separate genus (_Nitella_). But it may also be
seen in the stems and especially the young branchlets of any of the
other species; and as the granules of the protoplasm are large, the
phenomenon is more easily witnessed than in _Tradescantia_.

The fructification consists of two kinds of organs, viz. red _globules_
(Pl. VI. fig. 22) representing the anther-organ, and green capsules
(fig. 22), or _nucules_, corresponding to the ovaries. The structure of
the globules is very curious. Their transparent walls (fig. 20) consist
of eight somewhat triangular plates, each of which is composed of cells
radiating from a centre; and from the inside of each of these centres
arises a tubular cell extending to the middle of the globule, the
unattached ends giving origin to numerous colourless coiled filaments,
consisting of minute cells arranged end to end, each containing a very
minute coiled spiral fibre, to which are attached two exceedingly
slender cilia. These ciliated fibres are the spermatozoa. The capsules
or nucules (fig. 22), which are situated near the globules, are
urn-shaped, coated with spiral cells, and crowned with five shorter
cells. When the globules are ripe, they become ruptured by the
separation of the valves; and the spermatozoa, escaping from the cells
of the coiled filaments, swim about and enter a canal in the capsules to
fertilize the ovule contained within.

The _Charæ_ grow readily in a glass jar of fresh water, with a few
pebbles at the bottom; and if the plants be not overgrown with
Confervoids, the fructification will continue to be produced almost
throughout the year.

The circulation is best seen in the whorled branchlets, a portion of the
growing ends being placed in a live-box, or simply laid upon a slide in
water and covered with thin glass.

_Preservation._--The Algæ are best preserved in two ways,--the entire
fronds being dried upon paper under pressure, as directed for the Ferns;
and small portions, showing the minuter structures and fructification,
being mounted in chloride of calcium or glycerine. If it is required to
preserve the marine Algæ according to the first method, they should
first be immersed for a time in fresh water, to dissolve out the saline
matters derived from the sea-water, which would keep them damp and
ultimately spoil them. After these matters have been removed, the fresh
water should be changed, and pieces of paper placed beneath them while
suspended in the water; on withdrawing the paper carefully, keeping the
Algæ at the same time spread out, they may be made to retain the
required position; and when the water has drained away, and the
remaining moisture has mostly evaporated, they may be submitted to
pressure in a press.

The Confervoid Algæ may be conveniently spread out upon paper and
preserved in the same manner, as some of the distinguishing characters
are founded upon their appearance in the dry state, their adhesion to
the paper, &c. Moreover they can then at any time be minutely examined,
by the immersion of a small portion in water.



CHAPTER VIII.

LICHENS.


The Lichens are found growing upon the bark of trees, old palings, &c.
Those most easily seen with the naked eye form grey or coloured dryish
patches or pendulous tufts; while the smaller ones are singly easily
overlooked, from their minute size and close adhesion to the _mátrix_ or
body upon which they grow, forming, by their aggregation, the grey or
otherwise-coloured dry and brittle coatings of almost every tree or
decaying branch.

The Lichens derive their nourishment from the air, and not from the
matrix--in this respect differing from the Fungi, with some of which, as
we shall presently see, they agree in the structure of the fruit.

The structure of the Lichens is simple, no distinction of root, stem,
and leaves existing in them, although certain dry root-like fibres exist
in many of them, by which the plant is fixed to the matrix. The whole
consists mainly of a frond or _thal´lus_ (θαλλὀς, a leaf). This is
either raised above the surface of the matrix in a shrubby form, or
spread upon the surface as a flexible lobed layer (Pl. II. fig. 2), or
it is dry and brittle (crustaceous) and closely adherent (Pl. II. fig.
26).

The fructification consists of little saucers, disks, or streak-like
furrows, often of a different colour from the thallus, the structure of
which will be best illustrated by reference to a few common species.

PARMELIA´CEÆ.--_Parmélia parietína_ (Pl. II. fig. 2) is a very common
Lichen, found on the bark of trees, on old palings, &c. It is of an
orange-yellow colour, the thallus being flat, lobed, and scalloped
(crenate) at the margins. The structure of the thallus may serve to
represent that of most of the Lichens. It consists of three layers,--an
upper cortical or epidermic layer, which is continued over the margins
to the under surface of the thallus, to form the under layer; and
between these is the middle or medullary layer. The medullary layer
consists of interwoven fibres, which are more closely packed towards the
upper and under surfaces, so as to give them a cellular appearance. Near
the upper part of the medullary layer, a number of minute rounded green
cells exist, lying loosely in its meshes (Pl. II. fig. 4 _a_). These
green cells (gonidia) appear to correspond to the buds of the higher
plants, and, when detached from the plants, they are capable of growing
into new individuals.

On the upper surface of the thallus of _Parmelia_ (fig. 2) the
fructification may be observed. This consists of saucers or shields
(_apothécia_, ἁποθἡκη, a repository), formed of a raised and expanded
portion of the thallus (fig. 3), and containing the spores. The spores
are enclosed in closely set upright cells, or spore-sacs (figs. 4 _b_
and 5 _b_), called _as´ci_ (ἁσκὀς, a bottle) or thecæ; and intermingled
with the asci are filaments, enlarged and coloured at the ends
(_paraphyses_), which are probably abortive asci.

In systematic works upon the Lichens, the saucers and their contents are
included in the term apothecia, the saucer alone being called the
_excip´ulum_ (_excipulum_, a receiver); the mass of asci and paraphyses
forming the _nucleus_ or _thalámium_ (θἁλαμος, a bed).

The yellow spores are very minute, each ascus containing eight of them,
and they are divided by a transverse partition or septum.

Near the margins of the lobes of the thallus are small dark points.
These are the pouting mouths of little capsules (_spermogonia_) sunk in
the substance of the thallus, and containing numerous filaments,
terminated by very minute stick-shaped bodies (_spermatia_), which break
off and escape through the orifices of the capsules. These are probably
the representatives of the anthers of flowering plants and of the
antheridia of the ferns. They are, however, so difficult to find and
examine, that I must refer to the Dictionary for a further description
and figures of them.

LECIDIN´EÆ.--This family contains the genus _Cladónia_, three or four
species of which are common on boggy heaths, banks, &c., viz. _Cladonia
coccif´era_, the Scarlet Cup-moss (Pl. II. fig. 21); _C. pyxidáta_, the
Common Cup-moss (Pl. II. fig. 23); _C. vermiculáris_ (Pl. II. fig. 22);
and _C. rangiferína_, the Reindeer Moss (Pl. II. fig. 24). The thallus
of these Lichens forms little rounded irregularly overlapping scales,
with scalloped edges, overgrowing the surface upon which the Lichens are
found. The fruit-stalks, or _podétia_ (ποῦς, a foot), are hollow
(fistulose), and either simple and dilated into cups (Pl. II. fig. 23),
or branched with the corners or angles between the adjacent branches
perforated. The apothecia in the young state resemble those of
_Parmelia_ on a small scale; but as they approach maturity, the centre
becomes pushed up, so that the spore-layer is extended over the ends of
the stalks. In _C. coccifera_ and _pyxidata_ the cups are proliferous at
the margin; _i. e._, branches upon which the apothecia are placed arise
from it. The asci and paraphyses are very minute, but do not differ
essentially in structure from those of _Parmelia_. In _C. vermicularis_
the podetia are pointed and more solid than in the other species, the
apothecia forming very minute spots at their apices.

GRAPHID´EÆ.--To this family belong _Gráphis scripta_ (Pl. II. fig. 26)
and _Opeg´rapha betulína_ (Pl. II. fig. 30). These little Lichens are
easily overlooked, from the thin and but slightly raised thallus being
only visible to the naked eye as a discoloration of the bark of the
trees upon which they grow; while the fructification is very minute,
forming little black streaks or _lirel´læ_ (_lira_, a furrow),
irregularly arranged, and resembling somewhat the letters of some of the
Oriental alphabets.

In _Graphis scripta_ (fig. 26) the thallus is thin, somewhat membranous,
smoothish, shining, greyish white, and faintly bordered with black. The
lirellæ (fig. 27) are partly sunk in the bark, winding and narrow, some
being simple, others branched; and they are surrounded by a raised
border, formed by the thallus. The lirellæ are lined at the sides with a
black (carbonaceous) layer or _excip´ulum_, within which are situated
the asci and paraphyses. The spores (Pl. II. fig. 29) are 8-cleft, the
segments being again divided longitudinally into little spores or
sporidia.

In _Opegrapha betulina_ (Pl. II. fig. 30), which is found on the bark of
the birch-tree, the thallus is thin, dirty yellowish white, bordered
with black. The lirellæ (figs. 31, 32) are mostly simple, without a
raised border of the thallus, and the excipulum forms a complete lining
to them. The spores (fig. 33 _a_) are 3-cleft, and taper at the ends.

CALICI´EÆ.--_Calic´ium clavel´lum_ (Pl. II. fig. 6) is a pretty little
Lichen, growing upon old boards and farm-buildings. The thallus is
granular and greyish white. The apothecia (fig. 7) are stalked and
black, but of a lighter colour than the mass of spores forming the
nucleus. The spores are very minute, black, oblong, and divided by a
transverse septum.

The Lichens are divided into two Orders, according to whether the
apothecia are open before the spores are ripe, as in the species noticed
above, or whether the apothecia only open to discharge the ripe spores.
The first Order forms the _Gymnocar´pi_ (γυμνὀς, naked, καρπὀς, fruit);
the second forms the _Angiocar´pi_ (ἁγγεῖον, vessel, capsule).

_Preservation._--The Lichens are readily preserved, on account of their
dry nature; they need simply be kept in a dry place, and glued to pieces
of card. If room is an object, they may be dried under pressure, as in
the case of the flowering plants. When re-moistened, the minute
structures may be easily made out by sections. The smaller ones may be
mounted dry, in cells made of the wax cement (p. 16). The minute
structures keep well in chloride of calcium or glycerine.



CHAPTER IX.

FUNGI.


The Fungi form the lowest class of plants: as examples of them, may be
mentioned mushrooms, toadstools, puff-balls, the mould of paste, the
blue mould of cheese, &c. The more minute Fungi are very common, forming
beautiful microscopic objects, although they are rarely studied by the
microscopic observer.

Fungi live usually upon rotting or decaying vegetable substances, as
rotten wood, the dead leaves and stems of plants, &c.; but sometimes
they are found upon living plants, and some of them exist upon decaying
animal matters, and even in living animals.

Fungi exhibit no separation of root, stem, or leaves, as exists in the
higher plants; nor do they contain chlorophyll, the presence of which is
so generally associated with the idea of a plant. But they consist of
aggregations of mostly elongate cells, forming branched and interlacing
colourless fibres, buried like roots in the substance (matrix) upon
which they grow, and from which they derive their nourishment; this
portion of the Fungus is called the _mycélium_ (μὑκης, a fungus). The
portion of the Fungus projecting beyond the surface of the matrix is the
fructification; and this is the part usually called the fungus, the
mycelium being overlooked by a casual observer. So that here we have a
character distinguishing the Fungi from the Lichens, which derive their
nourishment from the air, and not from the matrix. The absence of the
green cells, or gonidia, forms another character by which the nearly
allied members of this class of plants can be distinguished.

PLATE VII. [PAGE 96.]

FUNGI.


Fig.

1. _Agaricus micaceus_: _a_, gills.

2. _Agaricus campestris_: _a_, spores; _b_, basidia.

3. _Physarum album_, on a piece of stick.

4. _Physarum album_, spores.

5. _Uredo segetum_, spores.

6. _Uredo caries_, spores.

7. _Uredo candida_, on leaf of Shepherd’s Purse (_Capsella_): _s_,
spores.

8. _Æcidium grossulariæ_, sorus.

9. _Æcidium grossulariæ_: _p_, spore-capsules (peridia); _s_,
anther-capsules (spermogonia).

10. _Nemaspora crocea_: _a_, spores.

11. _Torula casei._

12. _Torula herbarum_, on a piece of stick.

13. _Torula herbarum_, spores.

14. _Phragmidium bulbosum_, on bramble-leaf.

15. _Phragmidium bulbosum_, stylo-spores and paraphyses.

16. _Puccinia graminis_, on a piece of straw.

17. _Puccinia graminis_, spores.

18. _Sporocybe alternata_, filament and spores.

19. _Botrytis parasitica_, on Shepherd’s Purse.

20. _Botrytis parasitica_, spores and filaments.

21. _Rhinotrichum_, species of.

22. _Rhinotrichum_, heads of spores.

23. _Rhinotrichum_, spores detached.

24. _Rhinotrichum_, spores.

25. _Penicillium glaucum._

26. _Penicillium glaucum_, head of spores.

27. _Coremium leucopus._

28. _Tubercularia vulgaris._

29. _Tubercularia vulgaris_, divided receptacle.

30. _Tubercularia vulgaris_, filaments.

31. _Tubercularia vulgaris_, spores.

32. _Sphæria fragiformis._

33. _Trichothecium roseum_, on a piece of stick.

34. _Trichothecium roseum._

35. _Trichothecium roseum_, filaments and spores.


[Illustration: Plate VII.

W Bagg sculp

_London: John Van Voorst._]

The fructification of the Fungi occurs in two distinct forms, in one of
which the seeds or spores are naked, and situated at the ends of slender
cells or filaments, whilst in the other the spores are contained in
usually flask-like cells, called asci, similar to those occurring in the
Lichens. In a few Fungi, antheridial organs, called spermogonia, as in
the case of the Lichens, have also been detected. The Fungi are divided
into six Orders, from each of which a few species may be selected to
illustrate their structure more in detail.

HYMENOMYCE´TES (ὑμἠν, membrane, μὑκης, fungus). This is the highest
Order of Fungi, containing a large number of genera and species; as
examples of which may be mentioned the common Mushroom, Toadstools, &c.

Their general structure may be illustrated by the examination of the
common Mushroom (_Agar´icus campes´tris_); the species figured (Pl. VII.
fig. 1), however, being _Agaricus micáceus_, which is common at the root
of trees, the bottom of decaying posts, &c.

The vegetative part of the fungus consists of a cotton-like mycelium,
which is composed of slender, colourless, interwoven filaments,
popularly known as the spawn. The portion commonly called the mushroom
corresponds to the fructification, and consists of certain parts visible
to the naked eye. These are an expanded portion at the top, forming a
hemispherical cap, receptacle, or _píleus_ (_pileus_, a cap), and a
stalk, or _stípes_, upon which the cap is supported. On the under
surface of the cap are a number of nearly parallel, radiating,
dark-coloured plates or _gills_, somewhat resembling the gill-plates of
a fish. The dark colour of the gills arises from the presence of the
spores, which are coloured, although in some species they are white. The
surface of the gills, upon which the spores are situated, is called the
_hyménium_ (ὑμἠν, membrane). The spores (Pl. VII. fig. 2 _a_) are
microscopic and very minute, and are situated at the ends of little
stalks or points (Pl. VII. fig. 2 _b_), called _sterig´mata_ (στἡρυγμα,
a prop), which are four in number, and consist of prolongations of the
colourless cells of the hymenium; and these cells are the _basid´ia_
(βασἱδιον, a little base). The detection of the basidia requires great
care, as they are very minute and transparent; the best way to observe
them is to cut a very small portion from the uninjured edge of one of
the gills with a fine pair of scissors, and to examine it in water. If
the gills have been bruised, the spores are easily rubbed off, and their
connexion with the basidia destroyed.

In the young state of the Mushroom, the fructification appears as little
knobs upon the spawn or mycelium. Upon cutting these through
perpendicularly, the cap and stalk are found to be enclosed in a skin or
wrapper, called the _vol´va_ (_volva_, a wrapper), and the margin of the
cap is continuous with the surface of the stalk, the connecting membrane
forming the _veil_, or _vélum_ (_velum_, a veil). As the Fungus grows,
the cap rises and bursts the volva, which withers and disappears; and
the veil is torn through, the portion remaining in connexion with the
stalk encircling it as a collar or ring (_an´nulus_).

_Merúlius lach´rymans_, the Dry-rot Fungus, which belongs to the same
family (Agaricíni) as the Mushroom, deserves notice from its very
destructive action upon decaying timber. The filaments of the mycelium
may readily be detected in the rotting wood by examining a thin section
in water. The receptacle forms a yellowish-orange or brownish flattened
mass, some inches in breadth, with white downy margins; and the surface
exhibits folds arranged so as to form large irregular pores, instead of
gills as in the Mushroom.

In other families the cap and stalk appear fused together and
undistinguishable; or the fructification assumes the form of the cap or
the stalk only, and the hymenium does not form gills. Thus in the family
Polyporei (the members of which are common on the trunks of trees and on
rotten posts, and some of which are very large) the basidia are situated
upon the inner surface of tubes immersed in the under part of the mass,
their orifices forming minute pores, yet visible to the naked eye. In
other families the basidia are placed upon the outer surface of a
club-shaped or branched receptacle (Clavarini), or upon external
prickle-like points (Hydnei), &c.

In the family Tremellini the receptacle forms a gelatinous mass, and the
basidia are situated upon its surface, terminating the filaments of
which it is composed. One species, _Dacrymy´ces stillátus_, is very
common on fir posts in the winter and spring, forming little roundish,
yellowish-orange, cushion-like masses.

GASTEROMYCE´TES (γαστἠρ, belly, hollow, μὑκης, fungus).--This order of
Fungi, which contains the Puff-balls and many others not readily
procured, must be very briefly noticed. The spores are contained in a
capsule or perid´ium (πηρἱδιον, a little bag) which is often of large
size, and are situated upon a hymenium forming folds, partitions
(septa), or a lattice-work.

A small species, _Physárum album_ (Pl. VII. fig. 3), is often found
growing upon rotten stems of plants and decaying sticks. The capsules
are minute, grey, brittle, and black within, from the presence of the
spores (fig. 4), which are lens-shaped, and arise from the ends of short
filaments.

CONIOMYCE´TES (κὁνις, dust, μὑκης, fungus).--This Order contains some
beautiful microscopic Fungi, several of which are very common. Many of
them grow upon living plants, while others are found upon decaying
stems, sticks, &c. The mycelium consists of inconspicuous, fine
filaments, which run beneath the epidermis and bark of leaves and stems,
or exist in the intercellular passages, the fruit bursting through the
surface. The spores are short-stalked, forming _sty´lospores_ (στῦλος,
stalk, σπὁρος, seed) or _conid´ia_ (κονἱδιον, little dust). But there is
great confusion in the descriptions of the spores of the same Fungus by
different botanical authors, some describing the fruit (in Pl. VII. fig.
15, for instance) as composed of rows of spores, while others regard it
as forming a single septate (_septum_, a partition) or partitioned
spore.

_Tor´ula herbárum_ (Pl. VII. fig. 12) is very common on the decaying
stems of plants, especially those belonging to the Parsley order
(Umbelliferæ), forming greenish-black streaks or patches. The spores
(fig. 13) are grouped into chains or beaded (moniliform) rows, with very
short stalks, and these are crowded to form the black patches visible to
the naked eye. Under the microscope the spores appear of a brown colour.

_Torula cásei_ (Pl. VII. fig. 11) forms reddish or white patches upon
decaying cheese. It consists of branched, interwoven, tufted filaments
(_flocci_), with comparatively large spherical spores arranged in rows
at their ends.

_Nemas´pora crócea_ (Pl. VII. fig. 10) is a very curious member of this
Order, and is found upon decaying beech-sticks. It appears as an
orange-coloured tendril-like gelatinous mass of spores, bursting through
a little pore on the surface of the bark. The spores (fig. 10 _a_) are
very minute, slender, and curved, and under a high power appear jointed.

_Æcid´ium grossuláriæ_ (Pl. VII. fig. 8) is found very commonly on the
leaves of the gooseberry-bush. It forms to the naked eye oval or rounded
spots (_sori_), of a red colour; and on close examination, the spots
appear dotted with yellow points. Each point is the orifice of an open
capsule (_peridium_), which has burst through the epidermis of the leaf
(Pl. VII. fig. 9 _p_). The capsules are split or lacerated at the
margins, and form little cups containing the spores. The spores are very
minute, yellow, and are arranged in closely packed moniliform rows. The
red colour depends upon the altered chlorophyll of the leaf. On the
leaves containing the spore-capsules or peridia will be found smaller,
brownish-yellow capsules (_spermogonia_) partly imbedded in their
substance (Pl. VII. fig. 9 _s_). These contain minute filaments
(_sterigmata_), terminated by short rows of rounded cells (_spermatia_),
which are supposed to exert an antheridial function. The species of
_Æcidium_ are very numerous, and many of them are extremely common--as
those upon the nettle, the barberry, the dandelion, the wood-anemone,
the violet, and buttercups. The groups of capsules form exquisite opake
objects under a low power of the microscope.

_Phragmid´ium bulbósum_ (Pl. VII. fig. 14) is another very beautiful
coniomycetous Fungus. It forms little reddish, afterwards sooty dots
upon the under surface of the leaves of various species of Bramble
(_Rúbus_). The oblong spores (fig. 15) are from 2-to 4-septate, and
stalked, the stalks being swollen or bulbous at the base. The spores,
which appear brown when magnified, are covered with little knobs
(tuberculate) on the surface; and the uppermost little spore or
sporidium is terminated by a minute point (apiculate). Among the spores
are numerous barren filaments or paraphyses.

_Puccin´ia gram´inis_ (Pl. VII. fig. 16) is to be found everywhere upon
damp rotting straw, and upon grasses. It forms sooty irregular streaks,
consisting of densely crowded, one-partitioned (uniseptate) spores (fig.
17), which appear brown under the microscope. This Fungus is sometimes
called “mildew.” There are numerous other species of _Puccinia_ which
occur upon common plants.

_Urédo seg´etum_ is the “smut” of wheat, barley, and oats--a fungus too
well known to the farmer. It forms sooty masses, bursting through the
epidermis of the stalk and ears of the corn, and soiling the fingers
when handled. The spores (Pl. VII. fig. 5) are exceedingly minute, and
the stalks are so slender and loosely connected with them that they are
not readily detected. Under the microscope the spores appear brown and
faintly dotted, this appearance arising from a reticulated structure of
the surface, similar to that of the poppy-seed on a very small scale.

_Urédo cáries_ is the “bunt” of corn. It grows within the grain, filling
it with a sooty, fœtid mass. The spores (Pl. VII. fig. 6) are
considerably larger than those of the last species, and their surface is
distinctly reticulated. They are attached to the filaments of the
mycelium, as in _Uredo segetum_.

The spores of both these species of _Uredo_ may be found in most kinds
of flour and bread, especially in those of inferior quality.

_Urédo can´dida_ (Pl. VII. fig. 7) is another species, forming white
dots upon the leaves of the common Shepherd’s Purse (_Capsel´la bur´sa
pastor´is_)--which is easily recognized by the form and arrangement of
the pods (fig. 19). The spores (_s_) are rather large and white.

Other species of _Uredo_ are very common upon numerous species of weeds
or wild flowering plants; and they so closely resemble each other that,
when one is known, the others are easily recognized. Usually each
species occurs upon a distinct species of plant, as is the case with
parasites generally. In many of them the spots (_sori_) exhibit a thin
membrane covering the spores, which bursts down the middle, so as to
bear some resemblance to a capsule. But there is no true capsule, the
membrane consisting of the epidermis of the leaf or stalk of the plant,
which is raised and torn by the expansion of the growing fungus; so
that the peridium is spurious, as belonging to the matrix, and not to
the fungus. It may be mentioned here that the so-called species of
_Uredo_ are not truly distinct species, but are the forms of species of
_Puccinia_, _Phragmidium_, &c.; so that the latter genera have two kinds
of fruit, one of which is a _Uredo_, the other a _Puccinia_. But I must
refer to the Dictionary for further details upon this point.

HYPHOMYCE´TES (ὑφἁω, to weave, μὑκης, fungus). In this, the 4th Order of
Fungi, are contained many of the commonest moulds which are found
growing upon decaying substances, and sometimes upon living plants. The
mycelium creeps among the particles of the substance, or the elements of
the tissues, upon which the Fungus lives, in the form of slender threads
or filaments. The spores, which are either simple or partitioned
(septate), and naked, occur either singly or in rows at the ends of fine
interwoven cottony threads or _floc´ci_ (_floc´cus_, a flock of wool),
which are generally very evident to the naked eye. The threads
supporting the spores form the _ped´icels_ (_pedicel´lus_, a little
foot). In technical descriptions, these filaments, which are usually
composed of cells arranged end to end, are said to be _septate_ (Pl.
VII. fig. 26), and not jointed, as in the case of the filaments of the
_Confervæ_, which are constructed in a similar manner. When not septate,
the filaments are said to be _continuous_.

STILBA´CEI.--To this family belongs _Tuberculária vulgaris_ (Pl. VII.
fig. 28), which is found upon decaying sticks and branches of trees,
especially the lime-tree. It forms little firm red knobs or tubercles,
each of which is a receptacle. On making a section of a receptacle (Pl.
VII. fig. 29), the interior is seen to be paler than the bright red
surface, and a short broad stalk comes into view. The receptacle is
composed of crowded cell-filaments, so short near the base as rather to
resemble cellular tissue (fig. 30); but towards the surface the
filaments become extremely slender and branched; and each branch is
terminated by a minute oblong spore, or a short row of them (fig. 31).

If a stick with this Fungus upon it be kept for some time in a damp
place, short whitish fibres, branched at the ends, and visible to the
naked eye, will be seen sprouting from around the base of the receptacle
(Pl. VIII. fig. 1). These, when examined under the microscope, appear
composed of fine filaments (Pl. VIII. fig. 2), resembling those of
_Tubercularia_, and having the minute spores at the ends. After a
considerable time, the entire receptacle of the _Tubercularia_ becomes
resolved into these fibres. In this state the Fungus assumes the
characters of an _Isária_, a genus of a different family of Fungi
(_Isariacei_), so that we have here an _Isaria_-form of _Tubercularia_.

Sometimes the tubercles of the _Tubercularia_ become darker, almost
black, harder, and granular on the surface. On making a section of them
in this state, the whole of the under portion of the surface is found to
contain little roundish capsules, containing asci and spores, and it
constitutes _Sphæ´ria fragifor´mis_ (Pl. VII. fig. 32). As the _Sphæria_
is the more complex and highly organized condition of this Fungus, the
other two conditions must be regarded as forms, and not as species of
separate genera.

DEMATIE´I.--In this family the filaments upon which the spores are
placed are not compacted as in _Tubercularia_, but separate; and they
are of a dark brown or black colour.

_Sporoc´ybe alternáta_ (Pl. VII. fig. 18) is occasionally found upon
decaying vegetable substances, forming little black velvety spots or
patches. The mycelial filaments are exceedingly minute, septate,
tapering at the ends, and terminated by a little tuft of pear-shaped
cells, from which the black simple spores arise singly.

PLATE VIII. [PAGE 104.]

FUNGI.


Fig.

1. _Isaria_-form of _Tubercularia_.

2. _Isaria_-form of _Tubercularia_, filaments.

3. _Aspergillus glaucus._

4. _Aspergillus glaucus_, filaments and heads of spores; _a_, separate
spores.

5. _Aspergillus glaucus_, head of spores.

6. _Peziza omphalodes._

7. _Peziza stercorea._

8. _Peziza stercorea_, cup (receptacle).

9. _Peziza stercorea_, asci and paraphyses.

10. _Peziza stercorea_, divided receptacle.

11. _Peziza stercorea_, bristles.

12. _Dothidea typhina_, on leaf-stalk ofgrass.

13. _Dothidea typhina_, surface of patch (stroma).

14. _Dothidea typhina_, capsules (perithecia).

15. _Dothidea typhina_, ascus containing spores.

16. _Sphæria rubella_, on nettle-stem.

17. _Sphæria rubella_, asci.

18. _Sphæria rubella_, capsules (perithecia).

19. _Sphæria rubella_, ascus and spores.

20. _Sphæria bullata_, on piece of stick; 20 _a_, section of tubercle
(receptacle).

21. _Sphæria bullata_, asci and spores.

22. _Sphæria complanata_, on piece of stick.

23. _Sphæria complanata_, tubercles (receptacles).

24. _Dothidea ulmi_, on elm-leaf.

25. _Dothidea ulmi_, asci.

26. _Dothidea ulmi_, section of receptacle.

27. _Dothidea ulmi_, spores.

28. _Chætomium elatum_; 28 _a_, spores; 28 _b_, filaments.

29. _Chætomium elatum_, on piece of stick.

30. _Hysterium fraxini_, on piece of stick.

31. _Hysterium fraxini_, receptacle.

32. _Hysterium fraxini_, ascus with spores.

33. _Erysiphe guttata_, on hazel-leaf.

34. _Erysiphe guttata_, capsule.

35. _Erysiphe guttata_, capsule (conceptacle) with fulcra.

36. _Mucor mucedo_: _a_, columella; _s_, spores.

37. _Acrostalagmus_: _a_, spores.

38. Gall on oak-leaf.

39. Gall on oak-leaf.


[Illustration: Plate VIII.

_W Bagg sculp_

_London: John Van Voorst._]

MUCED´INES.--Many of the Fungi belonging to this family are extremely
common on decaying vegetable substances, and some are found upon living
plants, to which they are very injurious. To the naked eye they usually
appear as mouldy or cottony masses, either white, black, or coloured
blue, yellow, &c. The spores are attached singly or in rows to
branchlets arising from the ends of the filaments, so as to form little
heads.

_Bot´rytis parasit´ica_ (Pl. VII. fig. 19) is common upon the
flower-stalks of the Shepherd’s Purse, forming white mealy patches. The
fruit-stalks are comparatively large and thin-walled, the branchlets
being slender, mostly curved, and terminated each by a large, spherical,
smooth, simple, white spore.

_Botrytis vulgáris_ is also common on various decaying plants. Its
filaments are grey, and the branchlets lobe-like; the spores being
minute, spherical, either white or greenish, and placed simply at the
tips.

_Botrytis infes´tans_ is the potato-Fungus. It forms white spots upon
the under side of the leaves of the potato-plant, and by some authors is
considered to be the cause of the potato-disease. The filaments are
branched at the ends, and terminated by single oval spores, which are
apiculate at the free end, and contain minute little spores or sporidia.

_Oid´ium Tuck´eri_ is the well-known destructive grape-Fungus. It forms
white cottony masses upon the vine and its grapes, the fruit-stalks
being short and terminated by one or two end-to-end oblong spores. It
appears to be the Coniomycetous form of another Fungus (_Erysiphe_).

_Trichothécium róseum_ (Pl. VII. fig. 33) is found upon rotting sticks;
very frequently upon willow-baskets kept in a damp place. It forms
little rounded, slightly raised, pinkish spots, less than the size of a
pin’s head. The branched and septate foot-stalks (figs. 34, 35) are
terminated each by a little group of obovate spores, divided by a
transverse partition (uniseptate). Sometimes this little Fungus is quite
white, at others greenish; when perfectly ripe, the spores become
oblong.

_Penicil´lium glaúcum_ (Pl. VII. fig. 25) is the common Blue Mould found
upon decaying substances, as cheese, &c., the interwoven mycelial
filaments often forming large cakes or crusts upon the surface. The
septate fruit-stalks (fig. 26) are fork-branched at the ends, the
branchlets being terminated each by a row of very minute spherical
smooth spores. On some decaying substances, as apples, gum, &c., the
fruit-stalks are found aggregated into a thick stalk, the branchlets and
spores forming a rounded head, so that the whole resembles a little blue
mushroom (fig. 27). In this form the Fungus has been placed in a
distinct genus, and called _Corémium leúcopus_. In other species the
spores are pink and white.

This little Fungus is of special interest, on account of one form of it
constituting the yeast-plant, or yeast as it is commonly called. This
consists of rounded or oblong cells, which grow very rapidly in
fermenting liquids by budding--the large quantity of sugar and gluten
present favouring the vegetative or simple growing process, at the
expense of the fructifying process. But this is only an instance of what
we constantly find in flowering plants, the use of very rich soil
rendering flowers double, which is really reducing their organs to the
state of leaves. When the sugar has become exhausted, the cells of the
yeast become longer and thinner, as if starved; they then form a more
recognizable mycelium, which extends to the surface of the liquid, and
produces finally the fruit-stalks and the _Penicillium_ fruit. _
Aspergil´lus glaúcus_ (Pl. VIII. fig. 3) is an extremely common mould
upon cheese, jams, &c. It resembles the last in appearance to the naked
eye, except that it has rather a green tinge, the heads of fruit being
much more compact and rounded. The fruit-stalks (fig. 4) are large,
bulbous or inflated at the ends (fig. 5), and from the inflations arise
the crowded rows of spores. The spores are rounded, and rough (scabrous)
on the surface. On removing most of the spores from the head of fruit,
each row of spores is found to arise from a very short stalk.

Plate VII. fig. 21 represents a beautiful species of _Rhinot´richum_,
which is found upon decaying and sickly plants, and upon rotting sticks,
forming a minute grey mould. The fruit-stalks (fig. 22) are large,
sparingly branched, septate or jointed, appearing brownish under the
microscope. Their ends are branched, mostly biternate (fig. 23), _i. e._
each branch dividing into three branchlets, and these again into three
still finer ones. The ends of the branchlets are inflated, and coated
with little points, upon each of which a smooth white spore (fig. 24) is
placed.

ASCOMYCÉTES (ἁσκὀς, a bottle, μὑκης, fungus). The Fungi belonging to
this Order are found upon the stems and leaves of plants, and upon
decaying substances, as dung, &c. They are usually evident to the naked
eye, some even equalling the Hymenomycetous Fungi in size; and many of
them are brilliantly coloured. They are in general distinguishable with
facility from the Fungi of other Orders, by the arrangement of the
spores in colourless sacs or asci (Pl. VIII. fig. 9), resembling those
noticed in the case of the Lichens. These asci are usually enclosed in a
capsule or _perithécium_. The mycelium is usually buried in the matrix,
so as not to be conspicuous.

_Helvellácei._--To this family belongs the large genus _Pezíza_, some of
the species of which are beautifully coloured, yet scarcely microscopic.
Among these may be mentioned _Pezíza omphalódes_ (Pl. VIII. fig. 6),
which forms little red masses upon damp ceilings. It does not possess
the ordinary form of a _Peziza_, which is that of a cup fixed at the end
of a stalk, like a mushroom with the cup turned inside out, the asci
lining its interior.

_Peziza coccin´ea_ is not uncommon in woods. It is whitish outside, the
interior of the cup being of a brilliant scarlet colour. It is from half
an inch to an inch in height.

_Peziza stercor´ea_ (Pl. VIII. fig. 7) is often found upon dung. The
surface of the cup of this Fungus is granular and covered with bristles
(figs. 8 & 11). The cup is concave (fig. 10), and lined with the asci
(fig. 9), among which are simple paraphyses.

The _Pezizæ_ are excellent Ascomycetous Fungi for exhibiting the asci,
as they are more or less soft, and thus sections of them may be easily
prepared, or they may readily be picked to pieces with the mounted
needles.

TUBERACEI.--In this family is contained the Truffle (_Túber cibárium_).
The asci are situated upon the inner surfaces of the winding canals
traversing the substance of the fleshy fruit (_peridium_) of which the
truffle consists.

PHACIDIA´CEI.--To this family belongs _Hystérium frax´ini_ (Pl. VIII.
fig. 30), which is found upon ash-twigs. The drawn-out capsules or
perithecia (fig. 31) are black and elliptical, with a longitudinal
fissure or orifice, and contain the asci (fig. 32) with the spores.

SPHÆRIÁCEI.--_Dothid´ea typhína_ (Pl. VIII. fig. 12) is a common Fungus
upon the stems of living grasses. It forms an orange-coloured patch or
layer encircling the stem, and covered with little dots. On making a
section (fig. 14), it appears composed of a row of oblong or obovate
closely placed capsules (perithecia) immersed in and continuous with a
finely fibrous receptacular mass (_stróma_). The asci (fig. 15) are very
slender, arising in a tuft from the bottom of the capsules, and
containing eight still more slender spores. Except under a very high
power, the spores appear as interrupted lines running down the interior
of the asci. The little dots visible to the naked eye are the slightly
projecting mouths of the capsules, which are more distinctly seen in the
magnified portion of the Fungus (fig. 13). In the young state, this
Fungus is whitish.

This Fungus cannot be mistaken for a _Uredo_, two species of which occur
upon grasses--_Uredo lineáris_ forming yellowish-brown spots, and _Uredo
rubígo_ yellow spots.

_Dothidea ul´mi_ (Pl. VIII. fig. 24) forms black, slightly raised, and
somewhat star-shaped spots upon the upper surface of the leaves of the
elm. In a section (fig. 26) the cavities are seen, containing the very
delicate asci (fig. 25). The spores (fig. 27) are oval, with a minute
septum at one end.

_Sphæ´ria rubel´la_ (Pl. VIII. fig. 16) is extremely common on the dead
stems of the nettle, &c. In this Fungus the black bottle-like perithecia
(fig. 18), containing the asci and paraphyses (fig. 17), are at first
situated beneath the epidermis, through which they at length burst. The
spores (fig. 19 _a_) are spindle-shaped, and from four-to seven-septate.
When ripe, they escape by a hole or pore in the neck.

_Sphæ´ria complanáta_ (Pl. VIII. fig. 22) is another common species,
found in hedges, on dead sticks of the softer (herbaceous) plants, as
the parsley-order (Umbelliferæ). Here the minute capsules, which are
scattered over the stems, are at first rounded, then flattened on the
top (depressed), the neck being very minute (fig. 23). The spores in
this species are exceedingly minute, oblong, and not contained in asci.

_Sphæ´ria bulláta_ also belongs to this family. It occurs upon decaying
birch-sticks, presenting to the naked eye the appearance represented in
Pl. VIII. fig. 20. The black, raised tubercles (receptacles) in their
growth burst through the bark, splitting the epidermis. They consist of
a white stroma (fig. 20 _a_), in which the bottle-shaped capsules
(perithecia) are immersed, the necks projecting slightly above the
surface as little points (papillæ). The tufted spore-sacs or asci (fig.
21), with the thread-like paraphyses, are contained within the capsules;
and within the asci are the densely packed, very numerous and minute
curved spores.

Another species, _Sphæria discifor´mis_, is also common on birch-sticks.
It differs from the last in the tubercles being perfectly flat; the
spores are also longer, straight, and spindle-shaped (fusiform).

PERISPORÁCEI.--_Erys´iphe guttáta_ (Pl. VIII. fig. 33) is a member of
this family. It appears on the under side of the leaves of the common
hazel as a pale spot; and on closely examining it with the naked eye,
little black dots are seen scattered on the surface. These are the
capsules (conceptacles), which are seated upon straight white filaments.
The filaments (_fulcra_) are six or seven in number, and are placed
under the capsule, like the legs of a stool (fig. 34); they are rigid,
and swollen or inflated at the base (fig. 35). The asci are broad and
short, and contain only two spores.

_Erysiphe maculáris_ is the very destructive hop-mildew; and other
species are common on various plants.

_Chætómium elátum_ (Pl. VIII. fig. 29) resembles little tufts of brown
hairs, occurring upon decaying herbaceous stems. The capsule (fig. 28)
is crustaceous, and covered with interlaced, rough, branched hairs (fig.
28 _b_). The spores (fig. 28 _a_) are oval, with a little point at one
end (apiculate).

PHYSOMYCÉTES (φῦσα, bladder, μὑκης, fungus).--The Fungi belonging to
this order include some of the commonest moulds growing upon decaying
vegetable substances; while others are found upon leaves, &c. The flocci
are generally very evident; and the spores are contained in little
naked, bladder-like capsules (_peridíola_) at the ends of free
filaments.

MUCORINI.--In this family we have the common mould of paste, _Múcor
mucédo_ (Pl. VIII. fig. 36). It is easily recognized by the little
spherical capsules terminating the long and tufted fruit-stalks
(pedicels), which are perceptible to the naked eye. Each capsule
consists of a simple enlarged cell, the cavity of which is separated
from that of the stalk by a septum. They are white at first,
subsequently becoming brown and black. The minute crowded spores (fig.
36 _s_) are at first oblong, afterwards spherical. In the centre of the
capsule is a club-shaped body, or _columel´la_ (fig. 36 _a_), formed by
the elevation and inflation of the septum.

A beautiful little Fungus of this family, apparently referable to the
genus _Acrostalag´mus_ (Pl. VIII. fig. 37), is sometimes found upon soft
decaying stems. The main filaments are soft, smooth, and not septate.
The pedicels are very brittle, whorled, dichotomously branched,
scabrous, and terminated each by a little scabrous spherical vesicle
(fig. 37 _a_), containing two or three oblong spores.

ANTENNARIÉI.--In this family is _Racódium_ (or _Antennária_) _celláre_,
the Wine-cellar Fungus, forming the well-known cobweb-like masses
hanging from the walls, &c. The little black capsules are seated upon
slender septate filaments, and contain numerous round spores.

In examining leaves with the view of procuring Fungi, the reader will
most likely meet with the two kinds of bodies represented in Plate VIII.
figs. 38 & 39. These are not Fungi, but galls. They arise from an
abnormal growth of the leaf-structures, produced by the deposition of
the eggs of insects (_Cynipidæ_). The well-known oak-apple, and the red
hairy-looking body found upon hedge-roses, are both galls produced in
the same way.

_Examination and Preservation._--The examination of the Fungi scarcely
requires any special remarks. They should be viewed first as opake
objects under a low power; and then sections should be made, or the
textures separated with the mounted needles.

There is some difficulty in moistening the smaller filamentous Fungi
with water, which is requisite in the determination of the arrangement
of the spores upon the branches. Hence the best plan is to lay the
Fungus upon a slide, apply a cover, then to add a drop of spirit of wine
and afterwards a little water to the edge of the cover. When thus
wetted, the spores may be more or less removed with a wet hair-pencil,
when the ends of the branches will become perfectly distinct. In
examination of the dried smaller Fungi as the _Sphæriæ_, the capsules
should be macerated for a time in water.

The softer Fungi are very difficult of preservation in the entire state;
but the sections or minute structures may be mounted in chloride of
calcium or glycerine.

The harder and drier Fungi may be preserved by drying and gentle
pressure between coarse absorbent paper. They may then be glued to
pieces of paper and labelled, in the same manner as the flowering
plants. Specimens of the capsules, as of the _Sphæriæ_, &c., may also be
mounted in the dry state, the asci being preserved in the chloride of
calcium or glycerine, in which liquids most of the smaller Fungi will
keep extremely well.

PLATE IX. [PAGE 113.]

ANIMAL TISSUES, &c.


Fig.

1. Blood-corpuscles, Human.

2. Blood-corpuscles of Bird (Fowl).

3. Blood-corpuscles of Reptile (Frog).

4. Blood-corpuscles of Fish (Stickleback).

5. Hair of Bat.

6. Hair of Mouse.

7. Hair of Mouse.

8. Hair, human.

9. Hair, human.

10. Wool, fibre of.

11. Flax, fibres of.

12. Cotton, fibres of.

13. Silk, fibres of.

14. Feather, portion of: _a_, barbs; _b_, _c_, pinnæ.

15. Bone, section of: _a_, lacunæ.

16. Cartilage, section of.

17. Feather, downy.

18. Feather, downy: pinna.

19. Shell, pearly or nacreous portion of.

20. Muscle: _a_, cellular tissue; _b_, fibrillæ; _c_, bundle of
fibrillæ.

21. Tongue of Whelk: _a_, natural size.

22. Scale of Dace.

23. Scale of Perch.

24. Scale of Cod.

25. Spermatozoa of Chub.

26. _Flustra foliacea_: _a_, cells of; _b_, animal, with the tentacles
expanded.

27. _Flustra foliacea._

28. Shell of Oyster, brown portion of.

29. Shell of Oyster, prisms of.

30. _Cyclops quadricornis._

31. _Daphnia pulex_, female.

32. _Daphnia pulex_, head of male.

33. _Canthocamptus minutus_, an Entomostracan.

34. _Cypris tristriata._

35. _Cypris tristriata_, eggs of; 35 _a_, _b_, _c_, the same hatching.

36. _Acarus domesticus_ (Cheese-mite), female.

36*. Cilia of gills of Oyster.

37. _Trombidium fuliginosum_: _a_, pulp; _b_, mandible; _c_, foot; _d_,
natural size; _e_, hair; _f_, hair of _T. holosericeum_.

38. _Acarus domesticus_, male.

39. _Membranipora pilosa._


[Illustration: Plate IX.

W Bagg sculp

_London: John Van Voorst._]



CHAPTER X.

ANIMAL ELEMENTS AND TISSUES.


The tissues of which animals consist, like those of plants, are
primarily derived from cells; in fact the essential part of the egg or
óvum, from which all perfect animals originate consists at first only of
a simple cell, with its nucleus and nucleolus.

The animal cell-wall differs from that of the vegetable cell in its
softness and delicacy--also in its chemical composition,--the former
consisting of albúminous (_albúmen_, white of egg) matter, while the
latter is composed of cellular or vegetable-cell substance.

There is also a striking difference between vegetable and animal
tissues, in the circumstance that, while the former retain their
cellular condition to a very great extent, the cells of the latter are
frequently so altered by compression and fusion together, or are
obscured by the great development of the cell-contents, that the
cell-form is obliterated, or can only be discovered by the application
of chemical reagents; and in many instances, the relation of the tissues
to the cell can only be discovered by tracing the growth or development
of the latter from its earliest stages. Hence the examination of the
elements and tissues of animals is not well adapted for those who are
unpractised in the use of the microscope; and in treating of them, we
shall simply notice a few which are most easily examined, beginning with
those found in animals belonging to the subkingdom Vertebráta
(_ver´tebra_, a spine-bone).

MAMMÁLIA.--The animals belonging to this class suckle their young; and
their blood-vessels contain red blood.

_Blood._--This blood consists of a yellowish liquid, in which very
numerous red _blood-corpuscles_ or globules (Pl. IX. fig. 1) are
suspended, and to which the red colour is owing. The blood-corpuscles
are not globular, but discoidal, _i. e._ they are circular and
flattened, the sides being slightly sunk in. Their form is best seen as
they roll over on a slide, after the application of a glass cover. The
coloured corpuscles are cells; they appear yellowish red under the
microscope, the deep red colour of the blood depending upon the large
number of them seen at once and crowded together. It need scarcely be
stated that a drop of blood may easily be obtained, by puncturing the
wrist with a clean needle. The blood is contained in the blood-vessels.
These consist of the ar´teries, which convey the blood from the heart;
the veins, which return it to the heart; and a very fine set of
intermediate vessels, called the cap´illaries. If a little water be
added to a drop of blood on a slide, _colourless corpuscles_, rather
larger than the coloured disks, will be seen scattered among the latter.
These are the colourless or lymph-corpuscles of the blood. They are
truly spherical, and granular on the surface.

_Bone._--In examining a transverse section of a bone, one or several
very large cavities will be seen with the naked eye in the centre of the
section; these contain the marrow, or medulla. In the long bones, the
medullary cavity is single, and runs longitudinally down the bone;
whilst in the flat bones the cavities are numerous, forming cancelli.
Under the microscope, thin transverse sections of bone exhibit oval or
rounded holes, or foramina (Pl. IX. fig. 15), which are sections of
canals conveying blood-vessels through the bone; these are the
_Haver´sian canals_. Around the sections of these canals are seen
numerous concentric rings, indicating layers or lamellæ of bony matter.
The substance of bone presents numerous black, somewhat elongated bodies
(fig. 15 _a_), called the _lacúnæ_ (_lacúna_, a little hollow) or
bone-corpuscles, which are however hollow, therefore not truly
corpuscles, as they were formerly considered. Between the adjacent
lacunæ run numerous fine, dark, branched lines, consisting of very
minute canals, or _canalic´uli_. If the section of bone be viewed by
reflected light, the lacunæ and canaliculi will appear white. In the
dried bone they contain air.

The structure of bone is best seen when viewed as a transparent object
in the dry state; for when the section is immersed in liquid, the lacunæ
and canaliculi become filled up.

The size and form of the bone-corpuscles and canaliculi vary in
different animals, so much so that the Class or Order to which an animal
belongs may be determined by reference to these particulars.

Bone consists of earthy salts deposited in a finely granular form
throughout the substance of cartilage. By soaking a piece of bone in
vinegar or other dilute acid, the earthy salts will be dissolved, the
soft cartilage being left. But the structure of cartilage may be best
observed by making thin sections of the gristle covering the ends of
bones. It exhibits a bluish-white basis (Pl. IX. fig. 16), in which are
imbedded numerous cells or _cartilage-corpuscles_, often undergoing
cell-division. In some kinds of cartilage, the basis is composed of
fibres.

_Muscle._--On examining a piece of the red flesh of an animal under a
low power, the mass will exhibit a number of coarse, parallel,
longitudinal, dark lines (Pl. IX. fig. 20), the substance between these
lines being marked with cross or transverse striæ, or lines, and with
fine longitudinal lines. The coarse longitudinal lines indicate the
intervals between bundles of slender fibres, or _fibril´læ_, of which
muscle consists. The fibrillæ (fig. 20 _b_, _c_) are very difficult to
separate; but when perfectly separated, they are seen to be exceedingly
slender, and to consist of alternately light and dark portions in
regular series. When the fibrillæ of the bundles are in close
apposition, as in the natural muscle, the dark portions, being in the
same lines, by their coincidence form the transverse striæ. The bundles
into which they are combined are surrounded by a delicate skin or
membrane, with a little cellular tissue.

The structure of muscle may be observed in a piece of ham which has been
soaked for a day or two in spirit of wine, the mounted needles being
used to pick it to pieces.

The above-mentioned transversely striated muscular fibre is that found
in the voluntary muscles, or those under the influence of the will. But
there are other muscles in animals which are involuntary, or not subject
to the will; in these the fibrillar structure is absent, the muscular
tissue consisting of simple elongated and nucleated cells.

_Cel´lular tissue._--This fills the interstices between the other
tissues and organs of animals, in the same manner that the vegetable
parenchyma does those of plants. It is not, however, composed of cells,
but of very fine, soft, colourless, and wavy fibres (Pl. VIII. fig. 20
_a_), aggregated into bundles, which interlace so as to leave spaces or
aréolæ between them.

The cellular or areolar tissue may be found in a piece of beef or
mutton, in the intervals of the muscular fibres.

_Skin._--The skin is composed of cellular tissue, its outer surface
presenting a number of projecting blunt points, called _papil´læ_. It
contains a large number of blood-vessels; and when the capillaries are
well filled by injection with a coloured composition, it forms a
beautiful microscopic object.

The skin is covered by the epider´mis or cuticle, which consists of
several layers of cells. It is the epidermis which is raised and covers
the bladders formed by the action of a blister applied to the skin.

_Hair._--The hair consists of long solid filaments (Pl. IX. fig. 9), and
not of hollow tubes, as was formerly supposed. It presents varieties of
structure in different animals, which agree generally in animals
belonging to the same Orders.

Hairs are implanted in pits in the skin; each is swollen at the base to
form the bulb, which is seated upon a papilla of the skin, by which it
is formed or secreted. The hair is an epidermic formation, consisting of
epidermic cells more or less flattened and altered in shape by mutual
pressure.

The colour of the hair is usually seated in the outer or cortical
portion of the stem or shaft, and arises from the presence of
aggregations of minute granules of colouring-matter or _pigment_, as the
colouring-matter of animals is called: in the human hair it forms short
longitudinal stripes (fig. 9). In the central pith or medullary portion
of the hair the cellular structure is more open and distinct than in the
cortical portion, in which the cells are so compressed and consolidated
as only to exhibit the cell-structure after treatment with reagents; and
the medullary cells often contain air.

In grey or white hairs, the whiteness depends mainly upon the presence
of air in the cells of the pith. In the gnawing or rodent animals, as
the mouse or the rabbit, the pigment is partially at least situated in
the cells of the medulla.

In the hairs of many animals, the cuticular or surface-cells of the
shaft are distinctly imbricated (fig. 5), and form beautiful microscopic
objects.

The principal interest in the structure of the hair relates to the three
points above mentioned, viz. the position of the pigment, the
arrangement of the cuticular cells or scales, and that of the cells of
the pith.

The pigment is best examined in hairs moistened with a little spirit of
wine, which displaces the air from the cells of the pith, and renders
the hair transparent; a little water should be subsequently added. The
cuticular scales are also well shown by this proceeding. Towards the
root of the hairs in the mouse, they project beyond the margin, giving
it a toothed or dentate appearance; in the hair of the mole, the bat
(fig. 5), or the wolf, this dentation may also be seen. In the hairs of
some of the foreign bats, the scales are whorled, forming very beautiful
objects.

The cells of the pith (Pl. IX. fig. 7) also present interesting
varieties, being sometimes arranged in a single row, at others in two or
more rows (fig. 6). These are best seen in hairs recently immersed in
spirit or in oil of turpentine; for if the hair be too long soaked in
these liquids, the air will be entirely displaced by them. The cells of
the pith appear black by transmitted and white by reflected light, in
the dry hairs, from the presence of air. They may be well examined in
the hair of the mouse (figs. 6 & 7), or in that of the mole. Wool, which
is the hair of the sheep, consists of curled fibres (Pl. IX. fig. 10),
in which the imbricated arrangement of the surface-scales is very
distinctly seen.

In Pl. IX. figs. 10-13 the fibres of wool, flax or woody fibre, cotton,
and silk are represented together, to allow of comparison; for the
microscope is of great assistance in discriminating these substances
when existing in textile fabrics. The fibres of wool (fig. 10) are
distinguished by their solidity, wavyness, and the imbricated scales;
those of flax (fig. 11) by their thick walls, great length, acute ends,
and their knotty appearance at intervals. The fibres of cotton (fig. 12)
are soft, flaccid, flattened, and often twisted; and those of silk (fig.
13) are solid and very slender. By a little chemical testing, the
discrimination is made still more easy; but for an account of this I
must refer to the Dictionary.

BIRDS.--In the Class of Birds, the structure of the _feathers_ deserves
special notice. Feathers are epidermic formations, or consist of
aggregations of epidermic cells, yet so altered by compression and
fusion together that the cell-structure is in most parts difficult to
detect. In a feather three parts are distinguishable,--the transparent
cylindrical quill; its opake continuation, which is more or less
flattened at the sides, forming the shaft; and the vanes or beards,
which arise from the sides of the shaft, consisting of numerous closely
set, parallel, flattened fibres, called the barbs. The structure of the
barbs forms the interesting object to the microscopist. On examining a
piece of the coloured vane of a somewhat large feather (Pl. IX. fig.
14), a row of fine parallel colourless filaments (_pinnæ_) will be
observed, arising from the opposite sides, the filaments of one side
lying obliquely across those arising from the other; and while the
filaments or pinnæ of one side present a row of little teeth (fig. 14
_c_) near their base, those of the opposite side (fig. 14 _b_) are
provided with as many hooks near their apex, which curve over the teeth
to connect the barbs together. This curious arrangement is adapted to
keep the parts of the feather firmly united, and yet to allow of their
play and flexibility. To observe this structure, a portion of a vane
should be soaked in oil of turpentine, and mounted in balsam.

In the downy feathers (Pl. IX. fig. 17) the barbs are not furnished with
the pinnæ, but present simply whorls of minute spines (fig. 18).

The _bones_ of birds present the same general structure as that of
mammals, the lacunæ being, however, more numerous and smaller.

The _blood_ of birds (Pl. IX. fig. 2) differs entirely from that of
mammals, in the red corpuscles being oval instead of circular, and
convex instead of concave; and each contains a distinct oval and
granular nucleus.

REPTILES.--In reptiles, as the frog, toad, or water-lizard (_Tríton_),
the bone-corpuscles or lacunæ are larger and more numerous than in
either of the former classes; and the blood-corpuscles (Pl. IX. fig. 3)
are comparatively very large, oval, rather concave, and contain a large
granular nucleus.

The smooth water-newt or triton, properly called _Lissotríton
punctátus_, is a very interesting animal in a microscopic point of view.
It may be found in most ponds; and if several are removed in a net, and
kept in a large glass jar, with water-plants, they will live for a long
period. In the spring or early summer they will deposit their eggs upon
the aquatic plants, generally on the under surface of a leaf, which they
bend downwards, so as to protect them. The eggs or ova, are about half
the size of a pea, and consist of a sac containing a transparent liquid,
with a yellowish globule within. After a time these eggs will hatch, and
the larvæ or young newts must be removed from the water, otherwise the
parents will devour them.

If one of these larvæ, which resemble little fish in appearance, be
placed with a little water in the “live-box,” and the cap be very gently
pressed down, so as to fix the body of the animal, the circulation of
the blood may be very beautifully seen in either the fringe-like gills,
which are placed on each side of the neck, or in the tail, a low power
being used; at the same time the beautiful stellate pigment-cells of the
skin will be observed. The structure of the rudimentary spinal column,
which runs down the middle of the back, and consists of simple large
cartilage-cells, may also be made out, when the animal is dead, by a
little dissection with the aid of needles.

FISHES.--In the fourth class of vertebrate animals, which consists of
the fishes, we find interesting structures in the blood, the scales, and
the roe. The corpuscles of the blood (Pl. IX. fig. 4) differ from those
of the Mammalia, but agree with those of birds and reptiles, in being
oval instead of round. The scales of fishes (Pl. IX. figs. 22, 23) are
usually rounded or oval, as in most of our freshwater fishes, when they
are called cyc´loid (κὑκλος, circle); but sometimes they are toothed at
one end (fig. 23 _a_), forming cténoid (κτεἱς, a comb) scales, as in the
perch. Most scales exhibit a number of concentric rings, which are the
indications of laminæ; and many of them are lobed at the margin,
sometimes also having radiate furrows. In the centre are often seen
little rounded solid bodies, having somewhat the appearance of cells,
which are very well seen in the scales of the perch; and in some scales
these bodies are arranged in concentric rows throughout the substance,
as in those of the eel or the cod (fig. 24). The substance of which
scales consist is generally cartilaginous; in some of them, however,
true bony matter is present. Fish-scales are contained within the
substance of the skin, and not merely attached to it by one end, as
appears to be the case in many fishes. In most of our common fishes, as
the roach or perch, the scales project beyond the level of the skin; but
the projecting portion is covered by a thin layer of the skin; and when
the scales are scraped off, this layer, with its elegant stellate
pigment-cells, is usually found adherent to it. In some other fishes, as
the cod and eel, the scales are entirely sunk below the surface; and
these are commonly supposed to have no scales. They may, however, be
easily found by dissection, or by drying a piece of the skin under
pressure between two plates of glass, and mounting a portion in balsam.

The beautiful silvery lustre of the skin of fishes depends upon the
presence of innumerable very minute and thin crystals; these may be
well examined in the skin of a sprat.

The roe of fishes consists of the ova or eggs, and the spermatozoa,--the
ova being contained in the hard, the spermatozoa in the soft roe. The
eggs consist of a cell surrounded by one or two membranes; and the
latter are often traversed by numerous fine radial canals, or present a
funnel-shaped tube leading to the ovum. The spermatozoa of the soft roe
consist of exceedingly slender filaments (fig. 25), terminated at one
end by a kind of head. The reader will not fail to detect the analogy
between the ovum of the animal and that of the ovule of the plant; and
it need scarcely be stated that the spermatozoa of the animal fertilize
the ova, in the same manner that the pollen-tubes and spermatozoa of
plants fertilize the ovules existing in them. In the case of fishes, the
spermatozoa of the soft roe escaping into the water, and moved by the
ciliary action of the filament, enter the micropyle-like canals of the
ova, which are deposited by the fish upon the bottom of rivers.

The scales of fishes may be prepared for examination by scraping them
off and macerating them in water until the adherent portion of the skin
is softened and decomposed, so that it may be washed away. They should
be dried between glass plates, and viewed under a low power, as dry
transparent objects.

The structure of muscle can be more easily made out in fishes than in
other animals. A portion of the flesh should be macerated in spirit as
directed above.

MOLLUS´CA.--We shall now leave the vertebrate animals, and pass to the
subkingdom Mollusca, the marine kinds of which are popularly called
shellfish: three of their structures form interesting objects for
examination--the shell, the tongue, and the gills.

_Shell._--The general structure of the shell of the Mollusca may be
illustrated by reference to that of the oyster. Two kinds of
shell-substance are at once distinguishable in an oyster-shell, an outer
brown, and an inner pearly or nacreous. The brown portion exhibits under
the microscope the appearance of a cell-structure (Pl. IX. fig. 28), the
angular forms from mutual pressure being very distinct. The component
bodies of this portion are seen to be more or less elongated and
flattened in the side view, forming prisms (fig. 29). The structure of
the pearly part of the shell is more difficult of examination, and can
only be seen distinctly in ground and polished sections. In these, under
a high power, it exhibits numerous fine, somewhat parallel wavy lines
(fig. 19), which are the indications of thin layers, or laminæ, of which
it is composed.

Shell consists of a basis of animal matter in which carbonate of lime
(chalk) is deposited, the whole being poured out or secreted by the skin
or mantle of the mollusk.

Pearls, which possess the same structure as the nacreous part of shell,
consist of the nacre formed around some foreign body, as a grain of
sand, &c., by which the mantle has been wounded.

_Tongue._--The structure of the tongue of the Mollusca is very
interesting, on account of the curious teeth which are found upon it. It
may be illustrated by the common Whelk (_Buc´cinum unda´tum_), which is
sold at the street-stalls. As, to one unacquainted with the anatomy of
the Mollusca, there is some difficulty in finding the tongue, it may be
well to point out how it is to be found. If the shell containing the
animal be placed so that its orifice is directed upwards, the point or
apex of the spire being towards the reader, the lid (oper´culum) which
closes the shell will be at once evident. On drawing the animal from the
shell by means of the lid, the foot or portion which is applied by the
animal to the surface upon which it creeps will be seen. At the upper
part of this is the head, with its two horns (ten´tacles). Below the
roots or bases of the tentacles, and between them and the upper part of
the foot, is the little round mouth. On slitting this up with scissors,
a cavity will be opened, and in it will be seen a reddish tube (the
proboscis), about as large as a goose-quill, with an aperture at the
end. This must be carefully slit up, when the tongue, which is of about
the size of a crow-quill, will come into view. The tongue is moveable in
the proboscis, and can be protruded or withdrawn by the animal at will.
If the surface of the tongue be viewed under a handlens, the rows of
teeth will be seen at once. It is better not to pull the tongue out with
forceps, as the teeth are easily displaced and injured. The best plan is
to dissect away the muscular structures with forceps and a pair of
fine-pointed scissors, then to cut off the tongue at its root, and to
soak it in water for some hours, when the skin or epidermis containing
the teeth can be separated with the mounted needles under a simple lens
or microscope. After any loose particles have been washed away with a
hair pencil, the object may be spread flat on a slide, and dried between
two slides. The upper slide should then be removed, the tongue soaked in
oil of turpentine, and mounted in balsam with the least possible heat.

As thus prepared, the horny teeth (Pl. IX. fig. 21) are seen to be
arranged in rows, united by a colourless membrane, so as to form a long
ribbon. The teeth form three longitudinal parallel rows, a central and
two lateral. Each tooth, considering the separate pieces as constituting
distinct teeth, has little teeth or denticles at its lower edge. These
are curved inwards, four in number, and connected by a basal plate in
the side teeth; while the middle teeth have six or seven straight
denticles. These teeth serve to enable the animal to scrape or rasp the
algæ, and other matters forming their food, from the surfaces upon
which they grow. And if some water-snails are placed in a glass jar the
inside of which is covered with confervoid growths, the curious patterns
left after the action of the snails’ tongues will be found to present a
very curious appearance.

_Gills._--The gills or “beards” of the oyster or mussel exhibit very
strikingly the phenomenon of ciliary motion. The gills (branchiæ) are
respiratory organs, consisting of folds of the skin, covered with cilia,
by means of which the water in which the animal lives is set in motion,
and constantly changed to aërate the blood within them. The currents
thus induced serve also to bring the food which floats in the water
towards the mouth of the animal. By snipping off a thin portion of one
of the brown beards of a fresh oyster, laying it upon a slide, adding a
drop of the “liquor” contained within the shell, and lightly pressing a
cover upon the whole, the remarkable phenomenon to one who has not
before viewed it will be seen under a somewhat high power--about ¼-inch.
The whole field will appear in motion, and the lashing or whip-like
action of the cilia will be seen, especially towards the edges of the
bars (Pl. IX. fig. 36) of the gills. The rapid motion of any floating
particles present will also be noticed, showing the direction of the
currents of liquid, which, as the liquid is transparent, would not
otherwise be recognizable.

BRYOZÓA (βρὑον, moss, ξῶον, animal).--The animals included in this
Class, which belongs to the Mollusca, are mostly marine. They are
microscopic, and contained in horny or calcareous sacs or cells,
aggregated together to form polyp´idoms (_pol´ype_, and δῶμα, a house).
They are sometimes plant-like or leafy (Pl. IX. fig. 27), at others
filamentous and branched, or they form a layer or crust upon the objects
to which they are attached. The polypidoms, which are often some inches
in length, are frequently met with on the seashore, the cells (fig. 26
_a_) having slit-like valvular orifices. The bodies of the animals are
soft and polype-like, and are furnished at one end with a circle of
tentacles, covered with rows of cilia, by which the water is changed for
respiration, and particles of food are brought to the mouth. The
tentacles can be protruded or withdrawn at the will of the animal. The
Bryozoa are what are called compound animals, each individual body
having its own set of organs; yet the whole are connected together.

The two species figured are very common. _Flus´tra foliácea_ (Pl. IX.
fig. 27) is found everywhere upon the sea-shore. The polypidom has cells
upon both sides; and they are narrowed at one end, and rounded at the
other. _Membranip´ora pilósa_ (Pl. IX. fig. 39) occurs upon sea-weeds
and other marine bodies, forming a closely adherent layer. The orifices
of the cells are surrounded with teeth, and are usually furnished below
with a very long bristle--the polypidom appearing to the naked eye as a
white hairy crust. In the variety figured, the long bristles are
replaced by a spine; and this is not uncommon.

The polypidoms of the Bryozoa form interesting microscopic objects, the
cells being furnished with variously arranged spines and punctures or
dots. In some the cells are erect and arranged in rows upon the branches
of a plant-like stem, while in others they are scattered irregularly
over a creeping filament.

For examination they should be prepared by maceration in fresh water,
and drying between glass plates or sheets of paper, and either viewed as
opake objects or, after soaking in turpentine and mounting in balsam, as
transparent objects.

It may be remarked that the name Bryozoa for this class of Mollusca,
which was thoroughly established, has recently been changed in this
country to Polyzóa (πολὐς, many, ξῶον, animal), and that the name of
polypidom has been altered to polyzóary.



CHAPTER XI.

ARTICULATA (ARTIC´ULUS, A JOINT).


The animals belonging to this subkingdom are specially distinguished by
the body and limbs being jointed: as familiar instances, may be
mentioned the lobster, the wood-louse, spiders, insects, and worms.

Taking the class Crustacea, to which the two first animals belong, we
find interesting microscopic forms in the subclass Entomos´traca
(ἔντομον, insect; ὄστρακον, shell).

ENTOMOSTRACA.--The animals contained in this Order are met with in every
pool or pond, some of them inhabiting the sea. They are mostly minute,
yet visible to the naked eye, forming specks swimming actively or
leaping through the water; hence some of them have been called
water-fleas. The body of the animal is protected by a shell or
car´apace, which in some consists of a single piece (Pl. IX. fig. 30),
while in others it consists of two similar parts or valves (fig. 31), in
the latter case the joints of the body being indistinctly visible. The
head is furnished with usually two projecting feelers or antennæ
(_anten´na_, a sail-yard), one of which is uppermost or superior (Pl.
IX. figs. 30, 31, 34 _a_), the other lowermost or inferior (figs. 30 and
34 _b_); and these are often used for swimming. The antennæ are jointed,
and sometimes beautifully plúmose (_pluma_, a feather) or feathery, _i.
e._ furnished with rows of long and very slender filaments. There are
several pairs of jointed legs, some of which serve as jaws (foot-jaws),
while others are finely filamentous to serve for swimming and as
respiratory organs (branchial feet). The four species figured are very
common.

_Cypris tristriáta_ (Pl. IX. fig. 34) is found in ponds and ditches. The
carapace is bivalve, or has two valves, which are convex and oval; and
it is of a greenish colour, with three irregular dark stripes behind.
The superior antennæ (_a_) are jointed and finely feathery, the inferior
antennæ (_b_) having a tuft or pencil of fine filaments arising from
their anterior margin. The eye is single. The animal swims steadily and
freely through the water.

The eggs of _Cypris_ (Pl. IX. fig. 35) are often found in glasses of
water containing the animals. They are rounded or oblong, of a red
colour, glued together by an amorphous jelly, and adherent to pieces of
stick or the sides of the glass. They are enclosed in a thick shell,
which exhibits a cellular appearance in the surface view, and is
striated in the side view; so that the structure of the shell is
prismatic, as in that of the oyster. When the eggs escape from the
shell, they present the appearance represented in fig. 35 _a_, the body
of the young animal being enclosed in a transparent envelope, one end of
which forms a blunt protrusion; there is also a separate slender process
enclosing the superior antennæ. After a time, the envelope is cast off
(fig. 35 _b_), when the animal begins its active stage of life. The
cast-off envelopes (fig. 35 _c_), with the protruded portions wrinkled,
are often found in the sediment of water containing the animals. The
structure of these ova is that of what are called winter ova, which
agree with the resting-spores of the lower plants or the Algæ.

_Cy´clops quadricor´nis_ (Pl. IX. fig. 30) is another common species. In
this the body is closely surrounded by the jointed shell, as in a
lobster. The superior antennæ (_a_) are very long and many-jointed, each
joint having short bristles arising from it, while the inferior antennæ
are short and four-jointed. There is no separate head, this being
united to or consolidated with the first joint of the thor´ax or chest,
the head and thorax together comprising four joints. The remaining
joints enclose the belly, or abdómen, which has the appearance of a
tail; but the tail is constituted by the two last parallel pieces, which
are furnished with fine feathery filaments.

The female is most commonly met with, and is easily known by having the
egg-pouch, or ovary (_o_) external on each side, and filled with eggs or
ova. The little _Cyclops_ is readily recognized by its form and jerking
motion through the water.

_Daph´nia púlex_ (Pl. IX. fig. 31) is a very common Entomostracan, and
is very well adapted to illustrate the structure, on account of its size
and transparence. In this animal the body is loosely connected with a
bivalve shell, which, on careful examination, is seen to be reticulated
or marked with net-like lines. The superior antennæ (_a_) are very
small, placed under a small beak, and have at the end a minute tuft of
hairs. The inferior antennæ (_b_) resemble arms, being large and
branched; and by means of them the animal rows itself through the water.
The structure of the eye is curious, consisting of a number of round
lenses aggregated together, the fine muscular threads by which it is
moved being easily distinguished with a high power. The legs are
flattened, and furnished with elegant feathery sétæ (_seta_, a bristle),
serving as gills or branchiæ. They are constantly in motion, fanning the
water so as to change incessantly the portion with which they are in
contact. About the middle of the back is placed the little transparent
heart, with its colourless blood, which may be distinctly seen beating,
or contracting and dilating, in the living animal; and between the back
of the animal and the shell are seen the ova, which remain there until
they are hatched.

The genera and species of the Entomostraca are very numerous. Those
mentioned above will serve to illustrate the general structure of the
order. To distinguish the man´dibles (_mandib´ula_, a jaw) or proper
jaws, the foot-jaws, and the branchial legs, the animals must be
dissected in water with the mounted needles. The very delicate feathery
filaments of the branchiæ may be best observed when these organs are
dried on a slide.

The Entromostaca may be kept alive in a jar of water with water-plants
for a long period. They may be removed from the water for examination by
the dipping-tube, and are best observed in a live-box.

ARACH´NIDA (ἁρἁχνη, spider) is the Class of spiders, scorpions, and
mites.

_Aranéida._--This Order contains the more highly developed forms of the
Class, among which are the common spiders of houses and gardens; and
some of their structures are very curious and interesting.

The head of spiders is united or fused with the thorax, forming one
piece, which is called the _ceph´alothorax_ (κεφαλἠ, head, θὡραξ,
chest).

The claw-jaws, or _man´dibles_, are terminated by a curved and pointed
claw, with which the spiders hold their prey. It is traversed by a
slender canal, containing a slender tube or duct leading from a
poison-gland, and opening near its point; and when the insect prey is
transfixed by the mandible, the poison is pressed out and enters the
wound.

Near the root or base of the mandibles on each side is a jointed feeler,
or pal´pus; but spiders have no anten´næ. The eyes are simple, forming
separate round shining dots, and are called _ocel´li_ (_ocel´lus_, a
little eye); they are usually placed on the top of the head, and are
often arranged in a geometrical form, as a triangle, &c.

The legs are four pairs; they are hairy, and terminated by two or three
claws, which are fringed with minute teeth, or pec´tinate. These claws
serve to comb the fibres of the web, just as we comb our hair with a
common comb.

The _spinnerets_, with which spiders form their web, are very curious
organs. They are situated at the under and hind part of the body, and
consist of two or three cones, or papillæ, on each side. On the summits
of these papillæ are very numerous bristle-like tubes, through which the
secretion of certain glands passes; this secretion, when hardened by
exposure to the air, forms the fibres of the web.

On carefully examining a spider’s web, the radial fibres, or those which
pass from the centre to the circumference, will be found to be smooth,
these fibres serve to fix the web; while the cross fibres are covered
with numerous viscid globules, which serve to attach flies or other prey
to them. This difference of the fibres is best observed with a
hand-lens.

ACARÍNA, or the Order of Mites.--Here belongs cheese-mite, _Ac´arus
domes´ticus_ (Pl. IX. fig. 36). Its body is somewhat milky white, oval,
and furnished with feathery hairs. When viewed from beneath, there is
seen a transverse line, indicating the separation of the thorax from the
abdomen; and another line in front of this, with four minute tubercles,
from each of which arises a hair. The head is pointed and beak-like,
forming a _ros´trum_ (_rostrum_, a beak), consisting of two mandibles
pressed together; these can only be seen to be separate when dissected
apart with the mounted needles. Each mandible is chélate (χηλἠ,
forceps), or has the form of a lobster’s claw; and beneath the two
mandibles is a flat membranous under lip or labium, consolidated on each
side with a palp. The legs are four pairs, as in all the Arachnida; they
are pinkish, 6-jointed, and terminated by a leaflike sucker and a minute
claw.

The males (fig. 38) are smaller than the females, the fore legs being
much stouter, and furnished with a blunt tooth (fig. 38 _a_). The eggs
can often be distinguished within the body of the female (fig. 36);
they are oval and granular.

Another species of _Acarus_, _A. sac´chari_, is found abundantly in
ordinary moist sugar. If a little of the sugar be placed in a
wine-glass, some water added, and the mixture be stirred until the sugar
is dissolved, the _Acari_ will be found both in the sediment and
floating on the surface.

A somewhat larger member of the order occurs as a parasite upon a
species of Dung-beetle (_Geotrúpes stercorárius_) which is vulgarly
known as the Lousy Watchman. The beetle is black, shaded with purple,
about three-quarters of an inch long, and is found under cow-dung. The
mites cling pertinaciously to the under parts of the beetle, and can
easily be seen with the naked eye. They are whitish, with the mandibles,
the sucker, and two claws very distinct; and the palpi are unattached to
the labium, or free. These mites form the species _Gam´asus
coleoptrator´um_. Another species, _Gamasus telárius_, is the red spider
of the greenhouse.

_Trombid´ium fuliginósum_ (Pl. IX. fig. 37 _d_) is a common red spider
of gardens. It is of a scarlet colour, appearing velvety from the
presence of a dense coat of feathery hairs (fig. 37 _f_). The palpi of
this mite are large, free, the last joint but one (Pl. IX. fig. 37 _a_)
being furnished with a claw, while the last joint is obtuse, and
resembles a lateral appendage. The mandibles (fig. 37 _b_) are furnished
with a sharp curved claw. The legs are long, especially the anterior
pair, and terminated by two claws, with a delicate sucker-like appendage
(fig. 37 _c_).

Another species of _Trombidium_, _T. holoseric´eum_, greatly resembling
the last, is also found in gardens. It may be easily distinguished from
the last by the club-shaped hairs (fig. 37 _e_) existing upon the body.
The harvest-bug, which causes such irritation of the legs of persons who
frequent corn-fields in the autumn, is also a species of
_Trombidium_--_T. autumnále_.

PLATE X. [page 133.]

INSECTS.


Fig.

1. _Atropos pulsotorius_: * natural size.

2. _Aphis_ of Geranium: _a_, foot; _b_, anal tube; _c_, antenna.

3. Scales on wing of Apollo-butterfly.

4. _Lithobius forcipatus._

5. _Lithobius forcipatus_, head of: _a_, antennæ; _b_, mandibles; _c_,
labial palpi; _d_, labium.

6. _Dytiscus marginalis_, head of larva.

7. Young larva of _Dytiscus marginalis_.

8. Pupa of Gnat (_Culex pipiens_).

9. Larva of Gnat.

10. Head of Gnat, male.

11. Head of Gnat, female.

12. _Coccinella 7-punctata_ (large Lady-bird), labium of.

13. _Coccinella 7-punctata_, mandible of.

14. _Coccinella 7-punctata_, labrum of.

15. _Coccinella 7-punctata_, antenna of.

16. Coccinella 7-punctata, maxilla of: _a_, palp; _b_, _c_, lobes of
maxilla.

17. Head of _Musca domestica_ (House-fly): _a_, antenna; _b_, labial
palpi; _c_, proboscis.

18. Head of _Stomoxys calcitrans_: 18 _a_, antenna.

19. Scales of insects: _a_, scale of _Podura_; _b_, of House-moth
(_Tinea vestianella_); _c_, of _Podura_; _d_, of _Lepisma_; _e_, hair of
_Podura_; _f_, scale of Cabbage-Butterfly (_Pontia brassicæ_).

20. Head of Cabbage-Butterfly: _a_, antennæ broken off; _b_, palp; _c_,
tongue (antlia); _d_, club of antenna.

21. Head of human Flea (_Pulex irritans_), female (♀): _a_, palpi; _b_,
maxillæ.

22. Flea of the Rat (_Pulex muris_), male (♂).

23. _Pterostichus (Steropus) madidus_; 23 _a_, antenna.

24. Part of leg of _Pterostichus madidus_: _a_, tibia; _b_, tarsus.

25. Labrum of _Pterostichus madidus_.

26. Mandible of _Pterostichus madidus_.

27. Labium of _Pterostichus madidus_: a, mentum; b, labial palp.

28. Maxilla of Pterostichus madidus: _a_, claw; _b_, _c_, maxillary
palps.

29. Proboscis of House-fly.

30. Larva of Flea.

31. Larva of _Chironomos plumosus_.

32. Foot of House-fly.

33. Eye of House-fly.

34. Leg of Ant (_Formica fusca_); 34*, pectinate process.


[Illustration: Plate X.

W Bagg sculp

London: John Van Voorst.]

_Preparation._--The organs of the mouth, &c., of the Spiders are easily
prepared for examination, by carefully pulling them off with forceps or
the mounted needles, then drying them under pressure between two glass
slips, macerating in turpentine, and mounting in balsam. Those of the
Acarina should be dissected out with the needles, after the body has
been crushed in a drop of water on a slide, and the internal substance
has been gently washed away with a hair pencil. They may then be dried
on a slide, with a cover laid on, and turpentine applied to the edge of
the cover, balsam being added when most of this has evaporated. The
various parts may also be mounted in chloride of calcium or glycerine.

INSECTS.--The members of the class of Insects are extremely interesting
to the microscopic observer, not only on account of the beautiful
structures which they present, but also from these being comparatively
large, usually coloured, and easily distinguished under the lower
powers. Hence they form admirable objects for study to those who are but
little accustomed to the use of the microscope.

MYRIAP´ODA (μὑριος, myriad, ποῦς, foot).--This Order contains those
insects which are popularly known as the hundred-legs and millepedes; by
many zoologists they have been arranged in a distinct class.

The most common member of this Order is _Lithóbius forcipátus_ (Pl. X.
fig. 4), which is found under stones, in cellars, and among
garden-rubbish. It is of a yellowish-brown colour, with long,
many-jointed, gradually tapering or setáceous (_séta_, a bristle)
antennæ (figs. 4 _a_, 5 _a_), and two large and powerful mandibles (fig.
5 _b_) resembling those of the spiders. It has also a broad, notched,
and toothed lower lip, or labium (fig. 5 _d_), above which are two
toothed jaws, or maxillæ, and two lip-feelers, or labial palpi (_c_).
The eyes consist of a group of ocelli on each side (_e_). The body is
protected by alternately larger and smaller dorsal plates, which are
fifteen in number; and there are fifteen pairs of legs, which are
terminated by a single claw. On the sides of the body will be found some
oval dark-looking bodies, fringed with hairs; these are the spir´acles
(_spirac´ulum_, a breathing-hole) or breathing-pores. They form the
orifices of certain branched and transversely striated tubes, which are
distributed throughout the body; the tubes are called _trácheæ_
(_trachea_, the windpipe), and their walls contain an elastic spiral
fibre which keeps them open. These parts of the insect can only be
distinctly seen when the body has been slit up on the under side; and,
after washing away the animal matter with water by the aid of a hair
pencil, pressed between two slides with a clip, dried, soaked in
turpentine, and mounted in balsam.

THYSANÚRA (θὑσανοι, fringe, οὑρἀ, tail).--The insects belonging to the
genus _Podúra_, of this Order, are very minute and difficult to examine;
but they are specially interesting, on account of the structure of their
scales. They are common in gardens and cellars, under flower-pots, &c.,
and are about one-tenth of an inch long. They are of a brownish or
silvery-leaden colour, wingless, with six legs, and when touched they
leap like a flea. The leaping motion is produced by the action of the
tail, which is forked and bent under the body.

The body is usually covered with minute scales (Pl. X. fig. 19, _a_,
_c_), and these are used as test-objects. The structure of the scales
varies in the different genera and species; those usually used (fig. 19
_a_) are stated to belong to _Podúra plum´bea_; it appears, however,
that this is not correct. The scales sold as test-objects under this
name are covered with minute and short raised lines (fig. 19 _a_),
arranged in irregular but somewhat parallel wavy rows. It requires a
good microscope and a high power to show them distinctly, and they
should appear perfectly black and separate. The little lines are much
coarser in some scales than in others; so that there are easy and
difficult scales, as they are called.

The _Poduræ_ may be caught by holding a sheet of paper near their haunts
and disturbing them; and when they have jumped upon the paper, a slide
laid upon them and gently pressed will remove some of the scales for
examination.

The scales should be mounted as dry transparent objects; for if wetted,
they become very transparent, and the markings appear removed, which
however is not really the case.

The scales of _Lepis´ma saccharína_ (Pl. X. fig. 19 _d_), a member of
this Order, were formerly used as test-objects; but they are too easily
made out to serve for this purpose with modern microscopes. The insect
is not common. The scales (fig. 19 _d_) exhibit continuous nearly
parallel longitudinal lines or ribs.

ANOPLÚRA (ἄνοπλος, unarmed, οὑρἀ, tail).--This third Order of insects
consists of the Lice of the Mammalia and birds. They are minute,
resembling mites to the naked eye, but may be at once distinguished from
them by the distinct head and thorax and the presence of six instead of
eight legs.

Some of them are suctorial, _i. e._ have a short and slender tube, with
which they suck the blood of the animals of which they are parasites;
while others are mandibulate, or have mandibles, and also maxillæ, their
food consisting of portions of feathers, hairs, and scurf. The legs are
usually short and stout, and the claws large and powerful, to enable
them to hold firmly to the hairs, &c. The Anoplura are most abundant on
dirty and diseased animals.

SUCTOR´IA.--The fourth Order of insects consists of the genus
_Púlex_,--_Pulex irrítans_ being the human flea. Other species are
found upon different animals, as upon the dog, the rat (Pl. X. fig. 22),
the fowl, the pigeon, &c.

The head and the dark eye (fig. 21, head of the human flea) are very
evident. The antennæ or head-feelers are very minute and difficult to
find, being sunk in a little pit or fossa behind the eye; they may,
however, generally be recognized by the detection of the last joint,
which is pectinate or cut like a comb. The body, including the head,
consists of thirteen joints, one for the head, three for the chest or
thorax, and nine for the belly or abdómen,--it being understood that by
“joint” is meant a segment, and not the line of junction of two
segments. The indication of these joints is afforded by the horny
integument, which consists of a corresponding number of rings, forming
in fact the skeleton of the animal. This in the flea, as in all insects
and other Articulata, is external or cutaneous (_cútis_, skin), and
consists of the hardened skin, the peculiar animal substance of which it
is composed being called chitine (χιτὠν, tunic). These chitinous rings
overlap, and are composed of a dorsal or upper, and a ventral or lower
half; and near the middle of each is a row of hairs directed backwards.
Along the sides of the body of the insect may be seen a row of dots;
these are the spiracles or orifices of the breathing-tubes (tracheæ).

The legs are many-jointed, long, furnished with numerous spines, and
terminated by two slightly curved claws, each with a little blunt tooth
at its base. The claws are not perfectly smooth on the inside, but are
covered with slightly raised lines, like a file, so that a better hold
can be taken of bodies.

But the most interesting parts of the flea are those of the mouth, with
which it punctures the skin and sucks the blood. There are nine of
these; and they are best seen when the head of the flea is pulled off
with the mounted needles, and the parts spread out and mounted in
balsam, a high power being used to examine them. One of them forms a
long and slender bristle (seta) or tongue, furnished with distant minute
teeth. On each side of this is a flattened seta, with two rows of teeth
on the edges; these are the lancets, and when not in use these organs
are inclosed in two jointed sheaths. Outside these are two
representatives of jaws, or maxillæ (_b_), each having a jointed feeler
or palp (_a_) arising from it; the probable use of the palps being to
feel the position of the skin, so that the animal may be able to adjust
the lancets at a proper distance for puncture.

The eggs of the flea are often visible within the body of the parent;
and when this is crushed, they are more distinctly seen, of various
sizes, and contained within a long tube, which is the ovary or egg-bag.
The eggs are laid by the animals upon carpets, woollen garments, or in
the cracks of dirty floorboards; they are just perceptible to the eye as
white oblong specks, and they may always be found on the rug when a cat
is kept in the house. When hatched, they give rise to a minute white
worm-like maggot, or larva (fig. 30), having a 12-jointed body, with two
rudimentary antennæ and two slightly curved hooks appended to the last
joint. The mouth-organs of the larva are adapted for biting, and not for
sucking, as in the perfect animal, the jaws or maxillæ being distinctly
toothed. When they have acquired full growth, which takes place in about
twelve days in warm weather, they spin around themselves a little silky
cocoon, and become transformed into a chrysalis or pupa; and from this,
in about a fortnight, the perfect insect escapes.

DIP´TERA (δἰς, twice, πτερὀν, wing).--This, which forms the fifth Order
of Insects, consists of the two-winged insects, or flies, as the
house-fly, the blue-bottle, the gnats, &c.

MUS´CIDÆ.--The house-fly, and the blue-bottle or meat-fly, are both
species of the genus _Mus´ca_, belonging to this family, the former
being _Musca domes´tica_, the latter _Musca vomitor´ia_. Both these
insects are seen to be wonderfully constructed when minutely examined,
and they possess considerable resemblance in general structure.

On examining the head of the house-fly (Pl. X. fig. 17) under a low
power, and as an opake object, the observer will be struck with the
remarkable appearance presented by the two eyes, which are large, placed
one on each side of the fore part of the head, and composed of very
numerous little eyes closely packed together, or they are compound, as
it is called. The use of this compound structure is evidently to enable
the little animal to see in all directions without moving the head and
eyes. Each little eye has a lens to bring the rays of light emanating
from objects to a focus upon a nerve. The packing of the eyes together
gives rise to their angular form or their straight sides, each of the
little surfaces or facets being hexagonal, or bounded by six sides (fig.
33). In front of and between the eyes are seen the two small antennæ;
these have three joints, the third of which is larger than the rest
(fig. 17 _a_), and arising from near its base is a feathery bristle or
seta; these structures are best seen when the antennæ are pulled off
with a pair of forceps and mounted separately. Below the antennæ, and
extending downwards and forwards is the proboscis, or tongue, as it is
called, which can be entirely retracted within a pit in the fore part of
the head, or protruded at the will of the animal. This is a very
beautiful and complicated instrument, and is best examined when spread
out and separately mounted (Pl. X. fig. 29). It consists of a fleshy
tube, dilated at the end into two lobes, which are flattened beneath to
form a sucking-disk. The end is furnished with two solid horny lateral
branches to keep it expanded, and with two longitudinal tubes beneath,
running parallel, from the outer sides of which arise numerous nearly
parallel branches. These tubes and their branches are incomplete
beneath, and consist of imperfect rings, otherwise greatly resembling
tracheæ. On each side of the proboscis is a lip-feeler or labial palpus,
for the organ represents the labium of other insects. All these parts
are better seen in the proboscis of the blow-fly than in that of the
house-fly, on account of their larger size. The head, the thorax, and
the abdomen are very distinct in the fly, being separated from each
other by well-marked constrictions.

The legs are composed of five parts, each having a separate name. The
first piece or joint, which is that attached to the body, is called the
hip, or coxa; the next is a very small, somewhat triangular piece, and
is the trochan´ter; next comes the long and stout thigh, or fémur; this
being succeeded by the tib´ia, which, as in most insects, is furnished
with strong spines at the end; and lastly is the foot, or tar´sus,
consisting of five joints, the three last of which are represented in
the figure (Pl. X. fig. 32). At the end of the fifth or last joint of
the tarsus (fig. 32) (for it must be noted that the joints of the limbs
of insects are always numbered in order of distance from the body) are
two soft little cushions, or pulvil´li, which are covered on the under
surface with numerous hair-like bodies dilated at the end, acting as
suckers in enabling the fly to adhere to smooth surfaces. In addition to
these organs are two curved claws, and between them a sharp straight
spine.

The eggs of the fly are deposited upon heaps of decaying animal and
vegetable matters, as dungheaps, &c. The blow-fly deposits its eggs in
the same situations, but especially those where the animal matters are
most abundant: every one knows the eggs as deposited upon tainted meats,
when they are called flyblows. When the eggs are hatched, the _larvæ_
or maggots make their appearance. The larvæ of the blow-fly are well
known to the angler, who uses them for bait, and calls them gentles.

These larvæ exhibit some interesting points of structure. The jointed or
ringed condition of the body is distinct to the naked eye. The head is
provided with two rudimentary palpi, placed each upon a rounded papilla;
also with two brown curved and horny hooks or jaws. On the posterior end
of the body are two brown spots, which consist of spiracles, and have
three sieve-like oblong orifices; and at the anterior margin of each
segment of the body are very numerous little short spines, with the
points directed backwards. These answer the purpose, to some extent, of
legs; for when the larva is moving, and has forced itself through the
matter in which it burrows, the little spines prevent the body being
forced backwards as the head is pushed forwards and meets any
resistance.

To examine the structure of these larvæ, the gentles should be killed by
immersion for a time in warm water, and then dried by touching them with
blotting-paper. The hooks and palpi can be seen by holding the body in
the forceps as an opake object. To observe the spiracles, the end of the
body should be cut off, and the animal matter washed away in a
watch-glass with water and a hair pencil, then spread out, dried between
two slides, and mounted in balsam.

The larvæ of the house-fly and of the blow-fly very closely resemble
each other, so much so that the former are generally overlooked; hence
it is often wondered where flies come from, although they are so
numerous in every house. When the larvæ of these flies are fully
developed, they gradually assume a brown colour, the organs of the head
are retracted, and the skin becomes dry and hard. This is the state of
_pupa_, or chrysalis; and while remaining in this state of rest, the
development of the wings, legs, &c., takes place; so that when the
insect emerges from the shell of the chrysalis, it has attained its
highest state of development, and forms the _imágo_ or perfect insect.
It may be remarked here, that when insects pass through the three states
of larva, pupa, and imago, they are said to undergo complete
metamorphosis. There is not a more curious object than that presented by
the young fly contained in its case, as seen on carefully cutting away
portions from one end of the case of the chrysalis. The body and head
are quite white, with beautiful blood-red eyes.

Most persons must have noticed another kind of house-fly, having the
wings more widely separated than in the common fly, and moving more
slowly through the air. This is _Stomox´ys cal´citrans_. The proboscis
of this fly (Pl. X. fig. 18) differs from that of the house-fly in being
longer, more bent, and but little expanded at the end; also in being
provided with two long setæ, one forming a slender sharp lancet, the
other being somewhat stouter, and forming its sheath. The fly is thus
enabled to pierce the flesh and suck the blood.

CULIC´IDÆ.--We will now say a few words about the gnats, which are old
favourites for microscopic examination. The three states of larva, pupa,
and imago must be considered separately.

The imago or perfect form is well known. The males are easily
distinguished from the females by the difference in structure of the
antennæ, which in the males (Pl. X. fig. 10, head) are very beautifully
plumose or feathery, whilst in the females (fig. 11) the hairs are very
short--the long proboscis, or rostrum, or bundle of biting-organs
forming the striking feature; this difference is immediately evident to
the naked eye.

The antennæ of the males (fig. 10 _a_) consist of numerous small joints,
from each of which arises a ring of long hairs, giving the appearance of
a tuft on each side when flattened for view as a transparent object
under the microscope; at the end are two longer joints, the first having
a small ring of shorter hairs.

The proboscis of the female is a very complicated organ, consisting of
six separate bristle-like pieces or setæ, all nearly of the same length.
Two of these are somewhat curved near the end, and provided on the
inside with fine teeth; another pair consists of very thin
lancet-pointed instruments; then comes another lancet-pointed seta, very
sharp at the end, and traversed by a canal; next, a stouter and
darker-looking tube, slit up underneath, which serves to contain the two
toothed setæ; and lastly, a stout and broader sheath, also slit up
throughout its length beneath, in which all are packed. This sheath has
two lobes at the end, and has some resemblance to the proboscis of the
fly, of which organ it is the representative.

The gnat lays its eggs in water. The eggs are longish oval, with a kind
of neck at the upper end; they are glued together side by side in large
numbers, and form a boat-like mass, floating on the surface of the
water. The larvæ (Pl. X. fig. 9) are very commonly found skipping
through the water, or hanging, as it were, by the tail from the surface.
The head is very broad; and arising from each joint of the body are
tufts of hairs. Near the end of the body is a tube which communicates
with the tracheæ; and when the animal is quiet, this tube is brought to
the surface; so that when the animal appears hanging to the surface, it
is breathing. In addition to this method of respiration, there are other
respiratory organs attached to the last joint of the body, consisting of
leaf-like plates; hence these larvæ have an aquatic as well as an aërial
respiration: the respiratory or branchial plates also serve as a tail to
aid in swimming. Running down the back of the larva will be seen a thin
delicate vessel, dilated opposite the thorax, and beating or contracting
at regular intervals. This is the dorsal vessel, the dilatation
representing the heart of the higher animals; and it serves to propel
the colourless blood throughout the body.

After several moultings or castings of the skin, to allow of the growth
of the insect (for insects grow only in the larval state), the larva
becomes transformed into the pupa (Pl. X. fig. 8). In this state the
animals still move about in the water, and are often found suspended
from its surface by two respiratory tubes, which, however, are not
connected with the tail, but arise from the thorax; and the various
parts of the perfect insect may be seen through the case or skin, within
which they are closely packed.

When the pupa has attained its full development, the perfect insect
emerges from it, leaving the water to fly about and seek food.

TIPU´LIDÆ.--In the water of ponds and pools a young larva (Pl. X. fig.
31) will often be met with, which is that of _Chiron´omus plumósus_, a
largish gnat-like insect, belonging to the daddy-long-legs family
(Tipulidæ). This larva exhibits the usual thirteen segments, including
the head. In the young state the body is nearly colourless; but in the
mature larva it is of a blood-colour, and about an inch in length.
Beneath the first joint of the body are two foot-like processes covered
with hair; and at the end of the body are also two processes, surmounted
with hooks. The three last joints of the body are also furnished each
with a pair of fleshy processes, those of the first pair being very
short.

HYMENOP´TERA (ὑμἠν, membrane, πτερὀν, wing).--This, which forms the
sixth Order of insects, contains the bee, the wasp, the ant, &c.

In the Hymenoptera there is a curious contrivance for linking the two
wings on each side, so that they may form a single piece in the flight
of the insect. It consists of a row of hooks, placed upon the anterior
nerve of the hind wing, which play upon the folded-in corresponding edge
of the fore wing; the hooks, sliding along this edge, allow of freedom
of motion, although still holding the two wings together. This structure
may be well seen in the wings of the Humble Bee when mounted in balsam.

The sting of the wasp and bee is also a singular organ. In both insects
it is much alike, consisting of a sheath, slit up beneath, in which are
contained two long setæ, or lancets, with bent-back (recurved) teeth
near the end. These setæ are inserted into the flesh during the act of
stinging, and at the same time the poisonous secretion from two glands
is forced into the wound, which causes the severe pain resulting from
the sting.

In the wingless neuters of the common ant, attached to the end of the
tibia (Pl. X. fig. 34 _c_) is a beautiful pectinate process, somewhat
resembling a comb (fig. 34 _a_).

LEPIDOP´TERA (λεπἰς, scale, πτερὀν, wing).--This Order contains the
butterflies and the moths, the entire bodies of which are covered with
minute scales. When the insects are handled, these scales adhere to the
fingers as a fine dust; and on pressing a slide against the insects,
they may be removed for examination. They consist of a very slender and
short quill, by which they are attached, and a flattened plate of
various forms (Pl. X. fig. 19, _b_, _f_); it is, however, generally
narrower near the quill, and expanded towards the free end, where it is
often cut into lobes or toothlike segments. The scales are usually
covered with continuous longitudinal lines or ridges, with granules of
colouring-matter (pigment) situated between the two thin layers of which
the scales consist. In some of them the form is that of a filament,
either simple or branched at the end (fig. 19 _e_), when they resemble
minute hairs. In the males of the large white Cabbage Butterfly
(_Pi´eris brass´icæ_), certain of the scales of the wings are covered
with longitudinal rows of very minute dots (fig. 19 _f_), and have
little tassel-like bodies at the end. The males may be distinguished
from the females by the front wings having no black spots, while those
of the females have two upon each wing. When the scales are examined as
they exist upon the wings of the Lepidoptera, they are found to be
imbricated (Pl. X. fig. 3) or overlapping each other like the tiles on
the roof of a house.

The Lepidoptera suck the honey of flowers by means of a spiral tongue
(Pl. X. fig. 20 _c_) or ant´lia (_antlia_, a sucking-tube); this
consists of two halves, which represent the maxillæ of other insects;
and their margins are fringed with little tassel-like bodies, probably
organs of taste. The antennæ (fig. 20 _a_) are many-jointed, clubbed at
the ends (_d_) in the butterflies, and simple in the moths. The palpi
(fig. 20 _b_) are short and densely covered with scales. In the large
eyes the facets are very distinct and suitable for examination.

The larvæ are well known as caterpillars. They have six legs, as in the
perfect insects, but rudimentary and with single claws; also some
additional pairs of pro-legs, as they are called, with a crown of hooks,
towards the hind part of the body. The spiracles of caterpillars are
very favourable for observation.

NEUROP´TERA (νεῦρον, nerve, πτερὀν, wing).--This Order contains the
Dragon-flies (Libellúlidæ), the Day-fly (_Ephem´era_), &c., in which the
wings are usually so large and so beautifully netted. The species
figured (Pl. X. fig. 1), which is wingless, is very common in old books
and in collections of dried plants. It is whitish, mite-like, with
setaceous many-jointed antennæ, 3-jointed tarsi, and very broad thighs
(femora). Its name is _At´ropos pulsator´ius_.

HEMIP´TERA (ἤμισυς, half, πτερὀν, wing).--This Order contains the bugs
and other noxious insects. Those which we shall notice are the species
of _A´phis_, commonly known as Plant-lice and Green-fly, which are found
too frequently upon unhealthy plants. The species figured (Pl. X. fig.
2) is that of the geranium (_Pelargonium_). The head is small and
notched or emarginate. The body is oval and furnished behind with two
prolonged tubercles, these being covered with scales, giving them a
somewhat striated appearance (fig. 2 _b_). The antennæ are 6-jointed
(_c_), the second joint being very small, the last joint long, excavated
on one side, and ringed: these organs are reflexed over the body in the
natural state. There are two compound eyes, and three simple eyes or
ocelli, forming a triangle on the top of the head. The proboscis or
rostrum is bent under the body, 4-jointed, and contains three setæ, two
of them forming very slender lancets. The legs are long; the tarsi (fig.
2 _a_) 2-jointed, the first or basal joint being very minute, and the
last furnished with two claws.

In a colony of these insects, some are winged and some wingless; those
without wings being usually in the larva state, the pupæ having
rudimentary wings, and the males and females usually perfect wings.
_Aphis brass´icæ_ is the destructive turnip-fly.

COLEOP´TERA (κολεὀς, sheath).--This, which is the last Order to be
noticed, contains the Beetles, so easily recognized by their hard and
horny fore wings or wing-cases. The parts of the mouth in these insects
are exceedingly well adapted for examination; and as they are not fused
or consolidated with each other, they serve to illustrate the typical
constitution of the organs as existing in these animals.

_Pteros´tichus_ (_Ster´opus_) _mad´idus_ (Pl. X. fig. 23) is common in
cellars and gardens among vegetable rubbish. The body of this beetle is
shining black, the head projecting; the antennæ (fig. 23 _a_) are
filiform, and compressed towards the end. The thorax is somewhat
rounded, with a deep rough pit and a longitudinal stria at each
posterior angle. The wing-cases, or elytra, are longitudinally striated;
the wings, which in most beetles are concealed beneath the elytra when
the insects are at rest, being absent. The tibiæ of the fore legs are
notched on the inside (fig. 24 _a_), a fringe of hairs being situated in
the notch; the tarsi are 5-jointed (fig. 24 _b_), the first four joints
being triangular, the last elongate and terminated by two curved claws.
In the male the three first joints of the tarsi of the anterior legs
(fig. 24 _b_) are dilated and heart-shaped.

The parts of the mouth (which are named after an analogy with those of
the higher animals) consist of the following pieces:--An upper lip, or
_lábrum_ (fig. 25 _a_), which is squarish (quad´rate) and slightly
notched; a quadrate lower lip, or _lábium_ (fig. 27), with a process on
each side, and two 4-jointed lip-feelers or labial palpi (_b_); and
below the labium is a chin, or _mentum_ (fig. 27 _a_), with a projecting
bifid tooth: these parts form the roof and the floor of the mouth. Next
come the _man´dibles_ (fig. 26), one on each side, which are stout,
curved, and pointed; beneath which are the _maxil´læ_ (fig. 28), also
one on each side, and provided with a fixed claw (_a_), ciliated on the
inside, and furnished with two pairs of jaw-feelers, or _maxillary
palpi_, the inner (_b_) being 2-jointed, while the outer (_c_) are
4-jointed. It will be noticed that the jaws work laterally, or from side
to side, and not perpendicularly as in the higher animals.

These parts may be found in most beetles which the observer may submit
to examination, being however somewhat modified in different genera. We
may consider those existing in the Lady-birds, or species of
_Coccinel´la_, by way of comparison. The body in these insects is very
convex, and the head sunk deeply in the thorax. The antennæ (fig. 15)
are short, clubbed (clávate), and compressed. The thorax is short and
lúnate, or half-moon shaped. The mandibles (fig. 13) are curved, bifid
at the apex, and with a tooth on the inside near the base. The labrum
(fig. 14) is transverse, or broader than long. The labium (fig. 12) is
furnished with two palpi, which are 3-jointed. The maxillæ (fig. 16) are
two-lobed, the lobes (_b_, _c_) being ciliated, and the 4-jointed palpi
(_a_) have the last joint large and hatchet-shaped.

Coleopterous insects undergo complete metamorphosis, the larvæ being
commonly known as grubs. The larvæ of the aquatic beetles will often be
met with in the water of ponds or ditches, especially that of the common
large water-beetle (_Dytis´cus marginális_), or water-boatman as it is
called (Pl. X. fig. 7), and in various stages of growth. The structure
of the mouth-organs (fig. 6), which are, however, imperfect or
rudimentary in some parts, can be readily made out; and their names may
easily be found by comparison with what has been stated in regard to the
organs of the perfect beetle.

_Examination, &c._--The means of catching insects will readily occur to
the reader. A bag-net made of a curved piece of cane, to which is fitted
a bag made of net, will serve to catch those which trust to flight for
escape from their enemies, such as the Lepidoptera; and these may be
killed by firm pressure of the thorax between the finger and the thumb.
The running insects, as the beetles, may be caught in a spoon or with
forceps; and they may be killed by immersion in boiling water or in
camphorated spirit. In an excursion, most insects may be carried in a
well-corked bottle containing a little wool and a lump of camphor, which
stupifies them. When the insects are dead, the limbs should be extended
into the natural position by means of pins, the insect being transfixed
by a pin run through the thorax or one of the elytra and extending into
a sheet of cork. To preserve them, they may be kept in a box, the
bottom of which is covered with sheet cork, into which the pins are
stuck.

The smaller beetles, &c., which cannot be transfixed with a pin, may be
mounted as opake objects upon slips of card, the legs &c., being
carefully spread out, and gummed in position with a strong solution of
gum-tragacanth in boiling water. Many of the smaller Curculion´idæ or
diamond-beetles, in which the labium forms a rostrum or beak, with
elbowed or half-bent antennæ, form beautiful opake objects when thus
mounted, on account of the brilliant scales with which they are covered.

There are two ways of examining insects--either in the entire state as
opake objects or the separate parts mounted as transparent objects. In
the former case the pin with which the insect is transfixed should be
stuck into a slide made of cork, and this laid upon the stage, or the
pin may be held by the forceps. In this way, with the use of the side
condenser and a low power, the general form and arrangement of the parts
of the insect can be made out. The more minute details must be searched
for in the individual organs which have been picked off with forceps,
and mounted in balsam.

If it be required to submit the parts of a dried insect to examination,
this must be previously soaked in warm water for a time, as the legs,
&c., become very brittle when dry, and are thus easily injured.

ROTATOR´IA (_róta_, a wheel) or ROTIF´ERA (_rota_ and _fero_, to
bear).--The animals contained in this class are minute, being just
distinguishable to the naked eye as white specks. They are common in
long-kept infusions and among _Confervæ_ in the water of pools and
ditches. Their body is usually longer than broad, often presenting
indications of rings; and at or near the posterior end is frequently
found a prolongation resembling a tail, but terminated by two short
moveable thumb-like processes, rarely a sucker, which enable the
animals to cling to objects. The most characteristic organ, however, is
a kind of rounded or oval disk, placed at the anterior end of the body,
and furnished with cilia. When these are in active motion, the organ
appears as a revolving wheel, whence the name of Wheel-animalcules, by
which they are sometimes designated. The wheel-organ enables the animals
to swim through the water, and also brings their food to the mouth by
the currents which it produces. It is usually cleft into two or more
lobes, and can be retracted, as is commonly the case when the animals
are disturbed.

In many of these animals the body is more or less covered by a horny
shell or carapace; and in some it is fixed at the bottom of a tube,
within which it can be withdrawn. On the anterior part of the body are
frequently seen two or more red spots, which represent eyes. The
alimentary canal is mostly distinct, being indicated by the colour of
its contents, and it is lined with cilia. Towards its front portion is a
gizzard (Pl. XI. fig. 2 _a_) containing teeth, which are sometimes
attached to a jointed jaw-like framework; these are usually in active
motion. No heart or blood-vessels have been observed in the Rotatoria;
but on each side of the body in many of them is a long wavy tube,
containing at intervals minute ciliated bodies, the cilia propelling the
water through the tubes, and so exerting an aërating or respiratory
function. The reproduction of the Rotatoria takes place by the formation
of ova, which may often be distinguished within the body of the parent.

_Rot´ifer vulgáris_ (Pl. XI. fig. 2) is a common species. It has a
spindle-shaped body, which is capable of contraction almost into a ball.
The front or head end is sometimes protruded (fig. 2), at others
retracted and obscured by the exserted disk (fig. 2*); and beneath it is
a tentacle-like organ, supposed to represent an antenna (fig. 2 _b_).
The position of the jaws is indicated at _a_. The alimentary canal is
seen running down the body; and two ova exist, one on each side of it,
these being often recognizable by the existence of the eyes and jaws. At
the end of the body are two lateral processes, and a tail-like piece,
which can be withdrawn or protruded and is furnished with two moveable
portions or toes.

PLATE XI. [PAGE 150.]

ROTATORIA, INFUSORIA, &c.


Fig.

1. _Anguillula_ (_Dorylaimus_), species of.

2. _Rotifer vulgaris_: _a_, jaws and teeth; _b_, antenna; 2*,
wheel-organ expanded.

3. _Pterodina patina._

4. _Floscularia ornata._

5. _Hydra viridis._

6. _Arcella vulgaris._

7. _Arcella aculeata._

8. _Arcella aculeata_, shell with animal.

9. _Arcella dentata._

10. _Amœba diffluens._

12. _Actinophrys sol._

13. Sponge, fibres of; 13 _a_, _b_, _c_, spicules of Sponge.

14. _Sertularia pumila_, polypidom.

15. _Sertularia pumila_, polypidom with polypes.

16. _Monas lens._

17. _Cercomonas globulus._

18. _Cercomonas crassicauda._

19. _Heteromita ovata._

20. _Anthophysa Mülleri._

21. _Dinobryon sertularia._

22. _Trachelomonas volvocina._

23. _Chætoglena volvocina._

24. _Euglena viridis._

25. _Astasia hæmatodes._

26. _Enchelys nodulosa_; _a_, undergoing transverse division.

27. _Oxytricha gibba_; 27 _a_, side view.

28. _Paramecium aurelia_: _a_, contractile vesicle; _b_, a gastric
sacculus.

29. _Amphileptus fasciola._

30. _Colpoda cucullus_: _a_, contractile vesicle.

31. _Nassula elegans_: _a_, vesicle; 31 _b_, encysted form.

32. _Coleps hirtus._

33. _Vaginicola crystallina._

34. _Vorticella convallaria_: _a_, stalk spirally contracted; _b_, body
undergoing longitudinal division.

35. _Vorticella convallaria_ encysted and discharging the young brood.

36. _Vorticella convallaria_, body with nucleus (_a_).

37. _Chilodon cucullulus._

38. _Stentor polymorphus_: _a_, body extended; _b_, body contracted;
_c_, bodies aggregated around a globule of jelly; _d_, bodies adherent
to the side of a glass.

39. _Alyscum saltans._

40. _Podophrya fixa_, or the _Podophrya_-form of _Vorticella_.


[Illustration: Plate XI.

W Bagg sculp

London: John Van Voorst.]

_Pterodína pat´ina_ (Pl. XI. fig. 3).--This species has a shell or
carapace on the back, a two-lobed rotatory organ, two eyes, and a
slender wrinkled tail ciliated at the extremity. The curved alimentary
canal, and the two strong muscles inclined at an angle, are easily
distinguishable.

_Flosculária ornáta_ (Pl. XI. fig. 4) is a very beautiful member of the
Rotatoria, and is found adhering to _Confervæ_ and other water-plants.
The body is club-shaped, and contained in a transparent tube, the ringed
narrower portion being fixed to its base. The rotatory organ is divided
into five or six lobes, furnished with long, slender, radiating
tentacular filaments; these are not vibratile like ordinary cilia, but
can be slowly moved. In the contracted state, the filaments form a
pencil-like bundle.

_Examination, &c._--The Rotatoria are best examined in the living state,
the drop of water in which they are viewed being very small, so that
their movements may be impeded; and while they are struggling to escape,
the various parts of the body will come into view. Their preservation
has been attempted by drying on a slide; but when dead they become so
contracted and altered, that it is difficult to make out their
structure. Should the observer wish to record any observations on their
reproduction or habits, it will be well to preserve a specimen of the
jaws and teeth, as the species might be with certainty identified by
careful examination of their minute structure.

ENTOZÓA (ἑντὀς, within, ξῶον, animal).--This class consists of the
parasitic worms, as the Tape-worm (_Tænia_), the Thread-worm and
Round-worm (_As´caris_), which live within the bodies of man and
animals. It also includes the microscopic eel-like animalcules (species
of _Anguil´lula_) which are found in sour paste (_A. glútinis_), in
vinegar (_A. acéti_), and in blighted wheat (_A. trit´ici_). Some of the
species of allied genera are met with in damp moss and in the débris or
fragments of vegetable substances decaying in water. The general
appearance of the microscopic species is that of a minute colourless
eel, writhing in the water (Pl. XI. fig. 1). Their internal organs are
difficult to distinguish. The alimentary canal is usually evident, and
dilated into a kind of stomach, containing near its commencement some
rod-like or otherwise-formed teeth. In the species figured there are two
apparently tubular lancets, which are capable of protrusion, and
evidently serve to wound the prey.



CHAPTER XII.

RADIATA.


Descending in the scale of animal organization, we come next to the
subkingdom RADIÁTA, or that in which the parts are arranged in a radiate
manner around a centre. Of this there are three classes,--the
ECHINODER´MATA (ἑχῖνος, hedgehog, δἑρμα, skin), containing the
Sea-urchins (_Echínus_), Starfishes, &c., in which the skin is furnished
with hard calcareous projecting spines or curiously formed imbedded
calcareous corpuscles, forming a rudimentary skeleton; the ACALÉPHÆ
(ἁκαλἡφη, a nettle), or Sea-nettles; and the POL´YPI (πολὑς, many, ποῦς,
foot), to which we shall confine our notice. It may be remarked that the
last two classes have recently been united to form the single class
CŒLENTERÁTA (κοῖλον, hollow, ἔντερον, intestine).

POLYPI.--These animals are mostly marine. They are either single (Pl.
XI. fig. 5), or compound (Pl. XI. fig. 15), _i. e._ the bodies are
united; in the latter case the bodies being usually situated in horny
cells upon a branched polypidom. But in many of them, which do not occur
in this country, there is an internal solid calcareous skeleton, of
which coral is an example. The animal bodies are soft, and furnished at
the front end with a crown of tentacles (fig. 15 _a_); these are
contractile, and serve to enable the animals to catch their prey. The
horny, branched, and plant-like polypidoms are often found on the
seashore, and are popularly confounded with sea-weeds.

_Hy´dra vulgáris_ (Pl. XI. fig. 5) is a fresh-water species, which is
commonly met with among collections of water-plants, and may generally
be obtained by collecting some of these and placing them in a glass jar
of fresh water. When the water has stood for some hours, the Polypes
will be seen, on careful examination, adhering to the sides of the
glass. The body of the animals is cylindrical, hollow, and furnished
with from six to ten tentacles, arranged in a circle, in the centre of
which is the mouth. The tentacles are hollow, and communicate with the
cavity of the body. On examination with a high power, the tentacles will
be found to exhibit minute oval sacs, containing a long fibre coiled up
within them; and when the tentacles are touched by any foreign body, the
fibres are suddenly discharged. These are the stinging or urticating
organs. The _Hydræ_ move very slowly; but the body is very contractile,
and is often seen of various forms. When a minute animal, as an
Entomostracan, happens to come into contact with the tentacles, these
curve around it, holding it firmly, and finally bringing it to the
mouth. It is then forced into the cavity of the body of the animal,
where it is digested, the remains being discharged at the mouth. The
movements of the _Hydra_, when devouring its prey, form a very curious
and interesting spectacle. The _Hydræ_ are propagated by budding or
gemmation, also by the formation of capsules in the walls of the body,
containing ova and spermatozoa. The young Polypes formed by budding are
represented in the figure, adhering to the base of the parent.

_Sertulária púmila_ (Pl. XI. fig. 15) is a marine species, the polypidom
being frequently found adhering to _Fuci_ and other sea-weeds; it is
about half an inch long. The cells are opposite, pointed at the ends,
and with an oblique orifice. The tentacles are fourteen in this species.
In the summer large ovate cells are found, arising from the polypidom;
these contain the eggs, and are called ovisacs or ovig´erous vesicles.



CHAPTER XIII.

PROTOZÓA (πρῶτος, FIRST, ξῶον, ANIMAL).


The members of this subkingdom are the lowest in the scale of animal
organization, their bodies consisting of a soft gelatinous and
structureless mass, which has a remarkable tendency to form little
cavities or vacuoles in its substance, and is called _sar´code_ (σἀρξ,
flesh). They exhibit no organs, unless the cilia and certain variable
processes formed of the common substance of the body, and which form
their agents of locomotion, be considered as such,--this substance
exercising the combined functions of motion, sensation, and secretion,
for which separate organs exist in the higher animals.

RHIZOP´ODA (ῥἱξα, root, ποῦς, foot).--The animals belonging to this
class consist of the structureless colourless substance to which
reference has been made as sarcode, and they exhibit no organs. The
sarcodic body is slowly contractile, and portions of it can be protruded
at will in the form of irregular root-like processes, acting both as
legs for locomotion and as tentacles by which the animal grasps its
prey, which is then forced into the substance of the body, where it
becomes surrounded by the surface, and a cavity is formed, within which
it is digested.

_Amœ´ba dif´fluens_ (Pl. XI. fig. 10) is common in water in which
portions of plants have been kept for some time. When first placed on
the slide, the body appears as a minute, transparent, rounded mass of
jelly; but if observed for some time, it will be seen slowly to
protrude its root-like processes; and foreign bodies, as Diatomaceæ or
other minute Algæ, will often be found imbedded in its substance.

_Arcel´la vulgáris_ (Pl. XI. fig. 6) is found among _Confervæ_ in ponds
and ditch-water. It is contained in a hemispherical shell or carapace,
from the round orifice of which the lobed processes are protruded. The
shell is covered with minute pits.

_Arcel´la aculeáta_ (Pl. XI. fig. 7) has the convex shell furnished with
spines; fig. 8 represents the animal with its processes extended; while
_Arcella dentáta_ (Pl. XI. fig. 9) exhibits an angular or somewhat
toothed membranous shell. Both the latter species are met with in the
same localities as the first.

_Actínophrys sol_ (Pl. XI. fig. 12) is a very beautiful and excessively
delicate Rhizopod. The body is spherical, and covered with very delicate
and slender cilia-like processes. Its movements are exceedingly slow,
and can only be observed by prolonged watching. The body appears to be
reticulated, from the presence of numerous vacuoles.

Two large groups of genera and species of Rhizopoda, the animal bodies
possessing the above general characters, mostly with very slender
processes, exist, in one of which (the FORAMINIFERA) they are contained
in calcareous shells, often of elegant forms; while in the other (the
POLYCYSTINA) the shells are siliceous or composed of flint, both kinds
of shells being perforated with holes. These shells, which occur in the
fossil state in enormous numbers, sometimes forming mountain-masses, are
extremely beautiful objects for the microscope.

SPON´GIÆ.--This class contains the Sponges, almost all of which are
marine and foreign, and therefore not likely to come under observation
in the perfect state. The substance commonly called sponge is the horny
skeleton of the animal, consisting usually of rounded fibres (Pl. XI.
fig. 13), irregularly netted and interlacing. The surface of a sponge
exhibits minute pores and larger pouting orifices; the former of which
admit currents of water, to be discharged at the latter, both being the
mouths of continuous channels. The surfaces of the channels are lined
with sarcodic matter, which takes the form of ciliated amœbiform bodies,
by which the currents of liquid are produced.

The horny fibres of sponges are strengthened by little siliceous or
flinty bodies of various forms (Pl. XI. fig. 13 _a_, _b_, _c_), which
are imbedded in the substance of the fibres or attached to their
surface, and form very curious microscopic objects. They are called
_spic´ula_ (_spiculum_, a dart), being often of a pointed form. In some
sponges they are calcareous.

INFUSOR´IA.--The animals contained in this class are usually very
minute, being rarely even perceptible to the naked eye, except when
existing in very large numbers, so as to render the water milky, green,
or red. They are found in all kinds of water, but especially in stagnant
pools and in decomposing solutions or infusions of vegetable matters.
The true structure of their bodies is a matter of doubt, some authors
having considered them as being highly organized, while others have
regarded them as consisting of simple cells; and whether they are
correctly referred to the Protozoa must remain at present a matter of
doubt. The body is of various forms, as represented in Plate XI. figs.
16-40. In some of them it consists of a simple sarcodic mass, evidently
without any outer skin, as shown by its ready adhesion and laceration on
accidental contact with foreign bodies; while in others the surface is
regularly dotted with little depressions, or with nodules, so as to
resemble a definitely organized structure.

The most striking character of the Infusoria is the presence of
vibratile cilia, which are variously arranged; in some entirely
covering the body, irregularly or in regular rows, in others being
situated at definite parts only. By the action of the cilia they are
enabled to swim freely in the water, also to obtain their food, which
consists of minute Algæ or fragments of animal matter. In many of them
there is a special row or set of cilia, which, by the currents it
produces, urges the particles of food suspended in the water towards the
mouth. The cilia also act as respiratory organs, by changing the water
with which their bodies are in contact. In some of the species there are
stout bristles or setæ, by which they are enabled to crawl upon
water-plants.

On carefully examining the bodies of the Infusoria, rounded granular
spots will be seen, frequently containing minute Algæ, &c. (fig. 28
_b_). These spots are the digestive cavities, and have been called
_gastric sac´culi_; but whether they are definite sacs or mere
excavations, formed by the particles of food having been forced into the
softer internal substance of the body, has not been positively
determined. The sacculi may be filled artificially by mixing very fine
indigo, or carmine, on a slide with the water in which the Infusoria are
contained. A definite food-tube or alimentary canal has been detected in
a few of the Infusoria; but it cannot be shown to exist in the majority
of them.

A mouth exists in most of them, and is sometimes indicated by a row or
set of cilia somewhat larger than those existing upon other parts of the
body, and leading to or placed near it. The particles of food which have
entered the body are often seen to pass round it, as if circulating,
descending on one side and ascending on the other.

In addition to the gastric sacculi, certain clear transparent spots may
also be seen within the body, appearing light or dark according to the
adjustment of the focus. If these are attentively watched, they will be
seen to contract and finally disappear, becoming again distended and
vanishing at tolerably regular intervals. These are the _contractile
vesicles_ (figs. 27 _a_, 28 _a_, 37 _a_), and they contain a clear
liquid, the nature of which is uncertain.

In many of the Infusoria is a round or elongate granular body (figs. 31
_a_, 36 _a_), which is called the nucleus, the term having been applied
to it from a notion that the Infusoria consisted of simple cells. A
minute red spot is also often seen at the anterior end of the body,
which is supposed to represent an eye, and is called an eye-spot. The
Infusoria are propagated in several ways:--by budding or gemmation, new
beings sprouting out in a bud-like form, usually from the base of the
parent; by division, either transverse (fig. 26 _a_) or longitudinal
(fig. 34 _b_), of the body gradually into two parts, each of which
subsequently becomes a perfect animal; by encysting, the body
contracting into a globular form, and forming a firm coat around it, the
contents becoming resolved into a numerous progeny of young; and by
conjugation and the agency of spermatozoa and ova. We will now proceed
to the examination of a few species, arranging them in the order of the
families to which they belong.

MONAD´INA.--In this family the bodies of the Infusoria are very soft,
and without a skin or integument; they are also exceedingly minute, and
will not admit the particles of indigo.

_Mon´as lens_ (Pl. XI. fig. 16) is very minute, and commonly found in
old infusions. Its body is rounded and flattened, and granular on the
surface. At the front end of the body is a whip-like or flagel´liform
(_flagel´lum_, a whip) filament, differing from a cilium in being rigid
at the base and moveable at the end only, by which it is enabled to row
itself through the water with a wriggling motion.

_Cercom´onas glob´ulus_ (fig. 17) has a spherical body, with two
flagelliform filaments, one arising from the front, the other from the
end of the body. In _Cercomonas crassicau´da_ (fig. 18) the posterior
filament is replaced by a tail-like narrowing of the body.

_Heterom´ita ováta_ (fig. 19) has the body ovate, with two long anterior
flagelliform filaments, one of which is directed forwards, while the
other trails behind.

_Anthophy´sa mül´leri_ (fig. 20) has the monad bodies arranged in little
heads at the ends of an irregularly branched brown stalk. After a time
they become detached and revolve freely in the water.

DINOBRY´INA.--_Dinobry´on sertulária_ (Pl. XI. fig. 21) forms a minute
Sertularia-like polypidom, consisting of rows of cells, each containing
an oval monad with a single anterior filament. The two last species are
common in bog-water.

THECAMONAD´INA.--In these Infusoria the body is inclosed in a firm and
sometimes brittle shell or carapace.

_Trachelom´onas volvoc´ina_ (Pl. XI. fig. 22) has a spherical red shell,
the body being furnished with a single filament and a minute red
eye-spot; while _Chætogléna volvoc´ina_ (fig. 23) has an oblong shell,
covered with little spines.

EUGLÉNIA.--In this family the form of the body is constantly changing,
being at one time spherical, at another fusiform or ovate. It is covered
with a contractile skin or firmer external portion, and has one or more
flagelliform filaments for locomotion. The species are common in
stagnant pools, often colouring the water green or red.

_Eugléna vir´idis_ (Pl. XI. fig. 24) has a spindle-shaped body when
fully expanded, the ends being pale; and at the front end is a red
eye-spot.

_Astásia hæmatódes_ (fig. 25), which is probably a form of the
_Euglena_, is found in stagnant pools, which it renders red. It has no
eye-spot.

ENCHÉLIA.--These Infusoria are found in stagnant water and in
decomposing infusions. The body is covered with cilia variously
arranged, but there is no integument nor mouth.

_En´chelys nodulósa_ (Pl. XI. fig. 26) has a colourless, oblong,
irregularly nodular body, coated with very slender radiating cilia, and
often exhibits numerous vacuoles. It is frequently found undergoing
transverse division (fig. 26 _a_), the body becoming gradually
constricted until it separates into two parts, which become perfect
animals.

_Alys´cum sal´tans_ (fig. 39) has an ovoid-oblong, slightly furrowed
body, surrounded with radiating cilia, and has a side bundle of long
retractile cilia, by means of which it leaps from place to place in the
water.

KERÓNIA.--In this family the body is soft, irregularly ciliated, without
a special integument, but has an oblique row of vibratile cilia leading
to the mouth, and stouter cilia or bristles (setæ) on certain parts of
the body. The sacculi often contain Diatomaceæ, &c.

_Oxyt´richa gib´ba_ (Pl. XI. fig. 27) has a colourless, oblong body,
somewhat expanded in the middle, with setæ at the two ends. In the side
view (fig. 27 _a_), the body is seen to be convex above and flattened
beneath.

PARAMEC´INA.--The species belonging to this family have a soft, flexible
body, which is usually oblong and flattened beneath, with an integument
covered regularly with pits and rows of cilia.

_Col´poda cucul´lus_ (Pl. XI. fig. 30) has a slightly compressed body,
ciliated all over, and kidney-shaped or rounded on one side and notched
on the other, the surface exhibiting rows of nodules. The mouth is
situated at the bottom of the notch.

_Paramécium aurélia_ (fig. 28) has the body oblong or oblong-ovate, the
mouth being placed near the anterior third of its under part. This
infusorium is of comparatively large size, and is often found in
immense numbers in infusions, which it renders milky. It is admirably
adapted for showing the sacculi, which are easily filled with indigo.
The body exhibits two remarkable stellate organs, consisting of a
central contractile vesicle, surrounded by several radiately placed oval
vesicles, which may be seen to contract and dilate with great
regularity. The body is coated with very fine cilia.

_Amphilep´tus fascíola_ (fig. 29) is furnished with an elongate fusiform
or lanceolate flattened body, with a lateral oblique mouth.

_Chil´odon cucul´lulus_ (fig. 37) has an oblong thin body, irregularly
wavy on the sides; the mouth being situated obliquely in front of the
middle, and furnished with a cylinder of parallel rod-like teeth.

_Nas´sula el´egans_ (fig. 31) has the body ovoid or oblong, becoming
globular when contracted, the mouth being furnished with teeth as in
_Chilodon_. It is often found among Oscillatoriæ.

URCEOLARÍNA.--_Vorticel´la convallária_ (Pl. XI. fig. 34) is very
commonly met with in decomposing infusions. The bell-shaped body is
fixed at the end of a slender stalk, which is often seen to be extended
and then suddenly contracted into a spiral (fig. 34 _a_). The cilia are
arranged around a raised rim at the front of the body, and extend down a
fissure leading to the mouth. The sacculi of this infusorium may be
readily filled with indigo. The process of longitudinal division may
also often be observed, taking about an hour for its completion; and
when the new individual is about to separate from the parent, a ring of
cilia may be noticed to have sprung up around the base (fig. 36). The
encysting process is also often visible, the cilia disappearing, and the
body becoming globular and secreting a cyst around it; after a time the
contents become resolved into a number of embryos, which escape by the
bursting of the cyst (fig. 35). In some cases the _Vorticella_ assumes
the form of a _Podoph´rya_ (fig. 40), the surface becoming covered with
tentacle-like processes. This _Podophrya_ was formerly considered a
distinct species.

_Vaginic´ola crystal´lina_ (fig. 33) is contained in a crystalline tube,
from which the body can be protruded. The body is of variable form,
presenting when fully extended a trumpet shape. The cilia exist at the
anterior end, and extend down a lateral fissure as in _Vorticella_. It
is found attached to Confervæ in the water of ponds and bog-pools.

_Sten´tor polymor´phus_ (fig. 38) is a very beautiful trumpet-shaped
infusorium, the body being covered with spiral rows of cilia. The rim is
furnished with stouter cilia, its margins at the notch being spirally
turned inwards. This infusorium is often found in little groups attached
to a gelatinous mass (fig. 38 _c_); and it is met with also in a free or
unattached state.

COLEP´INA.--_Cóleps hir´tus_ (fig. 32) has a barrel-shaped carapace,
transversely and longitudinally furrowed, the furrows being occupied by
cilia. It has two or three spines behind, and ten or twelve at the front
end of the carapace. It is common among Confervæ, and is very voracious,
feeding upon dead Entomostraca, &c.; and if disturbed, at its meal by
moving the cover, it will soon return and resume feeding as before.

_Examination._--The Infusoria must be examined during life; for they are
so altered by preservative liquids that they cannot be well preserved.
The shells of those that are provided with them may be kept simply dried
upon a slide, and in this way a few will retain their form, and the
cilia of all may be more easily distinguished; the vacuoles may also
then be seen very distinctly. When they are confined in a small quantity
of water and are about to die, a curious phenomenon may be observed in
them, a number of oil-like sarcodic globules exuding from the body, and
within these, vacuoles may often be seen to form spontaneously.

The Infusoria may be collected in small phials; but it is difficult to
keep them, as they form the food of the Entomostraca, the Rotatoria, and
the larvæ of insects; so that their enemies are very numerous, and they
soon disappear.

       *       *       *       *       *

CLASSIFICATION.--Before leaving the subject of living bodies, it may be
well to make a few remarks upon their systematic relation as defined by
classification.

All natural bodies are referable to one of three great kingdoms, viz.
the Animal, the Vegetable, or the Mineral Kingdom. The bodies belonging
to the latter seldom come under the notice of the microscopic observer,
as they are mostly visible to the naked eye, and their minute structure
is the same as that of the larger masses. The general structure of the
members of the vegetable and animal kingdoms has been illustrated in the
preceding pages. These bodies are distinguished from those of the
mineral kingdom by their vital power of appropriating surrounding
matters to their own nutrition and growth--this power being exercised by
their organs, or, in the lowest forms, by any portion of their
substance. Hence animals and vegetables or plants are termed organic
bodies, while minerals are termed inorganic bodies, as having no organs;
and the material of which organic bodies consist is termed organic
matter, that of minerals being inorganic matter. But in both animals and
plants inorganic matter is mixed with the organic matter, having been
taken up or absorbed from the inorganic kingdom, although it does not
usually exist in its characteristic condition, which is that of
crystals, _i. e._ angular solids, as crystals of Epsom salts, &c. The
individual members of the animal and vegetable kingdoms are
systematically divided into certain groups, and these into successively
smaller groups until we arrive at the species. These groups are founded
upon the possession of certain points of resemblance by their members,
forming the distinguishing characters, and their names are significant
and definite. They usually run as follows, the larger and higher groups
standing first in order:--Kingdoms, Subkingdoms, Classes, Orders,
Families, Tribes, Genera, and Species. But it must be observed that the
term species has a different value from that of the other terms; for the
individuals of which the species consist are not only related by
resemblance of structure, but also by their origin--being supposed to
have derived their origin from a parent of original creation; while the
other groups have, as far as we know, no other relation than that of
similarity of structure.

It is obvious that the characters of the various groups might be founded
upon peculiarities of any kind. But on this point two methods must be
specially distinguished, in one of which the groups are founded upon the
sum of all the peculiarities, while in the other they are based upon the
structure of single parts or organs. In the former case the system is
called natural, in the latter artificial. And while the latter often
brings together beings which have perhaps but one or two points of
resemblance, and separates others which are closely related, the former
associates those which are really and naturally similar.

All the groups have special names, so that they may be referred to and
spoken of as in the case of common things; the names being composed of
Greek or Latin words, so that they may be intelligible to all nations;
and as these are dead languages, they will remain good for all time.

In mentioning the name of an animal or plant, the name of the genus is
always used with that of the species; thus, the name of Chickweed is
_Stellaria media_. Because there are mostly several species in a genus;
so that if the name of the genus only were used, the species meant would
be uncertain; and as there are often species of the same name in
different genera, if the name of the species only were used, the genus
meant would be doubtful.

The classification of animals and plants serves two important purposes:
one is that the structural peculiarities and affinities of the groups
may be contrasted and a knowledge of their absolute and their
differential characters acquired, and for this a natural system is
eminently serviceable; the second is that of enabling any animal or
plant to be simply distinguished from any other, for which an artificial
or analytical system is extremely useful.



CHAPTER XIV.

OPTICAL PRINCIPLES.


We shall now devote a few pages to the consideration of the nature of
light, and the optical principles involved in the construction and use
of the microscope. Two theories of light have been propounded. According
to one, light consists of minute particles emanating from self-luminous
bodies, as the sun, a candle, or a red-hot piece of iron; this is called
the corpuscular theory. According to the other, light consists of waves
or undulations like those of water or the ears of corn set in motion by
the wind, of the molecules of an extremely subtle and rarified elastic
matter, called ether, existing everywhere, and set in motion by the
causes which produce light; this is called the undulatory theory. The
consideration of the merits of these two theories would be foreign to
our purpose: suffice it to say that the evidence in favour of the
undulatory theory preponderates, so that the corpuscular theory is now
laid aside.

It will often be requisite to make use of the term ray of light, by
which must be understood the smallest bundle of luminous undulations
which can be separated from a mass of light--as by passing light through
a small hole in an opake body, or by any equivalent method.

The most casual observer must have noticed that the rays of light move
in straight lines; as when the sun’s rays are seen entering a dark room
through a small window or other aperture, their direction being then
distinctly visible; the manner in which ordinary shadows are formed also
illustrates the same fact.

_Refraction._--But when the rays in their passage impinge or are
incident upon and enter a transparent medium or material, of a different
density from that which they were at first traversing, their course
becomes altered, and the line of their direction broken, whence they are
said to be refracted. If the medium upon which the rays impinge be
denser than that through which they were at first passing, they will be
refracted towards a line perpendicular to the surface, or they will be
refracted towards the perpendicular, as it is expressed.

Thus, as shown in Pl. XII. fig. 1, the incident ray _i_, entering the
plate of glass, will be refracted at its surface in the direction _a r_,
towards the line _p_, which is perpendicular to the surface.

The extent to which the rays undergo refraction depends upon the degree
of density of the medium, and varies in the case of each individual
substance; but it follows a definite law. If, as in Pl. XII. fig. 2, a
circle be drawn around the point _b_, at which the ray _a_ is incident,
_b r_ representing the refracted ray, the lines _s i_ and _t r_, drawn
at right angles to the perpendicular _p_, will form respectively the
sines, as they are called, of the angles _s b i_ and _t b r_; _s i_
being the sine of the angle of incidence _s b i_, or the angle formed by
the incident ray with the perpendicular, and _t r_ the sine of the angle
of refraction _t b r_, or of that formed by the refracted ray with the
perpendicular. These sines, for brevity, are called the sines of
incidence and of refraction; and they bear a constant ratio or
proportion to each other. Taking the sine of refraction as the unit, or
as = 1, the value of the sine of incidence represents the refractive
index or the refractive power of the medium for a ray entering the
medium from a vacuum; or, the refractive power of air being extremely
small, the value of the sine of incidence may be considered as
representing the refractive power from air into the medium.

PLATE XII. [PAGE 168.]

OPTICAL PRINCIPLES.


Fig.

1. Refraction through a glass plate.

2. Law of refraction.

3. Reflexion from a plane surface.

4. Reflexion from a concave mirror.

5. Refraction at a curved surface.

6. A doubly convex lens.

7. A plano-convex lens.

8. A doubly concave lens.

9. A plano-concave lens.

10. A concavo-convex lens, or meniscus.

11. Refraction through a convex lens.

12. Relation of a convex lens to prisms.

13. Relation of a concave lens to prisms.

14. Refraction of parallel rays through a convex lens.

15. Refraction of converging rays through a convex lens.

16. Refraction of diverging rays through a convex lens.

17. Refraction of parallel rays through a concave lens.

18. Spherical aberration.

19. Dispersion and formation of a spectrum.

20. Chromatic aberration.

21. Formation of images in the eye.

22. Angle of vision.

23. Objects too near the eye.

24. Action of convex lens in vision.

25. Aplanatism.

26. Aberration produced by cover.

27. Course of rays through the microscope.

28. Achromatism.

29. Waves of light conspiring (_a_, _b_), and interfering (_b_, _c_).

30. Polarization: _t_, tourmaline; _d_, crystal; _s_, crystal of
calcareous spar.


[Illustration: Plate XII.

W Bagg sculp

London: John Van Voorst.]

Although the ratio of the sines is constant, the refractive index varies
in different media. Thus that of air is 1·0003; of water, 1·336; of
Canada balsam, 1·549; of crown glass, from which window-panes are made,
1·535; of flint glass, from which bottles are made, 1·6; of Faraday’s
heavy glass, composed of silicated borate of lead, 1·8; and of that
consisting of borate of lead, 2·0.

A knowledge of this “law of the sines” is of practical importance in
determining the direction which the rays will pursue when transmitted
through glass lenses, &c. the refractive index of which is known; or in
ascertaining the curve which should be given to their surfaces for
producing a particular refraction and focal length. Thus, supposing the
plate of glass in Pl. I. fig. 2 to consist of crown glass, the
refractive index of which is 1·5, the length of the sine of refraction,
_t r_, will be equal to one part or dimension, while the sine of
incidence, _s i_, is equal to one part and a half.

It must be remarked that when light is incident at a right angle to the
surface of the medium, no refraction takes place, the transmitted ray
pursuing its original course.

When a ray of light leaves a denser medium, such as glass, to enter a
rarer medium, such as air, it becomes refracted from the perpendicular.
In such case, the angle of refraction being greater than the angle of
incidence, its sine will also be greater than that of the latter; but
the ratio is still preserved.

_Reflexion._--When rays of light fall upon a plane surface, as the flat
surface of the mirror, a greater or less number of them are reflected,
and this according to a definite law, by which the angle of incidence,
or that formed by the incident ray with the perpendicular, is equal to
the angle of reflexion, or that formed by the reflected ray with the
same. Thus, as shown in Pl. XII. fig. 3, the angle _i b p_, formed by
the incident ray _i b_ with the perpendicular _p_, is equal to the angle
_p b r_, formed by the reflected ray _b r_ with the perpendicular _p b_.

If the body upon the surface of which the rays are incident be
transparent, some of the rays will be refracted and will pass through
it, whilst others will be reflected. The proportion of those reflected
is smallest when the rays are incident perpendicularly to the surface;
but this increases as the incident rays become more oblique, _i. e._ as
the angle of incidence becomes greater, although at no degree of
obliquity are the whole of the rays reflected. The case is different,
however, with those rays which enter the substance and impinge upon its
inner or second surface; for these at a particular angle of incidence
undergo total reflexion, so that none of the rays are transmitted at the
second surface. The angle of total reflexion is constant for the same
medium, but different for different media: thus in crown glass it is
equal to about 40°, in flint-glass 38°, &c.; and this internal reflexion
from the second surface of transparent media is more perfect than that
occurring at the surface of opake reflecting surfaces or mirrors.

If the reflecting surface be concave, as in Pl. XII. fig. 4, parallel
rays will be reflected to a focus _a_, nearer the mirror than the centre
of its curvature _b_, and this focus is called the principal focus;
while diverging rays are brought to a focus nearer the centre of
curvature; and converging rays form a focus further from the centre of
curvature.

_Lenses._--In most instances, as far as the microscope is concerned, the
surfaces of the glass through which the rays of light are transmitted
are not plane or flat, but curved-being either convex or concave, and
belonging to convex or concave lenses. In considering the course of rays
through curved surfaces, the refraction may be viewed as taking place at
a plane surface forming a tangent at the point of incidence of each
ray; or each curved surface may be regarded as consisting of a number of
minute plane surfaces placed at right angles to the perpendicular. Thus,
in Pl. XII. fig. 5, the ray _a_, incident at the point _b_ of the curved
surface, is refracted towards the perpendicular _p_, as if it had fallen
upon the plane surface represented by the tangent _t_. The forms of the
most common lenses are represented in Pl. XII. figs. 6-10;--fig. 6 being
doubly convex, or both surfaces being convex; fig. 7, plano-convex, or
one surface plane, the other convex; fig. 8, doubly concave, or both
surfaces being concave; fig. 9, plano-concave, one surface being plane,
the other concave; and fig. 10 is a meniscus, in which one surface is
convex and the other concave. The curved surfaces of lenses are usually
portions of spheres.

The manner in which the course of a ray may be traced through a lens is
illustrated by Pl. I. fig. 11, which requires no explanation after what
has been already stated.

To facilitate the comprehension of the general action of lenses, they
may be regarded as composed of two triangular prisms, with their bases
in contact in a convex lens, as in fig. 12; their apices being opposed
in a concave lens, as in fig. 13.

The point to which the rays converge after passing through a convex lens
is called the _focus_ (Pl. XII. fig. 14 _f_), the distance of which from
the centre of the lens, called the focal length, obviously depends upon
the direction of the incident rays. When these are parallel, which those
coming from distant objects may be considered to be, the focus at which
they meet is called the principal focus, or the focus for parallel rays:
thus, in Pl. XII. fig. 14, the parallel rays meet at _f_, which is the
principal focus.

If the incident rays are convergent, as in Pl. XII. fig. 15, the focus
_o_ will be situated nearer the lens than the principal focus, _f_. If,
on the other hand, they are divergent, as in Pl. XII. fig. 16, the
focus _f_ will be situated further from the lens than the principal
focus _o_. By concave lenses the incident rays are rendered divergent,
as in Pl. XII. fig. 17, as if they emanated from a point _f_, situated
on the same side of the lens as that upon which the rays are incident,
and called the virtual focus.

_Spherical aberration._--Although, as a general expression, we have
stated that the rays of light meet at a focus on passing through a
convex lens, this is not strictly correct. For, in ordinary convex
lenses, the marginal rays are more refracted than the central ones, and
meet at focal points nearer the lens than the latter, as shown in Pl.
XII. fig. 18. This important defect is called spherical aberration, and
arises from the lateral rays being incident upon more oblique portions
of the curved surface of the lens than the central rays. Hence objects
seen through such lenses appear misty and confused, the central and
lateral parts of a flat object not being visible at the same time; and
even when the marginal parts are visible, they appear distorted or
deformed.

Spherical aberration is greatest in the most convex lenses; and, in a
plano-convex lens, it is least when parallel rays enter at or emerge
from its convex surface.

In certain lenses, the convex surface of which has the form of a
parabola, a hyperbola, or an ellipse, the spherical aberration is
absent; but it is impossible to grind microscopic lenses of these forms
with absolute accuracy, so that the fact is of no practical value.

The form of simple convex lens most free from aberration is that in
which the curves of the two surfaces form parts of a sphere, the radii
of the curves being as 1 to 6; the focal length being rather less than
twice the length of the radius of the most convex surface. This form of
lens comes very near to a plano-convex lens, which is consequently the
best form for a simple lens.

_Dispersion, or Chromatic Aberration._--The rays of light have so far
been considered as simple. They are, however, in reality compound,
consisting of a number of primary-coloured rays, of which seven kinds
are easily distinguishable, viz. red, orange, yellow, green, blue,
indigo, and violet. The coloured rays of the sun are, as is well known,
often seen separated by the action of the triangular glass bars or
prisms forming the lustres of a chandelier; the separation arising from
the different refrangibility of the coloured rays, by which each is
refracted to a different degree from that of the others. This is shown
in Pl. XII. fig. 19, representing a ray of white light entering a
triangular prism, at the surface of which the paths of the rays become
different according to the degree of their refrangibility, whence they
emerge separately, forming a _spectrum_ at _v r_; the most refrangible
violet rays (_v_) being most refracted, the less refrangible red (_r_)
least so, the intermediate rays being refracted to intermediate degrees
according to their respective refrangibilities. This separation of the
coloured rays is called dispersion; and as different substances or media
disperse the coloured rays over a larger or smaller space, so as to
produce spectra of different lengths, they are said to possess different
dispersive powers. Thus the dispersive power of flint glass and balsam
are about equal, while that of crown glass is considerably less.

The extent to which dispersion is produced by the same medium also
depends upon the angle of the prism, being greater as the angle is
larger; increased obliquity of the incident light also increases the
dispersion, so that the spectrum produced by a small prism may be equal
to that produced by a larger one upon which the light is less obliquely
incident.

In consequence of the dispersion of light, rays passing through a
convex lens do not meet at a single point or focus (Pl. XII. fig. 20),
but form as many foci as there are coloured rays.

When the spectrum is received upon a convex lens, the coloured rays are
brought to a focus, and the light appears again white; for it is only
when the primary-coloured rays are parallel, and seen close together,
that they produce the impression of white or colourless light. The
spectra produced by different dispersive media not only differ in
length, but also in the breadth of the coloured spaces not being in the
same ratio to each other; hence the spectra are said to be irrational,
or the dispersion is said to be irrational.

_Vision._--The visibility of an object depends upon the rays of light
which emanate from each point of its surface presented to the eye being
brought to a focus upon the ret´ina or expansion of the nerve of sight
lining the inside of the back of the eye; so that, an image of each
point being impressed upon the retina, the sum of the images forms the
compound image of the entire object.

The manner in which the image is formed is shown in Pl. XII. fig. 21, in
which, to prevent confusion, the rays coming from three points of the
arrow only have been represented. The rays diverging from these three
points, _a b c_, form cones in contact by their bases; the apex of each
cone outside the eye being situated at the points _a b c_, the common
base of each being situated at the crystalline lens _x_, immediately
behind the pupil or rounded aperture in the coloured curtain of the eye,
called the iris, _i i_, and which limits the base of the cones. The
apices of the cones within the eye, _a´b´c´_, are formed by the rays
brought to foci upon the retina by the crystalline lens. The marginal
cones of rays coming from the object cross within the eye, so that the
uppermost rays from the object become lowermost upon the retina, and
thus an inverted image of the object is formed. This appears to the eye
to be erect, because the eye or rather the mind judges the parts of an
object to be situated in that direction in which the rays coming from
them are impressed upon it. Hence, as in fig. 21, the rays impressed
upon the retina at the lower part _a´_, appear to come from the upper
part of the cross, although they are lowermost in the image, and so on
for the other rays. For distinct vision the rays of each cone must be
parallel, or nearly so.

_Angle of Vision._--The marginal rays coming from the object cross at a
point corresponding to the centre of the pupil, and thus form an angle,
as seen in fig. 22, where the cones are omitted, to avoid confusion;
this angle is the angle of vision. Now the size which objects appear to
possess is measured by this angle, or by the linear magnitude of their
images (_i. e._ their size estimated in one direction, as of length or
breadth) upon the retina. When the object is distant, the angle and its
linear magnitude are small, and it appears small and distant; whilst if
it be large, or if small and brought near the eye, the reverse will be
the case.

_Magnification._--Hence an object may be made to appear larger, or may
be magnified, by increasing the linear magnitude of its image upon the
retina, which can be done by bringing it nearer the eye, as shown in
fig. 22, where the image of _b_ formed at _b´_ is larger than the image
of _a_ formed at _a´_. But when an object is brought nearer the eye than
about 8 or 10 inches (for the distance varies with different persons),
its image becomes indistinct and misty; and this because the rays
composing the cones are too divergent to meet at a focus upon the
retina, as shown in fig. 23. By interposing a convex lens, however,
between the eye and the object, the too divergent rays may be made to
meet at a focus upon the retina, as in fig. 24, the object at the same
time being rendered apparently larger or magnified, from the refracting
action of the lens upon the cones.

_Aplan´atism_ (_a_, not, πλανἁω, to wander).--The effect of spherical
aberration in rendering the image of an object seen through a lens
indistinct and misty may now be intelligible. In order that such image
may be distinct, the rays emanating from each point of the object must
converge at one spot upon the retina. But since, when spherical
aberration exists, the marginal rays are more refracted than the central
ones, they will meet at foci before those formed by the latter; and when
the foci of one set are coincident with the retina, so that the image
would otherwise be distinct, the latter is rendered confused and
indistinct by the rays of the other set.

In this consideration, we imply that there are only two sets of rays,
the central and the marginal; but the central and marginal rays are not
separate, for the rays possess every intermediate degree of obliquity,
hence the foci and images are really innumerable.

Now there are evidently two methods of destroying or correcting
spherical aberration, viz. by excluding the marginal rays, or by
altering their direction.

The exclusion of the marginal rays is often adopted; and is effected by
means of a diaphragm, or stop as it is called. This consists of a plate
of metal, with a round aperture in the middle, and it is placed behind
the lens; but it has the serious defect of diminishing considerably the
amount of light transmitted.

The alteration of the direction of the marginal rays is produced by
refraction, a thin plano-concave lens being placed in front of the
convex one (Pl. XII. fig. 25). The doubly convex lens is composed of
crown glass, and the concave lens of flint glass, which has a higher
refractive and dispersive power than crown glass. In this way we get a
compound lens, which, if the two lenses had the same refractive power,
would simply amount to a plano-concave lens with the marginal portions
removed. But as the concave lens consists of more highly refracting
material than the convex, if the curve and thickness of the two lenses
be properly adapted, the marginal portions of the concave correct the
too great convergence of the marginal rays produced by the convex lens,
and so the rays are brought to nearly the same focus. An idea of this
action may be obtained from fig. 25, the dotted lines indicating the
direction which the rays would take, if passing through the convex lens
only.

A lens in which the spherical aberration is corrected is said to be
aplanatic.

_Achrómatism._--Supposing the spherical aberration of a lens to be
corrected, there still remains the chromatic aberration (p. 173); for
although the central or mean coloured rays may meet at a focus, the
other coloured rays belonging to the same compound or ordinary ray will
meet at different foci, so that a series of coloured images of the
object will be formed at different distances from the lens; hence, at
whichever focus the object is viewed, it will appear coloured.

Now the coloured primary rays can only be made to coincide in direction,
so that the light parts of an object may appear white, by refraction.
And the correction is produced by the same plano-concave lens as that
which corrects the spherical aberration. But in this case the relative
dispersive powers of the media composing the convex and the concave
lenses form the point to be considered. If the dispersive power of the
media of which the convex and concave lenses are composed were the same,
the dispersive power of the convex lens would be in excess, and the
coloured rays in each compound ray could not become parallel. But by
forming the concave lens of a more highly dispersive medium, with a less
proportional mean refraction than the convex, when the curves of the
surfaces and the relative thickness of the lenses are properly adjusted,
the dispersive action of the concave lens may be made equal to that of
the convex; and being exerted in the opposite direction, the coloured
rays will become parallel and meet at a single focus.

This may be elucidated by considering the lenses as composed of prisms.
Thus, let fig. 28 represent the compound lens, the two halves of the
doubly convex lens acting as two triangular prisms (fig. 19) with their
bases opposed, converging the compound white rays _w w_, and dispersing
the coloured elementary rays, which would form spectra at _s s_. In the
plano-concave lens the triangular prisms may be considered as placed
with their apices towards each other, and so would tend to disperse the
coloured rays in the opposite direction, to form spectra at _t t_. Then,
supposing the dispersions to be equal and in opposite directions, the
coloured rays would become parallel and meet at a definite focus, the
colour being destroyed. At the same time, the spherical action of the
concave lens being opposite to that of the convex, the converging action
of the latter will be diminished, so that the focus of the compound lens
will be longer than that of the convex alone; but as the dispersive
power of the concave is greater relatively than that of the convex, the
mean refraction is less altered than the refraction or dispersion of the
separate coloured rays; so that the concave wholly opposes or corrects
the dispersion produced by the convex, while it only partially corrects
its mean refraction.

A lens in which the chromatic and spherical aberrations are corrected or
destroyed is commonly called achromatic; although the term properly
applies to the correction of the colour only.

If in a compound lens the chromatic aberration is only partially
corrected, so that the red rays still meet at a focus beyond the violet,
as in a simple uncorrected lens (fig. 20), the lens is said to be
under-corrected, or the aberration to be positive; while if the
correcting action of the plano-concave lens be too great, so that the
violet rays meet before the red, as in a simple concave lens, the lens
is said to be over-corrected, and the aberration is called negative.
Although the positive chromatic aberration of the extreme rays passing
through a convex lens may be corrected by the negative aberration of a
concave lens, there still remains a certain amount of uncorrected
colour, arising from the irrationality of the spectra of the two
refracting media. This evil cannot be overcome, and the remaining colour
is said to arise from the secondary spectrum.

The object-glasses of the microscope, consisting of the compound lenses,
have their aberrations balanced to a considerable extent on the above
principles--the lowest combination being under-corrected, while the
upper combinations are over-corrected; and, by suitable adaptation of
their distance from each other, further correction may be obtained, the
aberration of the object-glass altogether being, however, over-corrected
or negative.

The eye-piece consists of two simple plano-convex lenses, the upper or
eye-glass (fig. 27 _e_) having a shorter focus than the lower (_f_) or
field-glass, and the two placed at the distance of half the sum of their
focal lengths. The object-glass alone would form an enlarged and
reversed image of the object within the body of the microscope, the
cones of rays from each point of the object terminating at the larger
arrows in the figure (fig. 27). But the rays meeting the field-glass are
brought by it to a focus at the position of the smaller arrows, where
they form a reduced image; and, subsequently passing through the
eye-glass, they are so altered in direction as to enter the eye at a
greater angle, and to present a magnified image of the object.

The eye-piece produces several important effects. The refraction being
produced by two less convex lenses instead of one of greater convexity,
the spherical aberration is considerably reduced; and the convexities
of the lenses in the eye-piece being situated in an opposite direction
to that of those in the object-glass, the spherical aberration of the
former reverses and so neutralizes that of the latter. Also the
under-correction of the field-glass compensates the over-correction of
the object-glass--the blue rays which are refracted more than the red by
the field-glass, being thrown upon the eye-glass nearer its centre,
where the refraction is less, and thus the coloured rays become parallel
or nearly so on reaching the eye. Moreover the field-glass collects a
larger number of rays than the eye-glass could do alone, so that it
enlarges the field and increases its brightness.

In the best object-glasses the aberrations are so well balanced that the
mere covering an object with thin glass is sufficient to disturb the
balance and render very delicate markings either misty and coloured or
wholly invisible. The effect produced by a plate of glass may be
understood by reference to fig. 26, the rays being supposed to emanate
from the object at _a_; and it is evident that the refraction of the
glass so alters the direction of the rays that they will fall upon the
lower combination nearer the centre than if the cover were absent, and
thus negative aberration is produced. In the best object-glasses,
however, this aberration may almost entirely be removed, the lower
combination being susceptible of approximation by a screw movement to
the second or next above it, so that the ascending rays, being able to
continue their oblique course through the increased distance between the
object and the lower combination, may fall upon the same portions of the
latter that they did before the cover was applied.

POLARIZATION OF LIGHT.--In attempting to give a sketch of this curious
and difficult subject, we must suppose the reader to be in possession of
a natural crystal of calcareous spar, and either two Nicol’s prisms
(forming the ordinary polariscope) or two plates of the mineral called
tourmaline cut in the direction of the length or axis of the crystal.

Hitherto we have considered rays of light falling upon transparent
substances as simply refracted or reflected according to the ordinary
laws of refraction or reflexion. We have now to notice some curious
exceptions, forming the basis of many interesting phenomena, especially
in connexion with the microscope, in which these laws are more or less
deviated from. If we place a plate of tourmaline, cut as above directed,
upon or beneath the stage of the microscope, the light will pass through
it, appearing tinged with the green or brown colour natural to the
tourmaline; but on laying another slice upon the eye-piece, and turning
the latter round or rotating it, the light will be transmitted in
certain positions only, being partially or entirely arrested in others,
so that the field appears black. And, on careful examination, it will be
noticed that the change from black to white occurs at each quarter of a
rotation, being twice black and twice white in an entire rotation, the
changes occurring alternately. The same phenomena may also be exhibited
by substituting two Nicol’s prisms for the tourmalines.

Again, if we take a natural crystal of calcareous spar, and paste upon
one side of it a piece of black paper with a small hole in the middle,
on holding the crystal to the light or over a piece of white paper, with
the covered side next the light, two holes or two images of the hole
will be seen; and if the crystal without the paper be placed over some
print, the print will appear double. Hence the light passing through the
hole is twice or doubly refracted, one ray following the ordinary law of
refraction, while the other follows a different law, being retarded and
pursuing a longer course; and so the two rays are called respectively
the ordinary and extraordinary ray. And on viewing these through a
tourmaline or a Nicol’s prism, as in the experiment with the two
tourmalines, the images will become alternately visible and invisible,
just as was then the case with the entire mass of light.

The light which has undergone this singular change is said to be
polarized, because the rays appear to have acquired poles or sides. In
the above experiments the lower prism or tourmaline is called the
polarizer, because it polarizes the light, and the upper is called the
analyzer, because it analyzes or tests the light altered by the former.

An idea of the cause of this change may be obtained by reference to the
undulatory theory of light. Ordinary light consists of waves or
undulations taking place in planes at right angles to each other, or in
all planes; while in polarized light the undulations are all in one
plane or in parallel planes. This may perhaps be understood by
considering that books in a book-case are situated in parallel planes,
the shelves being in planes at right angles to the former. And by
imagining in polarizing substances the existence of some structure
acting like a grating, a notion can be obtained how the rays in the
different planes may be transmitted or intercepted. If the grating be so
placed that the bars (representing the planes of polarization) are
perpendicular, the books can pass between them; while if the grating be
turned round a quarter of a circle, they will become transverse, and the
books cannot pass, while the shelves could do so. Carrying on this
analogy, the tourmaline or Nicol’s prism polarizes the light by
transmitting only those rays whose undulations are in planes parallel to
the bars; while the analyzer allows these undulations to pass through it
when the direction of the planes coincides with that of the bars, but
interrupts them when their direction is at right angles to the bars. And
it is evident that the planes of polarization of the ordinary and
extraordinary rays are opposite, from the opposite action of the
analyzer upon them.

The power of doubly refracting and polarizing is not possessed by all
crystalline bodies, but only those belonging to other than the cubic
system; crystals belonging to this system neither doubly refract nor
polarize light. In all doubly refracting crystals there are one or more
lines or directions in which the light is not doubly refracted. These
are called the optic axes, and sometimes they coincide with the
geometric axis of the crystals, at others they do not; and they may be
regarded as positions or directions of equilibrium of certain molecular
forces existing within the crystal, which, acting in opposition,
neutralize each other.

If light, polarized by the polarizer, be transmitted through thin doubly
refracting crystals, and analyzed by the analyzer, splendid colours will
become visible; and on rotating separately either the polarizer or the
analyzer, at each quarter rotation the colours will change, being
complementary to those at first visible, or such as are requisite with
the first to make white light. We have seen (fig. 19) that white light
consists of seven coloured rays, or of three primary colours--red,
yellow, and blue, which, by superposition, form the others; and thus red
is complementary to green, which consists of blue and yellow, the two
sets of complementary colours appearing and vanishing as the light and
darkness did when the crystals were not used.

These colours are produced by interference. The compound rays of white
light (fig. 30 _l_) passing through the polarizer (_t_) are all
polarized in one plane; the crystal (_d_) depolarizes this light, _i.
e._ doubly refracts and resolves it into two sets of rays polarized in
planes at right angles to each other, forming the ordinary, O, and the
extraordinary ray, E. Each of these two sets of rays is resolved by the
analyzer (_s_) into two other sets, polarized in planes at right angles
to each other; so that in all there are four sets, two in one plane and
two in the other; and, the primary rays of the two sets in each plane
being in different stages or phases of undulation, in consequence of the
retardation of the extraordinary rays, the undulations of certain
coloured rays check and annihilate each other, while the remainder or
complementary conspire and pursue their course, producing the appearance
of colour, this effect being reversed at each quarter-revolution of the
analyzer.

An idea of what is meant by phases of undulation may be obtained by
reference to fig. 29, in which the undulations, _a_, _b_, are in similar
states or phases, and so conspire in action, while the wave _c_ is in a
different phase and half an undulation behind the others; hence it would
check or interfere with either of the other waves (_a_, _b_), the
etherial molecules of the two, which vibrate perpendicularly or at right
angles to the direction of the wave, acting to the same extent and in
opposite directions.

In fig. 30 the analyzer is represented as composed of a natural crystal
(rhomb) of calcareous spar, which transmits both sets of rays; but in
the ordinary analyzer or Nicol’s prism--which is made by dividing a
rhomb through the obtuse angles into two wedge-shaped pieces and
cementing them together again with balsam, only one set of rays is
transmitted at each quarter-revolution, the other being refracted out of
the field. In the case of the tourmaline, one of the sets of rays is
absorbed; so that the tourmaline, like the Nicol’s prism, is
single-imaged.

Thus the colours produced by polarization are the same as those of the
spectrum, but separated in a different way, both arising from the
elementary coloured rays of the compound white light. For while the
spectral rays are separated by dispersive refraction, the polarized
coloured rays are separated by the interference and annihilation of some
rays, the remainder passing on to produce the colours.

When the position of the depolarizing crystal is such that the plane of
the polarized light coincides with the direction of the optic axis or
axes, the light is not doubly refracted nor polarized in certain parts;
hence these parts appear white if the plane of the polarizer and
analyzer coincide, and black if they be crossed. A crystal cut at right
angles to its optic axis, with its length directed towards the polarizer
and analyzer, is in this position, and it exhibits alternately a black
and white cross, with one or two sets of concentric rings of
complementary colours at each quarter of a rotation.

To prepare crystals for examination by polarized light, a little Epsom
salt, nitre, or borax should be dissolved in water, a drop placed upon a
slide, and dried at a gentle heat. The crystals should then be mounted
in balsam, and viewed as transparent objects.

To see the cross and rings, the crystals should be sawn or cut across
transversely, the ends being polished on a strained piece of silk
moistened with water, and the sections mounted in balsam.

The property of doubly refracting and polarizing light is not confined
to crystalline substances, being also possessed by many organic bodies,
for the details of which I must refer to the article Polarization in the
‘Micrographic Dictionary.’



INDEX.


Abdomen, 129.

Aberration, chromatic, 173.

Aberration, spherical, 172.

Acalephæ, 153.

Acarus, 131.

Accumbent, 44.

Achromatic, 178.

Achromatic condenser, 5.

Achromatism, 177.

Acicular, 24.

Acotyledons, 43.

Acrocarpi, 55.

Acrostalagmus, 111.

Actinocyclus, 80.

Actinophrys, 156.

Adapter, 2.

Adulteration, 23.

Æcidium, 100.

Agaricus, 97.

Albumen, of seeds, 43.

----, of eggs, 113.

Algæ, 64.

Alyscum, 161.

Amœba, 155.

Amphileptus, 162.

Angiocarpi, 94.

Angle of vision, 175.

Anguillula, 152.

Angular, 23.

Animal tissues, 113.

Ankistrodesmus, 74.

Annular vessels, 27.

Annulus, of Ferns, 50.

----, of Fungi, 98.

----, of Mosses, 55.

Anoplura, 135.

Ant, 143.

Antennæ, of Entomostraca, 127.

Antennaria, 111.

Anthers, 37, 39.

Antheridia, of Algæ, 65.

----, of Ferns, 52.

Antheridia, of Mosses, 61.

Anthophysa, 160.

Antlia, 145.

Apex, 37.

Aphis, 146.

Apiculate, 101.

Aplanatic, 177.

Aplanatism, 176.

Apothecia, 92.

Apple-pulp, 19.

Apple, skin of, 41.

Arachnida, 130.

Arcella, 156.

Archegonia, of Ferns, 52.

----, of Mosses, 62.

Arcuate, 78.

Areolæ, 116.

Areolar tissue, 116.

Arrow-root, 23.

Arteries, 114.

Articulata, 127.

Asci, of Fungi, 97.

----, of Lichens, 92.

Ascomycetes, 107.

Ash, 25.

Aspergillus, 106.

Aspidium, 50.

Astasia, 160.

Atropos, 145.


Bacterium, 86.

Balsam, garden-, 25.

----, Canada, 12.

Bark, 36.

Basidia, 98.

Bast, 26.

Batrachospermeæ, 72.

Batrachospermum, 72.

Beards of Oyster, 125.

Bee, 143.

Beetles, 146.

Bermuda deposit, 81.

Bipinnatifid, 50.

Bivalve, 128.

Blood, 114.

----, of Birds, 119.

----, of Fishes, 121.

----, of Reptiles, 120.

Blood-corpuscles, 111.

Blood-worm (Chironomus), 143.

Blue-bottle, 137.

Blue mould, 106.

Bone, of Birds, 119.

----, of Reptiles, 120.

----, of Mammals, 114.

Bordered dots, 28.

Bothrenchyma, 28.

Botrytis, 105.

Branchiæ, 125.

Bread, 23.

Bryozoa, 125.

Bryum, 59.

Buds, 47.

Bug, 145.

Bull’s-eye, 6.

Bunt, 102.

Butterflies, 144.


Cabbage-butterfly, 145.

Calicium, 94.

Calyptra, 55.

Calyx, 37.

Calcareous, 66.

Camera lucida, 18.

Campylodiscus, 78.

Canaliculi, 115.

Cancelli, 114.

Capillaries, 114.

Capillary, 69.

Carapace, 127.

Caraway, 41.

Carpels, 41.

Cartilage, 115.

Caterpillars, 145.

Cell-contents, 21.

Cell-formation, 29.

Cells, 19.

Cell-wall, 19.

Cellular tissue, animal, 116.

---- tissue of plants, 20.

Cement, 16.

Cephalothorax, 130.

Ceramiaceæ, 69.

Ceramidia, 66.

Ceramium nodosum, 69.

---- rubrum, 70.

Cercomonas, 159.

Chætomium, 110.

Chætophoraceæ, 71.

Chara, 88.

Cheese-mite, 131.

Chelate, 131.

Chickweed, 37.

----, seeds of, 42.

Chilodon, 162.

Chin of Insects, 147.

Chironomus, 143.

Chitine, 136.

Chloride of calcium, 15.

Chlorococcum, 87.

Chlorophyll, 21.

Chrysalis, 141.

Chrysanthemum, 25.

Cilia, 52, 157.

Circulation in Plants, 31.

Cladonia, 93.

Cladophora, 71.

Classification, 164.

Clavarini, 99.

Clavate, 147.

Closterium, 74.

Coarse movement, 2.

Coccidia, 69.

Coccinella, 147.

Cocconeis, 79.

Cocoa-nut fibre, 26.

Cœlenterata, 153.

Coleochæte, 72.

Coleoptera, 146.

Coleps, 163.

Collomia, seeds of, 42.

Colpoda, 161.

Columella, of Mosses, 62.

----, of Mucor, 111.

Complementary colours, 183.

Compound animals, 126.

Concentric, 114.

Conceptacles, of Algæ, 65.

Conduplicate, 44.

Conferva bombycina, 71.

---- floccosa, 71.

Confervaceæ, 70.

Confervoideæ, 70.

Conidia, 100.

Coniferæ, 26.

Coniomycetes, 99.

Conjugation, 73.

Continuous, 103.

Convolvulus, pollen of, 39.

Coral, 153.

Corallina, 66.

Corallinaceæ, 66.

Corallines, 66.

Coremium, 106.

Corolla, 37.

Coscinodiscus, 81.

Cotton, 118.

Cotton-plant, 33.

Cotyledons, 43, 44.

Covers, 6.

Coxa, 139.

Crenate, 50.

Crocus, pollen-tubes of, 40.

Cruciferæ, 44.

Crustacea, 127.

Crustaceous, 91.

Cryptogamic plants, 48.

---- ----, seeds of, 43.

Ctenoid, 121.

Cucumber, 42.

Culex, 141.

Culm, 34.

Cuneate, 79.

Cup-moss, 93.

Curculionidæ, 149.

Cycloid, 121.

Cyclops, 128.

Cypris, 128.


Dacrymyces, 99.

Daphnia, 129.

Dasya, 68.

Day-flies, 145.

Deal, 26.

Delesseria, 69.

Dematiei, 104.

Desmidiaceæ, 73.

Diamond-beetles, 149.

Diaphragm, 3.

Diatoma, 78.

Diatomaceæ, 75.

----, fossil, 82.

----, markings on, 81.

----, preparation of, 82.

Dichotomous, 67.

Dicotyledons, leaves of, 35.

----, seeds of, 43.

----, stems of, 35.

Dicranum, 57.

Differentiation, 47.

Dimidiate, 55.

Dinobryina, 160.

Dinobryon, 160.

Diœcious, 38.

Dipping-tubes, 4.

Diptera, 137.

Dispersion, 178.

Division, spontaneous, 159.

Dothidea, 108.

Dragon-flies, 145.

Draparnaldia, 71.

Dry rot, 98.

Ducts, 27.

Dung-beetle, 132.

Dytiscus, 148.


Eccremocarpus, seeds of, 42.

Echinodermata, 153.

Echinus, 153.

Elbowed, 149.

Elder, 25.

Elements, vegetable, 19.

Elytra, 147.

Emarginate, 146.

Embryo, 42.

Embryo-sac, 46.

Embryonal vesicles, 46.

Enchelys, 161.

Encysting, 159.

Endogenous, 29.

Endogens, leaves of, 35.

----, seeds of, 44.

----, stems of, 44.

Enteromorpha, 87.

Entomostraca, 127.

Entozoa, 151.

Ephemera, 145.

Epidermis, animal, 117.

----, vegetable, 31.

Epithemia, 77.

Equisetum, 34.

Erector, 5.

Erysiphe, 110.

Euglena, 160.

Excipulum, 92.

Excurrent, 57.

Exogenous, 29.

Exogens, leaves of, 35.

----, seeds of, 43.

----, stems of, 35.

Eye-pieces, 4, 179.

Eyes, compound, 138.


Facets, 138.

Favellæ, 70.

Feathers, 119.

Feathery, 127.

Femur, 139.

Ferns, 49.

----, reproduction of, 51.

Fertilization, 45.

Fibro-cellular tissue, 25.

Fibro-vascular tissue, 31.

Field, 7.

Filament, 37.

----, flagelliform, 159.

Fine movement, 2.

Fish-crystals, 122.

Flagelliform, 159.

Flax, 26, 118.

Flea, 136.

Flocci, 103.

Florideæ, 66.

Floscularia, 151.

Flowers, 37.

Flustra, 126.

Fly, 138.

Fly-blows, 139.

Focus, 171.

Foot-jaws, 127.

Foramen, 45.

Foramina, 114.

Foraminifera, 156.

Forceps, 4.

Forked veins, 50.

Fovilla, 160.

Fragilaria, 78.

Fronds, 49.

Fruit, 40.

Frustules, 75.

Fucoideæ, 64.

Fucus, 65.

Fulcra, 110.

Funaria, 58.

Fungi, 96.

Funiculus, 45.

Fusiform, 110.


Galls, 111.

Gamasus, 132.

Gasteromycetes, 99.

Gemmation, 159.

Gentles, 140.

Geranium, leaf of, 21.

----, petals of, 20, 38.

Germinate, 43.

Gills, of Fungi, 97.

----, of Mollusca, 125.

Glands, 40.

Glandular, 31.

---- tissue, 26.

Glœocapsa, 88.

Glumes, 37.

Glycerine, 14.

Gnats, 141.

Gomphonema, 79.

Gonidia, of Algæ, 71.

----, of Lichens, 92.

Gonium pectorale, 83.

---- tranquillum, 84.

Grape-Fungus, 105.

Graphis, 94.

Green fly, 146.

Gristle, 115.

Groundsel, 33.

Guard-cells, 34.

Gymnocarpi, 94.

Gymnostomum, 56.

Gyrosigma angulatum, 80.

---- attenuatum, 79.


Hairs, 31.

----, animal, 117.

Hart’s-tongue Fern, 51.

Harvest-bug, 132.

Haversian canals, 114.

Helvellacei, 107.

Hemiptera, 145.

Hemp, 26.

Hermaphrodite, 38.

Heteromita, 160.

Hilum, 22.

Hoop, 76.

Humble Bee, 144.

Hundred-legs, 133.

Hyalotheca, 74.

Hydnei, 99.

Hydra, 153.

Hymenium, 98.

Hymenomycetes, 97.

Hymenoptera, 143.

Hyphomycetes, 103.

Hypnea, 69.

Hypnum, 60.

Hysterium, 108.


Illumination, 8.

Imago, 141.

Imbricate, 56.

Incumbent, 44.

Indusium, 51.

Inflated, 110.

Infusoria, 157.

Injection, 116.

Innovations, 55.

Insects, 133.

Intercellular passages, 20.

Intercellular spaces, 20.

Iodine, 22.

Isaria, 104.


Jania, 67.

Jaws, of Insects, 147.

Joints, of Insects, 136.

Jute, 26.


Knife, 4.


Labels, 17.

Labial palpi, 134.

Labium, 147.

Labrum, 147.

Lacunæ, 115.

Lady-bird, 147.

Lanceolate, 78.

Leaves, 31.

Legs, of insects, 139.

Lenses, 170.

Lentil, 23.

Lepidoptera, 144.

Lepisma, 134.

Liber, 26, 35.

Lice, 135.

Lichens, 91.

Lieberkuhn, 7.

Light, 167.

Lime, 25.

Lips, of insects, 147.

Lirellæ, 94.

Lithobius, 133.

Lithocystis, 67.

Live-box, 4.

London pride, anthers of, 39.

Longitudinal, 30.

Lunate, 147.

Lyngbya, 85.


Magnification, 175.

Magnifying power, 17.

Mammalia, 113.

Mandibles, of insects, 147.

----, of Spiders, 130.

Mantle, 123.

Marrow, 114.

Matrix, 91, 96.

Maxillæ, 147.

Medium, 168.

Medulla, 35.

Medullary sheath, 35.

Melanosporeæ, 64.

Melobesia, 67.

Melosira, 79.

Membrane, 31.

Membranipora, 126.

Mentum, 147.

Merulius, 98.

Micrasterias, 74.

Micropyle, of seeds, 46.

Microscope, 1.

----, simple, 5.

Midrib, 50.

Mignonette, seeds of, 42.

Mildew, 101.

Mirror, 2, 3.

Mitriform, 55.

Mollusca, 122.

Monadina, 159.

Monas, 159.

Moniliform, 31.

Monocotyledons, leaves of, 35.

----, seeds of, 43.

----, stems of, 36.

Monœcious, 38.

Mosses, 54.

Moths, 144.

Mould of paste, 111.

Moulds, 96.

Mounting, 10.

Mouth of insects, 147.

Mucedines, 105.

Mucor, 111.

Muscles, 115.

Mushroom, 97.

Mustard and cress, 43.

----, cotyledons of, 44.

Mycelium, 96.

Myriapoda, 133.


Nacre, 123.

Nassula, 162.

Needles, mounted, 4.

Nemaspora, 100.

Nerves, of leaves, 31.

Neuroptera, 145.

Newt, 120.

Nicol’s prism, 6, 184.

Nitella, 88.

Nitzschia, 78.

Nodding, 59.

Nodule, 79.

Nostoc, 86.

Nucleolus, 21.

Nucleus, 21.

----, of Infusoria, 159.

----, of ovule, 45.


Oak-apple, 111.

Oat, 23.

Object-glasses, 2.

Oblique, 30.

---- light, 9.

Obovate, 57.

Obtuse, 50.

Ocelli, of Spiders, 130.

Oidium, 105.

Opegrapha, 94.

Operculum, 55.

Optic axes, 183.

Orange, rind of, 40.

Organs, vegetable, 31.

Oscillatoria autumnalis, 84.

---- nigra, 85.

Oscillatoriaceæ. 84.

Ovary, of animals, 137.

---- of plants, 37, 40.

Ovate, 56.

Ovigerous vesicles, 154.

Ovisacs, 154.

Ovules, 45.

Ovum, 113.

Oxytricha, 161.

Oyster-shell, 123.


Paleæ, 37.

Palmella, 88.

Palmellaceæ, 87.

Palpi, of insects, 147.

Papilla, 39.

Papillæ, of skin, 116.

Papillose, 39.

Paramecium, 161.

Paraphyses, 61.

Parenchyma, 20.

Parmelia, 91.

Pear, 29.

Pearls, 123.

Pectinate, 69.

Pediastrum, 74.

Pedicels, 103.

Pencil, 128.

Penicillium, 106.

Perianth, 37.

Pericarp, 40.

Perichætial, 60.

Peridiola, 110.

Peridium, 99, 100.

Perigonial, 61.

Perispore, 65.

Peristome, 55.

Perithecium, 107.

Petals, 37, 38.

Peziza, 107.

Phases, 184.

Phragmidium, 101.

Physarum, 99.

Physomycetes, 110.

Pigment, 117.

Pileus, 97.

Pinnæ, 51.

Pinnate, 51.

Pinnatifid, 50.

Pinnularia, 79.

Pinnules, 51.

Pistil, 37.

Placenta, 45.

Pleurenchyma, 26.

Pleurocarpi, 55.

Pleurosigma, 79.

Plocamium, 69.

Plumose, 127.

Plum-stone, 29.

Plumule, 43.

Podetia, 93.

Podophrya, 162.

Podura, 134.

Polariscope, 6.

Polarization of light, 180.

Pollen, 37.

Pollen-granules, 39.

Pollen-tubes, 40.

Polycystina, 156.

Polypi, 153.

Polypidom, 125.

Polypodiaceæ, 50.

Polypodium, 50.

Polyporei, 99.

Polysiphonia, 68.

Polytrichum, 58.

Polyzoa, 126.

Polyzoary, 126.

Poppy, seeds of, 42.

Pore, 109.

Pores, in wood, 28.

Porous cells, 25.

Potato, 22.

Potato-fungus, 105.

Primine, 45.

Primordial utricle, 21.

Primrose, pollen of, 39.

Prismatic, 24.

Proboscis, 146.

Prolegs, 145.

Prosenchyma, 25.

Prothallium, 51.

Protoplasm, 21.

Protozoa, 155.

Pterodina, 151.

Puccinia, 101.

Puff-balls, 99.

Pulex, 135.

Pulvilli, 139.

Puncta, 79.

Pupa, 141.


Quadrate, 147.


Racodium, 111.

Radiata, 153.

Radicle, 43.

Ramenta, 49.

Raphides, 24.

Receptacles, of Algæ, 65.

----, of secretion, 40.

Red Spider, 132.

Reflexion, 169.

Refraction, 168.

Reindeer moss, 93.

Reproductive organs, 48.

Reptiles, 120.

Resting spores, 83.

Reticulated, 54, 129.

---- vessel, 27.

Rhabdonema, 80.

Rhinotrichum, 107.

Rhizome, 49.

Rhizopoda, 155.

Rhodomelaceæ, 67.

Rhodospermeæ, 66.

Rhodymeniaceæ, 69.

Rhubarb, 25.

Rice, 23.

Richmond earth, 82.

Rings, annual, 36.

Roe of fishes, 122.

Rootlets, 36.

Roots, 36.

Rostrate, 56.

Rostrum, 131.

Rotating disk, 6.

Rotation, 31.

Rotatoria, 149.

Rotifer, 150.

Rotifera, 149.


Sacculi, gastric, 158.

San Fiori deposit, 82.

Sarcode, 155.

Scalariform ducts, 49.

Scales, of fishes, 121.

----, of Podura, 134.

----, of insects, 144.

Scar of seeds, 46.

Scenedesmus, 74.

Schizogonium, 86.

Scolopendrium, 51.

Sea-weeds, 64.

Sections, 30.

Secundine, 45.

Seeds, 42.

Selenite, 24.

Sepals, 37.

Septa, 99.

Septate, 100.

Serrate, 51.

Sertularia, 154.

Sessile, 54.

Setaceous, 133.

Setæ, 129.

---- of Infusoria, 158.

Shell, 122.

Side condenser, 6.

Silk, 118.

Simple microscope, 5.

Sines, 168.

Siphonaceæ, 84.

Skin, 116.

Slides, 6.

Smut, 102.

Snapdragon, petals of, 39.

Snowberry, 42.

Sori, of Ferns, 50.

Spawn, of Fungi, 97.

Spectrum, 173.

Spermatia, 93.

Spermatozoa, of Algæ, 65.

----, of Ferns, 52.

----, of Fishes, 122.

----, of Mosses, 61.

Spermogonia, of Fungi, 101.

----, of Lichens, 92.

Sphæria, 109.

---- fragiformis, 104.

Sphagnum, 56.

Spicula, 157.

Spiders, 130.

Spiderwort, 31.

Spinnerets, 131.

Spiracles, 134.

Spiral cells, 25.

Spiral vessels, 27.

Spirit-lamp, 5.

Spirogyra, 72.

Spirulina, 85.

Sponges, 156.

Spongidæ, 156.

Sporangium, of Ferns, 54.

Spores, 50.

Sporocybe, 104.

Sporophylles, 69.

Stage, 2.

Stamens, 37.

Starch, 22.

Starfishes, 153.

Stellate, 31.

Stems, 35.

Stentor, 163.

Sterigmata, 98.

Steropus, 146.

Stichidia, 68.

Stigma, 37.

Stilbacei, 103.

Sting, of Wasp, 144.

Stinging-organs, 154.

Stings, 33.

Stipes, 76, 97.

Stomata, 34.

Stomoxys, 141.

Striated, 33.

Stroma, 108.

Struma, 58.

Style of plants, 37.

Stylospores, 100.

Suctorial, 135.

Suture, 73, 76.

Synedra, 78.

Synura, 83.


Tabellaria, 79.

Tabular, 83.

Tangential, 30.

Tape-worm, 152.

Tarsus, 139.

Tentacles, 124, 153.

Testa, 42.

Tetraspores, 167.

Thalamium, 92.

Thallus, 91.

Thecæ, 50.

Thorax, 129.

Thread-worm, 152.

Tibia, 139.

Tissues, vegetable, 19.

Toad-stools, 96.

Tongue, of Mollusca, 123.

Tortula, 57.

Torula, 100.

Tourmaline, 181.

Tous les mois, 24.

Trabeculæ, 59.

Tracheæ, 134.

Trachelomonas, 160.

Tradescantia, 31.

Transverse, 148.

---- section, 30.

Tremellini, 99.

Trichothecium, 105.

Triton, 120.

Trochanter, 139.

Trombidium, 132.

Truffle, 108.

Truncated, 27.

Tuberacei, 108.

Tubercularia, 103.

Tuberculate, 101.

Tubular cells, 25.

Tufts, 25.

Turnip-fly, 146.

Turpentine, 27.


Ulothrix, 86.

Ulva, 87.

Ulvaceæ, 86.

Umbelliferæ, 41.

Uncorrected lens, 178.

Uniseptate, 101.

Uredo, 102.

---- candida, 102.

---- caries, 102.

---- linearis, 109.

---- rubigo, 109.

---- segetum, 102.


Vaginicola, 163.

Vaginule, 62.

Valve, 76.

Vascular tissue, 27.

Vaucheria, 84.

Vegetable elements, 19.

---- organs, 31.

Veil, 98.

Veins, of animals, 114.

----, of leaves, 31, 35.

Velum, 98.

Vertebra, 113.

Vertebrata, 113.

Vesicles, contractile, 159.

----, embryonal, 46.

Vessels, 27.

Vibrio spirillum, 85.

Vision, 174.

Vittæ, of seeds, 40.

Volva, 98.

Volvocineæ, 83.

Volvox, 83.

Vorticella, 162.


Wallflower, anthers of, 39.

Wasp, 143.

Water-beetle, 148.

Web, spider’s, 131.

Wheat, cotyledon of, 45.

Wheat-flour, 23.

Wheel-animalcules, 150.

Whelk, 123.

Wine-cellar Fungus, 111.

Winged seeds, 42.

Winter ova, 128.

Wood-cells, 25.

Wood-louse, 127.

Woody fibre, 25.

---- tissue, 25.

Wool, 118.


Xanthidia, 75.


Yeast, 106.


Zygnema, 72.

Zygnemaceæ, 2.

Zoospores, 71.

TAYLOR AND FRANCIS, PRINTERS, RED LION COURT, FLEET STREET.





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