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Title: Common Objects of the Microscope
Author: Wood, J. G. (John George)
Language: English
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*** Start of this LibraryBlog Digital Book "Common Objects of the Microscope" ***
Transcriber’s notes:
Minor punctuation errors have been corrected silently (e.g. missing
full stops after abbreviated words such as Fig), as have the following
misspellings: Bretahing → Breathing, Pedicillaria → Pedicellaria,
Pedicelaria → Pedicellaria, Chœtonotus → Chætonotus, Spurganium
→ Sparganium, veiw → view. Unorthodox spelling and inconsistent
hyphenation has not been altered. Several wrongly numbered
cross-references to Plates and Figures have been corrected.
Plate VIII (and its accompanying key) was originally displayed
at the beginning of the book, before the Title Page, but has been
repositioned in the body of the text in correct numerical sequence.
COMMON OBJECTS OF
THE MICROSCOPE
BY THE LATE
Rev. J. G. WOOD, M.A., F.L.S., Etc.
AUTHOR OF
“COMMON OBJECTS OF THE COUNTRY” “COMMON OBJECTS OF THE SEA-SHORE”
“MY FEATHERED FRIENDS” ETC. ETC.
WITH ILLUSTRATIONS BY TUFFEN WEST
_SECOND EDITION, REVISED AND RE-WRITTEN, BY_
E. C. BOUSFIELD, L.R.C.P.(Lond.)
AUTHOR OF
“A GUIDE TO THE SCIENCE OF PHOTO-MICROGRAPHY”
WITH ADDITIONAL ILLUSTRATIONS BY THE REVISER
LONDON
GEORGE ROUTLEDGE AND SONS, Limited
BROADWAY HOUSE, LUDGATE HILL
1900
PREFACE TO THE SECOND EDITION
The task of revising and bringing up to date a work which has been the
guide, philosopher, and friend of thousands of commencing microscopists
has been, in the present case, one of no small difficulty. On the one
hand, there was the natural desire to interfere as little as possible
with the original work; and on the other, the necessity of rendering
available, to some extent at least, the enormous advance in every
department which has taken place in the thirty-six years which have
elapsed since the work was first offered to the public. The reviser has
done his best not only to fulfil these two objects, but to keep in view
the original purpose of the book.
In the popular department of pond-life especially, about fifty new
illustrations have been added, mostly from the reviser’s own notebook
sketches. The whole of the botanical part has been revised by one of
our first English authorities, and, in short, no effort has been spared
to make the work as accurate as its necessarily condensed form permits
of. It is hoped, therefore, that it may be found not less useful than
its predecessor by those for whom it is alone intended.
PREFACE TO THE FIRST EDITION
In my two previous handbooks, the “Common Objects” of the Sea-shore and
Country, I could but slightly glance at the minute beings which swarm
in every locality, or at the wonderful structures which are discovered
by the Microscope within or upon the creatures therein described. Since
that time a general demand has arisen for an elementary handbook upon
the Microscope and its practical appliance to the study of nature, and
in order to supply that want this little volume has been produced.
I must warn the reader that he is not to expect a work that will figure
and describe every object which may be found on the sea-shore or in the
fields, but merely one by which he will be enabled to guide himself
in microscopical research, and avoid the loss of time and patience
which is almost invariably the lot of the novice in these interesting
studies. Upwards of four hundred objects have been figured, including
many representatives of the animal, vegetable, and, mineral kingdoms,
and among them the reader will find types sufficient for his early
guidance.
Neither must he expect that any drawings can fully render the lovely
structures which are revealed by the microscope. Their form can be
given faithfully enough, and their colour can be indicated; but no pen,
pencil, or brush, however skilfully wielded, can reproduce the soft,
glowing radiance, the delicate pearly translucency, or the flashing
effulgence of living and ever-changing light with which God wills to
imbue even the smallest of His creatures, whose very existence has been
hidden for countless ages from the inquisitive research of man, and
whose wondrous beauty astonishes and delights the eye, and fills the
heart with awe and adoration.
Owing to the many claims on my time, I left the selection of the
objects to Mr. Tuffen West, who employed the greater part of a year
in collecting specimens for the express purpose, and whose well-known
fidelity and wide experience are the best guarantees that can be
offered to the public. To him I also owe many thanks for his kind
revision of the proof-sheets. My thanks are also due to Messrs. G. and
H. Brady, who lent many beautiful objects, and to Messrs. Baker, the
well-known opticians of Holborn, who liberally placed their whole stock
of slides and instruments at my disposal.
CONTENTS
CHAPTER I PAGE
Pleasures and Uses of Microscopy--Development of the
Microscope--Extemporised Apparatus 1
CHAPTER II
Elementary Principles of Optics--Simple Microscopes--Compound
Microscope--Accessory Apparatus--Cover-glasses--Troughs--
Condensers--Dissection--Dipping-tubes--Drawing--Measurement 7
CHAPTER III
Examination of Objects--Principles of Illumination--Mirror
and its Action--Substage Condenser--Use of Bull’s-eye--Opaque
Objects--Photography of Microscopic Objects 28
CHAPTER IV
Vegetable Cells and their Structure--Stellate Tissues--
Secondary Deposit--Ducts and Vessels--Wood-Cells--Stomata,
or Mouths of Plants--The Camera Lucida, and Mode of Using--
Spiral and Ringed Vessels--Hairs of Plants--Resins, Scents,
and Oils--Bark Cells 37
CHAPTER V
Starch, its Growth and Properties--Surface Cells of Petals--
Pollen and its Functions--Seeds 63
CHAPTER VI
Algæ and their Growth--Desmidiaceæ, where found--Diatoms,
their Flinty Deposit--Volvox--Mould, Blight, and Mildew--
Mosses and Ferns--Mare’s-Tail and the Spores--Common
Sea-weeds and their Growth 78
CHAPTER VII
Antennæ, their Structure and Use--Eyes, Compound and
Simple--Breathing Organs--Jaws and their Appendages--Legs,
Feet, and Suckers--Digestive Organs--Wings, Scales, and
Hairs--Eggs of Insects--Hair, Wool, Linen, Silk, and
Cotton--Scales of Fish--Feathers--Skin and its Structure--
Epithelium--Nails, Bone, and Teeth--Blood Corpuscles and
Circulation--Elastic Tissues--Muscle and Nerve 96
CHAPTER VIII
Pond-Life--Apparatus and Instructions for Collecting
Objects--Methods of Examination--Sponge--Infusoria 132
CHAPTER IX
Fresh-water Worms--Planarians--Hydra--Polyzoa--Rotifers
Chætonotus--Water-Bears 144
CHAPTER X
Marine Life--Sponges--Infusoria--Foraminifera--Radiolaria--
Hydroid Zoophytes--Polyzoa--Worms--Lingual Ribbons and
Gills of Mollusca--Star-Fishes and Sea-Urchins--Cuttle-Fish--
Corallines--Miscellaneous Objects 154
CHAPTER XI
Hints on the Preparation of Objects--Preservative Fluids--
Mounting Media--Treatment of Special Objects 168
CHAPTER XII
Section-Cutting--Staining 179
COMMON OBJECTS OF THE MICROSCOPE
CHAPTER I
Pleasures and Uses of Microscopy--Development of the
Microscope--Extemporised Apparatus.
Within the last half-century the use of the microscope, not only as
an instrument of scientific research, a tool in the hands of the
investigator of the finer organisation of the world of nature, nor even
as an adjunct to the apparatus of the chemist or the manufacturer, but
as a means of innocent and instructive recreation, has become so firmly
rooted amongst us that it seems hardly necessary to advocate its claims
to attention on any of these grounds.
So wonderful is the information which it affords, so indispensable is
it in many, if not in all, branches of scientific research, that not
only would the lover of nature be deprived of one of his most valued
sources of information and enjoyment, but some sciences would be
brought absolutely to a standstill if by any conceivable means the
microscope were to be withdrawn from their followers.
On the other hand, from every improvement in the construction of the
latter, a corresponding enlargement and enlightenment of the fields
reviewed by these sciences has taken place, and the beauty and interest
of the revelations made by its means has attracted an ever-increasing
host of earnest and intelligent volunteers, who have rendered yeoman
service to the cause of knowledge.
Moreover, so vast is the scope of the instrument, that whilst
discoveries in other fields of research are few and far between,
comparatively speaking, in microscopic science they are of everyday
occurrence, and the number of problems calling for solution by means of
the instrument in question is so vast that even the humblest worker may
be of the greatest assistance.
In the following pages we propose to carry out, as far as possible
with reference to the microscope, the system followed in the “Common
Objects of the Seashore and of the Country,” and to treat in as simple
a manner as may be of the marvellous structures which are found so
profusely in our fields, woods, streams, shores, and gardens. Moreover,
our observations will be restricted to an instrument of such a class as
to be inexpensively purchased and easily handled, and to those pieces
of supplementary apparatus which can be extemporised at small cost
of money and ingenuity by the observer himself, or obtained of the
opticians for a few shillings.
With the same view, the descriptions will be given in language as
simple and as free from technicalities as possible, though it must
be remembered that for many of the organisms and structures which we
shall have to describe there are none but scientific names; and since,
moreover, this little work is intended to furnish a stepping-stone
between the very elements of microscopic science, and those higher
developments of it which should be the aim of every worker, it would be
unwise to attempt to invent commonplace appellations for the purpose of
this book, and leave him to discover, when he came to consult works of
reference in any particular subject, that his “simplified” knowledge
had all to be unlearnt, and a new vocabulary acquired. Rather will
it be our purpose to use correct terms, and explain them, as far as
necessary, as we introduce them.
The commencing microscopist is strongly recommended, whilst not
confining his interest entirely to one branch of research or
observation, to adopt some one as his particular province.
The opportunities for discovery and original work, which are afforded
by all alike, will be more readily appreciated and utilised by
adopting such a plan than by a general and purposeless distribution of
effort. To mention one or two. The student of the fascinating field of
pond-life will find new organisms awaiting description by the hundred,
and of the old ones, life-histories to make out; if he be attracted
rather to the vegetable inhabitants of the same realm, the diatoms will
furnish him with the opportunity of studying, and perhaps solving,
the enigma of their spontaneous movement, and of watching their
development. The smaller fungi, and indeed the larger ones too, will
amply repay the closest and most laborious study of their habits of
life and processes of development. Since the first edition of this work
was published, the whole subject has been practically revolutionised,
and more remains to be done than has yet been accomplished.
In short, there is scarcely an organism, even of those best known
and most studied, which is so completely exhausted that persevering
investigation would reveal nothing new concerning it.
There can be little doubt but that if any worker, with moderate
instrumental means, but with an observant mind, were to set
determinately to work at the study of the commonest weed or the most
familiar insect, he, or she, would by patient labour accomplish work
which would not only be of value to science, but would redound to the
credit of the worker.
Something like finality appears to have been reached, at least for the
present, in the development of the microscope; and whilst it is beyond
the scope of this work to treat of the refined and costly apparatus
which is essential to useful work in certain departments of research,
the result has, on the whole, been highly favourable to the worker of
moderate means and ambitions, since lenses are now accessible, at the
cost of a few shillings, comparatively speaking, which could not have
been purchased at all when this work first appeared. It is with such
appliances that we have here to deal. The worker whose finances are
restricted must be contented to extemporise for himself many pieces
of apparatus, and will find pleasure and occupation in doing so. And
let him remember, for his encouragement, that many such home-made
appliances will fulfil their purpose quite as well as the imposing
paraphernalia of glittering brass and glass which decorates the table
of the wealthy amateur. It is not the man who possesses the best or
most costly apparatus, but the one who best understands the use of that
which he possesses, that will make the most successful microscopist. A
good observer will discover, with only the aid of a pocket-magnifier,
secrets of Nature which have escaped the notice of a whole army of
dilettante microscopists, in spite of the advantages which, as regards
instruments, the latter may enjoy.
It is for those who desire to be of the former class that this book
is written, and in the course of the following pages instances will
be given in which the exercise of a small amount of ingenuity and the
expenditure of a few pence will be found equivalent to the purchase of
costly and complicated apparatus.
An enormous amount of valuable work was done in the earliest days
of microscopy, when the best apparatus available was a single lens,
composed of the bead formed by fusing the drawn-out end of a rod
of glass. Inserted into a plate of metal, or a piece of card, such
a primitive instrument was capable of affording a large amount of
information. Similar instruments are to be purchased for a few pence
at the present day, and are not without their use for purposes of
immediate examination of material. A very common form is a glass
marble, ground flat on one side, and mounted in a tube. The material to
be examined is placed upon the flat side, and is seen magnified to an
extent inversely proportional to the diameter of the sphere of glass.
CHAPTER II
Elementary Principles of Optics--Simple Microscopes--Compound
Microscope--Accessory Apparatus--Cover-glasses--Troughs--
Condensers--Dissection--Dipping-tubes--Drawing--Measurement.
Before proceeding to deal with the microscope itself, it may be useful
to give a short summary of the optical laws upon which its working
depends.
To go into the minutiæ of the matter here would be out of place, but it
will be found very helpful, especially in the matter of illumination,
to acquire some knowledge of, and facility in applying, these
elementary principles. We shall confine our remarks to convex lenses,
as being the form to which all the combinations in the microscope may
be ultimately reduced.
Every convex lens has one “principal” focus, and an infinite number
of “conjugate” foci. The principal focus is the distance at which it
brings together in one point the rays which fall upon it parallel to
its axis, as shown in Fig. 1, in which _A_ is the axis of the lens _L_,
and the rays _RR_ are brought together in the principal focus _P_. Thus
a ready means of finding the focal length of any lens is to see at what
distance it forms an image of the sun, or of any other distant object,
upon a screen, such as a piece of smooth white cardboard. In the figure
this distance will be _PL_. Conversely, if the source of light be at
_P_, a parallel beam of light will be emitted from the lens.
[Illustration: Fig. 1.]
The focal length may, however, be found in another way. When an object
is placed at a distance from a lens equal to twice the principal focal
length of the latter, an image of the object is formed at the same
distance upon the other side of the lens, inverted in position, but of
the same dimensions as the original object. The object and image then
occupy the equal conjugate foci of the lens, so that by causing them
to assume these relative positions, and halving the distance at which
either of them is from the lens, the focal length of the latter is
known.
These points will be seen on reference to Fig. 2, in which _L_ being
the lens, and _P_ the principal focus, as before, rays from the point
_C_ are brought together at the conjugate focus _C'_, at the same
distance on the other side of _L_. In this case it manifestly does not
matter whether the object be at one or the other of these points.
[Illustration: Fig. 2.]
So far we have been dealing with points on the line of the axis of the
lens. The facts mentioned apply equally, however, to rays entering the
lens at an angle to the axis, only that in this case they diverge or
converge, correspondingly, upon the other side. It is evident, from
Fig. 1, that no image is formed of a point situated at the distance
of the principal focus; but Fig. 3, which is really an extension
of Fig. 2, shows how the rays passing along secondary axes form an
inverted image of the same size as the object, when the latter is
situated at twice the focal length of the lens from this last. To
avoid confusion, the bounding lines only are shown, but similar lines
might be drawn from each and every point of the object; and if the
lines _ALA'_, _BL'B'_ be supposed to be balanced at _L_ and _L'_
respectively, they will indicate the points at which the corresponding
parts of the object and image will be situated along the lines _AB_,
_B'A'_ respectively. Moreover, rays pass from every part of the object
to every part of the lens, so that we must imagine the cones _LAL'_,
_LA'L'_ to be filled with rays diverging on one side of the lens and
converging on the other.
The image so formed is a “real” image,--that is to say, it can be
thrown upon a screen.
[Illustration: Fig. 3.]
The microscopic image, on the other hand, is a virtual image, which can
be viewed by the eye but cannot be thus projected, for it is formed by
an object placed nearer to the lens than the principal focal length of
the latter, so that the rays diverge, instead of converging, as they
leave the lens, and the eye looks, as it were, back along the path in
which the rays appear to travel, and so sees an enlarged image situated
in the air, farther away than the object, as shown in Fig. 4. In this
case, as the diagram shows, the image is upright, not inverted.
Images of the latter class are those formed by simple microscopes, of
the kind described in the previous chapter. In the compound microscope
the initial image, formed by the object-glass, is further magnified by
another set of lenses, forming the eye-piece, by which the diverging
rays of the virtual image are brought together to a focus at the
eye-point; and when viewed directly, the eye sees an imaginary image,
as in a simple microscope, whilst, when the rays are allowed to fall
upon a screen, they form a real image of the object, larger or smaller,
as the screen is farther from or nearer to the eye-point.
[Illustration: Fig. 4.]
These remarks must suffice for our present purpose. Those who are
sufficiently interested in the subject to desire to know more of the
delicate corrections to which these broad principles are subjected in
practice, that objectives may give images which are clear and free from
colour, to say nothing of other faults, will do well to consult some
such work as Lommel’s _Optics_, in the International Science Series.
In following a work such as the present one, the simple microscope,
in some form or other, will be found almost indispensable. It will be
required for examining raw material, such as leaves or other parts of
plants, for gatherings of life in fresh or salt water, for dissections.
With such powers as those with which we shall have to deal, it will
rarely happen that, for example, a bottle of water in which no life is
visible will be worth the carrying-home; whilst, on the other hand, a
few months’ practice will render it not only possible, but easy, not
only to recognise the presence, but to identify the genus, and often
even the species, of the forms of life present. Moreover, these low
powers, affording a general view of the object, allow the relation
to each other of the details revealed by the power of the compound
microscope to be much more easily grasped. A rough example may suffice
to illustrate this. A penny is a sufficiently evident object to the
naked eye, but it will require a sharp one to follow the details in
Britannia’s shield, whilst the minute scratches or the bloom upon the
surface would be invisible in detail without optical aid. Conversely,
however, it would be rash to conclude from an examination of a portion
of the surface with the microscope alone that the portion in view was
a sample of the whole surface. The more the surface is magnified,
the less are the details grasped as a whole, and for this reason the
observer is strongly recommended to make out all that he can of an
object with a simple magnifier before resorting to the microscope.
For general purposes, the intending observer cannot do better than
supply himself with a common pocket-magnifier, with one, two, or three
lenses, preferably the last, as although the absolute performance
is not so accurate, the very considerable range of power available
by using the lenses singly, or in various combinations, is of great
advantage. Such a magnifier may be obtained from Baker for about
three-and-sixpence, or, with the addition of a powerful Coddington lens
(Fig. 5) in the same mount, for nine shillings more. Aplanatic lenses,
such as the one shown in section in Fig. 6, with a much flatter field
of vision, but of one power only each, cost about fifteen shillings,
and a simple stand, which adapts them for dissecting purposes, may
be obtained of the same maker for half a crown, or may easily be
extemporised from a cork sliding stiffly on an iron rod set in a heavy
foot, the cork carrying a loop of wire terminating in a ring which
carries the lens.
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
So much may suffice for the simple microscope. We pass on now to the
consideration of the instrument which forms the subject of the present
work, an instrument which, whilst moderate in price, is yet capable
of doing a large amount of useful and valuable work in the hands of
a careful owner, and of affording him a vast amount of pleasure and
recreation, even if these be his only objects in the purchase, though
it is difficult to understand that, an insight being once attained into
the revelations effected by the instrument, without a desire being
excited in any intelligent mind to pursue the subject as a study as
well as a delightful relaxation. The microscope described, and adopted
as his text by the author of this work, is still made, and has shared
to a very considerable extent in the general improvement of optical
apparatus which has taken place during the last thirty years. It is to
be obtained from Baker, 244 High Holborn, and is provided with most
of the apparatus which will be found indispensable by the beginner,
costing, with a case in which to keep it, the modest sum of three
guineas.
[Illustration: Fig. 7.]
If this instrument represent the limit of the purchaser’s power of
purse, he may very well make it answer his purpose for a considerable
time. The same instrument, however, with separate objectives of good
quality, together with a bull’s-eye condenser (an almost indispensable
adjunct), a plane mirror in addition to a concave one, and a simple but
efficient form of substage condenser, may be obtained for £5, 12s. 6d.,
and the extra outlay will be well repaid by the advantage in working
which is gained by the possession of the additional apparatus.
[Illustration: Fig. 8.]
A still better stand, and one which is good enough for almost any
class of work, is that shown in Fig. 8, which is known as the
“Portable” microscope. In this instrument the body is made up of
two tubes, so that the length may be varied at will, and this gives
a very considerable range of magnification without changing the
object-glass, a great convenience in practice; whilst the legs fold
up for convenience of carriage, so that the whole instrument, with
all necessary appliances, may be readily packed in a corner of a
portmanteau for transport to the country or seaside.
The objectives supplied with the simplest form of microscope above
referred to are combinations of three powers in one, and the power is
varied by using one, two, or three of these in combination. This form
of objective is very good, as far as it goes, though it is impossible
to correct such a combination with the accuracy which is possible in
manufacturing one of a fixed focal length.
Perhaps the best thing for the beginner to do would be to purchase the
combination first, and, later on, to dispose of it and buy separate
objectives of, say, one-inch, half-inch, and quarter-inch focal
lengths. It may be explained here, that when a lens is spoken of as
having a certain focal length, it is meant that the magnification
obtained by its use is the same, at a distance of ten inches from the
eye, as would be obtained by using a simple sphere of glass of the same
focal length at the same distance. This, of course, is simply a matter
of theory, for such lenses are never used actually.
[Illustration: Fig. 9.]
Of accessory apparatus, we may mention first the stage forceps (Fig. 9,
_a_). These are made to fit into a hole upon the stage, so as to be
capable of being turned about in any direction, with an object in their
grasp, and for some purposes, especially such as the examination of a
thin object, say the edge of a leaf, they are extremely useful.
[Illustration: Fig. 10.]
The live box, in which drops of water or portions of water-plants,
or the like, may be examined, will be found of great service. By
adjustment of the cap upon the cylinder, with proper attention to
the thickness of the cover-glass in the cap, any required amount of
pressure, from that merely sufficient to fix a restless object to an
amount sufficient to crush a resistent tissue, may easily be applied,
whilst the result of the operation is watched through the microscope.
This proceeding is greatly facilitated if the cap of the live-box be
slotted spirally, with a stud on the cylinder, so that a half-turn of
the cap brings the glasses into contact. By this means the pressure may
be adjusted with the greatest nicety.
In examining delicate objects, such as large infusoria, which
invariably commit suicide when pressure is applied, a good plan is to
restrict their movements by placing a few threads of cotton-wool, well
pulled out, in the live-box with the drop of water.
A variety of instruments has been invented for the same purpose, of
which Beck’s parallel compressorium may be mentioned as the most
efficient, though it is somewhat complicated, and consequently
expensive, costing about twenty-five shillings.
A few glass slips and cover-glasses will also be required. The latter
had better be those known as “No. 2,” since the beginner will find it
almost impossible to clean the thinner ones satisfactorily without
a large percentage of fractures. The safest way is to boil the thin
glass circles in dilute nitric acid (half acid, half water) for a
few minutes, wash well in several waters, first tap-water and then
distilled, and finally to place the covers in methylated spirit. When
required for use, the spirit may be burnt off by applying a light, the
cover-glass, held in a pair of forceps, being in no way injured by the
process.
In addition to the glass slides, the observer will find it advisable to
be provided with a few glass troughs, of various thicknesses, in which
portions of water-plants, having organisms attached to them, may be
examined. Confined in the live-box, many of the organisms ordinarily
found under such circumstances can rarely be induced to unfold their
beauties, whilst in the comparative freedom of the trough they do so
readily. The troughs may be purchased, or may be made of any desired
shape or size by cutting strips of glass of a thickness corresponding
to the depth desired, cementing these to a glass slide somewhat larger
than the ordinary one, and cementing over the frame so formed a piece
of thin glass, No. 3; the best material to use as cement being marine
glue of the best quality, or, failing this, Prout’s elastic glue, which
is much cheaper, but also less satisfactory. The glass surface must
be made, in either case, sufficiently hot to ensure thorough adhesion
of the cement, as the use of any solvent entails long waiting, and
considerable risk of poisoning the organisms. A useful practical hint
in the use of these troughs is that the corners, at the top, should be
greased slightly, otherwise the water finds its way out by capillary
attraction, to the detriment of the stage of the microscope.
Of optical accessories, the bull’s-eye is almost the most valuable. So
much may be effected by its means alone, in practised hands, that it is
well worth the while of the beginner to master its use thoroughly, and
the methods of doing so will be explained in the succeeding chapter.
The substage condenser, too, even in its most simple form, is an
invaluable adjunct, even though it be only a hemisphere of glass,
half an inch or so in diameter, mounted in a rough sliding jacket to
fit underneath the stage. Such an instrument, properly fitted, will
cost about fifteen shillings, but the ingenious worker will easily
extemporise one for himself.
[Illustration: Fig. 11.]
Many plants and animals require to be dissected to a certain extent
before the details of their structure can be made out, and for this
purpose the naked eye alone will rarely serve. The ordinary pocket
magnifier, however, if mounted as described in a preceding chapter,
will greatly facilitate matters, and the light may be focused upon the
object by means of the bull’s-eye condenser, as shown in Fig. 11. In
the figure the latter is represented as used in conjunction with the
lamp, but daylight is preferable if it be available, the strain upon
the eyes being very much less than with artificial light. Two blocks
of wood, about four inches high, will form convenient rests for the
hands, a plate of glass being placed upon the blocks to support the
dish, and a mirror being put in the interspace at an angle of 45° or
so if required. A piece of black paper may be laid upon the mirror if
reflected light alone is to be used.
As all delicate structures are dissected under fluid, a shallow dish
is required. For this purpose nothing is better than one of the dishes
used for developing photographic negatives. The bottom of the dish is
occupied by a flat cork, to which a piece of flat lead is attached
below, and the object having been pinned on to the cork in the required
position, the fluid is carefully run in. This fluid will naturally vary
according to the results desired to be obtained, but it must not be
plain water, which so alters all cellular structures as to practically
make them unrecognisable under the microscope. Nothing could be better
than a 5 per cent. solution of formalin, were it not for the somewhat
irritating odour of this fluid, since it at once fixes the cells and
preserves the figure. For many purposes a solution of salt, in the
proportion of a saltspoonful of the latter to a pint of water, will
answer well for short dissections. For more prolonged ones, a mixture
of spirit-and-water, one part of the former to two of the latter,
answers well, especially for vegetable structures. When the dilution
is first made, the fluid becomes milky, unless pure spirit be used,
but with a little trouble the Revenue authorities may be induced to
give permission for the use of pure methylated spirit, which answers
every purpose. The trouble then is that not less than five gallons can
be purchased, an _embarras de richesses_ for the average microscopist,
but, after all, the spirit is extremely cheap, and does not deteriorate
by keeping.
When the dissection in either of these media is completed, spirit
should be gradually added to bring the strength up to 50 per cent., in
which the preparation may remain for a day or two, after which it is
gradually brought into pure spirit, or into water again, according to
the medium in which it is to be mounted.
[Illustration: Fig. 12.]
As to the tools required, they are neither numerous nor expensive.
Fine-pointed but strong forceps (Fig. 9, _c_), curved and straight;
a couple of pairs of scissors, one strong and straight, the other
more delicate, and having curved blades, and a few needles of various
thicknesses and curves, are the chief ones. The latter may be made by
inserting ordinary needles, for three-fourths of their length, into
sticks of straight-grained deal (ordinary firewood answers best), and
thereafter bending them as required. A better plan, however, is to be
provided with a few of the needle-holders shown in Fig. 9, _b_. These
are very simple and inexpensive, and in them broken needles are readily
replaced by others. Dipping-tubes, such as are shown in Fig. 12, will
also be extremely useful for many purposes. These are very easily made
by heating the centre of a piece of soft glass tubing of the required
size, and, when it is quite red-hot, drawing the ends apart. The fine
tube in the centre should now be divided by scratching it with a fine
triangular file, and the scratch may of course be made at such a
point as to afford a tube of the required fineness. The edges should
be smoothed by holding them in the flame until they just run (not
melt, or the tube will close). These tubes can often be made to supply
the place of a glass syringe. They may be used either for sucking up
fluid or for transferring it, placing the finger over the wide end,
allowing the tube to fill by displacement of air, and then re-closing
it with the finger. This last method is especially useful for
transferring small objects from one receptacle to another. In speaking
of the dissection of objects, it should have been mentioned that the
microscope itself may, under careful handling, be made to serve very
well, only, as the image is reversed, it is almost impossible to work
without using a prism to re-erect the image. Such a prism is shown in
Fig. 13. The microscope is placed vertically, and the observer, looking
straight into the prism, sees all the parts of the image in their
natural positions. This appliance is extremely useful for the purpose
of selecting small objects, and arranging them on slides in any desired
manner. A few words may be added as to the reproduction of the images
of objects.
[Illustration: Fig. 13.]
The beginner is strongly recommended to practise himself in this
from the outset. Even a rough sketch is worth pages of description,
especially if the magnification used be appended; and even though
the worker may be devoid of artistic talent, he will find that with
practice he will acquire a very considerable amount of facility in
giving truthful outlines at least of the objects which he views.
Various aids have been devised for the purpose of assisting in the
process. The simplest and cheapest of these consists of a cork cut so
as to fit round the eye-piece. Into the cork are stuck two pins, at an
angle of 45° to the plane of the cork, and, the microscope being placed
horizontally, a thin cover-glass is placed upon the two pins, the light
being arranged and the object focused after the microscope is inclined.
On looking vertically down upon the cover-glass, a bright spot of light
will be seen, and as the eye is brought down into close proximity with
it the spot will expand and allow the observer to see the whole of the
image without looking into the microscope. If a sheet of paper be now
placed upon the table at the place occupied by the image so projected,
the whole of the details will be clearly seen, as will also the point
of a pencil placed upon the paper in the centre of the field of view;
and, after a little practice, it will be found easy to trace round the
chief details of the object. Two points require attention. The first is
that if the light upon the paper be stronger than that in the apparent
field of the microscope, the image will not be well seen, or if the
paper be too feebly lighted, it will be difficult to keep the point of
the pencil in view. The light from the microscope is thrown into the
eye, and the view of the image upon the paper is the effect of a mental
act, the eye looking out in the direction from which the rays appear
to come. The paper has therefore to be illuminated independently, and
half the battle lies in the adjustment of the relative brightness of
image and paper. The second point is, that it is essential to fix one
particular point in the image as the starting-point of the drawing, and
this being first depicted, the image and drawing of this point must be
kept always coincident, or the drawing will be distorted, since the
smallest movement of the eye alters the relations of the whole. The
reflector must be placed at an angle of 45°, or the field will be oval
instead of circular. The simple form of apparatus just described has
one drawback, inasmuch as the reflection is double, the front and back
of the cover-glass both acting as reflectors. The image from the latter
being much the more feeble of the two, care in illumination will do
much to eliminate this difficulty; but there are various other forms
in which the defect in question is got rid of. The present writer has
worked with all of them, from the simple neutral tint reflector of
Beale to the elaborate and costly apparatus of Zeiss, and, upon the
whole, thinks that he prefers the cover-glass to them all.
A very simple plan, not so mechanical as the last-named, consists in
the use of “drawing-squares,” which are delicate lines ruled upon a
piece of thin glass, and dropped into the eye-piece so that the lines
rest upon the diaphragm of the eye-piece, and therefore are in focus
at the same time as the object. By the use of these, in combination
with paper similarly ruled, a diagram of any required size can be drawn
with very great facility. The squares, if compared with a micrometer,
will furnish an exact standard of magnitude for each object-glass
employed. The micrometer is a piece of thin glass upon which are ruled
minute divisions of an inch or a millimeter. Suppose the micrometer to
be placed under the microscope when the squares are in the eye-piece,
and it be found that each division corresponds with one square of
the latter, then, if the micrometric division be one one-hundredth
of an inch, and the squares upon the paper measure one inch, it is
clear that the drawing will represent the object magnified a hundred
“diameters”; if two divisions of the micrometer correspond to three
squares, the amplification will be a hundred and fifty diameters; if
three divisions correspond to two squares, sixty-six diameters, and
so on. If a draw-tube be used, it will be necessary to know the value
of the squares at each inch of the length, if they are to be used for
measuring magnification.
CHAPTER III
Examination of Objects--Principles of Illumination--Mirror
and its Action--Substage Condenser--Use of Bull’s-eye--Opaque
Objects--Photography of Microscopic Objects.
So much depends upon a right method of employing the microscope, as
regards both comfort and accuracy, that we propose to devote a little
space to the consideration of the subject.
Let us first warn the intending observer against the use of powers
higher than are required to bring out the details of the object. Mere
magnification is of very little use: it increases the difficulties both
of illumination and of manipulation, and, as already said, interferes
with that grasp of the object which it is most desirable to obtain.
Rather let the beginner lay himself out to get the very most he can out
of his lowest powers, and he will find that, by so doing, he will be
able far better to avail himself of the higher ones when their use is
indispensable.
The essential means to this end is a mastery of the principles of
illumination, which we now proceed to describe.
We suppose the microscope to be inclined at an angle of about 70° to
the horizontal, with a low-power objective attached to it, a one-inch
by preference. Opposite to the microscope, and about a foot away from
it, is a lamp with the edge of the flame presented to the microscope,
the concave mirror of which is so arranged as to receive the rays from
the flame and direct them up the tube of the microscope. Upon the stage
is placed a piece of ground-glass, and the mirror-arm is now to be
moved up or down upon its support until the ground-glass receives the
maximum of illumination, which it will do when the lamp-flame is at one
conjugate focus of the mirror and the ground-glass at the other. The
focus will not be an image of the flame, but a bar of light.
If an object be now placed upon the stage, instead of the ground-glass,
and the objective focused upon it, it will, if the mirror be properly
adjusted, be brilliantly illuminated.
It will be understood that every concave mirror has a focus, and
converges the rays which fall upon it to this focus, behaving exactly
like a convex lens. The principal focus of a concave mirror is its
radius of curvature, and this is not difficult to determine. Place side
by side a deep cardboard box and the lamp, so that the concave mirror
may send the rays back, along a path only slightly inclined to that by
which they reached it, to the bottom of the box. The lamp and box being
equidistant from the mirror, it is evident that when the mirror forms
an image of the former upon the latter equal to the flame in size, we
have the equivalent of the equal conjugate foci shown in Fig. 2. Now
move the box to the distance from the mirror which corresponds to the
distance of the stage of the microscope from the mirror when the latter
is in position upon the microscope, and then move the lamp to or fro
until the mirror casts a sharp image of the flame upon the bottom of
the box, which is not to be moved. The lamp distance so found will be
the correct one for working with the concave mirror. The writer is led
to lay special stress upon this matter, from the fact that he almost
invariably finds that the mirror is arranged to be used for parallel
rays, _i.e._ for daylight, and is therefore fixed far too close to the
stage to be available for correct or advantageous working with the
lamp, unless, indeed, the bull’s-eye condenser be used, as hereinafter
described, to parallelise the rays from the lamp.
Work done with the concave mirror can, however, under the most
favourable conditions, only be looked upon as a _pis aller_. The
advantages gained by the use of some substage condenser, even the most
simple, in conjunction with the plane mirror, or even without any
mirror at all, are so manifold that the beginner is strongly urged
to provide himself with some form or other of it, and we now proceed
to describe the way in which this should be used to produce the best
effect.
To reduce the problem to its most simple elements, turn the mirror
altogether out of the way, and place the microscope upon a block at
such a height as shall be convenient for observation, and shall allow
the rays from the lamp, placed in a line with it on the table, to shine
directly into the tube of the microscope. Ascertain that this is so
by removing both objective and eye-piece and looking down the tube,
when the flame should be seen in the centre, edgewise. Now replace
the eye-piece, and screw on to the tube the one-inch combination or
objective. Place upon the stage an object, preferably a round diatom or
an echinus-spine, and focus it as sharply as possible. Now place the
substage condenser in its jacket, and slide it up and down until the
image of the object is bisected by the image of the flame.
The centre of the object will now be brilliantly illuminated by rays
travelling in the proper direction for yielding the best results.
The object is situated at the common focus of the microscope and the
condenser, and, whatever means of illumination be adopted, this is the
result which should always be aimed at.
Satisfactory as this critical arrangement is, however, from a
scientific point of view, it has its drawbacks from an artistic and
æsthetic one. It is not pleasant, for most purposes, to have merely the
centre of an object lighted up, and we have now to consider how the
image of the edge of the flame may be so expanded as to fill the field
without sacrificing more than a very small fraction of the accuracy of
the arrangement just attained.
Referring to Fig. 1, we see that if we place the lamp at the principal
focus of a lens, it will emit a bundle of parallel rays equal in
diameter to the diameter of the lens. This is the key of the position.
We cannot place the lamp at an infinite distance from the substage
condenser, but we can supply the latter with rays approximately
parallel, so that it shall bring them to a focus upon the object
at very nearly its own principal focus. This we do by means of the
bull’s-eye condenser. Place the latter, with its flat side toward the
edge of the flame, and at its principal focal distance (the method of
determining which has already been described) from the latter, so that
the bundle of parallel rays which issue from it may pass up to the
substage condenser. On examining the object again, it will be found
that, after slight adjustments of the position of the bull’s-eye have
been made, the object lies in the centre of an evenly and brilliantly
lighted field.
It may be necessary to place the bull’s-eye a little farther from or
nearer to the lamp, or to move it a little to one side or the other,
but when it is at the correct distance, and on the central line between
the lamp and the substage condenser, at right angles to this line,
the effects will be as described. It may help in securing this result
if we mention that when the bull’s-eye is too far from the lamp, the
image of the flame is a spindle-shaped one; whilst, when the distance
between the two is too short, _i.e._ less than the principal focal
length of the lens, the field is crossed by a bar or light, the ends of
which are joined by a ring, whilst on either side of the bar there is a
semi-circular dark space.
We have hitherto supposed the objects viewed to be transparent, but
there are many, of great interest, which are opaque, and call for other
means of illumination. Of these there are several. The simplest and, in
many ways, the best is to use the bull’s-eye condenser to bring to a
focus upon the object the rays of light from some source placed above
the stage of the microscope. If light can be obtained from the sun
itself, no lens will be needed to concentrate it; and indeed, if this
were done, there would be considerable risk of burning the object. The
light from a white cloud, however, with the help of the bull’s-eye,
answers admirably. At night-time an artificial source of light, the
more intense and the more distant the better, is required. For most
cases, and with powers not higher than one inch, a good paraffin lamp,
placed about two feet away from the stage, and on one side of it, so as
to be about a foot above the level of the object, will give all that is
needed. Such a lamp is shown in Fig. 14. Low magnifications are, as a
rule, all that is called for in this method.
Lieberkuhn’s condensers are useful aids, but are somewhat expensive.
They are concave mirrors, which are so adjusted to the objective that
the latter and the reflector come into focus together, the light being
sent in from below, or from one side.
One other method of illumination must be mentioned before leaving the
topic, and this is the illumination of objects upon a “dark field.”
With suitable subjects, and when carefully managed, there is no method
which gives more beautiful effects, and it has the great advantage of
allowing the object to be brilliantly lighted, without the strain to
the eyes which is involved in such lighting by the usual method of
direct illumination.
[Illustration: Fig. 14.]
It consists essentially in allowing the light to fall upon the object
from below, at such an angle that none of it can enter the objective
directly. Thus the concave mirror, turned as far as possible to one
side, and reflecting on to the object the rays from the lamp placed
upon the opposite side, will give very fair results with low powers;
this plan, however, is capable of but very limited application. Again,
a disc of black paper may be stuck on to the middle of the bull’s-eye,
and the latter be placed below the stage between it and the mirror. In
this case everything depends upon the size of the disc, which, if too
small, will not give a black ground, and if too large will cut off all
light from the object.
The best and only really satisfactory plan is to arrange the
illumination with the substage condenser, as previously described, and
then to place below the lens of the latter a central stop of a suitable
size, which can only be determined by trial. When this has been done
the object will be seen brilliantly illuminated upon a field of velvety
blackness. Such stops are supplied with the condenser.
We have devoted a considerable portion of space to this question, since
it is, of all others, the most important to a successful, satisfactory,
and reliable manipulation of the microscope; but even now, only the
main points of the subject have been touched upon, and the worker
will find it necessary to supplement the information given by actual
experiment. A few failures, rightly considered, will afford a great
amount of information, but those who desire to go thoroughly into the
matter are recommended to consult the present writer’s _Guide to the
Science of Photomicrography_, where it is treated at much greater
length, as an essential part of the subject-matter of the book.
It may be added here, that no method of reproducing the images of
objects is on the whole so satisfactory as the photographic one; and
whilst a lengthened reference to the topic would be out of place in
a work of the character of the present one, the one just mentioned
will be found to contain all that is necessary to enable the beginner
to produce results which, for faithfulness and beauty, far excel any
drawing, whilst they have the additional advantage that they can, if
required, be exhibited to hundreds simultaneously.
CHAPTER IV
Vegetable Cells and their Structure--Stellate Tissues--Secondary
Deposit--Ducts and Vessels--Wood-Cells--Stomata, or Mouths of
Plants--The Camera Lucida, and Mode of Using--Spiral and Ringed
Vessels--Hairs of Plants--Resins, Scents, and Oils--Bark Cells.
We will now suppose the young observer to have obtained a microscope
and learned the use of its various parts, and will proceed to work
with it. As with one or two exceptions, which are only given for the
purpose of further illustrating some curious structure, the whole
of the objects figured in this work can be obtained without any
difficulty, the best plan will be for the reader to procure the plants,
insects, etc., from which the objects are taken, and follow the book
with the microscope at hand. It is by far the best mode of obtaining a
systematic knowledge of the matter, as the quantity of objects which
can be placed under a microscope is so vast that, without some guide,
the tyro flounders hopelessly in the sea of unknown mysteries, and
often becomes so bewildered that he gives up the study in despair of
ever gaining any true knowledge of it. I would therefore recommend the
reader to work out the subjects which are here mentioned, and then
to launch out for himself on the voyage of discovery. I speak from
experience, having myself known the difficulties under which a young
and inexperienced observer has to labour in so wide a field, without
any guide to help him to set about his work in a systematic manner.
The objects that can be most easily obtained are those of a vegetable
nature, as even in London there is not a square, an old wall, a
greenhouse, a florist’s window, or even a greengrocer’s shop, that will
not afford an exhaustless supply of microscopic employment. Even the
humble vegetables that make their daily appearance on the dinner-table
are highly interesting; and in a crumb of potato, a morsel of greens,
or a fragment of carrot, the enthusiastic observer will find occupation
for many hours.
Following the best examples, we will commence at the beginning, and see
how the vegetable structure is built up of tiny particles, technically
called “cells.”
That the various portions of every vegetable should be referred to
the simple cell is a matter of some surprise to one who has had no
opportunity of examining the vegetable structure, and indeed it does
seem more than remarkable that the tough, coarse bark, the hard
wood, the soft pith, the green leaves, the delicate flowers, the
almost invisible hairs, and the pulpy fruit, should all start from
the same point, and owe their origin to the simple vegetable cell.
This, however, is the case; and by means of a few objects chosen from
different portions of the vegetable kingdom, we shall obtain some
definite idea of this curious phenomenon.
I.
FIG.
1. Strawberry, cellular tissue
2. Buttercup leaf, internal layer
3. Privet, Seed Coat, showing star-shaped cells
4. Rush, Star-shaped cells
5. Mistletoe, cells with ringed fibre
6. Cells from interior of Lilac bud
7. Bur-reed (_Sparganium_), square cells from leaf
8. Six-sided cells, from stem of Lily
9. Angular dotted cells, rind of Gourd
10. Elongated ringed cells, anther of Narcissus
11. Irregular star-like tissue, pith of Bulrush
12. Six-sided cells, pith of elder
13. Young cells from Wheat
14. Do. rootlets of Wheat
15. Wood-cells, Elder
16. Glandular markings and resin, “Cedar” pencil
17. Do. Yew
18. Scalariform tissue, Stalk of Fern
19. Dotted Duct, Willow
20. Do. Stalk of Wheat
21. Wood-cell, Chrysanthemum
22. Do. Lime-tree
23. Dotted Duct, Carrot
24. Cone-bearing wood, Deal
25. Cells, outer coat, Gourd
26. Ducts, Elm
27. Cellular tissue, Stalk of Chickweed
28. Holly-berry, outer coat
[Illustration: I.]
On Plate I. Fig. 1, may be seen three cells of a somewhat globular
form, taken from the common strawberry. Any one wishing to examine
these cells for himself may readily do so by cutting a very thin slice
from the fruit, putting it on a slide, covering it with a piece of thin
glass (which may be cheaply bought at the optician’s, together with the
glass slides on which the objects are laid), and placing it under a
power of two hundred diameters. Should the slice be rather too thick,
it may be placed in the live-box and well squeezed, when the cells will
exhibit their forms very distinctly. In their primary form the cells
seem to be spherical; but as in many cases they are pressed together,
and in others are formed simply by the process of subdivision, the
spherical form is not very often seen. The strawberry, being a soft and
pulpy fruit, permits the cells to assume a tolerably regular form, and
they consequently are more or less globular.
Where the cells are of nearly equal size, and are subjected to equal
pressure in every direction, they force each other into twelve-sided
figures, having the appearance under the microscope of flat six-sided
forms. Fig. 8, in the same Plate, taken from the stem of a lily, is
a good example of this form of cell, and many others may be found in
various familiar objects.
We must here pause for a moment to define a cell before we proceed
further.
The cell is a close sac or bag formed of a substance called from its
function “cellulose,” and containing certain semi-fluid contents as
long as it retains its life. In the interior of the cell may generally
be found a little dark spot, termed the “núcleus,” and which may be
seen in Fig. 1, to which we have already referred. The object of the
nucleus is rather a bone of contention among the learned, but the best
authorities on this subject consider it to be the vital centre of the
cells, to and from which tends the circulation of the protoplasm, and
which is intimately connected with the growth and reproduction of the
cell. On looking a little more closely at the nucleus, we shall find it
marked with several small light spots, which are termed “nucléoli.”
On the same Plate (Fig. 2) is a pretty group of cells taken from the
internal layer of the buttercup leaf, and chosen because they exhibit
the series of tiny and brilliant green dots to which the colour of the
leaf is due. The technical name for this substance is “chlorophyll,”
or “leaf-green,” and it may always be found thus dotted in the leaves
of different plants, the dots being very variable in size, number, and
arrangement. A very fine object for the exhibition of this point is the
leaf of _Anácharis_, the “Canadian timber-weed,” to be found in almost
every brook and river. It also shows admirably the circulation of the
protoplasm in the cell.
In the centre of the same Plate (Fig. 12) is a group of cells from
the pith of the elder-tree. This specimen is notable for the number
of little “pits” which may be seen scattered across the walls of the
cells, and which resemble holes when placed under the microscope.
In order to test the truth of this appearance, the specimen was
coloured blue by the action of iodine and dilute sulphuric acid, when
it was found that the blue tint spread over the pits as well as the
cell-walls, showing that the membrane is continuous over the pits.
Fig. 7 exhibits another form of cell, taken from the Spargánium, or
bur-reed. These cells are tolerably equal in size, and have assumed a
square shape. They are obtained from the lower part of the leaf. The
reader who has any knowledge of entomology will not fail to observe the
similarity in form between the six-sided and square cells of plants and
the hexagonal and square facets of the compound eyes of insects and
crustaceans. In a future page these will be separately described.
Sometimes the cells take most singular and unexpected shapes, several
examples of which will be briefly noticed.
In certain loosely made tissues, such as are found in the rushes and
similar plants, the walls of the cells grow very irregularly, so
that they push out a number of arms which meet each other in every
direction, and assume the peculiar form which is termed “stellate,”
or star-shaped tissue. Fig. 3 shows a specimen of stellate tissue
taken from the seed-coat of the privet, and rather deeply coloured,
exhibiting clearly the beautiful manner in which the arms of the
various stars meet each other. A smaller group of stellate cells taken
from the stem of a large rush, and exemplifying the peculiarities of
the structure, are seen in Fig. 4.
The reader will at once see that this mode of formation leaves a vast
number of interstices, and gives great strength with little expenditure
of material. In water-plants, such as the reeds, this property is
extremely valuable, as they must be greatly lighter than the water in
which they live, and at the same time must be endowed with considerable
strength in order to resist its pressure.
A less marked example of stellate tissue is given in Fig. 11, where
the cells are extremely irregular, in their form, and do not coalesce
throughout. This specimen is taken from the pithy part of a bulrush.
There are very many other plants from which the stellate cells may be
obtained, among which the orange affords very good examples, in the
so-called “white” that lies under the yellow rind, a section of which
may be made with a very sharp razor, and placed in the field of the
microscope.
Looking toward the bottom of the Plate, and referring to Fig. 27, the
reader will observe a series of nine elongated cells, placed end to
end, and dotted profusely with chlorophyll. These are obtained from
the stalk of the common chickweed. Another example of the elongated
cell is seen in Fig. 14, which is a magnified representation of the
rootlets of wheat. Here the cells will be seen set end to end, and each
containing its nucleus. On the left hand of the rootlet (Fig. 13) is a
group of cells taken from the lowest part of the stem of a wheat plant
which had been watered with a solution of carmine, and had taken up a
considerable amount of the colouring substance. Many experiments on
this subject were made by the Rev. Lord S. G. Osborne, and may be seen
at full length in the pages of the _Microscopical Journal_, the subject
being too large to receive proper treatment in the very limited space
which can here be given to it. It must be added that later researches
have caused the results here described to be gravely disputed.
Fig. 9 on the same Plate exhibits two notable peculiarities--the
irregularity of the cells and the copiously pitted deposit with which
they are covered. The irregularity of the cells is mostly produced by
the way in which the multiplication takes place, namely, by division
of the original cell into two or more new ones, so that each of these
takes the shape which it assumed when a component part of the parent
cell. In this case the cells are necessarily very irregular, and when
they are compressed from all sides they form solid figures of many
sides, which, when cut through, present a flat surface marked with a
variety of irregular outlines. This specimen is taken from the rind of
a gourd.
The “pitted” structure which is so well shown in this figure is caused
by a layer of matter which is deposited in the cell and thickens its
walls, and which is perforated with a number of very minute holes
called “pits.” This substance is called “secondary deposit.” That these
pits do not extend through the real cell-wall has already been shown
in Fig. 12.
This secondary deposit assumes various forms. In some cases it is
deposited in rings round the cell, and is clearly placed there for
the purpose of strengthening the general structure. Such an example
may be found in the mistletoe (Fig. 5), where the secondary deposit
has formed itself into clear and bold rings that evidently give
considerable strength to the delicate walls which they support. Fig. 10
shows another good instance of similar structure; differing from the
preceding specimen in being much longer and containing a greater number
of rings. This object is taken from an anther of the narcissus. Among
the many plants from which similar objects may be obtained, the yew is
perhaps one of the most prolific, as ringed wood-cells are abundant
in its formation, and probably aid greatly in giving to the wood the
strength and elasticity which have long made it so valuable in the
manufacture of bows.
Before taking leave of the cells and their remarkable forms, we will
just notice one example which has been drawn in Fig. 6. This is a
congeries of cells, containing their nuclei, starting originally end to
end, but swelling and dividing at the top. This is a very young group
of cells (a young hair, in fact) from the inner part of a lilac bud,
and is here introduced for the purpose of showing the great similarity
of all vegetable cells in their earliest stages of existence.
Having now examined the principal forms of cells, we arrive at the
“vessels,” a term which is applied to those long and delicate tubes
which are formed of a number of cells set end to end, their walls of
separation being absorbed.
In Fig. 19 the reader will find a curious example of the “pitted
vessel,” so called from the multitude of little markings which cover
its walls, and are arranged in a spiral order. Like the pits and rings
already mentioned, the dots are composed of secondary deposit in the
interior of the tube, and vary very greatly in number, function, and
dimensions. This example is taken from the wood of the willow, and is
remarkable for the extreme closeness with which the dots are packed
together.
Immediately on the right hand of the preceding figure may be seen
another example of a dotted vessel (Fig. 20), taken from a wheat stem.
In this instance the cells are not nearly so long, but are wider than
in the preceding example, and are marked in much the same way with a
spiral series of dots. About the middle of the topmost cell is shown
the short branch by which it communicates with the neighbouring vessel.
Fig. 23 exhibits a vessel taken from the common carrot, in which the
secondary deposit is placed in such a manner as to resemble a net of
irregular meshes wrapped tightly round the vessel. For this reason
it is termed a “netted vessel.” A very curious instance of these
structures is given in Fig. 26, at the bottom of the Plate, where
are represented two small vessels from the wood of the elm. One of
them--that on the left hand--is wholly marked with spiral deposit,
the turns being complete; while, in the other instance, the spiral is
comparatively imperfect, and the cell-walls are marked with pits. If
the reader would like to examine these structures more attentively, he
will find plenty of them in many familiar garden vegetables, such as
the common radish, which is very prolific in these interesting portions
of vegetable nature.
There is another remarkable form in which this secondary deposit is
sometimes arranged that is well worthy of our notice. An example
of this structure is given in Fig. 18, taken from the stalk of the
common fern or brake. It is also found in very great perfection in the
vine. On inspecting the illustration, the reader will observe that
the deposit is arranged in successive bars or steps, like those of a
winding staircase. In allusion to the ladder-like appearance of this
formation, it is called “scalariform” (Latin, _scala_, a ladder).
In the wood of the yew, to which allusion has already been made, there
is a very peculiar structure, a series of pits found only in those
trees that bear cones, and therefore termed the coniferous pitted
structure. Fig. 16 is a section of a common cedar pencil, the wood,
however, not being that of the true cedar, but of a species of fragrant
Juniper. This specimen shows the peculiar formation which has just been
mentioned.
Any piece of deal or pine will exhibit the same peculiarities in a
very marked manner, as is seen in Fig. 24. A specimen may be readily
obtained by making a very thin shaving with a sharp plane. In this
example the deposit has taken a partially spiral form, and the numerous
circular pits with which it is marked are only in single rows. In
several other specimens of coniferous woods, such as the Araucaria, or
Norfolk Island pine, there are two or three rows of pits.
A peculiarly elegant example of this spiral deposit may be seen in the
wood of the common yew (Fig. 17). If an exceedingly thin section of
this wood be made, the very remarkable appearance will be shown which
is exhibited in the illustration. The deposit has not only assumed the
perfectly spiral form, but there are two complete spirals, arranged
at some little distance from each other, and producing a very pretty
effect when seen through a good lens.
The pointed, elongated shape of the wood-cells is very well shown
in the common elder-tree (see Fig. 15). In this instance the cells
are without markings, but in general they are dotted like Fig. 21,
an example cut from the woody part of the chrysanthemum stalk. This
affords a very good instance of the wood-cell, as its length is
considerable, and both ends are perfect in shape. On the right hand
of the figure is a drawing of the wood-cell found in the lime-tree
(Fig. 22), remarkable for the extremely delicate spiral markings with
which it is adorned. In these wood-cells the secondary deposit is so
plentiful that the original membranous character of the cell-walls
is entirely lost, and they become elongated and nearly solid cases,
having but a very small cavity in their centre. It is to this deposit
that the hardness of wood is owing, and the reader will easily see
the reason why the old wood is so much harder than the young and new
shoots. In order to permit the passage of the fluids which maintain
the life of the part, it is needful that the cell-wall be left thin
and permeable in certain places, and this object is attained either
by the “pits” described on page 43, or by the
intervals between the spiral deposit.
At the right-hand bottom corner of Plate I. (Fig. 28) may be seen
a prettily marked object, which is of some interest. It is a slice
stripped from the outer coat of the holly-berry, and is given for the
purpose of illustrating the method by which plants are enabled to
breathe the atmospheric air on which they depend as much as ourselves,
though their respiration is slower. Among the mass of net-like cells
may be seen three curious objects, bearing a rather close resemblance
to split kidneys. These are the mouths, or “stómata,” as they are
scientifically called.
In the centre of the mouths may be seen a dark spot, which is the
aperture through which the air communicates with the passages between
the cells in the interior of the structure. In the flowering plants
their shape is generally rounded, though they sometimes take a squared
form, and they regularly occur at the meeting of several surface
cells. The two kidney-shaped cells which form the “mouth” are the
“guard-cells,” so called from their function, since, by their change of
form, they cause the mouth to open or shut, according to the needs of
the plant. In young plants these guard-cells are very little below the
surface of the leaf or skin, but in others they are sunk quite beneath
the layer of cells forming the outer coat of the tissue. There are
other cases where they are slightly elevated above the surface.
Stomata are found chiefly in the green portions of plants, and are
most plentiful on the under side of leaves. It is, however, worthy of
notice, that when an aquatic leaf floats on the water, the mouths are
only to be found on the upper surface. These curious and interesting
objects are to be seen in many structures where we should hardly think
of looking for them; for instance, they may be found existing on the
delicate skin which envelops the kernel of the common walnut. As might
be expected, their dimensions vary with the character of the leaf on
which they exist, being large upon the soft and pulpy leaves, and
smaller upon those of a hard and leathery consistence. The reader will
find ample amusement, and will gain great practical knowledge of the
subject, by taking a plant, say a tuft of groundsel, and stripping off
portions of the external skin or “epidermis” from the leaf or stem,
etc., so as to note the different sizes and shapes of the stomata.
On the opposite bottom corner of Plate I. Fig. 25, is an example of
a stoma taken from the outer skin of a gourd, and here given for the
purpose of showing the curious manner in which the cells are arranged
about the mouth, no less than seven cells being placed round the
single mouth, and the others arranged in a partially circular form
around them.
Turning to Plate II., we find several other examples of stomata, the
first of which (Fig. 1) is obtained from the under surface of the
buttercup leaf, by stripping off the external skin, or “epidermis,” as
it is scientifically termed. The reader will here notice the slightly
waved outlines of the cell-walls, together with the abundant spots
of chlorophyll with which the leaf is coloured. In this example the
stomata appear open. Their closure or expansion depends chiefly on the
state of the weather; and, as a general rule, they are open by day and
closed at night.
A remarkably pretty example of stomata and elongated cells is to be
obtained from the leaf of the common iris, and may be prepared for the
microscope by simply tearing off a strip of the epidermis from the
under side of the leaf, laying it on a slide, putting a little water on
it, and covering it with a piece of thin glass. (See Plate II. Fig. 2.)
There are a number of longitudinal bands running along the leaf where
these cells and stomata appear. The latter are not placed at regular
intervals, for it often happens that the whole field of the microscope
will be filled with cells without a single stoma, whilst elsewhere a
group of three or four may be seen clustered closely together.
Fig. 3 on the same Plate exhibits a specimen of the beautifully waved
cells, without mouths, which are found on the upper surface of the ivy
leaf. These are difficult to arrange from the fresh leaf, but are
easily shown by steeping the leaf in water for some time, and then
tearing away the cuticle. The same process may be adopted with many
leaves and cuticles, and in some cases the immersion must be continued
for many days, and the process of decomposition aided by a very little
nitric acid in the water, or by boiling.
On the same Plate are three examples of spiral and ringed vessels,
types of an endless variety of these beautiful and interesting
structures. Fig. 4 is a specimen of a spiral vessel taken from the
lily, and is a beautiful example of a double spire. The deposit which
forms this spiral is very strong, and it is to the vast number of
these vessels that the stalk owes its well-known elasticity. In many
cases the spiral vessels are sufficiently strong to be visible to the
naked eye, and to bear uncoiling. For example, if a leaf-stalk of
geranium be broken across, and the two fragments gently drawn asunder,
a great number of threads, drawn from the spiral vessels, will be seen
connecting the broken ends. In this case the delicate membranous walls
of the vessel are torn apart, and the stronger fibre which is coiled
spirally within it unrolls itself in proportion to the force employed.
In many cases these fibres are so strong that they will sustain the
weight of an inch or so of the stalk.
In Fig. 5 is seen a still more bold and complex form of this curious
structure; being a coil of five threads, laid closely against each
other, and forming, while remaining in their natural position, an
almost continuous tube. This specimen is taken from the root of the
water lily, and requires some little care to exhibit its structure
properly.
Every student of nature must be greatly struck with the analogies
between different portions of the visible creation. These spiral
structures which we have just examined are almost identical in
appearance, and to some extent in their function, with the threads that
are coiled within the breathing tubes of insects. This is in both cases
twofold, namely, to give support and elasticity to a delicate membrane,
and to preserve the tube in its proper form, despite the bending to
which it may be subjected. When we come to the anatomy of the insect in
a future page we shall see this structure further exemplified.
In some cases the deposit, instead of forming a spiral coil, is
arranged in a series of rings, and the vessel is then termed
“annulated.” A very good example of this formation is given in Fig. 6,
which is a sketch of such a vessel, taken from a stalk of the common
rhubarb. To see these ringed vessels properly, the simplest plan is to
boil the rhubarb until it is quite soft, then to break down the pulpy
mass until it is flattened, to take some of the most promising portions
with the forceps, lay them on the slide and press them down with a thin
glass cover. They will not be found scattered at random through the
fibres, which elsewhere present only a congeries of elongated cells,
but are seen grouped together in bundles, and with a little trouble may
be well isolated, and the pulpy mass worked away so as to show them
in their full beauty. As may be seen in the illustration, the number
of the rings and their arrangement is extremely variable. A better,
but somewhat more troublesome, plan is to cut longitudinal sections of
the stem, as described in our concluding chapter, when not only the
various forms of cells and vessels, but their relations to each other,
will be well shown. The numerous crystals of oxalate of lime, which
make rhubarb so injurious a food for certain persons, will also be well
seen. These crystals are called “raphides,” and are to be found in very
many plants in different forms.
II.
FIG.
1. Cuticle, Buttercup leaf
2. Do. Iris
3. Do. Ivy leaf
4. Spiral vessel, Lily
5. Do. root, (rhizome) Water Lily
6. Ringed vessel, Rhubarb
7. Chaff, after burning
8. Bifid hair, Arabis
9. Hair, Marvel of Peru
10. End of hair, leaf of Hollyhock
11. Hair, Sowthistle leaf
12. Do. Tobacco
13. Do. Southernwood
14. Group of hairs, Hollyhock leaf
15. Hair, Yellow Snapdragon
16. Do. Moneywort
17. Hair, Geum
18. Do. Flower of Heartsease
19. Do. Dockleaf
20. Do. Throat of Pansy
21. Do. Dead-nettle Flower
22. Do. Groundsel
23. Cell, Beech-nut
24. Do. Pine cone
25. Vitta, Caraway Seed
26. Cork
27. Hair, Flower of Garden Verbena
28. Do. fruit of Plane
29. Do. do.
30. Do. do.
31. Do. Lobelia
32. Do. Cabbage
33. Do. Dead-nettle flower
34. Do. Garden Verbena flower
35. Fruit-hair, Dandelion
36. Hair, Thistle leaf
37. Do. Cactus
38. Do. do.
39. Do. Virginian Spider-wort
40. Do. Lavender
41. Section, Lavender leaf, Hairs and perfume-gland
42. Section, Orange Peel
43. Sting of Nettle
44. Hair, Marigold flower
45. Do. Ivy
[Illustration: II.]
* * * * *
The hairs of plants form very interesting objects, and are instructive
to the student, as they afford valuable indications of the mode in
which plants grow. They are all appendages of and arise from the skin
or epidermis; and although their simplest form is that of a projecting
and elongated cell, the variety of shapes which are assumed by these
organs is inexhaustible. On Plate II. are examples of some of the more
striking forms, which will be briefly described.
The simple hair is well shown in Figs. 18, 19, and 32, the first being
from the flower of the heartsease, the second from a dock-leaf, and the
third from a cabbage. In Fig. 18 the hair is seen to be but a single
projecting cell, consisting only of a wall and the contents. In Fig. 19
the hair has become more decided in shape, having assumed a somewhat
dome-like form; and in Fig. 32 it has become considerably elongated,
and may at once be recognised as a true hair.
In Fig. 8 is a curious example of a hair taken from the white Arabis,
one of the cruciferous flowers, which is remarkable for the manner
in which it divides into two branches, each spreading in opposite
directions. Another example of a forked hair is seen in Fig. 13, but in
this instance the hair is composed of a chain of cells, the three lower
forming the stem of the hair, and the two upper being lengthened into
the lateral branches. This hair is taken from the common southernwood.
In most cases of long hairs, the peculiar elongation is formed by a
chain of cells, varying greatly in length and development. Several
examples of these hairs will be seen on the same Plate.
Fig. 9 is a beaded hair from the Marvel of Peru, which is composed of
a number of separate cells placed end to end, and connected by slender
threads in a manner that strongly reminds the observer of a chain of
beads strung loosely together, so as to show the thread by which they
are connected with each other. Another good example is seen at Fig. 11,
in a hair taken from the leaf of the sowthistle. In this case the
beads are strung closely together, and when placed under a rather high
power of the microscope have a beautifully white and pearly aspect.
The leaf must be dry and quite fresh, and the hairs seen against the
green of the leaf. Fig. 39 represents another beaded hair taken from
the Virginian Spiderwort, or Tradescantia. This hair is found upon the
stamens, and is remarkable for the beautifully beaded outline, the
fine colouring, and the spiral markings with which each cell is adorned.
A still further modification of these many-celled hairs is found in
several plants, where the hairs are formed by a row of ordinarily
shaped cells, with the exception of the topmost cell, which is suddenly
elongated into a whip-like form. Fig. 22 represents a hair of this
kind, taken from the common groundsel; and Fig. 36 is a still more
curious instance, found upon the leaf of the thistle. The reader may
have noticed the peculiar white “fluffy” appearance of the thistle leaf
when it is wet after a shower of rain. This appearance is produced
by the long lash-like ends of the hairs, which are bent down by the
weight of the moisture, and lie almost at right angles with the thicker
portions of the hair.
An interesting form of hair is seen in the “sting” of the common
nettle. This may readily be examined by holding a leaf edgewise in the
stage forceps, and laying it under the field of the microscope. In
order to get the proper focus throughout the hair, the finger should
be kept upon the screw movement, and the hair brought gradually into
focus from its top to its base. The general structure of this hair is
not unlike that which characterises the fang of a venomous serpent. The
acrid fluid which causes the pain is situated in the enlarged base of
the hair, and is forced through the long straight tubular extremity by
means of the pressure exerted when the sting enters the skin. At the
very extremity of the perfect sting is a slight bulb-like swelling,
which serves to confine the acrid juice, and which is broken off on the
least pressure. The sting is seen in Fig. 43.
The extremities of many hairs present very curious forms, some being
long and slender, as in the examples already mentioned, while others
are tipped with knobs, bulbs, clubs, or rosettes in endless variety.
Fig. 12 is a hair of the tobacco leaf, exhibiting the two-celled gland
at the tip, containing the peculiar principle of the plant, known by
the name of “nicotine.” The reader will see how easy it is to detect
adulteration of tobacco by means of the microscope. The leaves most
generally used for this purpose are the dock and the cabbage, so that
if a very little portion of leaf be examined the character of the hairs
will at once inform the observer whether he is looking at the real
article or its substitute.
Fig. 15 is a hair from the flower of the common yellow snapdragon,
which is remarkable for the peculiar shape of the enlarged extremity,
and for the spiral markings with which it is decorated. Fig. 16 is a
curious little knobbed hair found upon the moneywort, and Fig. 17 is an
example of a double-knobbed hair taken from the Geum. Fig. 34 affords
a very curious instance of a glandular hair, the stem being built up
of cells disposed in a very peculiar fashion, and the extremity being
developed into a beautiful rosette-shaped head. This hair came from
the Garden Verbena.
Curiously branched hairs are not at all uncommon, and some very good
and easily obtained examples are given on Plate II.
Fig. 28 is one of the multitude of branched hairs that surround the
well-known fruit of the plane-tree, the branches being formed by some
of the cells pointing outward. These hairs do not assume precisely the
same shape; for Fig. 29 exhibits another hair from the same locality,
on which the spikes are differently arranged, and Fig. 30 is a sketch
of another such hair, where the branches have become so numerous and so
well developed that they are quite as conspicuous as the parent stem.
One of the most curious and interesting forms of hair is that which
is found upon the lavender leaf, and which gives it the peculiar
bloom-like appearance on the surface.
This hair is represented in Figs. 40 and 41. On Fig. 40 the hair is
shown as it appears when looking directly upon the leaf, and in Fig. 41
a section of the leaf is given, showing the mode in which the hairs
grow into an upright stem, and then throw out horizontal branches in
every direction. Between the two upright hairs, and sheltered under
their branches, may be seen a glandular appendage not unlike that which
is shown in Fig. 16. This is the reservoir containing the perfume, and
it is evidently placed under the spreading branches for the benefit
of their shelter. On looking upon the leaf by reflected light the
hairs are beautifully shown, extending their arms on all sides; and
the globular perfume cells may be seen scattered plentifully about,
gleaming like pearls through the hair-branches under which they repose.
They will be found more numerous on the under side of the leaf.
This object will serve to answer a question which the reader has
probably put to himself ere this, namely, Where are the fragrant
resins, scents, and oils stored? On Plate I. Fig. 16, will be seen the
reply to the first question; Fig. 41 of the present Plate has answered
the second question, and Fig. 42 will answer the third. This figure
represents a section of the rind of an orange, the flattened cells
above constituting the delicate yellow skin, and the great spherical
object in the centre being the reservoir in which the fragrant
essential oil is stored. The covering is so delicate that it is easily
broken, so that even by handling an orange some of the scent is sure
to come off on the hands, and when the peel is stripped off and bent
double, the reservoirs burst in myriads, and fling their contents to
a wonderful distance. This may be easily seen by squeezing a piece of
orange peel opposite a lighted candle, and noting the distance over
which the oil will pass before reaching the flame, and bursting into
little flashes of light. Other examples are given on the same plate.
Returning to the barbed hairs, we may see in Fig. 35 a highly magnified
view of the “pappus” hair of a dandelion, _i.e._ the hairs which
fringe the arms of the parachute-like appendage which is attached
to the seed. The whole apparatus will be seen more fully on Plate
III. Figs. 44, 45, 46. This hair is composed of a double layer of
elongated cells lying closely against each other, and having the ends
of each cell jutting out from the original line. A simpler form of
a double-celled, or more properly a “duplex” hair, will be seen in
Fig. 44. This is one of the hairs from the flower of the marigold and
has none of the projecting ends to the cells.
In some instances the cell-walls of the hairs become greatly hardened
by secondary deposit, and the hairs are then known as spines. Two
examples of these are seen in Figs. 37 and 38, the former being picked
from the Indian fig-cactus, and well known to those persons who have
been foolish enough to handle the fig roughly before feeling it. The
wounds which these spines will inflict are said to be very painful,
and have been compared to those produced by the sting of the wasp.
The latter hair is taken from the Opuntia. These spines must not be
confounded with thorns; which latter are modified branches.
Fig. 10 represents the extreme tip of a hair from the hollyhock leaf,
subjected to a lens of very high power.
Many hairs assume a star-like appearance, an aspect which may be
produced in different ways. Sometimes a number of simple hairs start
from the same base, and by radiating in different directions produce
the stellate effect. An example of this kind of hair may be seen in
Fig. 14, which is a group of hairs from the hollyhock leaf. There is
another mode of producing the star-shape which may be seen in Fig. 45,
a hair taken from the leaf of the ivy. Very fine examples may also be
found upon the leaf of Deutzia scabra.
Hairs are often covered with curious little branches or protuberances,
and present many other peculiarities of form which throw a considerable
light upon certain problems in scientific microscopy.
Fig. 33 represents a hair of two cells taken from the flower of the
well-known dead-nettle, which is remarkable for the number of knobs
scattered over its surface. A similar mode of marking is seen in
Fig. 31, a club-shaped hair covered with external projections, found
in the flower of the Lobelia. In order to exhibit these markings well,
a power of two hundred diameters is needed. Fig. 21 shows this dotting
in another hair from the dead-nettle, where the cell is drawn out to a
great length, but is still covered with these markings.
Fig. 20 is an example of a very curious hair taken from the throat of
the pansy. This hair may readily be obtained by pulling out one of the
petals, when the hairs will be seen at its base. Under the microscope
it has a particularly beautiful appearance, looking just like a glass
walking-stick covered with knobs, not unlike those huge, knobby
club-like sticks in which some farmers delight, where the projections
have been formed by the pressure of a honeysuckle or other climbing
plant.
A hair of a similar character, but even more curious, is found in the
same part of the flower of the Garden Verbena (see Fig. 27), and is not
only beautifully translucent, but is coloured according to the tint of
the flower from which it is taken. Its whole length is covered with
large projections, the joints much resembling the antennæ of certain
insects; and each projection is profusely spotted with little dots,
formed by elevation of the outer skin or cuticle. These are of some
value in determining the structure of certain appearances upon petals
and other portions of the flowers, and may be compared with Figs. 33 to
35 on Plate III.
Fig. 26 offers an example of the square cells which usually form the
bark of trees. This is a transverse section of cork, and perfectly
exhibits the form of bark cells. The reader is very strongly advised
to cut a delicate section of the bark of various trees, a matter very
easily accomplished with the aid of a sharp razor and a steady hand.
Fig. 24 is a transverse section through one of the scales of a
pine-cone, and is here given for the purpose of showing the numerous
resin-filled cells which it displays. This may be compared with Fig. 16
of Plate I. Fig. 25 is a part of one of the “vittæ,” or oil reservoirs,
from the fruit of the caraway, showing the cells containing the
globules of caraway oil. This is rather a curious object, because the
specimen from which it was taken was boiled in nitric acid, and yet
retained some of the oil globules. Immediately above it may be seen
(Fig. 23) a transverse section of the beechnut, showing a cell with
its layers of secondary deposit.
In the cuticle of the grasses and the mare’s-tails is deposited a large
amount of pure flint. So plentiful is this substance, and so equally
is it distributed, that it can be separated by heat or acids from the
vegetable parts of the plant, and will still preserve the form of the
original cuticle, with its cell-walls, stomata, and hairs perfectly
well defined.
Fig. 7, Plate II., represents a piece of wheat chaff, or “bran,” that
has been kept at a white heat for some time, and then mounted in
Canada balsam. I prepared the specimen from which the drawing was made
by laying the chaff on a piece of platinum, and holding it over the
spirit-lamp. A good example of the silex or flint in wheat is often
given by the remains of a straw fire, where the stems may be seen still
retaining their tubular form but fused together into a hard glassy
mass. It is this substance that cuts the fingers of those who handle
the wild grasses too roughly, the edges of the blades being serrated
with flinty teeth, just like the obsidian swords of the ancient
Mexicans, or the shark’s-tooth falchion of the New Zealander.
These are but short and meagre accounts of a very few objects, but
space will not permit of further elucidation, and the purpose of this
little work is not to exhaust the subjects of which it treats, but to
incite the reader to undertake investigation on his own account, and to
make his task easier than if he had done it unaided.
CHAPTER V
Starch, its Growth and Properties--Surface Cells of Petals--Pollen
and its Functions--Seeds.
The white substance so dear to the laundries under the name of starch
is found in a vast variety of plants, being distributed more widely
than most of the products which are found in the interior of vegetable
cells.
The starch grains are of very variable size even in the same plant,
and their form is as variable as their size, though there is a general
resemblance in those of the same plant which allows of their being
fairly easily identified after a moderate amount of practice. Sometimes
the grains are found loosely packed in the interior of the cells, and
are then easily recognised as starch grains by their peculiar form
and the delicate lines with which they are marked; but in many places
they are pressed so closely together that they assume an hexagonal
shape under the microscope, and bear a close resemblance to ordinary
twelve-sided cells. In other plants, again, the grains never advance
beyond the very minute form in which they seem to commence their
existence; and in some, such as the common oat, a great number of very
little granules are compacted together so as to resemble one large
grain.
There are several methods of detecting starch in those cases where
its presence is doubtful; and the two modes that are usually employed
are polarised light and the iodide of potassium. When polarised
light is employed--a subject on which we shall have something to say
presently--the starch grains assume the characteristic “black-cross,”
and when a plate of selenite is placed immediately beneath the slide
containing the starch grains, they glow with all the colours of the
rainbow. The second plan is to treat them with a very weak solution
of iodine and iodide of potassium, and in this case the iodine has
the effect on the starch granules of staining them blue. They are so
susceptible of this reaction that when the liquid is too strong the
grains actually become black from the amount of iodine which they
imbibe.
Nothing is easier than to procure starch granules in the highest
perfection. Take a raw potato, and with a razor cut a very thin
slice from its interior, the direction of the cut not being of the
slightest importance. Put this delicate slice upon a slide, drop a
little water upon it, cover it with a piece of thin glass, give it a
good squeeze, and place it under a power of a hundred or a hundred and
fifty diameters. Any part of the slice, provided that it be very thin,
will then present the appearance shown in Plate III. Fig. 9, where an
ordinary cell of potato is seen filled loosely with starch grains of
different sizes. Around the edges of the slice a vast number of starch
granules will be seen, which have been squeezed out of their cells by
pressure, and are now floating freely in the water. As cold water has
no perceptible effect upon starch, the grains are not altered in form
by the moisture, and can be examined at leisure.
III.
FIG.
1. Laurel leaf, transverse section
2. Starch, Wheat
3. Do. from Pudding
4. Do. Potato
5. Outer Skin, Capsicum pod
6. Starch, Parsnip
7. Do. Arrow Root, West Indian
8. Do. “Tousles Mois”
9. Do. in cell of Potato
10. Do. Indian Corn
11. Do. Sago
12. Do. Tapioca
13. Root, Yellow Water-Lily
14. Starch, Rice
15. Do. Horsebean
16. Do. Oat
17. Pollen, Snowdrop
18. Do. Wallflower
19. Do. Willow Herb, a pollen tube
20. Do. Violet
21. Do. Musk Plant
22. Do. Apple
23. Do. Dandelion
24. Do. Sowthistle
25. Do. Lily
26. Do. Heath
27. Do. Heath, another species
28. Pollen, Furze
29. Do. Tulip
30. Petal, Pelargonium
31. Do. Periwinkle
32. Do. Golden Balsam
33. Do. Snapdragon
34. Do. Primrose
35. Do. Scarlet Geranium
36. Pollen, Crocus
37. Do. Hollyhock
38. Fruit, Galium, Goosegrass
39. A hook of ditto more magnified
40. Seed, Red Valerian
41. Portion of Parachute of same, more magnified
42. Seed, Foxglove
43. Do. Sunspurge
44. Parachute, Dandelion seed
45. Seed, Dandelion
46. Do. Hair of Parachute
47. Do. Yellow Snapdragon
48. Do. Mullein
49. Do. Robin Hood
50. Do. Bur-reed
51. Do. Willow Herb
52. Do. Musk Mallow
[Illustration: III.]
On focusing with great care, the surface of each granule will be seen
to be covered with very minute dark lines, arranged in a manner which
can be readily comprehended from Fig. 4, which represents two granules
of potato starch as they appear when removed from the cell in which
they took their origin. All the lines evidently refer to the little
dark spots at the end of the granule, called technically the “hilum,”
and represent the limits of successive layers of material deposited
one after another. The lines in question are very much better seen if
the substage condenser be used with a small central stop, so as to
obtain partial dark-field illumination. Otherwise they are often very
difficult of detection.
In the earliest stages of their growth the starch granules appear to be
destitute of these markings, or at all events they are so few and so
delicate as not to be visible even with the most perfect instruments,
and it is not until the granules assume a comparatively large size that
the external markings become distinctly perceptible.
We will now glance at the examples of starch which are given in the
Plate, and which are a very few out of the many that might be figured.
Fig. 2 represents the starch of wheat, the upper grain being seen in
front, the one immediately below it in profile, and the two others
being examples of smaller grains. Fig. 6 is a specimen of a very minute
form of starch, where the granules do not seem to advance beyond
their earliest stage. This specimen is obtained from the parsnip; and
although the magnifying power is very great, the dimensions of the
granules are exceedingly small, and except by a very practised eye they
would not be recognisable as starch grains.
Fig. 3 is a good example of a starch grain of wheat, exemplifying the
change that takes place by the combined effects of heat and moisture.
It has already been observed that cold water exercises little, if
any, perceptible influence upon starch; but it will be seen from
the illustration that hot water has a very powerful effect. When
subjected to the action of water at a temperature over 140° Fahr.,
the granule swells rapidly, and at last bursts, the contents escaping
in a gelatinous mass, and the external membrane collapsing into the
form which is shown in Fig. 3, which was taken out of a piece of hot
pudding. A similar form of wheat starch may also be detected in bread,
accompanied, unfortunately, by several other substances not generally
presumed to be component parts of the “staff of life.”
In Fig. 7 are represented some grains of starch from West Indian
arrowroot, and Fig. 8 exhibits the largest kind of starch grain known,
obtained from the tuber of a species of canna, supposed to be _C.
edúlis_, a plant similar in characteristics to the arrowroot. The
popular name of this starch is “Tous les Mois,” and under that title
it may be obtained from the opticians, or chemists.
Fig. 10 shows the starch granules from Indian corn, as they appear
before they are compressed into the honeycomb-like structure which
has already been mentioned. Even in that state, however, if they are
treated with iodine, they exhibit the characteristics of starch in a
very perfect manner. Fig. 11 is starch from sago, and Fig. 12 from
tapioca, and in both these instances the several grains have been
injured by the heat employed in preparing the respective substances for
the market.
Fig. 13 exhibits the granules obtained from the root of the water-lily,
and Fig. 14 is a good example of the manner in which the starch
granules of rice are pressed together so as to alter the shape and
puzzle a novice. Fig. 16 is the compound granule of the oat, which
has already been mentioned, together with some of the simple granules
separated from the mass; and Fig. 15 is an example of the starch grains
obtained from the underground stem of the horse-bean. It is worthy of
mention that the close adhesion of the rice starch into those masses is
the cause of the peculiar grittiness which distinguishes rice flour to
the touch.
Whilst very easily acted on by heat, starch-granules are very resistent
to certain other reagents. Weak alkalies, in watery solution, readily
attack them, but by treating portions of plants with caustic potash
dissolved in strong spirit, the woody and other parts may be dissolved
away; and after repeated washing with spirit the starch may be mounted.
This, however, must never be in any glycerine medium, except that given
on p. 172.
* * * * *
In Plate III. Fig. 1, may be seen a curious little drawing, which is
a sketch of the laurel-leaf cut transversely, and showing the entire
thickness of the leaf. Along the top may be seen the delicate layer
of “varnish” with which the surface of the leaf is covered, and which
serves to give to the foliage its peculiar polish. This varnish is
nothing more than the translucent matter which binds all the cells
together, and which is poured out very liberally upon the surface of
the leaf. The lower part of this section exhibits the cells of which
the leaf is built, and towards the left hand may be seen a cut end of
one of the veins of the leaf, more rightly called a wood-cell.
We will now examine a few examples of surface cells.
Fig. 5 is a portion of epidermis stripped from a Capsicum pod,
exhibiting the remains of the nuclei in the centre of each cell,
together with the great thickening of the wall-cells and the numerous
pores for the transmission of fluid. This is a very pretty specimen for
the microscope, as it retains its bright red colour, and even in old
and dried pods exhibits the characteristic markings.
In the centre of the Plate may be seen a wheel-like arrangement of
the peculiar cells found on the petals of six different flowers, all
easily obtainable, and mounted without difficulty.
Fig. 30 is the petal of a geranium (Pelargonium), a very common object
on purchased slides. It is a most lovely subject for the microscope,
whether it be examined with a low or a high power,--in the former
instance exhibiting a most beautiful “stippling” of pink, white, and
black, and in the latter showing the six-sided cells with their curious
markings.
In the centre of each cell is seen a radiating arrangement of dark
lines with a light spot in the middle, looking very like the mountains
on a map. These lines were long thought to be hairs; but Mr. Tuffen
West, in an interesting and elaborate paper on the subject, has shown
their true nature. From his observations it seems that the beautiful
velvety aspect of flower petals is owing to these arrangements of the
surface cells, and that their rich brilliancy of colour is due to the
same cause. The centre of each cell-wall is elevated as if pushed up by
a pointed instrument from the under side of the wall, and in different
flowers this elevation assumes different forms. Sometimes it is merely
a slight wart on the surface, sometimes it becomes a dome, while in
other instances it is so developed as to resemble a hair. Indeed,
Mr. West has concluded that these elevations are nothing more than
rudimentary hairs.
The dark radiating lines are shown by the same authority to be formed
by wrinkling of the membrane forming the walls of the elevated centre,
and not to be composed of “secondary deposit,” as has generally been
supposed.
Fig. 31 represents the petal of the common periwinkle, differing from
that of the geranium by the straight sides of the cell-walls, which do
not present the toothed appearance so conspicuous in the former flower.
A number of little tooth-like projections may be seen on the interior
of the cells, their bases affixed to the walls and their points tending
toward the centre, and these teeth are, according to Mr. West, formed
of secondary deposit.
In Fig. 32 is shown the petal of the common garden balsam, where the
cells are elegantly waved on their outlines, and have plain walls.
The petal of the primrose is seen in Fig. 34, and that of the yellow
snapdragon in Fig. 33; in the latter instance the surface cells assume
a most remarkable shape, running out into a variety of zigzag outlines
that quite bewilders the eye when the object is first placed under the
microscope. Fig. 35 is the petal of the common scarlet geranium.
In several instances these petals are too thick to be examined without
some preparation, and glycerine will be found well adapted for that
purpose. The young microscopist must, however, beware of forming his
ideas from preparations of dried leaves, petals, or hairs, and should
always procure them in their fresh state whenever he desires to make
out their structure. Even a fading petal should not be used, and if the
flowers are gathered for the occasion, their stalks should be placed
in water, so as to give a series of leaves and petals as fresh as
possible.
* * * * *
We now pass from the petal of the flower to the pollen, that coloured
dust, generally yellow or white, which is found upon the stamens, and
which is very plentiful in many flowers, such as the lily and the
hollyhock.
This substance is found only upon the stamens or anthers of full-blown
flowers (the anthers being the male organs), and is intended for the
purpose of enabling the female portion of the flower to produce fertile
seeds. In form the pollen grains are wonderfully diverse, affording
an endless variety of beautiful shapes. In some cases the exterior is
smooth and marked only with minute dots, but in many instances the
outer wall of the pollen grain is covered with spikes, or decorated
with stripes or belts. A few examples of the commonest forms of pollen
will be found on Plate III.
Fig. 17 is the pollen of the snowdrop, which, as will be seen, is
covered with dots and marked with a definite slit along its length.
The dots are simply tubercles in the outer coat of the grain, and
are presumed to be formed for the purpose of strengthening the
membrane, otherwise too delicate, upon the same principle which gives
to “corrugated” iron such strength in proportion to the amount of
material. Fig. 18 is the pollen of the wall-flower, shown in two views,
and having many of the same characteristics as that of the snowdrop.
Fig. 19 is the pollen of the willow-herb, and is here given as an
illustration of the manner in which the pollen aids in the germination
of plants.
In order to understand its action, we must first examine its structure.
All pollen-grains are furnished with some means by which their contents
when thoroughly ripened can be expelled. In some cases this end is
accomplished by sundry little holes called pores; in others, certain
tiny lids are pushed up by the contained matter; and in some, as in
the present instance, the walls are thinned in certain places so as to
yield to the internal pressure.
When a ripe pollen-grain falls upon the stigma of a flower, it
immediately begins to swell, and seems to “sprout” like a potato in
a damp cellar, sending out a slender “pollen-tube” from one or other
of the apertures already mentioned. In Fig. 19 a pollen-tube is seen
issuing from one of the projections, and illustrates the process better
than can be achieved by mere verbal description. The pollen-tubes
insinuate themselves between the cells of the stigmas, and, continually
elongating, worm their way down the “style” until they come in contact
with the “ovules.” By very careful dissection of a fertilised stigma,
the beautiful sight of the pollen-tubes winding along the tissues of
the style may be observed under a high power of the microscope.
The pollen-tube is nothing more than the interior coat of the grain,
very much developed, and filled with a substance technically named
“fovilla,” composed of “protoplasm” (the semi-liquid substance which
is found in the interior of cells), very minute starch grains, and some
apparently oily globules.
In order to examine the structure of the pollen-grains properly, they
should be examined under various circumstances--some dry, others placed
in water to which a little sugar has been added, others in oil, and it
will often be found useful to try the effect of different acids upon
them.
Fig. 20 is the pollen of the common violet, and is easily recognisable
by its peculiar shape and markings. Fig. 21 is the pollen of the
musk-plant, and is notable for the curious mode in which its surface
is belted with wide and deep bands, running spirally round the
circumference. Fig. 22 exhibits the pollen of the apple, and Fig. 23
affords a very curious example of the raised markings upon the surface
of the dandelion pollen. In Fig. 24 there are also some very wonderful
markings, but they are disposed after a different fashion, forming a
sort of network upon the surface, and leaving several large free spaces
between the meshes. The pollen of the lily is shown in Fig. 25, and is
a good example of a pollen-grain covered with the minute dottings which
have already been described.
Figs. 26 and 27 show two varieties of compound pollen, found in two
species of heath. These compound pollen-grains are not of unfrequent
occurrence, and are accounted for in the following manner.
The pollen is formed in certain cavities within the anthers, by means
of the continual subdivision of the “parent-cells” from which it is
developed. In many cases the form of the grain is clearly owing to the
direction in which these cells have divided, but there is no great
certainty on this subject. It will be seen, therefore, that if the
process of subdivision be suddenly arrested, the grains will be found
adhering to each other in groups of greater or smaller size, according
to the character of the species and the amount of subdivision that has
taken place. The reader must, however, bear in mind that the whole
subject is as yet rather obscure, and that further discovery may throw
doubt on many theories which at present are accepted as established.
Fig. 28 shows the pollen of the furze, in which are seen the
longitudinal slits and the numerous dots on the surface; and Fig. 29
is the curiously shaped pollen of the tulip. The two large yellow
globular figures at each side of the Plate represent the pollen of
two common flowers; Fig. 36 being that of the crocus, and Fig. 37 a
pollen-grain of the hollyhock. As may be seen from the illustration,
the latter is of considerable size, and is covered with very numerous
projections. These serve to raise the grain from a level surface, over
which it rolls with a surprising ease of motion, so much so indeed
that if a little of this substance be placed on a slide and a piece
of thin glass laid over it, the glass slips off as soon as it is in
the least inclined, and forces the observer to fix it with paper or
cement before he can place it on the inclined stage of the microscope.
The little projections have a very curious effect under a high
power, and require careful focusing to observe them properly; for the
diameter of the grain is so large that the focus must be altered to
suit each individual projection. Their office is, probably, to aid in
fertilisation.
* * * * *
The seeds of plants are even easier of examination than the pollen, and
in most cases require nothing but a pocket lens and a needle for making
out their general structure. The smaller seeds, however, must be placed
under the microscope, many of them exhibiting very curious forms. The
external coat of seeds is often of great interest, and needs to be
dissected off before it can be rightly examined. The simplest plan in
such a case is to boil the seed well, press it while still warm into
a plate of wax, and then dissect with a pair of needles, forceps, and
scissors under water. Many seeds may also be mounted in cells as dry
objects, after being thoroughly dried themselves.
A few examples of the seeds of common plants are given at the bottom of
Plate III.
Fig. 38 exhibits the fruit, popularly called the seed, of the common
goosegrass, or Galium, which is remarkable for the array of hooklets
with which it is covered. Immediately above the figure may be seen a
drawing of one of the hooks much magnified, showing its sharp curve
(Fig. 39). It is worthy of remark that the hook is not a simple curved
hair, but a structure composed of a number of cells terminating in a
hook.
Fig. 40 shows the seed, or rather the fruit, of the common red
valerian, and is introduced for the purpose of showing its plumed
extremity, which acts as a parachute, and causes it to be carried
about by the wind until it meets with a proper resting-place. It is
also notable for the series of strong longitudinal ribs which support
its external structure. On Fig. 41 is shown a portion of one of the
parachute hairs much more magnified.
The seed of the common dandelion, so dear to children in their
play-hours, when they amuse themselves by puffing at the white
plumy globes which tip the ripe dandelion flower-stalks, is a very
interesting object even to their parents, on account of its beautiful
structure, and the wonderful way in which it is adapted to the place
which it fills. Fig. 45 represents the seed portion of one of these
objects, together with a part of the parachute stem, the remainder of
that appendage being shown lying across the broken stem.
The shape of the seed is not unlike that of the valerian, but it is
easily distinguished from that object by the series of sharp spikes
which fringe its upper end, and which serve to anchor the seed firmly
as soon as it touches the ground. From this end of the seed proceeds
a long slender shaft, crowned at its summit by a radiating plume of
delicate hairs, each of which is plentifully jagged on its surface, as
may be seen in Fig. 46, which shows a small portion of one of these
hairs greatly magnified. These jagged points are evidently intended to
serve the same purpose as the spikes below, and to arrest the progress
of the seed as soon as it has found a convenient spot.
Fig. 42 is the seed of the foxglove, and Fig. 43 the seed of the
sunspurge, or milkwort. Fig. 47 shows the seed of the yellow
snapdragon; remarkable for the membranous wing with which the seed
is surrounded, and which is composed of cells with partially spiral
markings. When viewed edgewise, it looks something like Saturn with
his ring, or, to use a more homely but perhaps a more intelligible
simile, like a marble set in the middle of a penny. Fig. 48 is a seed
of mullein, covered with net-like markings on its external surface.
These are probably to increase the strength of the external coat, and
are generally found in the more minute seeds.
On Fig. 50 is shown a seed of the burr-reed; a structure which is
remarkable for the extraordinary projection of the four outer ribs, and
their powerful armature of reverted barbs. Fig. 51 shows another form
of parachute seed, found in the willow-herb, where the parachute is
not expanded nearly so widely as that of the valerian; neither is it
set upon a long slender stem like that of the dandelion, but proceeds
at once from the top of the seed, widening towards the extremity,
and having a very comet-like appearance. Two more seeds only remain,
Fig. 49 being the seed of Robin Hood, and the other, Fig. 52, that of
the muskmallow, being given in consequence of the thick coat of hairs
with which it is covered.
Many seeds can be well examined when mounted in Canada balsam.
CHAPTER VI
Algæ and their Growth--Desmidiaceæ, where found--Diatoms, their
Flinty Deposit--Volvox--Mould, Blight, and Mildew--Mosses and
Ferns--Mare’s-Tail and the Spores--Common Sea-weeds and their Growth.
On Plate IV. will be seen many examples of the curious vegetables
called respectively algæ and fungi, which exhibit some of the lowest
forms of vegetable life, and are remarkable for their almost universal
presence in all parts of this globe, and also almost all conditions of
cold, heat, or climate. Many of them are well known under the popular
name of sea-weeds, others are equally familiar under the titles of
“mould,” “blight,” or “mildew,” while many of the minuter kinds exhibit
such capability of motion, and such apparent symptoms of volition, that
they have long been described as microscopic animalcules, and thought
to belong to the animal rather than to the vegetable kingdoms.
Fig. 1 represents one of the very lowest forms of vegetable life, being
known to the man of science as the Palmella, and to the general public
as “gory dew.” It may be seen on almost any damp wall, extending in red
patches of various sizes, looking just as if some blood had been dashed
on the wall, and allowed to dry there. With a tolerably powerful
lens this substance can be resolved into the exceedingly minute cells
depicted in the figure. Generally, these cells are single, but in many
instances they are double, owing to the process of subdivision by which
the plant grows, if such a term may be used.
Fig. 2 affords an example of another very low form of vegetable, the
Palmoglæa, that green slimy substance which is so common on damp
stones. When placed under the microscope, this plant is resolvable
into a multitude of green cells, each being surrounded with a kind of
gelatinous substance. The mode of growth of this plant is very simple.
A line appears across one of the cells, and after a while it assumes a
kind of hour-glass aspect, as if a string had been tied tightly round
its middle. By degrees the cell fairly divides into two parts, and then
each part becomes surrounded with its own layer of gelatine, so as to
form two separate cells, placed end to end.
One of the figures, that on the right hand, represents the various
processes of “conjugation,” _i.e._ the union and fusion together of two
cells. Each cell throws out a little projection; these meet together,
and then uniting, form a sort of isthmus connecting the two main
bodies. This rapidly widens, until the two cells become fused into one
large body. The whole subject of conjugation is very interesting, and
is treated at great length in the _Micrographic Dictionary_ of Messrs.
Griffith and Henfrey, a work to which the reader is referred for
further information on many of the subjects that, in this small work,
can receive but a very hasty treatment.
Few persons would suppose that the slug-like object on Fig. 3, the
little rounded globules with a pair of hair-like appendages, and the
round disc with a dark centre, are only different forms of the same
organism. Such, however, is the case, and these are three of the
modifications which the Protococcus undergoes. This vegetable may be
seen floating like green froth on the surface of rain-water.
On collecting some of this froth and putting it under the microscope,
it is seen to consist of a vast number of little green bodies, moving
briskly about in all directions, and guiding their course with such
apparent exercise of volition that they might very readily be taken for
animals. It may be noticed that the colour of the plant is sometimes
red, and in that state it has been called the Hæmatococcus.
The “still” state of this plant is shown in the round disc. After a
while the interior substance splits into two portions; these again
subdivide, and the process is repeated until sixteen or thirty-two
cells become developed out of the single parent-cell. These little ones
then escape, and, being furnished with two long “cilia” or thread-like
appendages, whirl themselves merrily through the water. When they have
spent some time in this state, growing all the while, they lose their
cilia, become clothed with a strong envelope, and pass into the still
stage from which they had previously emerged. This curious process is
repeated in endless succession, and causes a very rapid growth of the
plant. The moving bodies are technically called zoospores, or living
spores, and are found in many other plants besides those of the lowest
order.
IV.
FIG.
1. Gory Dew, Palmella cruenta
2. Palmoglæa macrococca
3. Protococcus pluvialis,
_a_, in its motile,
_b_, in its fixed state,
_c_, zoospores
4. Closterium
5. Ditto, end more magnified
6. Pediastrum
7. Scenedesmus
8. Oscillatoria
9. Spirogyra
10. Tyndaridea
11. Do. spore
12. Sphærozosma
13. Chlorococcus
14. Scenedesmus
15. Pediastrum, to show cells
16. Ankistrodesmus
17. Cosmarium
18. Desmidium
19. Cosmarium, formation of Resting Spore
20. Cocconema lanceolatum
21. Diatoma vulgare
Do. larger frustules, at the side
22. Volvox globator
Do. single green body, above
23. Synedra
24. Gomphonema acuminatum
Do. larger frustules, below
25. Yeast
26. Sarcina ventriculi
27. Eunotia diadema
28. Melosira varians
Do. two bleached frustules
29. Cocconeis pediculus
30. Achnanthes exilis
31. Navicula amphisbœna
32. Uredo, “Red-rust” of corn
33. Puccinea, Mildew of corn
34. Botrytis, mould on grapes
Do. Sporules, beside it
35. Do. parasitica, Potato blight
36. Ectocarpus siliculosus
37. Ulva latissima
38. Polypodium
Do. single spore, below
39. Moss capsule, Hypnum
40. Mare’s tail, Equisetum, _a_
Do. do. _b_ and _c_
41. Porphyra laciniata
[Illustration: IV.]
On Fig. 13 is delineated a very minute plant, called from its colour
Chlorococcus. It may be found upon tree-trunks, walls, etc., in the
form of green dust, and has recently been found to take part in forming
the first stage of lichens.
A large and interesting family of the “confervoid algæ,” as these low
forms of vegetable life are termed, is the Desmidiaceæ, called in more
common parlance desmids. A few examples of this family are given in
Plate IV.
They may be found in water, always preferring the cleanest and the
brightest pools, mostly congregating in masses of green film at the
bottom of the water, or investing the stems of plants. Their removal
is not very easy, but is best accomplished by very carefully taking up
this green slippery substance in a spoon, and straining the water away
through fine muslin. They may also be separated by allowing a ring,
covered with muslin, to float upon the surface of the water collected
in a jar, for, being great lovers of light, they assemble where it
is most abundant. An opaque jar should be used. For preservation,
glycerine-gelatine seems to be the best fluid. A very full and accurate
description of these plants may be found in Ralfs’ _British Desmidieæ_.
Fig. 4 represents one of the species of Closterium, more than twenty
of which are known. These beautiful objects can be obtained from the
bottom of almost every clear pool, and are of some interest on account
of the circulating currents that may be seen within the living plants.
A high power is required to see this phenomenon clearly. The Closteria
are reproduced in various ways. Mostly they divide across the centre,
being joined for a while by two half-cells. Sometimes they reproduce
by means of conjugation, the process being almost entirely conducted
on the convex sides. Fig. 5 represents the end of a Closterium, much
magnified in order to show the actively moving bodies contained within
it.
Fig. 16 is a supposed desmid, called Ankistrodesmus, and presumed to be
an earlier stage of Closterium.
Fig. 6 is a very pretty desmid called the Pediastrum, and valuable to
the microscopist as exhibiting a curious mode of reproduction. The
figure shows a perfect plant composed of a number of cells arranged
systematically in a star-like shape; Fig. 15 is the same species
without the colouring matter, in order to show the shape of the cells.
The Pediastrum reproduces by continual subdivision of the contents of
each cell into a number of smaller cells, termed “gonidia” on account
of their function on the perpetuation of the species. When a sufficient
number has been formed, they burst through the envelope of the original
cell, taking with them a portion of its internal layer, so as to form
a vesicle, in which they move actively. In a few minutes they arrange
themselves in a circle, and after a while they gradually assume the
perfect form, the whole process occupying about two days. Fig. 18
exhibits an example of the genus Desmidium. In this genus the cells are
either square or triangular in their form, having two teeth at their
angles, and twisted regularly throughout their length, causing the wavy
or oblique lines which distinguish them. The plants of this genus are
common, and may be found almost in any water. I may as well mention
that I have obtained nearly all the preceding species, together with
many others, from a little pond on Blackheath.
Fig. 7 is another desmid called Scenedesmus, in which the cells are
arranged in rows of from two to ten in number, the cell at each
extremity being often furnished with a pair of bristle-like appendages.
Fig. 14 is another species of the same plant, and both may be found in
the water supplied for drinking in London, as well as in any pond.
A common species of desmid is seen at Fig. 12, called Sphærozosma,
looking much like a row of stomata set chainwise together. It
multiplies by self-division.
Fig. 17 is a specimen of desmid named Cosmarium, plentifully found
in ponds on heaths and commons, and having a very pretty appearance
in the microscope, with its glittering green centre and beautifully
transparent envelope. The manner in which the Cosmarium conjugates is
very remarkable, and is shown at Fig. 19.
The two conjugating cells become very deeply cleft, and by degrees
separate, suffering the contents to pour out freely, and, as at present
appears, without any envelope to protect them. The mass, however,
soon acquires an envelope of its own, and by degrees assumes a dark
reddish-brown tint. It is now termed a sporangium, and is covered with
a vast number of projections, which in this genus are forked at their
tip, but in others, which also form sporangia, are simply pointed. The
Closteria conjugate after a somewhat similar manner, and it is not
unfrequent to find a pair in this condition, but in their case the
sporangium is quite smooth on its surface.
Another very remarkable family of confervoid algæ is that which is
known under the name of Oscillatoriæ, from the oscillating movement
of the plant. They are always long and filamentous in character, and
may be seen moving up and down with a curious irregularity of motion.
Their growth is extremely rapid, and may be watched under a tolerably
powerful lens, thus giving many valuable hints as to the mode by
which these plants are reproduced. One of the commonest species is
represented at Fig. 8.
Figs. 9, 10, and 11 are examples of another family, called technically
the Zygnemaceæ, because they are so constantly yoked together by
conjugation. They all consist of a series of cylindrical cells, set end
to end, and having their green contents arranged in similar patterns.
Two of the most common and typical species are here given.
Fig. 9 is the Spirogyra, so called from the spiral arrangement of
the chlorophyll; and Fig. 10 is the Tyndaridea, or Zygnema, as it is
called by some writers. A casual inspection will show how easy it is
to distinguish the one from the other. Fig. 11 represents a portion of
the Tyndaridea during the process of conjugation, showing the tube of
connection between the cells and one of the spores.
* * * * *
We now arrive at the diatoms, so called because of their method of
reproduction, in which it appears as if a cut were made right along the
original cell. The commonest of these plants is the Diatóma vulgáre,
seen in Fig. 21 as it appears while growing. The reproduction of this
plant is effected by splitting down the centre, each half increasing
to the full size of the original cell; and in almost every specimen of
water taken from a pond, examples of this diatom undergoing the process
of division will be distinguished. It also grows by conjugation. The
diatoms are remarkable for the delicate shell or flinty matter which
forms the cell skeleton, and which will retain its shape even after
intense heat and the action of nitric acid. While the diatoms are
alive, swimming through the water, their beautiful markings are clearly
distinct, glittering as if the form were spun from crystalline glass.
Just above the figure, and to the right hand, are two outlines of
single cells of this diatom, the one showing the front view and the
other the profile.
Fig. 20 is an example of a diatom--Cocconéma lanceolátum--furnished
with a stalk. The left-hand branch sustains a “frustule” exhibiting the
front view, while the other is seen sideways.
Another common diatom is shown in Fig. 23, and is known by the name of
Synedra. This constitutes a very large genus, containing about seventy
known species. In this genus the frustules are at first arranged upon
a sort of cushion, but in course of time they mostly break away from
their attachment. In some species they radiate in every direction from
the cushion, like the spikes of the ancient cavalier’s mace.
Fig. 24 is another stalked diatom called Gomphonéma acuminátum, found
commonly in ponds and ditches. There are nearly forty species belonging
to this genus. A pair of frustules are also shown which exhibit the
beautiful flinty outline without the coloured contents (technically
called endochrome).
Fig. 27 is a side view of a beautiful diatom, called Eunótia diadéma
from its diadem-like form. There are many species of this genus. When
seen upon the upper surface, it looks at first sight like a mere row of
cells with a band running along them; but by careful arrangement of the
light its true form may easily be made out.
Fig. 28 represents a very common fresh-water diatom, named Melosíra
várians. The plants of this genus look like a cylindrical rod composed
of a variable number of segments, mostly cylindrical, but sometimes
disc-shaped or rounded. An end view of one of the frustules is seen
at the left hand, still coloured with its dots of “endochrome,” and
showing the cylindrical shape. Immediately above is a figure of another
frustule seen under both aspects with the endochrome removed.
A rather curious species of diatom, called Cocconeïs pedículus, is
seen at Fig. 29 as it appears on the surface of common water-cress.
Sometimes the frustules, which in all cases are single, are crowded
very closely upon each other and almost wholly hide the substance on
which they repose. Fig. 30 is another diatom of a flag-like shape,
named Achnanthes, having a long slender filament attached to one end
of the lower frustule, representing the flag-staff. There are many
wonderful species of such diatoms, some running almost end to end like
a bundle of sticks, and therefore called Bacillária; others spreading
out like a number of fans, such as the genus Licmophora; while some
assume a beautiful wheel-like aspect, of which the genus Meridion
affords an excellent example.
A very remarkable, and not uncommon, fresh-water diatom is the
Bacillária paradóxa. It looks, when at rest, like a broad brown ribbon
of varying length. The diatoms lie across the ribbon, on edge, and
slide upon each other exactly like the ladders of a fire-escape, so
that the broad ribbon is converted into a fine long thread, which
speedily closes up again into the original ribbon, and so _da capo_.
The reason for this movement, and how it is effected, is absolutely
unknown; indeed, nothing certain is known as to the way in which
diatoms move, nor has ever a probable guess yet been made.
The last of the diatoms which we shall be able to mention in this
work is that represented on Fig. 31. The members of this genus have
the name of Navícula, on account of their boat-like shape and their
habit of gliding through the water in a canoe-like fashion. There are
many species of this genus, all of which are notable for the graceful
and varied courses formed by their outlines, and the extreme delicacy
of their markings. In many species the markings are so extremely
minute that they can only be made out with the highest powers of the
microscope and the most careful illumination, so that they serve as
test objects whereby the performance of a microscope can be judged by a
practical man.
* * * * *
The large spherical figure in the centre of Plate IV. represents an
example of a family belonging to the confervoid algæ, and known by the
name of Volvox globator. There seems to be but one species known.
This singular plant has been greatly bandied about between the
vegetable and animal kingdoms, but seems now to be satisfactorily
settled among the vegetables. In the summer it may be found in pools
of water, sufficiently large to be visible to the naked eye, like a
little green speck proceeding slowly through the water. When a moderate
power is used, it appears as shown in the figure, and always contains
within its body a number of smaller individuals, which after a while
burst through the envelope of the parent and start upon an independent
existence. On a closer examination, a further generation may be
discovered even within the bodies of the children. The whole surface is
profusely covered with little green bodies, each being furnished with
a pair of movable cilia, by means of which the whole organism is moved
through the water. These bodies are analogous to the zoospores already
mentioned, and are connected with each other by a network of filaments.
Reproduction also takes place by conjugation as in other algæ. A
more magnified representation of one of the green bodies is shown
immediately above the larger figure. The volvox is apt to die soon when
confined in a bottle.
Fig. 25 is the common yeast-plant, consisting simply of a chain of
cells, which increase by budding, and only form spores when they have
exhausted the nutriment in the fluid in which they live. Fig. 26 is
a curious object, whose scientific name is Sárcina ventrículi. It is
found in the human stomach. Similar forms are often to be found in the
air; for instance, a piece of cocoa-nut will exhibit this, and many
other kinds of Bacteria and moulds, after a few days’ exposure to the
air, preferably in a dark cupboard.
We now come upon a few of the blights and mildews. A very interesting
series of forms is first to be alluded to. Upon the bramble-leaf may
often be found spots, at first red, then orange, then reddish black.
These are known as Œcidium berberidis. Fig. 32 shows the “red-rust”
of wheat, the Urédo; and Fig. 33 is the mildew of corn, known as
Puccinia. The interest lies in the fact that these three forms are
successive stages in the life-history of the same plant. Another
species of Urédo, together with a Phragmídium, once thought to be
another kind of fungus, is seen on a rose-leaf on Plate V. Fig. 1.
On Fig. 10, however, of the same Plate, the Phragmídium may be seen
proceeding from Urédo, thus proving them to be but two states of the
same plant. There is room for any amount of observation and work in
connection with the life-histories of many of these fungi.
Another species of Puccinia, found on the thistle, is shown on Plate
V. Fig. 7. Fig. 34 is the mould found upon decaying grapes, and called
therefrom, or from the clustered spores, Botrýtis. Some of the detached
spores are seen by its side. Fig. 35 is another species of the same
genus, termed Botrýtis parasítica, and is the cause of the well-known
“potato-disease.”
The mosses and ferns afford an endless variety of interesting objects
to the microscopist; but as their numbers are so vast, and the details
of their structure so elaborate, they can only be casually noticed in
the present work. Fig. 38 represents a spore-case of the Polypodium,
one of the ferns, as it appears while in the act of bursting and
scattering the contents around. One of the spores is seen more
magnified below. The spore-cases of many ferns may be seen bursting
under the microscope, and have a very curious appearance, writhing and
twisting like worms, and then suddenly filling the field with a cloud
of spores. Fig. 9, Plate V., is a piece of the brown, chaff-like, scaly
structure found at the base of the stalk of male fern cells, showing
the manner in which a flat membrane is formed. Fig. 39 is a capsule of
the Hypnum, one of the mosses, showing the beautiful double fringe with
which its edge is crowned. Fig. 2, Plate V., is the capsule of another
moss, Polytríchum, to show the toothed rim; on the right hand is one of
the teeth much more magnified.
Fig. 3, Plate V., is the capsule of the Jungermannia, one of the
liverworts, showing the “elaters” bursting out on every side, and
scattering the spores. Fig. 4 is a single elater much magnified,
showing it to be a spirally coiled filament, that, by sudden expansion,
shoots out the spores just as a child’s toy-gun discharges the arrow.
Fig. 5 is a part of the leaf of the Sphagnum moss, common in fresh
water, showing the curious spiral arrangement of secondary fibre which
is found in the cells, as well as the circular pores which are found
in each cell at a certain stage of growth. Just below, and to the
left hand, is a single cell greatly magnified, in order to show these
peculiarities more strongly. Fig. 8 is part of a leaf of Jungermannia,
showing the dotted cells.
Fig. 6, Plate V., is a part of a rootlet of moss, showing how it is
formed of cells elongated and joined end to end.
On the common mare’s-tail, or Equisétum, may be seen a very remarkable
arrangement for scattering the spores. On the last joint of the stem
is a process called a fruit-spike, being a pointed head around which
are set a number of little bodies just like garden-tables, with their
tops outward. One of these bodies is seen in Fig. 40. From the top of
the table depend a number of tiny pouches, which are called sporangia;
these lie closely against each other, and contain the spores. At the
proper moment these pouches burst from the inside, and fling out the
spores, which then look like round balls with irregular surfaces, as
shown in Fig. 40, _c_. This irregularity is caused by four elastic
filaments, knobbed at the end, which are originally coiled tightly
round the body of the spore, but by rapidly untwisting themselves cause
the spore to leap about, and so aid in the distribution. A spore with
uncoiled filaments is seen at Fig. 40, _b_. By breathing on them they
may be made to repeat this process at will.
Fig. 36 is a common little sea-weed, called Ectocarpus siliculósus,
that is found parasitically adhering to large plants, and is figured in
order to show the manner in which the extremities of the branches are
developed into sporangia. Fig. 37 is a piece of the common green laver,
Ulva latíssima, showing the green masses that are ultimately converted
into zoospores, and by their extraordinary fertility cause the plant to
grow with such rapid luxuriance wherever the conditions are favourable.
Every possessor of a marine aquarium knows how rapidly the glass sides
become covered with growing masses of this plant. The smaller figure
above is a section of the same plant, showing that it is composed of a
double plate of cellular tissue.
Fig. 41 is a piece of purple laver or “sloke,” Porphýra laciniáta, to
show the manner in which the cells are arranged in groups of four,
technically named “tetraspores.” This plant has only one layer of cells.
On Plate V. may be seen a number of curious details of the higher algæ.
Fig. 11 is the Sphacelária, so called from the curious capsule cells
found at the end of the branches, and termed sphacelæ. This portion
of the plant is shown more magnified in Fig. 12. Another sea-weed is
represented in Fig. 13, in order to show the manner in which the fruit
is arranged; and a portion of the same plant is given on a larger scale
at Fig. 14.
A very pretty little sea-weed called Cerámium is shown at Fig. 15; and
a portion showing the fruit much more magnified is drawn at Fig. 22.
Fig. 23 is a little alga called Myrionéma, growing parasitically on the
preceding plant.
Fig. 16 is a section of a capsule belonging to the Hálydris siliquósa,
showing the manner in which the fruit is arranged; and Fig. 17 shows
one of the spores more magnified.
Fig. 18 shows the Polysiphónia parasítica, a rather common species of
a very extensive genus of sea-weeds, containing nearly three hundred
species. Fig. 19 is a portion of the stem of the same plant, cut across
in order to show the curious mode in which it is built up of a number
of longitudinal cells, surrounding a central cell of large dimensions,
so that a section of this plant has the aspect of a rosette when placed
under the microscope. A capsule or “ceramídium” of the same plant is
shown at Fig. 20, for the purpose of exhibiting the pear-shaped spores,
and the mode of their escape from the parent-cell previous to their
own development into fresh plants. The same plant has another form
of reproduction, shown in Fig. 21, where the “tetraspores” are seen
imbedded in the substance of the branches. There is yet a third mode
of reproduction by means of “antheridia,” or elongated white tufts at
the extremities of the branches. The cells produced by these tufts
fertilise the rudimentary capsules, and so fulfil the function of the
pollen in flowering plants.
Fig. 25 is the Cladóphora, a green alga, figured to illustrate its mode
of growth; and Fig. 26 represents one of the red sea-weeds, Ptilóta
élegans, beautifully feathered, and with a small portion shown also
on a larger scale, in order to show its structure more fully. A good
contrast to this species is seen on Fig. 27, and the mode in which the
long, slender, filamentary fronds are built up of many-sided cells is
seen just to the left hand of the upper frond. Fig. 24 is a portion of
the lovely Delesséria sanguínea, given in order to show the formation
of the cells, as also the arrangement by which the indistinct nervures
are formed.
V.
FIG.
1. Rose Leaf, with fungus
2. Moss capsule, Polytrichum
3. Jungermannia, capsule
4. Do. an elater more magnified
5. Leaf of Moss, Sphagnum
6. Rootlet, Moss
7. Puccinia, from Thistle
8. Jungermannia, leaf
9. Scale from stalk of male fern
10. Uredo
11. Sphacelaria filicina
12. Do. top, more magnified
13. Seaweed, showing fruit
14. Do. fruit, more magnified
15. Ceramium
16. Capsule, Halidrys
17. Spore of do.
18. Polysiphonia parasitica
19. Do. stem, more magnified
20. Do. Capsule, tetraspores escaping
21. Do. fruit, another form
22. Ceramium, fruit
23. Myrionema, parasitic Seaweed
24. Delesseria sanguinea, Frond
25. Cladophora
26. Ptilota elegans
27. Enteromorpha clathrata
28. Nitophyllum laceratum
[Illustration: V.]
The figure on the bottom left-hand corner of Plate V. is a portion of
the pretty Nitophyllum lacerátum, a plant belonging to the same family
as the preceding one. The specimen here represented has a gathering
of spores upon the frond, in which state the frond is said to be “in
fruit.”
Fig. 27 represents a portion of the common sea-grass (_Enteromorpha_),
so common on rocks and stones between the range of high and low water.
On the left hand of the figure, and near the top, is a small piece of
the same plant much more magnified, in order to show the form of its
cells.
CHAPTER VII
Antennæ, their Structure and Use--Eyes, Compound and
Simple--Breathing Organs--Jaws and their Appendages--Legs, Feet,
and Suckers--Digestive Organs--Wings, Scales, and Hairs--Eggs
of Insects--Hair, Wool, Linen, Silk, and Cotton--Scales of
Fish--Feathers--Skin and its Structure--Epithelium--Nails, Bone, and
Teeth--Blood Corpuscles and Circulation--Elastic Tissues--Muscle and
Nerve.
We now take leave of the vegetables for a time, and turn our attention
to the animal kingdom.
On Plate VI. may be seen many beautiful examples of animal structures,
most of them being taken from the insect tribes. We will begin with the
antennæ, or horns, as they are popularly termed, of the insect.
The forms of these organs are as varied as those of the insects to
which they belong, and they are so well defined that a single antenna
will, in almost every instance, enable a good entomologist to designate
the genus to which the insect belonged. The functions of the antennæ
are not satisfactorily ascertained. They are certainly often used as
organs of speech, as may be seen when two ants meet each other, cross
their antennæ, and then start off simultaneously to some task which is
too much for a single ant. This pretty scene may be witnessed on any
fine day in a wood, and a very animated series of conversations may
readily be elicited by laying a stick across their paths, or putting a
dead mouse or large insect in their way.
I once saw a very curious scene of this kind take place at an ant’s
nest near Hastings. A great daddy long-legs had, unfortunately for
itself, settled on the nest, and was immediately “pinned” by an ant or
two at each leg, so effectually that all its struggles availed nothing.
Help was, however, needed, and away ran four or five ants in different
directions, intercepting every comrade they met, and by a touch of the
antennæ sending them off in the proper direction. A large number of the
wise insects soon crowded round the poor victim, whose fate was rapidly
sealed. Every ant took its proper place, just like a gang of labourers
under the orders of their foreman; and by dint of pushing and pulling,
the long-legged insect was dragged to one of the entrances of the nest,
and speedily disappeared.
Many of the ichneumon-flies may also be seen quivering their antennæ
with eager zeal, and evidently using them as feelers, to ascertain the
presence of the insect in which they intend to lay their eggs; and many
other similar instances will be familiar to anyone who has been in the
habit of watching insects and their ways.
It is, however, most likely that the antennæ serve other purposes than
that which has just been mentioned, and many entomologists are of
opinion that they serve as organs of hearing.
Fig. 15, Plate VI., represents a part of one of the joints belonging to
the antennæ of the common house-fly; it is seen to be covered with a
multitude of little depressions, some being small, and others very much
larger. A section of the same antenna, but on a larger scale, is shown
by Fig. 16, in order to exhibit the real form of these depressions.
Nerves have been traced to these curious cavities, which evidently
serve some very useful purpose, some authors thinking them to belong to
the sense of smell, and others to that of hearing. Perhaps they may be
the avenues of some sensation not possessed by the human race, and of
which we are therefore ignorant. Fig. 17 represents a section of the
antennæ of an ichneumon-fly, to show the structure of these organs of
sense.
We will now glance cursorily at the forms of antennæ which are depicted
in the Plate.
Fig. 1 is the antenna of the common cricket, which consists of a vast
number of little joints, each a trifle smaller than the preceding one,
the whole forming a long, thread-like organ. Fig. 2 is taken from the
grasshopper, and shows that the joints are larger in the middle than at
either end.
Figs. 3 and 5 are from two minute species of cocktailed beetles
(_Staphylínidæ_), which swarm throughout the summer months, and even in
the winter may be found in profusion under stones and moss. The insect
from which Fig. 5 was taken is so small that it is almost invisible
to the naked eye, and was captured on the wing by waving a sheet of
gummed paper under the shade of a tree. These are the tiresome little
insects that so often get into the eye in the summer, and cause such
pain and inconvenience until they are removed.
Fig. 4 shows the antenna of the tortoise beetle (_Cássida_), so common
on many leaves, and remarkable for its likeness to the reptile from
which it derives its popular name. Fig. 3 is from one of the weevils,
and shows the extremely long basal joint of the antennæ of these
beetles, as well as the clubbed extremity. Fig. 7 is the beautifully
notched antenna of the cardinal beetle (_Pyrochróa_), and Fig. 11 is
the fan-like one of the common cockchafer. This specimen is taken from
a male insect, and the reader will find his trouble repaid on mounting
one of these antennæ as a permanent object.
Fig. 12 is an antenna from one of the common ground beetles (_Cárabus_)
looking like a string of elongated pears, from the form of the joints.
The reader will see that in beetles he is sure to find eleven joints in
the antennæ.
Fig. 10 is the entire antenna of a fly (_Syrphus_), one of those pretty
flies which may be seen hovering over one spot for a minute, and then
darting off like lightning to hang over another. The large joint is the
one on which are found those curious depressions that have already been
mentioned. Fig. 8 is one of the antennæ of a tortoise-shell butterfly
(_Vanessa_), showing the slender, knobbed form which butterfly
antennæ assume; and Figs. 13 and 14 are specimens of moths’ antennæ,
showing how they always terminate in a point. Fig. 13 is the beautiful
feathery antenna of the ermine moth (_Spilosóma_); and Fig. 14 is the
toothed one of the tiger moth (_Arctia caja_). In all these feathered
and toothed antennæ of moths, the male insects have them much more
developed than the female, probably for the purpose of enabling them
to detect the presence of their mates, a property which some possess
in wonderful perfection. The male oak-egger moth, for example, can be
obtained in any number by putting a female into a box with a perforated
lid, placing the box in a room, and opening the window. In the course
of the evening seven or eight males are seen to make their appearance,
and they are so anxious to get at their intended mate that they will
suffer themselves to be taken by hand.
Fig. 9 is an antenna of the male gnat, a most beautiful object,
remarkable for the delicate transparency of the joints, and the
exquisitely fine feathering with which they are adorned.
We now arrive at the eyes of the insects, all of which are very
beautiful, and many singularly full of interest.
In the centre of Plate VI. may be seen the front view of the head
of a bee, showing both kinds of eyes, three simple eyes arranged
triangularly in the centre, and two large masses, compound eyes, at the
sides.
The simple eyes, termed “ocelli,” are from one to three in number, and
usually arranged in a triangular form between the two compound eyes.
Externally they look merely like shining rounded projections, and can
be seen to great advantage in the dragon-flies. The compound eyes may
be considered as aggregations of simple eyes, set closely together,
and each assuming a more or less perfect six-sided form. Their number
varies very greatly; in some insects, such as the common fly, there are
about four thousand of these simple eyes in one compound one, in the
ant only fifty, in the dragon-fly about twelve thousand, and in one of
the beetles more than twenty-five thousand.
Fig. 18 shows a portion of the compound eye of the Atalanta butterfly,
and Fig. 20 the same organ of the death’s-head moth. A number of
the protecting hairs may be seen still adhering to the eye of the
butterfly. Fig. 22 is a remarkably good specimen of the eye of a fly
(_Helióphilus_), showing the facets, nearly square, the tubes to
which they are attached, and portions of the optic nerves. Fig. 23
is part of the compound eye of a lobster, showing the facets quite
square. All these drawings were taken by the camera lucida from my own
preparations, so that I can answer for their authenticity.
On Plate VIII. Figs. 6 and 12, the reader will find two more examples
of eyes, these being taken from the spiders. Fig. 6 is an example of
the eight eyes of the well-known zebra spider, so common on our garden
walls and similar situations, hunting incessantly after flies and other
prey, and capturing them by a sudden pounce. The eyes are like the
ocelli of insects, and are simple in their construction. The number,
arrangement, and situation of the eyes is extremely varied in spiders,
and serves as one of the readiest modes of distinguishing the species.
Fig. 12, Plate VIII., represents one of the curious eyes of the common
harvest spider, perched on a prominence or “watch-tower” (as it has
been aptly named), for the purpose of enabling the creature to take a
more comprehensive view of surrounding objects.
* * * * *
Returning to Plate VI., in Fig. 21 we see a curiously branched
appearance, something like the hollow root of a tree, and covered with
delicate spiral markings. This is part of the breathing apparatus of
the silkworm, extracted and prepared by myself for the purpose of
showing the manner in which the tubes branch off from the “spiracle”
or external breathing-hole, a row of which may be seen along the
sides of insects, together with the beautiful spiral filament which
is wound round each tube for the purpose of strengthening it. One of
these spiracles may be seen in the neck of the gnat (Fig. 27). Another
spiracle, more enlarged, may be seen on Plate VII. Fig. 34, taken from
the wireworm, _i.e._ the larva of the skipjack beetle (_Eláter_), to
show the apparatus for excluding dust and admitting air. The object of
the spiral coil is very evident, for as these breathing-tubes extend
throughout the whole body and limbs, they would fail to perform their
office when the limbs were bent, unless for some especial provision.
This is achieved by the winding of a very strong but slender filament
between the membranes of which the tube is composed, so that it always
remains open for the passage of air throughout all the bends to which
it may be subjected. Flexible tubes for gas and similar purposes are
made after the same fashion, spiral metal wire being coiled within
the india-rubber pipe. A little piece of this thread is seen unwound
at the end of a small branch towards the top, and this thread is so
strong that it retains its elasticity when pulled away from the tube,
and springs back into its spiral form. I have succeeded in unwinding
a considerable length of this filament from the breathing-tube of a
humble bee.
Fig. 28 represents the two curious tubercles upon the hinder quarters
of the common green-blight, or Aphis, so very common on our garden
plants, as well as on many trees and other vegetables. From the tips of
these tubercles exudes a sweet colourless fluid, which, after it has
fallen upon the leaves, is popularly known by the name of honey-dew.
Ants are very fond of this substance, and are in the habit of haunting
the trees upon which the aphides live, for the purpose of sucking the
honey-dew as it exudes from their bodies. A drop of this liquid may be
seen on the extremity of the lower tubercle.
The head of the same insect may be seen in Fig. 24, where the reader
may observe the bright scarlet eye, and the long beak with which the
aphis punctures the leaves and sucks the sap. Fig. 29 is the head of
the sheep-tick, exhibiting the organ by which it pierces the skin of
the creature on which it lives. Fig. 25 is the head of another curious
parasite found upon the tortoise, and remarkable for the powerful
hooked apparatus which projects in front of the head.
Turning to Plate VII. Fig. 4, we find the head of a ground beetle
(_Cárabus_), valuable as exhibiting the whole of the organs of the head
and mouth.
Immediately above the compound eyes are seen the roots of the antennæ,
those organs themselves being cut away. Above there are two pairs of
similarly constructed organs termed the “maxillary palpi,” because
they belong to the lesser jaws or maxillæ, seen just within the pair
of great curved jaws called the mandibles, which are extended in so
threatening a manner. The “labial palpi,” so called because they
belong to the “labium,” or under lip, are seen just within the others;
the tongue is seen between the maxillæ, and the chin or “mentum”
forms a defence for the base of the maxillæ and the palpi. A careful
examination of a beetle’s mouth with the aid of a pocket lens is very
instructive as well as interesting.
Fig. 1 on the same Plate shows the jaws of the hive bee, where the same
organs are seen modified into many curious shapes. In the centre may be
seen the tongue, elongated into a flexible and hair-covered instrument,
used for licking the honey from the interior of flowers. At each side
of the tongue are the labial palpi, having their outermost joints very
small, and the others extremely large, the latter acting as a kind
of sheath for the tongue. Outside the labial palpi are the maxillæ,
separated in the specimen, but capable of being laid closely upon each
other, and outside all are the mandibles.
VI.
FIG.
1. Antenna, Cricket
2. Do. Grasshopper
3. Do. Staphylinus
4. Do. Cassida
5. Do. Staphylinus
6. Do. Weevil
7. Do. Pyrochroa
8. Do. Butterfly, Tortoiseshell
9. Do. Gnat, male
10. Do. Syrphus
11. Do. Cockchafer, male
12. Do. Ground Beetle
13. Do. Ermine Moth
14. Do. Tiger Moth
15. Do. Blowfly
16. Do. do. section
17. Do. Ichneumon
18. Eye of Butterfly, Atalanta
19. Eyes, Bee
20. Eye, Death’s Head Moth
21. Breathing-tube, Silkworm
22. Eye, Heliophilus
23. Do. Lobster
24. Do. Aphis of Geranium
25. Head, Parasite of Tortoise
26. Hind leg, Aphis of Geranium
27. Head, Gnat
28. “Paps” of Aphis
29. Head, Sheep-tick
30. Foot, Tipula
[Illustration: VI.]
The curiously elongated head of the scorpion-fly (_Panorpa_), seen
at Fig. 7, affords another example of the remarkable manner in which
these organs are developed in different insects. Another elongated
head, belonging to the daddy long-legs, is seen in Plate VI. Fig. 27,
and well shows the compound eyes, the antennæ, and the palpi. Fig. 2
represents the coiled tongue of the Atalanta butterfly; it is composed
of the maxillæ, very greatly developed, and appearing as if each
had originally been flat, and then rolled up so as to make about
three-fourths of a tube. A number of projections are seen towards the
tip, and one of these little bodies is shown on a larger scale at
Fig. 3. These curious organs have probably some connection with the
sense of taste. Along the edges of the semi-tubes are arranged a number
of very tiny hooks, by means of which the insect can unite the edges at
will.
Fig. 11, in the centre of the Plate, shows one of the most curious
examples of insect structure, the proboscis or trunk of the common
bluebottle-fly. The maxillary palpi covered with bristles are seen
projecting at each side, and upon the centre are three lancet-like
appendages, two small and one large, which are used for perforating
various substances on which the insect feeds. The great double disc
at the end is composed of the lower lip greatly developed, and is
filled with a most complex arrangement of sucking-tubes, in order to
enable it to fulfil its proper functions. The numerous tubes which
radiate towards the circumference are strengthened by a vast number
of partial rings of strong filamentary substance, like that which we
have already seen in the breathing-tube of the silkworm. Some of these
partial rings are seen on Fig. 12, a little above. The mode in which
the horny matter composing the rings is arranged upon the tubes is most
wonderful, and requires a tolerably high power to show it. The fine
hairs upon the proboscis itself afford most admirable practice for the
young microscopist. They should, when properly lighted and focused, be
quite black and sharp. Any errors of manipulation will cause them to be
“fuzzy.”
Fig. 5 shows the tongue of the common cricket, a most elegantly formed
organ, having a number of radiating bands covered with zigzag lines,
due to the triangular plates of strengthening substance with which they
are furnished, instead of the rings. A portion more highly magnified
is shown at Fig. 6, exhibiting the manner in which the branches are
arranged.
* * * * *
The legs of insects now claim our attention.
Fig. 9, Plate VII., shows the “pro-leg” of a caterpillar. The pro-legs
are situated on the hinder parts of the caterpillar, and, being set in
pairs, take a wonderfully firm hold of a branch or twig by pressure
toward each other. Around the pro-legs are arranged a series of sharp
hooks, set with their points inwards, for greater power in holding.
Fig. 10 represents one of the hooks more magnified.
Fig. 15 is the lower portion of the many-jointed legs of the
long-legged spider (_Phalángium_), the whole structure looking very
like the antenna of the cricket. Fig. 17 is the leg of the glow-worm,
showing the single claw with which it is armed. Fig. 26 shows the foot
of the flea, furnished with two simple claws. Fig. 16 is the foot of
the Trombídium, a genus of parasitic creatures, to which the well-known
harvest-bug belongs. Fig. 26, Plate VI., shows the leg of the green
Aphis of the geranium, exhibiting the double claw, and the pad or
cushion, which probably serves the same purpose as the pads found upon
the feet of many other insects. Fig. 8 is the lower portion of the leg
of the ant, showing the two claws and the curious pad in the centre,
by means of which the insect is able to walk upon slippery surfaces.
The Típula has a foot also furnished with a single pad (see Plate VI.
Fig. 30). This organ is seen under a very high power to be covered with
long hair-like appendages, each having a little disc at the end, and
probably secreting some glutinous fluid which will enable the creature
to hold on to perpendicular and smooth surfaces. Many of my readers
will doubtless have noticed the common fly, towards the end of autumn,
walking stiffly upon the walls, and evidently detaching each foot with
great difficulty, age and infirmity having made the insect unable to
lift its feet with the requisite force.
Fig. 21 is the foot of one of the ichneumon-flies (_Ophíon_), the
hairy fringe being apparently for the purpose of enabling it to hold
firmly to the caterpillar in which it is depositing its eggs, and
which wriggles so violently under the infliction that it would soon
throw its tormentor had not some special means been provided for the
purpose of enabling the latter to keep its hold. Fig. 20 is a beautiful
example of a padded foot, taken from the little red parasitic creature
so plentifully found upon the dor or dung beetle (_Geotrúpes_), and of
which the afflicted insect is said to rid itself by lying on its back
near an ant’s nest, and waiting until the ants carry off its tormentors.
Fig. 18 is the foot of the common yellow dung-fly (plentiful in pasture
lands), having two claws and two pads; and Fig. 19 shows the three pads
and two claws found in the foot of the hornet-fly (_Ásilus_).
Few microscopic objects call forth such general and deserved admiration
as the fore-foot of the male water-beetle (_Dytiscus_), when properly
prepared and mounted, for which see Fig. 13.
On examining this preparation under the microscope, it is seen that
three of the joints are greatly expanded, and that the whole of their
under surface is covered profusely with certain wonderful projections,
which are known to act as suckers. One of them is exceedingly large,
and occupies a very considerable space, its hairs radiating like the
rays of the heraldic sun. Another is also large, but scarcely half
the diameter of the former, and the remainder are small, and mounted
on the extremities of delicate foot-stalks, looking something like
wide-mouthed trumpets. In the specimen from which the drawing was taken
the smaller suckers are well shown, as they protrude from the margin of
the foot.
One of the larger suckers is seen more magnified on Fig. 14.
Plate VIII. Fig. 1, exemplifies the manner in which the muscles of
insects do their work, being well attached in the limbs to the central
tendon, and pulling “with a will” in one direction, thus giving very
great strength. This leg is taken from the water boatman (_Notonecta_),
and has been mounted in Canada balsam.
On Plate VII. Fig. 29, may be seen a curiously formed creature. This
is the larva of the tortoise beetle (_Cássida_), the skin having been
flattened and mounted in Canada balsam. The spiracles are visible
along the sides, and at the end is seen a dark fork-like structure.
This is one of the peculiarities of this creature, and is employed for
the purpose of carrying the refuse of its food, which is always piled
upon its back, and retained in its place by the forked spines, aided
probably by the numerous smaller spines that project from the side.
Fig. 33 shows part of the stomach and gastric teeth of the grasshopper.
This structure may be seen to perfection in the “gizzard,” as it is
called, of the great green locust of England (_Ácrida viridíssima_).
The organ looks like a sudden swelling of the œsophagus, and when slit
longitudinally under water, the teeth may be seen in rows set side by
side, and evidently having a great grinding power. The common house
cricket has a similar organ of remarkable beauty. Just above (Fig. 27)
is the corresponding structure in the hive bee, three of the teeth
being shown separately at Fig. 28.
* * * * *
We now cast a rapid glance at the wings of insects.
They have no analogy, except in their use, with the wings of birds, as
they are not modifications of existing limbs, but entirely separate
organs. They consist of two membranes united at their edges, and
traversed and supported by sundry hollow branches or “nervures,” which
admit air, and serve as useful guides to entomologists for separating
the insects into their genera. Indeed, the general character of the
wings has long been employed as the means of dividing the insect race
into their different orders, as may be seen in any work on entomology.
The typical number of wings is four, but it often happens that two are
almost wholly absent, or that the uppermost pair are thickened into a
shelly kind of substance which renders them useless for flight; while
in many insects, such as the ground beetles and others, the upper wings
become hardened into firm coverings for the body, and the lower pair
are shrivelled and useless.
Fig. 22 shows two of the wings of a humble bee, together with their
nervures, and the peculiar system by which the upper and lower pair are
united together at the will of the insect. At the upper edge of the
lower wing, and nearly at its extremity, may be seen a row of very tiny
hooks, shown on a larger scale at Fig. 25. These hooklets hitch into
the strengthened membrane of the upper wing, which is seen immediately
above them, and so conjoin the two together. The curious wing-hooks of
the Aphis may be seen on Fig. 24, very highly magnified.
Fig. 31 is the wing of the midge (_Psychóda_), that odd little insect
which is seen hopping and popping about on the windows of outhouses
and similar localities, and is so hard to catch. The whole wing is
plentifully covered with elongated scales, and is a most lovely object
under any power of the microscope. These scales run along the nervures
and edges of the wings, and part of a nervure is shown more highly
magnified at Fig. 32.
At Fig. 23 is shown the wing of one of the hemipterous insects, common
along the banks of ditches and in shady lanes, and known by the name
of Cíxius. It is remarkable for the numerous spots which stud the
nervures, one being always found at each forking, and the others being
very irregularly disposed.
Fig. 30 is one of the balancers or “haltéres” of the house-fly. These
organs are found in all the two-winged insects, and are evidently
modifications of the second pair of wings. They are covered with
little vesicles, and protected at their base by scales. Some writers
suppose that the sense of smell resides in these organs. Whatever other
purpose they may serve, they clearly aid in the flight, as, if the
insect be deprived of one or both of the balancers, it has the greatest
difficulty in steering itself through the air.
The wings of insects are mostly covered with hairs or scales, several
examples of which are given in Plate VIII. Fig. 4 shows one of the
scales of the Adippe or fritillary butterfly, exhibiting the double
membrane--part of which has been torn away--and the beautiful lines of
dots with which it is marked. The structure of the scales is further
shown by a torn specimen of tiger moth scale seen on Fig. 16. On
many scales these dots assume a “watered” aspect when the focus or
illumination changes, an example of which may be seen in Fig. 15, a
scale of the peacock butterfly.
Fig. 11 is one of the ordinary scales of the azure blue butterfly,
and Fig. 10 shows one of the curious “battledore” scales of the same
insect, with its rows of distinct dottings. Fig. 14 is one of the
prettily tufted scales of the orange-tip butterfly, and Fig. 8 is the
splendid branched scale of the death’s-head moth. Fig. 19 shows a scale
of the sugar-runner (_Lepisma saccharína_), a little silvery creature
with glistening skin, and long bristles at the head and tail, that is
found running about cupboards, window-sills, and similar places. It
is not easy to catch with the fingers, as it slips through them like
oil; but by holding a cover-glass in a pair of forceps, and pressing it
upon one of the little creatures, a number of the scales may be caused
to adhere to it, and these should be mounted dry for examination. The
gnats also possess very pretty scales, with the ribs projecting beyond
the membrane.
VII.
FIG.
1. Tongue, Hive Bee
2. Do. Tortoiseshell Butterfly
3. Do. do. one of the barrel-shaped bodies
4. Head, Violet Ground Beetle (Carabus)
5. Tongue, Cricket
6. Do. do.
7. Head, Scorpion Fly (Panorpa)
8. Leg, Ant
9. Proleg, Caterpillar
10. Do. do. single hook
11. Proboscis, Fly
12. Do. do. “modified trachea”
13. Part of Foreleg of Water Beetle (Acilius)
14. Do. large sucker
15. Leg, long-legged Spider (Phalangium)
16. Do. Harvest-bug (Trombidium)
17. Do. Glow-worm
18. Do. Dung fly
19. Do. Asilus
20. Do. Acarus of Dor-beetle
21. Claws and Pad, Ophion
22. Wings, Humble Bee
23. Do.
24. Wing hooks, hind wing of Aphis
25. Wing hooks, Humble Bee
26. Foot, Flea
27. Stomach and gastric teeth, Bee
28. Three teeth of do.
29. Cast skin, Larva of Tortoise Beetle (Cassida)
30. Balancer, Blow fly
31. Wing, Midge (Psychoda)
32. Do. do. part of a nervure with scales
33. Stomach and gastric teeth, Grasshopper
34. Spiracle, Wire-worm
[Illustration: VII.]
Fig. 21 is a scale from the common spring-tail (_Podúra plúmbea_), a
little creature which is found plentifully in cellars and other damp
places, skipping about with great activity. Some flour scattered on a
piece of paper is a sure trap for these little beings. Fig. 3 is one
of the scales taken from the back of the celebrated diamond beetle,
showing the cause of the magnificent gem-like aspect of that insect. We
have in England many beetles of the same family--the weevils--which,
although much smaller, are quite as splendid when exhibited under a
microscope by reflected light. The wing-case or “elytron” of a little
green weevil, very common in the hedges, may be seen on Plate XII.
Fig. 10.
The reader will observe that all these scales are furnished with little
root-like appendages, by means of which they are affixed to the insect.
Fig. 13 shows a portion of the wing of the azure blue butterfly, from
which nearly all the scales have been removed, for the purpose of
exhibiting the pits or depressions in which they had formerly been
fastened, and one or two of the scales are left still adherent to their
places. The scales are arranged in equal rows like the slates of a
housetop, as may be seen on Fig. 18, which represents part of the same
wing, to show the scales overlapping each other, and the elegant form
which they take near the edges of the wing, so as to form a delicate
fringe. The long hair-like down which covers the legs and bodies of the
moths and butterflies (which are called Lepidóptera, or scale-winged
insects, in consequence of this peculiarity), is seen under the
microscope to be composed of scales very much elongated, as is shown in
Fig. 17, a portion taken from the leg of a tiger moth.
* * * * *
The eggs of insects are all very beautiful, and three of the most
curious forms are given on Plate VIII.
Fig. 2 is the empty egg of the gad-fly, as it appears when fastened to
a hair of the horse. Fig. 5 represents the pretty ribbed egg of the
common tortoise-shell butterfly; and Fig. 7 is the very beautiful egg
of the very horrid bed-bug, worthy of notice on account of the curious
lid with which its extremity is closed, by means of which the young
larva creeps out as soon as it is hatched.
The feathers of birds, and the fur of animals, will furnish many
examples of the eggs of parasites, some of which are of extreme beauty.
The feather or hair may be mounted in a cell without disturbing the
eggs, which should, however, be heated sufficiently to kill the embryo
if present.
Fig. 9 shows the penetrating portions of the sting of the wasp. The
two barbed stings, which seem to be the minute prototypes of the
many-barbed spears of the South Sea islanders, are seen lying one at
each side of their sheath, and a single barb is drawn a little to the
left on a very much larger scale. It is by reason of these barbs that
the sting is always left adhering to the wound, and is generally drawn
wholly out of the insect, causing its death in a short while.
The sting is only found in female insects, and is supposed to be
analogous to the “ovipositor” of other insects, _i.e._ the instrument
by which the eggs are deposited in their places. Fig. 20 shows the
curious egg-placing apparatus of one of the saw-flies. The backs of
these “saws” work in grooves, and they work alternately, so that the
fly takes but a very short time in cutting a slit in the young bark
of a tender shoot, and laying her eggs in the slit. When she has
completed one of these channels, she sets to work upon another, and in
the early spring the young branches of the gooseberry bushes may be
seen plentifully covered with these grooves and the eggs. When hatched,
black caterpillar-like grubs from the eggs issue, and devastate the
bushes sadly, turning in process of time into blackish flies, which are
seen hovering in numbers over the gooseberries, and may be killed by
thousands.
* * * * *
The scales and hairs of other animals deserve great attention. Fig. 23
is a single hair of the human beard, as it often appears when tied in
a knot--by Queen Mab and her fairies, according to Mercutio. Fig. 22
is a portion of the same hair as it appears when splitting at its
extremity. The structure of the hair is not, however, so well seen in
this object as in that represented on Fig. 24, which is a beautiful
example of white human hair that once adorned the head of the victor
of Waterloo. It formed one of a tiny lock given to me by a friend, and
is so admirable an example of human hair, that I forthwith mounted
it for the microscope. In this hair the cells may be seen extending
down its centre, and the peculiar roughened surface produced by the
flattened cells which are arranged around its circumference are also
seen. By steeping in caustic potash, these scales can be separated,
but generally they lie along the hair in such a manner that if the
hair be drawn through the fingers from base to point, their projecting
ends permit it to pass freely; whilst if it be drawn in the reverse
direction, they cause it to feel very harsh to the touch.
In the sheep’s wool (Fig. 30) this structure is much more developed,
and gives to the fibres the “felting” power that causes them to
interlace so firmly with each other, and enables cloth--when really
made of wool--to be cut without unravelling. Fig. 37 is the smooth hair
of the badger; and Fig. 34 is the curious hair of the red deer, which
looks as if it had been covered with a delicate net.
Fig. 28 is the soft, grey, wool-like hair of the rat; and Fig. 29
is one of the larger hairs that protrude so plentifully, and form
the glistening brown coat of that animal. Fig. 38 is the curiously
knobbed hair of the long-eared bat, the knobs being formed of
protuberant scales that can easily be scraped off. Fig. 31 shows a
hair of the common mole; and Fig. 32 is one of the long hairs of the
rabbit. Fig. 27 is a flat hair of the dormouse, slightly twisted, the
difference in the breadth showing where the twist has taken place. The
hair of the mouse is beautifully ribbed, so as to look like a ladder.
Fig. 26 is one of the very long hairs that so thickly clothe the tiger
moth caterpillar; and Fig. 25 is a beautifully branched hair taken from
the common humble bee.
All hairs should be examined by polarised light, with a plate of
selenite, when most gorgeous colour effects may be obtained.
The four fibres mostly used in the manufacture of apparel are: wool,
Fig. 30, which has already been described; linen, Fig. 39; cotton,
Fig. 40; and silk, Fig. 41. The structure of each is very well marked
and easily made out with the microscope; so that an adulterated article
can readily be detected by a practised eye. Cotton is the most common
adulteration of silk and linen fabrics, and may at once be detected
by its flat twisted fibre. Silk is always composed of two parallel
threads, each proceeding from one of the spinnerets of the caterpillar,
and it may be here remarked that if these threads are not quite
parallel the silk is of bad quality. Silken fibre is always covered,
when new, with a kind of varnish, usually of a bright orange colour,
which gives the undressed “floss” silk its peculiar hue, but which is
soluble and easily washed away in the course of manufacture.
Figs. 35 and 36 are the small and large hairs of that magnificent
creature, the sea mouse (_Aphrodíte aculeáta_), whose covering,
although it lies in the mud, glows with every hue of the rainbow, and
in a brilliant light is almost painfully dazzling to the eye.
VIII.
FIG.
1. Boat-fly, leg
2. Gadfly, empty egg
3. Diamond Beetle, scale
4. Scale, Fritillary, Adippe
5. Egg, Tortoiseshell Butterfly
6. Head and Eyes, Zebra Spider
7. Eyes, Bed-Bug
8. Scale, Death’s-Head Moth
9. Sting, Wasp
10. Scale, battledore, Azure blue
11. Do. ordinary scale
12. Eye, Harvest Spider
13. Wing Membrane, Azure Blue
14. Scale, Anthocera cardaminis
15. Do. Peacock Butterfly
16. Do. Tiger Moth
17. Do. Thigh of Tiger Moth
18. Wing and Scales, Azure Blue
19. Scale, Lepisma
20. Saws, Sawfly
21. Scale, Podura
22. Hair, Black Human
23. Do. Human Beard
24. Do. do. aged
25. Do. Humble Bee
26. Do. Tiger Moth, Larva
27. Do. Dormouse
28. Do. Rat
29. Do. do. long hair
30. Do. Sheep
31. Do. Mole
32. Do. Rabbit
33. Scale, Greenbone Pike
34. Hair, Red Deer
35. Do. fine, Sea Mouse
36. Do. do. large
37. Do. do. Badger
38. Do. do. long-eared Bat
39. Fibre, Linen
40. Do. Cotton
41. Do. Silk
42. Scale, Perch
43. Do. do.
[Illustration: VIII.]
The scales of some of the fishes are shown on Plate VIII., in order
to exhibit their mode of growth by successive layers. The scales are
always enveloped in membranous sacs, and in some cases, as in the
eel, they do not project beyond the surface, and require some little
observation to detect them. A scale of an eel is shown on Plate XI.
Fig. 14, and is a magnificent object under polarised light. Fig. 33
is a scale of the greenbone pike; and Figs. 42 and 43 are scales of
the perch, showing the roots by which they are held in their places.
The roach, dace, bleak, and many other similar fish have a beautiful
silvery substance on the under surface of the scales, which was greatly
used in the manufacture of artificial pearls, glass beads being thinly
coated in the interior with the glittering substance, and then filled
in with wax. A piece of sole-skin, when preserved in Canada balsam and
placed under the microscope, is a very beautiful object.
More examples of hairs, and other processes from the skin, together
with the structure of the skin itself, of bone, of blood, and the mode
in which it circulates, are given on Plate X.
In all important points of their structure the feathers of birds
are similar to the hairs of animals, and are developed in a similar
manner. They are all composed of a quill portion, in which the pith
is contained, and of a shaft, which carries the vane, together with
its barbs. The form of each of these portions varies much, even in
different parts of the same bird, and the same feather has almost
always two kinds of barbs; one close and firm, and the other loose,
floating, and downy. If a small feather be plucked from the breast
or back of a sparrow or any other small bird, the upper part of the
feather is seen to be close and firm, while the lower is loose and
downy, the upper part being evidently intended to lie closely on the
body and keep out the wet, while the lower portion affords a soft and
warm protection to the skin.
Fig. 12, Plate X., shows the feather of a peacock, wherein the barbs
are very slightly fringed and lie quite loosely side by side. Fig. 18
is part of the same structure, in a duck’s feather, wherein are seen
the curious hooks which enable each vane to take a firm hold of its
neighbour, the whole feather being thus rendered firm, compact, and
capable of repelling water. The reader will not fail to notice the
remarkable analogy between these hooks and those which connect the
wings of the bee.
Fig. 17 is a part of the shaft of a young feather taken from the
canary, given for the purpose of showing the form of the cells
of which the pith is composed. Fig. 20 is part of the down from a
sparrow’s feather, showing its peculiar structure; and Fig. 21 is a
portion of one of the long drooping feathers of the cock’s tail.
Fig. 13 exhibits a transverse section of one of the large hairs or
spines from the hedgehog, and shows the disposition of the firm,
horn-like exterior, and the arrangement of the cells. Sections of
various kinds of hair are interesting objects, and are easily made by
tying a bundle of them together, soaking them in gum, hardening in
spirit, and then cutting thin slices with a razor. A little glycerine
will dissolve the gum, and the sections of hair will be well shown.
Unless some such precaution be taken, the elasticity of the hair will
cause the tiny sections to fly in all directions, and there will be no
hope of recovering them.
Several examples of the skin are also given. Fig. 27 is a section
through the skin of the human finger, including the whole of one of the
little ridges which are seen upon the extremity of every finger, and
half of two others. The cuticle, epidermis, or scarf-skin, as it is
indifferently termed, is formed of cells or scales, much flattened and
horny in the upper layers, rounder and plumper below. The true skin,
or “cutis,” is fibrous in structure, and lies immediately beneath, the
two together constituting the skin, properly so called. Beneath lies a
layer of tissue filled with fatty globules, and containing the glands
by which the perspiration is secreted.
One of the tubes or channels by which these glands are enabled to
pour their contents to the outside of the body, and, if they be kept
perfectly clean, to disperse them into the air, is seen running up
the centre of the figure, and terminating in a cup-shaped orifice on
the surface of the cuticle. On the palm of the hand very nearly three
thousand of these ducts lie within the compass of a square inch, and
more than a thousand in every square inch of the arm and other portions
of the body, so that the multitude of these valuable organs may be well
estimated, together with the absolute necessity for keeping the skin
perfectly clean in order to enjoy full health.
Fig. 1 shows a specimen of epidermis taken from the skin of a frog,
exhibiting the flattened cells which constitute that structure, and
the oval or slightly elongated nuclei, of which each cell has one. In
Fig. 32, a portion of a bat’s wing, the arrangement of the pigment
is remarkably pretty. Immediately above, at Fig. 31, is some of the
pigment taken from the back of the human eye-ball. The shape of the
pigment cells is well shown. Similar specimens may easily be obtained
from the back of a sheep’s eye which has been hardened in spirit, or
from that of a boiled fish. Fig. 33 shows the pigment in the shell of
the prawn.
* * * * *
On various parts of animal structures, such as the lining of internal
cavities, the interior of the mouth, and other similar portions of
the body, the cells are developed into a special form, which is
called “Epithélium,” and which corresponds to the epidermis of the
exterior surface of the body. The cells which form this substance are
of different shapes, according to their locality. On the tongue, for
example (for which see Fig. 11), they are flattened, and exhibit their
nucleus, in which the nucléolus may be discovered with a little care.
Cells of this kind are rounded, as in the case just mentioned, or
angular, and in either case they are termed squamous (_i.e._, scaly)
epithelium. Sometimes they are like a number of cylinders, cones, or
pyramids, ranged closely together, and are then called cylindrical
epithelium. Sometimes the free ends of cylindrical epithelium are
furnished with a number of vibrating filaments or cilia, and in this
case the structure is called “ciliated” epithelium. Cylindrical
epithelium may be found in the ducts of the glands which open into
the intestines, as well as in the glands that secrete tears; and
ciliated epithelium is seen largely in the windpipe, the interior of
the nose, etc. A specimen taken from the nose is seen at Fig. 15. A
beautiful example of ciliated epithelium is to be found in the gills
of the mussel. A portion of one of the yellowish bands which lie along
the edge of the shell on the opening side is carefully removed with
sharp scissors, and examined in the shell-liquor, being protected
from pressure by placing a piece of paper beneath each end of the
cover-glass. Such a preparation is shown in Plate IX. Fig. 39, but no
drawing can give an idea of its wonderful beauty and interest. The
cilia will continue to move for a long time after removal from the
shell.
* * * * *
Bone in its various stages is figured on Plate X.
Fig. 9 is a good example of human bone, and is a thin transverse
section taken from the thigh. When cut across, bone exhibits a whitish
structure filled with little dottings that become more numerous towards
the centre, and are almost invisible towards the circumference. In the
centre of the bone there is a cavity, which contains marrow in the
mammalia and air in the birds. When placed under a microscope, bone
presents the appearance shown in the illustration.
The large aperture in the centre is one of innumerable tubes that run
along the bone, and serve to allow a passage to the vessels which
convey blood from one part of the bone to another. They are technically
called Haversian canals, and if a longitudinal section be made they
will be found running tolerably parallel, and communicating freely
with each other. Around each Haversian canal may be seen a number of
little black spots with lines radiating in all directions, and looking
something like flattened insects. These are termed bone-cells or
“lacúnæ,” and the little black lines are called “canalículi.” In the
living state they contain cells which are concerned in the growth of
the bone, and these may be made evident by softening fresh bone with
acid, cutting sections of it, and staining. When viewed by transmitted
light the lacunæ and canaliculi are black; but when seen by dark-field
illumination the Haversian canals become black, and the lacunæ are
white.
As these canaliculi exist equally in every direction, it is impossible
to make a section of bone without cutting myriads of them across; and
when a high power is employed they look like little dots scattered over
the surface. A very pretty object can be made of the bone taken from a
young animal which has been fed with madder, as the colour gets into
the bone and settles chiefly round the Haversian canal. A young pig is
a very good subject, so is a rabbit.
Fig. 16 is a similar section cut from the leg-bone of an ostrich.
The development of bone is beautifully shown in Fig. 30, a delicate
slice taken from a pig’s rib. Above may be seen the gristle or
cartilage, with the numerous rows of cells; below is the formed bone,
with one of the Haversian canals and its contents; while between the
two may be seen the cartilage-cells gathering together and arranging
themselves into form. The cartilage-cells are well shown in Fig. 28,
which is a portion of the cup which had contained the eye of a haddock.
The horn-like substances at the end of our fingers, which we call the
nails, are composed of innumerable flattened cells. These cells are
generally so fused together as to be quite indistinguishable even with
a microscope, but can be rendered visible by soaking a section of nail
in liquor potassæ, which causes the cells to swell up and resume to a
degree their original rounded form.
It is worthy of remark that the animal form is built up of cells, as is
the case with the vegetables, although the cells are not so variable
in shape. They generally may be found to contain well-marked nuclei,
two or more of the latter being often found within a single cell, and
in many cases the tiny nucleoli are also visible. Good examples of
these cells may be obtained from the yolk of an egg, and by careful
management they may be traced throughout every part of the animal form.
The teeth have many of the constituents of bone, and in some of their
parts are made after precisely the same fashion. When cut, the teeth
are seen to consist of a hard substance, called enamel, which coats
their upper surfaces, of dentine, or ivory, within the enamel, and of
“cement,” which surrounds the fangs. In Fig. 26, Plate X., which is
a longitudinal section of the human “eye” tooth, is seen the ivory
occupying the greater part of the tooth, coated by the enamel at the
top and the cement at the bottom. In the centre of each tooth there is
a cavity, which is plentifully filled with a pulpy substance by which
the tooth is nourished, and which conveys the nerves which endow it
with sensation. A traverse section of the same tooth is seen in Fig. 25.
The enamel is made of little elongated prisms, all pointing to the
centre of the tooth. When viewed transversely, their ends are of a
somewhat hexagonal shape, something like an irregular honeycomb. The
dentine is composed of a substance pierced with myriads of minute
tubes. They require a rather high power--say 300 diameters--to show
them properly. The cement is found at the root of the fangs, and is
best shown in the tooth of an aged individual, when it assumes very
clearly the character of bone.
Sections may be made by sawing a slice in the required direction,
polishing one side, and cementing it with old Canada balsam to a slide.
It may then be filed down to nearly the required thinness, finished by
carefully rubbing with a hone, and polished with buff leather. Canada
balsam may then be dropped upon it, and a glass cover pressed firmly
down.
Sections of young bone form magnificent objects for the polariser.
Fig. 29 is a section cut from one of the palate teeth of the ray
(_Myliobátes_).
A rather important element in the structure of animals is the “elastic
ligament,” which is found in the back of the neck and other parts of
the body, especially about the spine. It is made of a vast number of
fibres of variable shape and length, branching and communicating,
arranged generally in bundles, and remarkable for containing very few
vessels, and no nerves at all. At Fig. 14 may be seen an example of
elastic ligament, popularly called “paxwax,” taken from the neck of a
sheep.
The white fibrous tissue by which all the parts of the body are bound
together is seen at Fig. 10; and at Fig. 11 is a beautiful example of
the “ultimate fibres” of the crystalline lens of a sturgeon’s eye.
The muscles of animals are of two kinds, the one termed the striped,
and the other the unstriped. Of these, the latter belongs to organs
which work independently of will, such as the stomach, etc., while
the former belongs to those portions of the body which are subject to
voluntary motion, such as the arm and the leg. The unstriped muscle
is very simple, consisting merely of long spindle-shaped cells, but
the striped or voluntary muscle is of more complex construction. Every
voluntary muscle consists of myriads of tiny fibres, bound together in
little bundles, enveloped in a kind of sheath. Fig. 24 is an example of
this muscular fibre, taken from beef. When soaked in spirit, it often
splits into a number of discs, the edges of which are marked by the
transverse lines.
A fibre of nerve is drawn at Fig. 23, and is given for the purpose of
showing the manner in which the nerve is contained in and protected
by its sheath, just like a telegraph-wire in its coverings. Just
above is a transverse section of the same fibre, showing the same
arrangement from another point of view, and also illustrating the
curious phenomenon, that when nerve-fibres are treated with carmine the
centre takes up the colouring matter, while the sheath remains white as
before. The best way of studying nerves is to decapitate a frog, and
cut off a piece of one of the nerves, which, like fine silk threads,
come out between the joints of the spine inside the abdomen. By careful
teasing out it is easy to obtain preparations showing all the above
points, and, in addition, the folding-in of the internal sheath which
correspond to the insulators of a telegraph-line.
The blood of animals is analogous in its office to the sap of plants,
but differs greatly from it under the microscope. In sap there seem to
be no microscopic characters, except that when a branch is cut, as in
the vine, the flowing sap may contain certain substances formed in the
wounded cells, such as chlorophyll, starch, and raphides; but the blood
is known to be an exceedingly complex substance both in a microscopic
and a chemical point of view. When a little fresh blood is placed under
the microscope, it is seen to consist of a colourless fluid filled with
numerous little bodies, commonly called “blood-globules,” varying very
greatly in size and shape, according to the animal from which they
were taken. Those of the reptiles are very large, as may be seen at
Fig. 4, Plate X., which represents a blood corpuscle of the Proteus.
In this curious reptile the globules are so large that they may be
distinguished during its life by means of a common pocket lens.
In the vertebrated animals these corpuscles are red, and give to the
blood its peculiar tint. They are accompanied by certain colourless
corpuscles, spherical in form, which are sometimes, as in man, larger
than the red globules, and in others, as in the siren and the newt,
considerably smaller. The general view of the red corpuscles has
sufficient character to enable the practised observer to name the
class of animal from which it was taken, and in some cases they are
so distinctive that even the genus can be ascertained with tolerable
certainty. In point of size, the reptiles have the largest and the
mammalia the smallest, those of the Proteus and the musk-deer being
perhaps the most decidedly opposed to each other in this respect.
IX.
FIG.
1. Amœba diffluens
2. Arcella
3. Sun animalcule
4. Miliolina
5. Paramœcium
6. Chilodon subdividing
7. Melicerta ringens
8. Spicula of Sponge, Grantia
9. Noctiluca miliaris
10. Rotifer vulgaris
11. Do. jaws
12. Sponge animalcule
13. Sertularia operculata
14. Sponge, Grantia
15. Sertularia operculata, with ovicells
16. Actinia, showing weapons
17. Do. base of weapon more magnified
18. Sponge granule, ciliated
19. Anguinaria anguina
20. Spicules of sponge from Oyster Shell
21. Head of Snake-headed Zoophyte
22. Bugula avicularia
23. Polyzoön, Eretea
24. Do. Notamia
25. Zoea, Young of Crab
26. Hydra tuba
27. Medusa, cast off from above
28. Naked-eyed Medusa, Thaumantias
29. Compound Eye, Medusa
30. Larva, Snake Star
31. Water Flea
32. Serpula, Pushing Pole
33. Comatula, early stage of Starfish
34. Carbonate of Lime, artificial
35. Sea Urchin, transverse section of spine
36. Serpula, bundle of spears
37. Sun-star, part of skin
38. Oyster shell in different stages
39. Cilia on mussel
[Illustration: IX.]
In shape, those of the mammalia are circular discs, mostly with a
concave centre, though the camel has oval ones; those of the birds are
more or less oval and convex; those of the reptiles are decidedly oval,
very thin, and have the nucleus projecting; and those of the fishes are
oval and mostly convex. During the process of coagulation the blood
corpuscles run together into a series of rows, just as if a heap of
pence had been piled on each other and then pushed down, so that each
penny overlaps its next neighbour.
These objects are illustrated by six examples on Plate X. Fig. 2 is
human blood, showing one of the white corpuscles; Fig. 3 is the blood
of the pigeon; Fig. 4, of the _Proteus anguínus_; Fig. 5, of the
tortoise; Fig. 6, of the frog, showing the projecting nucleus; and
Fig. 7, of the roach. The blood possesses many curious properties,
which cannot be described in these few and simple pages.
In the centre of Plate X. is a large circular figure representing
the membrane of a frog’s foot as seen through the microscope, and
exhibiting the circulation of the blood. The mode of arranging the foot
so as to exhibit the object without hurting the frog is simple enough.
Take an oblong slip of wood,--my own was made in five minutes out of
the top of a cigar-box,--bore a hole about an inch in diameter near
one end, and cut a number of little slits all round the edge of the
wooden slip. Then get a small linen bag, put the frog into it, and dip
him into water to keep him comfortable. When he is wanted, pull one of
his hind feet out of the bag, draw the neck tight enough to prevent
him from pulling his foot back again, but not sufficiently tight to
stop the circulation. Have a tape fastened to the end of the bag, and
tie it down to the wooden slide. Then fasten a thread to each of his
toes, bring the foot well over the centre of the hole, stretch the toes
well apart, and keep them in their places by hitching the threads into
the notches on the edge of the wooden strip. Perhaps an easier plan
is to secure the threads by drops of sealing-wax when in the desired
position. Push a glass slide carefully between the foot and the wood,
so as to let the membrane rest upon the glass, and be careful to keep
it well wetted. If the frog kick, as he will most likely do, pass a
thin tape over the middle of the leg, and tie him gently down to the
slide.
Bring the glass into focus, and the foot will present the appearance
so well depicted in the engraving. The veins and arteries are seen
spreading over the whole of the membrane, the larger arteries being
often accompanied by a nerve, as seen in the illustration. Through all
these channels the blood continually pours with a rather irregular
motion, caused most probably by the peculiar position of the reptile.
It is a most wonderful sight, of which the observer is never tired,
and which seems almost more interesting every time that it is beheld.
The corpuscles go pushing and jostling one another in the oddest
fashion, just like a British crowd entering an exhibition, each one
seeming to be elbowing its way to the best place. To see them turning
the corners is very amusing, for they always seem as if they never
could get round the smaller vessels, and yet invariably accomplish the
task with perfect ease, turning about and steering themselves as if
possessed of volition, and insinuating their ends when they could not
pass crosswise.
By putting various substances, such as spirit or salt, upon the
foot, the rapidity of the circulation at the spot can be greatly
increased or reduced at will, or even stopped altogether for a while,
and the phenomenon of inflammation and its gradual natural cure be
beautifully illustrated. The numerous black spots upon the surface are
pigment-cells.
The tails of young fish also afford excellent objects under the
microscope, as the circulation can be seen nearly as well as in the
frog’s foot. The gills of tadpoles can also be arranged upon the stage
with a little care, and the same organs in the young of the common newt
will also exhibit the circulation in a favourable manner. The frog,
however, is perhaps the best, as it can be arranged on the “frog-plate”
without difficulty, and the creature may be kept for months by placing
it in a cool, damp spot, and feeding it with flies, little slugs, and
similar creatures.
CHAPTER VIII
Pond-Life--Apparatus and Instructions for Collecting Objects--Methods
of Examination--Sponge--Infusoria.
Of all departments of microscopic research the most fascinating and
the most popular is that which deals with what is known by the generic
name of “pond-life.” The minute forms of the animal creation included
in this term are of such exquisite beauty, and allow the processes of
their life-history to be followed with such facility, from the cradle
(when they have one) to the grave (which is very often the body of
another, larger, organism), that there is none which has attracted
more observers. Indeed, the first application of the microscope, by
Leeuwenhoek, early in the seventeenth century, was to the observation
of these forms of life.
X.
FIG.
1. Skin, Frog
2. Blood, Human
3. Do. Pigeon
4. Do. Proteus
5. Do. Tortoise
6. Do. Frog
7. Do. Fish
8. Human nail
9. Bone, Human
10. White fibrous tissue
11. Epithelial cells from tongue
12. Feather, Peacock
13. Spine, Hedgehog, transverse section
14. Pax-wax
15. Epithelial cells from nose
16. Bone, Ostrich
17. Feather, Shaft of Canary’s
18. Do. Wild Duck
19. Circulation of blood, Frog’s foot
20. Feather, Sparrow
21. Do. Cock’s tail
22. Fibre, crystalline lens of fish
23. Nerve
24. Muscle, Meat
25. Tooth, transverse section
26. Do. Longitudinal section
27. Sweat duct
28. Eye of Haddock
29. Myliobates, palate
30. Gristle, Pig
31. Pigment, Human eye
32. Do. Wing of Bat
33. Do. Shell of Prawn
[Illustration: X.]
A few words may be said, in the first place, as to the outfit. A very
useful part of it is a walking-stick, to which can be attached either
a net for capturing the larger forms of life, or a hook for collecting
the weeds, to which many forms of great interest and beauty are
attached (Fig. 15). The stick is telescopic, and can also have attached
to it a bottle, which, put into the water at any desired spot,--say,
amongst a clump of weeds, or near the bottom, upside down, and then
suddenly reversed,--will bring away samples of the inhabitants of the
neighbourhood. When these are sparsely distributed through the water,
the latter may be concentrated by the use of a bottle round the neck of
which is firmly tied a coarse calico bag, funnel-shaped, and supported
by a wire ring, somewhat as shown in the illustration. Muslin is,
however, too coarse for many organisms. This net is immersed in the
water so that the ring is just above the surface, and one bottleful
after another poured through. The water strains off, the organisms are
left behind. The immersion is necessary to reduce the pressure to which
delicate organisms would otherwise be subjected. When the bottle is
full, or sufficiently concentrated as to its contents, the latter are
poured into one of the ordinary collecting-bottles, of which half a
dozen at least should always be taken.
[Illustration: Fig. 15.]
On reaching home, and as often as possible on the way, the corks should
be removed, as these organisms soon use up the air in the water.
For examination a glass trough of considerable size, say three inches
in length, half an inch in depth, and two inches in height, should be
half filled with the water, and examined with the pocket magnifier.
With a little practice it will be found easy to take up not only
the larger organisms, but even very minute ones, with one of the
dipping-tubes with a long tapering point already referred to. The
organism, when “spotted,” is followed by eye and tube, the finger
being held over the mouth of the latter, and at the critical moment
the finger is removed, and the organism swept into the tube by the
in-rushing water. Now wipe off the excess with a clean handkerchief,
“spot” the organism in the tube again, and carefully absorb the
superfluous water with a piece of blotting paper; and finally, gently
but sharply blow the remainder on to the plate of the live-box, put on
the cover, and examine with a one-inch power. If, as often happens,
the organism sticks to the side of the tube, a little more water must
be drawn in, and the process repeated. The use of the cotton-wool trap
spoken of previously will often be very helpful in the examination of
actively moving organisms.
In the case of weeds, a small portion should be placed in the trough
and carefully examined from end to end, first with the pocket lens and
then with the one-inch power. Let us now consider the objects most
likely to be met with.
A piece of stick may be coated with a white layer, feeling rough to the
touch, and full of small holes. The chances are that this will be a
piece of fresh-water sponge, _Spongilla fluviatílis_, and by dark-field
illumination particles may be seen to enter at some orifices and be
ejected at others. With a very high power and a very thin section,
properly prepared, these holes will be seen to be the mouths of
channels which are lined by the most delicate organisms possible, each
having a minute body crowned with a tiny crystal cup, in the middle of
which is a long cilium, or flagellum, as it is here called (Plate XIII.
Fig. 1). The currents are produced by the combined action of these
flagella. In point of fact, the sponge is a colony of minute animals
working harmoniously for the common good. If the specimen be found in
winter the sponge will be full of tiny balls, the “gemmules” of the
next season’s growth. The roughness is due to the flinty spicules,
which are at once the scaffolding and the protection of the sponge, and
by boiling the sponge in a mixture of nitric acid and water (half and
half) these spicules will be set free, and may be washed, allowed to
settle, washed again, dried, and mounted in balsam. The gemmules are
coated by very beautiful spicules, consisting of two wheels connected
by a rod. These may be treated in the same way. The life-history of the
common sponge is as yet but imperfectly known.
Perhaps the lowest form of life is the _Amœba_, shown in Plate IX.
Fig. 1, a mere lump of jelly, which flows along, and when it comes
into contact with any likely subject for digestion flows round it,
encloses it, absorbs what it can from it, and leaves it behind. A near
relative of the Amœba is the _Arcella_ (Fig. 2), which is simply an
Amœba with a shell. Being unable to swim, these organisms are naturally
to be most often found at the bottom of the collecting bottle, and
it is always advisable to take up a portion of the débris with a
dipping tube, which is then held upright on a slide with the finger
upon it until the dirt settles on to the slide, when it is removed,
a cover-glass put upon the dirt, and a quarter-inch power used for
examination. Many forms will be discovered in this way which would
otherwise escape observation.
Another curious organism, of great size (comparatively) and extreme
beauty, is the sun animalcule (_Actínophrys_), which has a round body
and long tentacles (Fig. 3), to which free-swimming organisms adhere,
and by the combined action of the neighbouring ones are drawn to the
body and received into it; one cannot say swallowed.
Fig. 6, Plate IX., shows the curious arithmetical process whereby
the Infusoria multiply by division, a groove appearing at one point,
rapidly deepening, and finally separating the animal completely into
two. The species is the _Chílodon_, a flattened creature, ciliated all
over, having a set of teeth arranged in the form of a tube, and at its
fore-part a kind of membranous lip. A similar phenomenon, in an earlier
stage, is shown in Fig. 26, Plate XIII., the organism in this case
being _Euplótes_.
It has been said that sponges are colonies of extremely minute
organisms, each furnished with a membranous collar or funnel, the whole
looking like an exquisite wine-glass without a foot. These organisms
are not always grouped in colonies, however. Many are free-growing,
and may be found attached to the stems of water-plants, but they are
extremely minute, and will hardly be noticed until the microscopist
has acquired considerable experience, nor even then--with such an
instrument as we have postulated--will he see more than a tiny pear,
with a straight line, the margin of the cup, on each side of its
summit. The flagellum will be quite invisible.
Some similar organisms may, nevertheless, be found which, though
still minute, are within the range of a properly managed quarter-inch
objective. Such an one, of extreme beauty, is the _Dinobrýon_ shown in
Plate XIII. Fig. 3. Each “zoöid,” as the separate animals are called,
among the Infusoria, or each generation of zoöids, stands upon its
parent and has two flagella. When alarmed, the zoöid sinks to the
bottom of its cell, and withdraws its flagella. In Fig. 2 (_Eugléna_)
we have a similar zoöid, but of far greater size, and free-swimming.
It is a very common object, and possesses a red eye-speck close to the
“contractile vesicle.” All Infusoria have the latter, some a great
number, as in Fig. 9. The vesicle contracts at regular intervals, and
is then simply blotted out, but reforms in the same place, so that it
is probably the heart or the urinary bladder of these minute animals.
The lovely rosette shown in Fig. 4 is the _Synura_, a spherical colony
of zoöids, each of which has two flagella, and is in addition clothed
with rows of cilia. A beautiful sight it is to watch these colonies
rolling through the field of view. Not uncommon, especially in brackish
water, is the _Peridinium_ (Fig. 5), with its plate armour, long
flagellum, and girdle of cilia. A gigantic species of the same family
is common in sea-water, and will be easily recognised by its body, not
much larger than that of Peridinium, being furnished with three long
arms, curiously bent. It is called _Ceratium_, and is sometimes present
in such abundance as to thicken the water, near the surface of which it
swims.
We now come to a class of Infusoria which is characterised by the
possession of a complete covering of cilia, arranged in rows all over
the body. The number of these is enormous; we can only glance at a
few types, by mastering which the observer will, at all events, know
whereabouts he is. The first we will take is the _Coleps_ (Fig. 6),
a very common kind, whose body is marked by a series of geometrical
lines, so that the organism looks very much like an elongated
geographical globe. These markings are on the tunic, which is of a
brownish colour. Very different is the _Trachelocerca_ (Fig. 7), with
its long flexible neck, which is in constant movement from side to side
as the creature swims along. As seen in the figure, the neck is clear
and the head has a fringe of longer cilia.
The _Trachelius_ (Fig. 8) is perhaps the largest of all the Infusoria,
being readily visible to even an inexperienced eye. Its body is richly
furnished with contractile vesicles, and the protoplasm is curiously
reticulated. We may here remark that the Trachelius is especially
prompt in doing what most of these organisms do when put under pressure
in a live-box, namely, in performing a kind of _harakiri_. The outline
first becomes irregular, then the body rapidly swells and finally comes
to pieces, the fragments dancing mockingly away under the influence of
their still-moving cilia. The remedy is to use the cotton-wool trap and
the lightest possible pressure.
A very elegant organism is shown in the bottom right-hand corner of the
Plate (Fig. 25). It is the _Loxophyllum_, and has a strongly marked
contractile vesicle.
Another large form is _Amphileptus_ (Fig. 9), already referred to as
having a large number of contractile vesicles arranged in a regular
row; and more massive still is _Bursaria_ (Fig. 10), a very curious
organism, very much like a purse indeed, and possessing a wonderful
arrangement of cilia inside the funnel. These are arranged like a
ladder, a series of rows of short stiff cilia, which move at short
intervals in unison, and tend to sweep down into the cavity any
small particles of food. This arrangement is here described for the
first time, and appears to be quite unlike anything else among the
Infusoria. Not unlike Bursaria, but having no ladder, and being
furnished with a delicate membranous pouch in front of the slit of the
purse, is _Condylostoma_, which we shrewdly suspect to be the young
form of Bursaria. This is a point which requires elucidation.
One of the most beautiful of all these forms is shown in Fig. 11,
_Folliculina_, a type of a large group characterised by the possession
of a transparent case, of extremely elegant form, within which the
animal retreats on the slightest alarm.
Fearless and independent, as becomes its size, is the trumpet-shaped
_Stentor_ (Fig. 12), which may easily be seen when present, as it is in
almost every good gathering of water-weed. The particular form drawn
(_S. Mülleri_) does not make a case, but many members of the genus
do, and it is very common to see a stem almost covered with them.
Such a sight, once seen under dark-field illumination, will never be
forgotten. The method of multiplication of the Stentors (by division)
is extremely easy to watch, and very instructive.
A curious organism is _Trichodina_ (Fig. 13), which, though a
free-swimmer, is always parasitic upon the body of some higher animal.
We have found it sometimes upon Hydra, and always in hundreds upon the
stickleback. The next group of Infusoria is distinguished by the body’s
being only ciliated at particular points, usually round the mouth, or
what acts as such. The first form is Vorticella (Fig. 14), a beautiful
vase-like creature upon a stem. Down the stem runs a muscular fibre,
and on the least shock the fibre contracts and draws the stem into a
beautiful spiral, whilst the cilia are drawn in, and the zoöid assumes
the appearance of a ball at the end of a watch-spring. An exquisite
sight is a colony of Vorticellæ, for these actions are always going on,
as, for example, when one member of the family touches another, which
is quite sufficient to provoke the contraction.
XI.
POLARIZED LIGHT.
FIG.
1. Carbonate of Lime
2. Starfish
3. Thistle down
4. Starch, Wheat
5. Do. Potato
6. Prawn-shell
7. Starch, “Tous les mois”
8. Bone, cancellous
9. Gun-cotton
10. Cow’s hair
11. Hoof, donkey, longitudinal
12. Do. transverse
13. Nitre, Crystals
14. Scale, Eel
15. Wing, Water-Boatman
16. Chlorate of Potash, Crystals
17. Cellularia reptans
18. Star-shaped hair, Stalk of Yellow Water-Lily
19. Teeth, Palate of Whelk
20. Zoophyte, Bowerbankia
21. Raphides, _i.e._ crystalline formations in
vegetable cells, Bulb of Hyacinth
22. Do. Rhubarb
23. Sulphate of Magnesia, Crystals
24. Bone, Skate
25. Cherrystone, transverse section
26. Sugar, Crystals in honey
27. Tendon, Ox
28. Calcareous plates. Tooth of Echinus
[Illustration: XI.]
Many compound tree-like forms of Vorticella are known, one of which,
_Carchesium_ (Fig. 15), may serve as a type of all. In the case of this
organism, the colony contracts in sections on a moderate shock; in the
second, _Zoothamnium_, as a whole; whilst in _Epistylis_ the stalks are
rigid, and the individuals contract singly. When the shock is violent,
the appearance presented by the two former is that shown in Fig. 16. In
all three cases the colonies are usually so large that they are visible
as trees to the naked eye, and some members of the group are extremely
common. Moreover, they are often parasitic, as, for example, upon
Cyclops, which is frequently loaded with them.
Another compound form is _Ophrydium_, a colony of which (not unusually
large) is shown of the natural size in Fig. 18, with a single zoöid,
magnified, by the side of it, in Fig. 19.
Lastly, we have an exquisite group of organisms related to Vorticella,
but possessing a transparent envelope, the forms of which are most
varied, but always graceful. _Vaginicola_ (Fig. 17) is a good example
of this, and _Cothurnia_ (Fig. 20) still more so. Many of these
organisms, too, are furnished with a plate, attached either to the
head or to the body, which plate, when they withdraw into their cases,
closes the latter perfectly, as in the case of the exquisite _Pyxicola_
(Fig. 21).
A very interesting but singularly obtrusive organism is the
_Stylonychia_ (Figs. 22, 23). How often has it happened to us to have
an interesting object nicely in the field of view, and then to have it
knocked out of sight by the blundering incursion of this burly fellow,
who runs so rapidly by means of his “styles” that he gives nothing
time to get out of the way. He is of interest to us, however, as the
representative of a class in which the body is not ciliated, or very
partially and slightly so, usually round the mouth. We have frequently
found Stylonychia, in company with Vorticella and _Paramœcium_ (Plate
IX. Fig. 6), in the water in which flowers have been standing for a few
days; sometimes the numbers are so great as to make the water quite
milky.
One more form must conclude this short sketch of the great Infusorial
family. It is the _Acineta_ (Fig. 24), which, attached by its
foot-stalk, and devoid of cilia, patiently waits, with outspread arms,
to receive and embrace smaller members of the family as they dance
merrily about. Alas! its embrace is as fatal as that of the image of
the Virgin which bore beneath its robe spikes and daggers, for the
victim struggles vainly to escape, and the nourishment from its body is
rapidly absorbed.
And here we take our leave of a group which, simple as is the
construction of the animals which it includes (for every one, great
and small alike, is composed of a single cell), is yet full of beauty
and interest. He who wishes to pursue the matter further will find in
Saville Kent’s _Manual of the Infusoria_ a perfect mine of information,
to which we gladly acknowledge our indebtedness, both now and in time
past.
CHAPTER IX
Fresh-water Worms--Planarians--Hydra--Polyzoa--Rotifers--Chætonotus--
Water-Bears.
The fresh-water worms form a large and well-defined group, and a few
words regarding them may be useful.
They are very common, and very difficult to find information about,
most of the work relating to them having been done in Germany. At the
same time, they are so highly organised and so transparent that the
process of their life-history may be easily followed.
One large group has the peculiarity of multiplying by division, the
last joints or segments being devoted to the formation of the new
individual. At one time of the year the ordinary sexual process of
reproduction takes the place of this method, and each worm is then
surrounded by a belt such as may be seen in the common earthworm under
similar conditions. Further information on this subject is greatly
needed.
The type is the common _Naïs_, which has a body of thirty segments or
more, two eye-specks on the head, and a double row of bristles along
the back; whilst below, each segment carries strong hooked bristles,
nearly buried in the body, by means of which the worm crawls. Inside
the mouth is a large proboscis, which can be protruded, and this leads
into a stomach which is merely an enlargement of the intestine which
succeeds it. The circulation of the blood (which is colourless) can be
easily watched. It begins at the tail with a contraction of the dorsal
vessel, passes up to the head, and then down below the intestine to the
tail again. The intestine is ciliated inside, and it is by a current
of water carried into the intestine by these cilia that the blood is
aërated.
In the next genus, _Dero_, this is clearly seen, for the tail (Plate
XIV. Fig. 1) is opened out into a wide shield, from which rise four,
six, or even eight finger-like processes. These parts are all ciliated,
and contain a network of blood-vessels. The worm lives in a case which
it builds in the mud, and the way to find it is to put some of the mud
into a glass beaker with water, and allow it to stand. If there be
members of this family in it, their tails will be seen protruding above
the water. Pour out the mud sharply, fill up with water, and allow the
dirt to subside, and the worms may then be made to leave their cases by
pressure by a camel hair pencil on the lower end of the tube, and may
be caught with the dipping tube and placed in the live-box. They have
no eyes, otherwise the general outline of the body closely resembles
that of Naïs.
_Slavína_ (Fig. 2) has a row of touch-organs, like pimples, round
each segment, and is a dirty looking creature, with an inordinately
long first pair of bristles, but this reaches its acme in _Pristina_
(Fig. 3) (sometimes, though wrongly, called _Stylaria_) _parasita_,
which has three long sets of bristles upon the back, and keeps these in
constant wing-like motion. The true _Stylaria_ has a long trunk, set
right in the head, and tubular (Fig. 6); it grows to a considerable
length, and when in the stage of fission it is very funny to see
the two proboscides waving about, one on the middle, as well as the
original one at the head. There is also a form with a shorter proboscis
of the same kind.
_Bohemilla_ has a tremendous array of saw-like bristles upon the back,
whilst _Chætogaster_ has none at all in this position, and few below.
_Æolosoma_ has merely tufts of hair instead of bristles, and swims
freely. It is easily recognised by the red, yellow, or green pigment
spots in its skin, and by the ciliated head. Rarest of all the family
is the one which connects it with the ordinary _Tubifex_, the red worm
which lives in masses in the mud of brooks and ponds, the waving tails
protruding above the water, and being instantly withdrawn when a foot
is stamped upon the bank. Their Naid cousin is _Naidium_, and has red
blood, but multiplies by fission, which Tubifex does not.
Another group of worms is the _Planarians_, small leech-like worms,
black, white, or brown, which are rarely absent from a gathering. The
would-be investigator will find in them an abundant field for work, as
they are but very imperfectly known or studied.
The great enemy of all these worms is the _Hydra_, a good idea of
which may be formed from Plate IX. Fig. 13. There are three species,
all of which are fairly common. They capture their prey in exactly the
same way as sea-anemones and the marine hydroid forms, so numerous and
varied.
Nor must we omit to notice the exquisitely beautiful Polyzoa, such
as _Lophopus_ (Plate XIV. Fig. 4), with its ciliated tentacles and
transparent social home; _Fredericella_ (Fig. 5), with its graceful
stems, and their still more graceful inhabitants; and the wonderful
_Cristatella_, whose colonies form bodies which crawl over the stems
of water plants. But for grace, beauty, and variety, the Rotifers
assuredly outshine all their fellow inhabitants of our ponds and
streams.
We can only take a few types, and of all these the most common is the
common Rotifer (Plate IX. Fig. 10). It is there shown in the act of
swimming, but it can withdraw its “wheels” and creep like a leech,
protruding its foot as it does so. It is distinguished by the two
eye-spots on the proboscis from _Philodina_, in which they are on the
breast, and _Callidina_, which has none. When at ease in its mind, the
animal protrudes its wheels, and by their action draws in particles
of food, these passing down to the incessantly moving jaws, which act
like a mill and crush the food before it passes on to be digested. The
movement of the jaws may even be seen in the young Rotifer whilst still
in the egg within the body of the parent, and as the egg reaches its
full development other eggs again are visible within it, so that we may
have three generations in one individual. The males of most of the
Rotifera are unknown. Those that are known are very lowly organised,
having only the ciliary wreath and the reproductive organs, and are
only found at certain seasons of the year. For the remainder of the
time parthenogenesis is the rule, just as among the Aphides. We select
a few individuals for illustration as types. Those who wish to pursue
this study further must be referred to the monumental work of Hudson
and Gosse.
The common Rotifer, already referred to, may be taken as the type
of the Bdelloida, or leech-like class, so called from their mode of
“looping” themselves along. The group is a comparatively small one in
comparison with the next, the Ploïma, or free-swimmers. We can only
select from the vast variety a few species, first of those classed
as illoricated, from their being without a _loríca_, or case, and
then of the loricated, which possess it. A very large and common
form is _Hydátina_ (Plate XIV. Fig. 7), which lives by choice in the
reddish pools of water found often by the roadside. It shows the whole
organisation of the class magnificently; the ciliary wreath on the
head, with the striped muscles which draw the latter back, the powerful
jaws, the digestive canal with its crop and intestine, the ovary
with the developing eggs, the water-vascular system with the curious
vibratile tags, and finally, the cloaca, which receives the waste of
the body and expels it at intervals.
Another form, also common, especially in clear water, is _Synchæta_
(Fig. 8), very much like a kite or peg-top in shape, which has the
power of attaching itself by a glutinous thread, and spinning round at
a tremendous rate. Then there is the gigantic _Asplanchna_ (Fig. 9),
which has no opening below, so that the waste must be discharged by the
mouth; and curious _Sacculus_, which gorges itself with chlorophyll
until it looks like a green bag with a string round it, but clear and
sparkling. Of the _Notommatæ_ there is a whole host, but we can only
mention the beautiful _N. Aurita_ (Fig. 10), with an eye of a beautiful
violet colour, composed of several spherules massed together, and two
curious ear-like processes on the head, from which it takes its name.
Some of the Ploïma have powers of leaping which must be noticed. The
_Triarthra_ (Fig. 11) has three arms, or what we may call such, which
it can stretch out suddenly and leap to a considerable distance, whilst
in _Polyarthra_ the arms become a whole cluster of broad saw-like
bristles.
We pass on to note a few species of the mail-clad or loricated
Rotifers, chief among which the great _Euchlanis_ (Fig. 12), a
noble-looking fellow, calls for our attention, his great size rendering
him easily visible to the naked eye. It is difficult to avoid using the
masculine gender, but, of course, all those figured and described are
of the gentler sex. _Salpina_, too (Fig. 14), with its box-like lorica,
armed with spines at each of the upper angles, and having three below,
is quite easily recognised, and very common. _Brachionus_ (Fig. 13)
has a shield-shaped case, well furnished with spines, symmetrically
arranged at the top, and an opening below for the flexible wrinkled
tail, like the trunk of an elephant. _Pterodina_ (Fig. 15) has a
similar tail, but a round case, and the head is much more like that
of the common Rotifer when extended. _Anuræa_ (Fig. 16), on the other
hand, has no tail, and its case is shaped like a butcher’s tray, with
a handle at each corner. _Dinocharis_ (Fig. 17) has a roof-like case,
with long spines on the root of the tail, and a forked stiff foot.
_Noteus_ (Fig. 18) is much like Pterodina, except in its foot, which
more nearly resembles that of Dinocharis.
The list might be indefinitely extended, but sufficient has probably
been said to enable the tyro to find his bearings in this large,
beautiful, and interesting class.
We pass on to notice in conclusion two or three of the fixed forms,
of which a beautiful example is the _Melicerta ringens_ (Plate IX.
Fig. 7), whose building operations have a never-ending charm. Particles
of débris are accumulated in a curious little cavity in the chin, in
which they are whirled round, and mixed with a secretion which binds
them together, and when a brick is made the head is bent down and the
brick applied to the desired spot with mathematical regularity. By
supplying fine particles of innocuous colouring matters, the Melicerta
may be made to build a variegated case. The most remarkable specimen
known is the one figured in Hudson and Gosse’s work, which was found
by the present writer in a specimen of water from which he had already
obtained five-and-twenty species of various kinds of Rotifer; the water
was collected by an inexperienced person, and there was only a pint
of it. It had, moreover, been kept for three weeks, and the moral of
that is, to preserve one’s gatherings, and keep an aquarium into which
they may be poured when done with for the moment. New forms will often
develop with startling rapidity, their eggs having been present in the
original gathering. The young form of Melicerta, shown in Plate XIV.
Fig. 20, is strangely unlike its mother, and much more nearly resembles
its father.
Another group of extreme beauty is the Flosculariæ (Fig. 19), several
species of which are very common. They will be easily known by their
appearance, which resembles a shaving brush when closed; whilst, when
opening, the shaving brush resembles a cloud of delicate shimmering
threads, which at last stand out straight, radiating all round the head
of the creature, and forming the trap by means of which it catches its
prey. Finally, there is the lovely _Stephanoceros_ (not, unfortunately,
very common), with its five symmetrically placed and gracefully curved
arms, perhaps the most lovely of all Rotifers, with its exquisitely
transparent body, sparkling with masses of green and golden brown.
He who finds this has a treasure indeed, and will be encouraged to
prosecute his studies in this “Fairyland of Microscopy.”
Two irregular forms call for a word of remark. The first is
_Chætonotus_ (Plate XIII. Fig. 27), which stands on the borderland of
the Infusoria and the Rotifers, neglected as a rule by the students
of both; and the second the _Tardigrada_ (Plate XIV. Fig. 21), or
water-bears, which have feet like those of the red wriggling larva of
_Chironomus_, whose silky tubes are common enough on submerged walls
and on the stems of plants, these feet consisting of a mass of radially
arranged hooklets, which can be protruded or withdrawn at will; whilst
the head of the water-bear is far more like that of a louse, pointed
and hard, and suited for burrowing about, as the animal does, among the
rubbish at the bottom of the bottle. Both the genera just referred to
will repay careful study, as little is known of their life-history or
development.
A few words must be devoted, in conclusion, to the Entomostraca,
those shrimp-like animals which, like their marine relatives, act as
scavengers to the community. Fig. 22 is a portrait of _Cypris_, a not
very handsome form, but one very commonly found. Its shell is opaque,
so that the internal organs are difficult to observe. Far different
in this respect is the beautiful _Daphnia_, the water-flea _par
excellence_, whose carapace is of crystalline clearness, so that every
movement of every one of the internal organs may be followed with the
greatest facility. There are many species of the genus, and some of
them are very common, so that the opportunity of examining these lovely
objects is easily obtained. Plate XIV. Fig. 23, shows the most common
of all the class under notice, the _Cyclops_, so named from the fact
that, like the fabled giants of classical literature, it has a single
eye in the middle of its forehead. It is often loaded with Infusoria,
especially Vorticella and Epistylis, already described, to such an
extent that its movements are greatly hampered.
XII.
FIG.
1. Tubercle, Sun-star
2. Zoophyte, Gemellaria
3. Cuttle bone
4. Plate of ditto from above
5. Zoophyte, Antennularia
6. Pedicellaria, skin of Starfish
7. Shell, Foraminifer
8. Snake-star, disc from below
9. Pedicellaria, Echinus
10. Wing-case, Weevil
11. Coralline
12. Spine, Echinus
13. Foraminifer, Polystomella
14. Do. Truncatulina
15. Do. Polymorphina
16. Do. Miliolina
17. Gold dust, with quartz
18. Foraminifer, Lagena vulgaris
19. Pouches, Skin of Rat’s tail
20. Foraminifer, Biloculina ringens
21. Ore, Copper
22. Zoophyte, Membranipora pilosa
23. Human skin, injected
24. Coal, Longitudinal section
25. Do. Transverse section
26. Lung, Frog
[Illustration: XII.]
We have not space to figure more of these creatures, but other forms
will be found not inferior in interest to those mentioned. The most
curious of all are those which earn a dishonest and lazy living by
attaching themselves to the bodies of other and larger animals, chiefly
fish. One of the largest is the _Argulus_, the bane of aquarium
keepers, which is of considerable size, and attacks gold-fish, and in
fact almost any fish to which it can obtain access.
The gills of the stickleback will furnish examples of the curious
_Ergasilus_, which consists chiefly of an enormous pair of hooks and
two long egg-bags, the latter, in varying form, being carried by many
of the Entomostraca.
Upon the fins of the same fish will be found the remarkable
_Gyrodactylus_, a worm-like animal which attaches itself by a large
umbrella-like foot, in the centre of which are two huge claws. The
head is split down the middle for some distance. We may mention, in
concluding our notice of the external and involuntary guests of the
unlucky stickleback, that its skin is usually frequented by hosts
of the Trichodina described in the last chapter. Of the internal
parasites, want of space forbids us to speak.
CHAPTER X
Marine Life--Sponges--Infusoria--Foraminifera--Radiolaria--Hydroid
Zoophytes--Polyzoa--Worms--Lingual Ribbons and Gills of
Mollusca--Star-Fishes and Sea-Urchins--Cuttle-Fish--
Corallines--Miscellaneous Objects.
Great as is the range of objects presented to the student of
fresh-water life, the latter field is limited indeed as compared with
that afforded by the sea. The Infusoria and Rotifers furnished by the
latter are, indeed, much fewer in number and variety, but the vast host
of sponges, polyzoa, hydroids, crustacea, molluscs, ascidians, and
worms, to say nothing of the wealth of vegetable life, renders the sea
the happy hunting-ground of the microscopist.
Whether it be along the edge of the water, as the tide retreats,
especially after a gale; or in the rock-pools; or, perhaps best of
all, upon those portions of the shore left uncovered only by the
lowest spring-tides, the harvest is simply inexhaustible. Stones
turned up will exhibit a world in miniature. Encrusted with green or
pink sponges, or with gelatinous masses of ascidians, fringed at its
edges with hydroids, coated above with polyzoa, a single one will
often supply more work than could be got through in a week of steady
application.
A description of the fresh-water sponge already given may serve very
well to indicate the general outlines of the organisation of the marine
ones too. The spicules of the latter are, however, not always flinty;
very often, as in the case of _Grantia_ (Plate IX. Figs. 8 and 14),
they are calcareous, a point which can be settled by the application of
a little nitric acid and water. If lime be present there will be strong
effervescence, and the separation of the spicules can only be effected
by gently warming a portion of the sponge in caustic potash solution,
pouring the resulting mass into water, and allowing the spicules to
settle. The washing and settling must be repeated several times, and a
portion of the deposit may then be taken up with a dipping-tube, spread
upon a slide and dried, and then covered in balsam solution. The forms
are endless, and the same sponge will often supply three or four, or
even more. Among them may be seen accurate likenesses of pins, needles,
marlin-spikes, cucumbers, grappling-hooks, fish-hooks, porters’-hooks,
calthrops, knife-rests, fish-spears, barbed arrows, spiked globes,
war-clubs, boomerangs, life-preservers, and many other indescribable
forms. The flinty forms must be prepared by boiling, as described in
speaking of the mounting of diatoms in Chapter XI., except that, of
course, only one settlement is required after thorough washing.
Every one who has been by or on the sea on a fine summer night must
have noticed the bright flashes of light that appear whenever its
surface is disturbed; the wake of a boat, for example, leaving a
luminous track as far as the eye can reach. This phosphorescence is
caused by many animals resident in the sea, but chiefly by the little
creature represented at Fig. 9, the _Noctilúca_, myriads of which may
be found in a pail of water dipped at random from the glowing waves. A
tooth of this creature more magnified is shown immediately above.
A large group of microscopic organisms is known to zoologists under
the name of Foraminifera, on account of the numerous holes in their
beautiful shells, most of which are composed of carbonate of lime,
though some are horny and others are composed of aggregations of minute
grains of sand, the forms in one class often closely imitating those in
another. It is of the shells of these minute animals that the “white
cliffs of old England” are very largely composed, and those who desire
to understand the part which these tiny creatures have played, and are
playing, in geology, will do well to study Huxley’s fascinating essay
on “A Piece of Chalk.”
XIII.
FIG.
1. Grantia compressa
2. Euglena viridis
3. Dinobryon sertularia
4. Synura uvella
5. Peridinium tabulatum
6. Coleps hirtus
7. Trachelocerca viridis
8. Trachelius ovum
9. Amphileptus gigas
10. Bursaria Mülleri
11. Folliculina elegans
12. Stentor polymorphus
13. Trichodina pediculus
14. Vorticella nebulifera
15. Zoothamnium arbuscula
16. Do. do. contracted
17. Vaginicola crystallina
18. Ophrydium versatile (colony)
19. Do. do. (single zoöid)
20. Cothurnia imberbis
21. Pyxicola affinis
22–23. Stylonychia mytilus
24. Acineta grandis
25. Loxophyllum meleagris
26. Euplotes charon (dividing)
27. Chætonotus larus
[Illustration: XIII.]
The inhabitants of these shells are Amœbæ, mere masses of jelly,
and some forms may be found sliding over the weeds in almost every
rock-pool. The anchor-mud, already spoken of, always contains some,
and many forms may be found in the sand from sponges, which should be
passed through a series of wire sieves, of increasing fineness, and
the residuum in each case be examined dry under a one-inch power. The
shells may be picked up with a needle which has been slightly greased
by being passed over the hair, and they may be mounted by sticking them
to the slide with thin starch paste, putting on a cover-glass properly
supported, and then running turpentine under the cover-glass, heating
to expel the air, and finally filling up with balsam. Or, as opaque
objects, they may be mounted in a cell dry. The forms are endless, but
all are beautiful, and a few examples are given in Plate IX. Fig. 4
(_Miliolína_), and Plate XII. Fig. 7, which is a portion of the shell
to show the holes, Fig. 13 (_Polystomella_), Fig. 14 (_Truncatulína_),
Fig. 15 (_Polymorphína_), Fig. 16 (_Miliolína_, partly fossilised),
Fig. 18 (_Lagéna_). and Fig. 20 (_Biloculína_).
Allied to these are the lovely Radiolaria, whose shells, constructed
on a similar plan, are composed of flint. They are found in remarkable
profusion in the deposit from Cambridge, Barbados, but also in a
living state at even enormous depths in the ocean. The present writer
has obtained many forms from _Challenger_ soundings, and the great
authority on this subject is Haeckel’s report in the official accounts
of the expedition of the above-named vessel.
The Hydroid Zoophytes are represented by several examples. These
creatures are soft and almost gelatinous, and are furnished with
tentacles or lobes by which they can catch and retain their prey.
In order to support their tender structure they are endowed with a
horny skeleton, sometimes outside and sometimes inside them, which is
called the polypidom. They are very common on our coasts, where they
may be found thrown on the shore, or may be dredged up from the deeper
portions of the sea.
Fig. 13 is a portion of one of the commonest genera, _Sertularia_,
showing one of the inhabitants projecting its tentacles from its
domicile. Fig. 15 is the same species, given to show the egg-cells.
This, as well as other zoophytes, is generally classed among the
sea-weeds in the shops that throng all watering-places.
The form just referred to is a near relative of the Hydra, already
described, and belongs to the same great family as the sea-anemones.
One form, shown in Fig. 26, is the _Hydra Tuba_, long thought to be a
distinct animal, but now known to be the young form of a jelly-fish,
or Medusa. The Hydra Tuba throws off joints at intervals, each of
which becomes a perfect jelly-fish. One of them is shown in Fig. 27.
Fig. 28 represents a very small and pretty Medusa, the Thaumantias.
When this animal is touched or startled, each of the purple globules
round the edge flashes into light, producing a most beautiful and
singular appearance. Fig. 29 exhibits the so-called compound eye of
another species of Medusa, though it would appear that these are really
connected with the nervous system of the animal, and have to do with
the pulsating contractions of the bell by which it is propelled through
the water.
In my _Common Objects of the Sea-Shore_ the Actíniæ, or Sea-Anemones,
are treated of at some length. At Fig. 16 is shown part of a tentacle
flinging out the poison-darts by which it secures its prey; and Fig. 17
is a more magnified view of one of these darts and its case.
Much more might be said under this head, but we must pass on to another
group, which, whilst possessing a certain general resemblance to the
hydroid zoophytes, differs utterly from them in internal organisation.
We have already referred to the fresh-water polyzoa. The marine forms
are vastly more numerous, and more easily found, since not only
pieces of weed upon which they grow are to be found upon every beach,
but whole masses of leaf-like colonies, forming what is known as
horn-wrack, may be plentifully found. Instead of the tentacles armed
with sting-cells, like the anemone’s, possessed by the Hydrozoa, the
Polyzoa have arms clothed with active cilia, by which the food is swept
into the mouth, passing on into the stomach, and then through the
intestine to another opening.
Fig. 19 is a very curious zoophyte called _Anguinaria_, or snake-head,
on account of its shape, the end of the polypidom resembling the head
of the snake, and the tentacles looking like its tongue as they are
thrust forward and rapidly withdrawn. Fig. 21 is the same creature
on an enlarged scale, and just below is one of its tentacles still
more magnified. Fig. 23 is the ladies’-slipper zoophyte (_Eretea_);
and Fig. 24 is called the tobacco-pipe or shepherd’s-purse zoophyte
(_Notamia_).
Fig. 22 is a portion of the _Bugula_, with one of the curious
“birds’-head” processes. These appendages have the most absurd likeness
to a bird’s head, the beak opening and shutting with a smart snap (so
smart, indeed, that the ear instinctively tries to catch the sound),
and the head nodding backward and forward just as if the bird were
pecking up its food. On Plate XII. Fig. 2, is a pretty zoophyte called
_Gemellaria_, on account of the double or twin-like form of the cells;
and Fig. 5 represents the _Antennularia_, so called on account of its
resemblance to the antennæ of an insect. Fig. 22 is an example of a
pretty zoophyte found parasitic on many sea-weeds, and known by the
name of _Membranipora_. Two more specimens of zoophytes may be seen
on Plate XI. as they appear under polarised light. Fig. 17 is the
_Cellularia reptans_; and Fig. 20 is the _Bowerbankia_, one form of
which occurs in fresh water.
Among the worms we may refer to the beautiful little _Spirorbis_,
whose tiny coiled spiral tube may be found attached to almost every
sea-weed, and which, when placed in a trough of sea-water, protrudes
its beautiful crown of plumes. In chalk or other soft rocks, again,
the tubes of _Spio_, with its two long waving tentacles, may be found
by hundreds. Then there are the centipede-like worms, which may be
found under nearly every stone, and which belong to the great family of
Nereids, provided with formidable jaws and stiff bristles of various
forms. The Serpulæ are allied to the Spirorbis already mentioned. Parts
of the so-called feet of one of these worms are shown in Fig. 36,
where the spears or “pushing-poles” are seen gathered into bundles, as
during life. One of them, on a larger scale, is shown in Fig. 32. The
gorgeous hairs of Aphrodite have already been alluded to.
In the sea the few species of Crustacea which fresh water offers to
the observer in the shape of Cyclops and its allies become thousands,
and their changes during development are numerous and puzzling. Who,
for example, would suppose that the young stage of the Cyclops was
indistinguishable in habits, and almost in form, from that of the
barnacle which adheres to the rocks? Yet such is the case, and there
are other metamorphoses even more startling. Fig. 25 is the larva of
the common crab, once thought to be a separate species, and described
as such under the name of _Zoæa_.
The Mollusca proper will not afford us many objects, except in the form
of their lingual ribbon, which may be extracted from the mouth, gently
heated in _liquor potassæ_, and mounted in balsam after well washing,
when the rows of teeth form splendid objects by polarised light. The
palate of a whelk is shown in Plate XI. Fig. 19.
Again, the gills of the mussel will afford a beautiful illustration of
ciliary action. If a portion of the thin plates which lie along the
edge of the shell be examined in a little of the liquor, the action may
be splendidly seen, and watched for a long time (Fig. 39).
The structure of shell, _e.g._ oyster-shell, is well shown in three
examples: Fig. 34 is a group of artificial crystals of carbonate of
lime; and on Figs. 38 and 39 may be seen part of an oyster-shell,
showing how it is composed of similar crystals aggregated together.
Their appearance under polarised light may be seen on Plate XI. Figs. 1
and 6.
We now pass on to the Echinoderms, including the star-fishes and
sea-urchins.
The old story of the goose-bearing tree is an example of how truth may
be stranger than fiction. For if the fable had said that the mother
goose laid eggs which grew into trees, budded and flowered, and then
produced new geese, it would not have been one whit a stranger tale
than the truth. Plate IX. Fig. 33, shows the young state of one of
the common star-fishes (_Comátula_), which in its early days is like
a plant with a stalk, but afterwards breaks loose and becomes the
wandering sea-star which we all know so well. In this process there
is just the reverse of that which characterises the barnacles and
sponges, where the young are at first free and then become fixed for
the remainder of their lives. Fig. 30 is the young of another kind of
star-fish, the long-armed Ophiúris, or snake-star.
Fig. 37 is a portion of the skin of the common sun-star (_Solaster_),
showing the single large spine surrounded by a circle of smaller
spines, supposed to be organs of touch, together with two or three
of the curious appendages called pedicellariæ. These are found on
star-fishes and Echini, and bear a close resemblance in many respects
to the bird-head appendages of the zoophytes. They are fixed on
foot-stalks, some very long and others very short, and have jaws which
open and shut regularly. Their use is doubtful, unless it be to act as
police, and by their continual movements to prevent the spores of algæ,
or the young of various marine animals, from effecting a lodgment on
the skin. A group, of pedicellariæ from a star-fish is shown on a large
scale on Plate XII. Fig. 6, and Fig. 9 of the same Plate shows the
pedicellariæ of the Echinus.
Upon the exterior of the Echini, or sea-urchins, are a vast number of
spines having a very beautiful structure, as may be seen by Fig. 35,
Plate IX., which is part of a transverse section of one of these
spines. An entire spine is shown on Plate XII. Fig. 12, and shows the
ball-and-socket joint on which it moves, and the membranous muscle that
moves it. Fig. 8 is the disc of the snake-star as seen from below.
Fig. 1 is a portion of skin of the sun-star, to show one of the curious
madrepore-like tubercles which are found upon this common star-fish.
Fig. 3 is a portion of cuttle “bone,” very slightly magnified, in order
to show the beautiful pillar-like form of its structure; and Fig. 4
is the same object seen from above. When ground very thin this is a
magnificent object for the polariscope.
One or two miscellaneous objects now come before our notice. Fig. 11
is one of those curious marine plants, the Corallines, which are
remarkable for depositing a large amount of chalky matter among their
tissues, so as to leave a complete cast in white chalk when the
coloured living portion of the plant dies. The species of this example
is _Jania rubens_.
Fig. 19 is part of the pouch-like inflation of the skin, and the hairs
found upon the rat’s tail, which is a curious object as bearing so
close a similitude to Fig. 22, the sea-mat zoophyte. Fig. 23 is a
portion of the skin taken from the finger, which has been injected
with a coloured preparation in order to show the manner in which the
minute blood-vessels or “capillaries” are distributed; and Fig. 26 is a
portion of a frog’s lung, also injected.
The process of injection is a rather difficult one, and requires
considerable anatomical knowledge. The principle is simple enough,
being merely to fill the blood-vessels with a coloured substance, so as
to exhibit their form as they appear while distended with blood during
the life of the animal. It sometimes happens that when an animal is
killed suddenly without effusion of blood, as is often seen in the case
of a mouse caught in a spring trap, the minute vessels of the lungs and
other organs become so filled with coagulated blood as to form what is
called a natural injection, ready for the microscope.
Before leaving the subject I must ask the reader to refer again for a
moment to the frog’s foot on Plate X., and to notice the arrangement
of the dark pigment spots. It is well known that when frogs live in a
clear sandy pond, well exposed to the rays of the sun, their skins are
bright yellow, and that when their residence is in a shady locality,
especially if sheltered by heavy overhanging banks, they are of a deep
blackish-brown colour. Moreover, under the influence of fear they will
often change colour instantaneously. The cause of this curious fact is
explained by the microscope.
Under the effects of sunlight the pigment granules are gathered
together into small rounded spots, as seen on the left hand of the
figure, leaving the skin of its own bright yellow hue. In the shade the
pigment granules spread themselves so as to cover almost the entire
skin and to produce the dark brown colour. In the intermediate state
they assume the bold stellate form in which they are shown on the right
hand of the round spots. Very remarkable forms of these cells may be
found in the skin of the cuttle-fish.
Figs. 24 and 25 are two examples of coal, the former being a
longitudinal and the latter a transverse section, given in order to
show its woody character. Fig. 17 is a specimen of gold-dust intermixed
with crystals of quartz sand, brought from Australia; and Fig. 21 is a
small piece of copper-ore.
Every possessor of a microscope should, as soon as he can afford it,
add to his instrument the beautiful apparatus for polarising light.
The optical explanation of this phenomenon is far too abstruse for
these pages, but the practical application of the apparatus is very
simple. It consists of two prisms, one of which, called the polariser,
is fastened by a catch just below the stage; and the other, called an
analyser, is placed above the eye-piece. In order to aid those bodies
whose polarising powers are but weak, a thin plate of selenite is
generally placed on the stage immediately below the object. The colours
exhibited by this instrument are gorgeous in the extreme, as may be
seen by Plate XI., which affords a most feeble representation of the
glowing tints exhibited by the objects there depicted. The value of the
polariser is very great, as it often enables observers to distinguish,
by means of their different polarising properties, one class of objects
from another.
If the expense of a polarising apparatus be too great for the means
of the microscopist, he may manufacture a substitute for it by taking
several thin plates of glass, arranging them in a paper tube so that
the light may meet the surface of the lowest one at an angle of about
52°, and placing the bundle above the eye-piece to act as an analyser;
whilst, by using a plate of glass, and so arranging the lamp that the
light falls upon it at the above angle, and is reflected up the tube
of the microscope, he will find on rotating the extemporised analyser
that the phenomena of polarisation are to a great extent reproduced;
whilst by splitting an extremely thin film from the surface of a sheet
of mica, such as is employed for making smoke-screens above glass
globes, he will have a substitute for the selenite by means of which
alone can the gorgeous effects be produced. The extemporised apparatus
will not, of course, give such perfect effects, but this is sometimes
an advantage, and the present writer has used the same means with
considerable success in photographing starch-granules.
XIV.
FIG.
1. Dero latissima
2. Slavina serpentina
3. Pristina longiseta
4. Lophopus crystallinus
5. Fredericella sultana
6. Stylaria proboscidea (head)
7. Hydatina senta
8. Synchœta mordax
9. Asplanchna Brightwellii
10. Notommata aurita
11. Triarthra longiseta
12. Euchlanis triquetra
13. Brachionus amphiceros
14. Salpina mucronata
15. Pterodina patina
16. Anurœa brevispina
17. Dinocharis tetractis
18. Noteus quadricornis
19. Floscularia ornata
20. Young Melicerta
21. Macrobiotus (sp.?)
22. Cypris fusca
23. Cyclops quadricornis
[Illustration: XIV.]
CHAPTER XI
Hints on the Preparation of Objects--Preservative Fluids--Mounting
Media--Treatment of Special Objects.
The microscopist who relies altogether on the dealer for his permanent
preparations may expend a good deal of money, but the satisfaction
which he derives from his hobby will be very inferior to that
experienced by the worker who endeavours to secure, for exhibition or
for reference, specimens of the objects which he finds most interesting
and instructive to himself.
It will be our endeavour in the following pages to give a summary of
the elementary principles upon which reliance is to be placed, though
it must be clearly understood that the technique of the subject,
already occupying a vast amount of literature, is extending day by day,
so that it is impossible to deal exhaustively even with one single
section of it. Reference must be made, for further information, to
such publications as the _Journal of the Royal Microscopical Society_,
or that of the Quekett Club, or to the monographs on the various
departments. Davies’ work on the general subject will also be found
useful by the beginner.
Taking first the question of reagents, we may mention five which leave
the cells of a tissue as nearly as possible in the natural condition,
but fit for permanent preservation. The first of these, in order of
importance and of general applicability, is alcohol, represented for
most purposes by methylated spirit, which contains about 84 per cent.
of absolute alcohol, though, unfortunately for our purpose, there is
a certain quantity of mineral naphtha in it in addition. This last
has the effect of making it go milky upon dilution with water, which
is a considerable disadvantage, though the milkiness disappears to
some extent on standing, and it is rarely worth the while of the
ordinary microscopist to go through the formalities necessary to obtain
permission to purchase unmineralised spirit, which cannot be had in
quantities of less than five gallons (as it is only to be had from the
distillers under an Excise permit), and distillers may not supply less.
Four parts of methylated spirit with one of water forms the classical
“70 per cent.” alcohol, the most generally useful of all fluids for
hardening and preserving purposes. A considerable quantity of this
fluid should always be available.
Whatever other fluid may be used to begin with, spirit must almost
always be used to finish the process, and fit the tissue for
section-cutting and staining.
Of purely preservative, or fixative, fluids, we may mention “formalin,”
a 40 per cent. solution of formic aldehyde, which is rapidly coming to
the front, as indeed it deserves to do. It is but slightly poisonous,
if at all, and leaves in the tissue nothing which requires subsequent
removal before proceeding to harden for section-work, whilst it is an
admirable preservative of cell-form.
Another admirable but highly poisonous reagent is corrosive sublimate,
in saturated solution, with 2 per cent. of acetic acid.
A fourth is osmic acid, used in 1 per cent. solution. This is a highly
valuable reagent, but extremely expensive, very poisonous, and giving
off fumes which are most irritating to the eyes.
The fifth, a very gentle, but in many respects very satisfactory one,
is picric acid in saturated solution. Tissues preserved in this medium
must not be washed out with water, as it enters into very feeble
combination with protoplasm, and the cells swell and disintegrate as
the reagent is dissolved out.
Of mounting media we may mention glycerine, glycerine jelly (made
by dissolving starch in glycerine with the aid of heat), and Canada
balsam, dissolved in xylol or benzole. The Canada balsam must be dried
hard by evaporation over a water-bath, and dissolved as wanted. Under
no circumstances should raw balsam be used, as it takes years to set
hard, and turns of a deep yellow colour in the process.
Chloroform is frequently used as a solvent, but it has the disadvantage
of attacking and extracting a large number of the aniline dyes used for
staining structures, an objection from which the mineral solvents are
free.
We will now proceed to go through the objects already referred to, and
indicate the method of preservation.
For the study of the cell-structures of plants the portion to be
examined is to be placed in spirit of about 30 per cent. strength,
which is changed after twenty-four hours for 40 per cent., after a
further twenty-four hours for 55 per cent., and finally, as regards
our present purpose, in 70 per cent. spirit, in which it may remain
until required for section-cutting. The effect of this treatment is
to extract the bulk of the water from the tissue, with the minimum of
shrinkage of the cells, the latter being preserved in their natural
relations to surrounding parts.
In some cases, however, it is desirable to examine and preserve
delicate structures, or parts, or dissections, in a medium which allows
of the retention of the greater part of the natural moisture, and in
such a case the tissue is immersed in glycerine diluted very much in
the same way as the alcohol in the last process, but with very much
longer intervals between the alterations of strength, until it reaches
pure glycerine, in which it remains for a considerable time, as the
exchange between the tissue and the dense fluid surrounding it goes on
very slowly.
A combination of the two methods is also possible, the spirit-hardening
being carried out for a portion of the time, and the tissue being
thereafter transferred to glycerine, diluted or pure.
The object of using glycerine at all is merely that it has a much
lower refractive index than balsam, so that delicate structures may
sometimes be better seen in the former medium, but balsam is to be
preferred wherever it is possible to use it, _i.e._ almost always.
The writer has not mounted a preparation in glycerine or a medium
containing it for many years, nor, with proper staining, does he think
it can ever be necessary to do so, except in the case of dissections
in which the glycerine can be slowly run in without disturbing the
arrangement, as spirit would be pretty sure to do. The harder portions
of plants, woody stems, shells of fruit, or the like, require different
treatment, and must, as a rule, be allowed to dry thoroughly before
being cut.
Starch granules are somewhat troublesome to mount satisfactorily.
The writer has tried many methods, and, on the whole, prefers a
glycerin-gelatin medium, which keeps for an almost indefinite time, and
may be made as follows: Thirty grains of gelatine (Nelson’s “brilliant”
or other transparent gelatine is to be preferred) are allowed to soak
in water, and the swollen gelatine is drained, and dissolved in the
water which it has absorbed, by the aid of a gentle heat. An equal
bulk of pure glycerine is then added. In using, a small portion is
transferred to a slide with the point of a knife and melted, a small
quantity of starch granules added, and stirred into it with a needle.
The cover-glass is then laid up on the still-fluid drop, pressed gently
down so that the drop is extended to the margin of the cover, and the
whole allowed to cool. It is then to be painted round with several
layers of Brunswick black, or Hollis’s glue, or zinc-white cement, to
prevent evaporation,--Hollis’s glue being perhaps the best medium for
the purpose.
Petals, or other parts of which it is desired to obtain a surface view,
must be mounted in cells, which may be made by the use of button-moulds
of suitable size, cemented to the glass slide with marine glue. The
slide must be free from grease, as the tissue must be fixed in position
by the use of gum, and allowed to dry thoroughly before closing the
cell, or the cover-glass will be bedewed with moisture when the cell
is closed. The best plan is, after air-drying for a couple of days,
to place the preparation on a metal plate over a beaker of boiling
water for an hour or more, and then to close the cell immediately with
Brunswick black, maintaining the heat at first to ensure rapid drying,
and then slowly withdrawing it. When cool, another coat should be
given, and rather thick covers should be used, as these preparations
are never required to be examined with high powers.
To mount pollen-grains, they should be sprinkled upon the surface of a
slide which has been previously moistened with thin gum, and allowed
to dry until it has become just “tacky”; the drying is then completed
by gentle heat and a drop of balsam placed upon the grains, with a
cover-glass over all. Bubbles will probably form, but with Canada
balsam this is not of the slightest importance, as they always come
out of their own accord, and balsam mounts should never be closed with
cement of any kind until thoroughly dry.
Air-bubbles in other media may be eliminated by the use of the
air-pump shown in Fig. 16, which may be obtained from Baker at a very
reasonable rate, and which is useful not only for that purpose, but for
accelerating the drying of moist tissues. To do this, there is placed
upon the plate of the pump a porcelain dish containing strong sulphuric
acid, and upon this is placed a little triangle of platinum wire, which
serves to support the preparation. The air is now exhausted; the tissue
parts with moisture to supply its place, and this moisture is in turn
greedily absorbed by the sulphuric acid, so that drying is rapid and
continuous, as well as very thorough, whilst the process has the great
advantage of dispensing entirely with the use of heat.
[Illustration: Fig. 16.]
Portions of many of the delicate algæ may be mounted in glycerine,
having previously been soaked in it as already described; whilst the
unicellular forms, such as desmids and diatoms, may be preserved in
almost exactly the natural condition by simply mounting them in a
saturated solution of picric acid.
Probably formalin, in a solution of 10 per cent. strength, would answer
the purpose equally well, but the writer has not tried it. It is
hardly necessary to say that, with such extremely fluid media, great
care is required in closing the cell. A thin layer of Hollis’s glue
should be first painted on, to secure the cover in position, and when
this is thoroughly dry, several successive layers must be added in the
same way.
It may be said here, that it is advisable in all cases to use circular
cover-glasses, as far as possible, as they lend themselves with great
facility to a mechanically accurate closure. This slide is placed
upon a turn-table, carefully adjusted until the cover is seen to be
central when rotated, and a brush, preferably a small camel-hair
pencil, charged with the desired fluid, but not in large excess, is
held against the junction of the slide and cover, whilst the table
is rapidly spun. A little experience will teach better than any
description what amount of fluid there should be in the brush, and how
thick the cement should be. If too thick, it will drag off the cover;
if too thin, it will flow over the latter and over the slide.
The preparation of diatom-skeletons as permanent objects is easy.
Consisting, as they do, of pure silex, or flint,--_i.e._, practically
glass,--they resist long boiling in acids, so that there is little
difficulty in isolating them from any organic matter with which they
are mingled. It is generally recommended to treat them with strong
nitric acid. This is a mistake. The acid acts much more powerfully
and less violently when diluted with an equal bulk of water, and it
is in an acid so diluted that portions of water-plants, or other
diatomaceous material, should be boiled in a glass beaker until all the
organic matter is dissolved. The beaker should be covered with a glass
plate, to prevent dissipation of the acid fumes. When the process is
complete, usually in about half an hour, the contents of the beaker are
thoroughly stirred with a glass rod, poured rapidly off into a larger
bulk of cold water, and allowed to settle for another half-hour. The
process is then repeated with a smaller bulk of water, several times,
to allow the removal of the last traces of acid, and finally with
distilled water. The separation of the diatoms into grades is effected
by settlement. The final result is poured into a tall glass vessel,
and allowed to settle for, at first, a minute, the supernatant fluid
again poured off, and allowed to settle for two minutes, and so on, the
period being gradually increased, and each sediment preserved apart.
The first will probably only be sand, but the proportion of diatoms
will increase with each separation, though there will always be a
certain proportion of sand of such a size as to settle at the same rate
as the diatoms. Marine plants especially will furnish a rich harvest by
treatment as described.
Solid diatomaceous deposits, such as kiesel-guhr, mountain-meal, and
especially the famous Oamaru deposit from New Zealand, demand different
treatment, and perhaps the best way is to disintegrate the mass, either
by boiling with Sunlight soap (though the alkali attacks the flint to
some extent) or to mix the mass with a super-saturated solution of
acetate of soda (made by saturating water with the crystals whilst
boiling), and by successive coolings, heatings, and stirrings to cause
the process of crystallisation to break up the mass, which it will do
very thoroughly. The diatoms are then separated by sedimentation, as
above described.
A small portion of the deposit may now be spread thinly on a glass
slide, allowed to dry thoroughly, be treated with balsam, and covered.
If it be desired to select individual diatoms, this must be done under
the microscope, by means of a bristle fixed in a handle either with
glue or sealing-wax. The diatom selected will adhere to the bristle
if the latter be slightly greasy, and should then be transferred to
a slightly adhesive slide, coated either with thin solution of white
shellac, or with thin gum nearly dry. When the forms desired are
mounted, the preparation should be covered in balsam. The process is by
no means as easily effected as described, however.
The preparation of insects, or parts of insects, as microscopic objects
is a tedious and difficult task. The main point is the trouble of
softening the integument and eliminating the colour.
The latter can, in any case, be only partially effected. The beginner
would do well to begin with a fairly easy form, such as the worker-ant.
A good supply of these insects may be placed in a bottle of liquor
potassæ, and left there for at least some days until they begin to
become clear and limp. From time to time a specimen may be taken,
well washed with several waters, then with acetic acid and water of
a strength of about 10 per cent., then with weak spirit, about 50 per
cent. An attempt may then be made to arrange the insect upon a slide,
spreading out the legs so as to exhibit them to the best advantage, and
when this has been done a cover-glass may be put on, supported in such
a way as to prevent absolute pressure. The spirit is then withdrawn
by means of a piece of filtering-paper cut to a point, and strong
spirit added. This is again succeeded by absolute alcohol, then by a
mixture of turpentine and crystal carbolic acid in equal proportions,
and finally the cover-glass is carefully lifted, and some thick balsam
solution dropped on, the limbs finally arranged by means of warm
needles, and the cover-glass carefully replaced and pressed gently down
by means of a clip, which may be obtained for a few pence. The whole
is then set aside to harden, the deficiency caused by evaporation made
good, the balsam allowed to dry, and the preparation finally painted
round.
The contents of the body, in large insects, must be removed, and this
is effected during the washing in water by gentle pressure with a
camel-hair brush, the process being aided, if necessary, by a small
incision made through the integument at the root of the tail. Sections
of insects require very special methods, which will hardly fall within
the scope of this work.
CHAPTER XII
Section-Cutting--Staining
No method of examination can equal, for general applicability and
usefulness, that of section-work. The relations of the parts to each
other being preserved, it is possible to draw conclusions as to their
actual relations which no other mode allows of, and we shall devote
this concluding chapter to some account of the methods to be employed
to this end.
The apparatus required is not necessarily complicated. Reduced to its
elements, it consists essentially only of a razor to cut the sections
and a dish to receive them. It but seldom happens, however, that the
relations of the parts in sufficiently thin sections can be preserved
by such a rough-and-ready method, and frequently the object to be cut
is of such small dimensions as to render it impossible to deal with it
in this way. It is therefore necessary to “imbed” it, so as to obtain
a handle by which to hold it, in such a way that it shall be equally
supported in all directions. Moreover, since the human hand can only
in exceptional cases be brought to such a pitch of skill as to cut a
series of sections, or even single ones, of the needful delicacy, some
mechanical means of raising the object through a definite distance is
highly desirable. The writer has cut many thousands of sections with
the “free hand,” but the personal equation is a large one, and is not
always the same in the same person. For single sections the method
will, with practice, succeed very well, but some means of securing
a number of sections of more or less the same thickness is usually
required.
Let us deal with the imbedding first.
If it be desired to imbed a tissue which has merely been fixed
with formalin, the block should be immersed in strong gum (made by
saturating water with picked gum arabic, white and clean) for several
days. It is then taken out and, without draining, transferred to the
plate of a freezing microtome, and the sections cut from the frozen
block, and mounted in glycerine at once.
This plan is of limited usefulness, since it allows of very little
differentiation of the tissue elements, and that only optical.
To get the best results, some plan of staining must be adopted. Perhaps
the simplest, and certainly a very excellent one, is as follows. After
the tissue has been passed from the hardening, or fixing, fluid into
the successive alcohols, as described, it is placed in the following
solution. Take about forty grains of carmine and eighty grains of
borax, dissolve in about an ounce of water, add to the mixture an ounce
of methylated spirit, and let it stand for some time with frequent
shaking; about a week will be sufficient, and the process of solution
may be hastened by gentle warming at intervals. The clear upper portion
is then poured off, and into this the block of tissue is dropped, and
allowed to remain until thoroughly penetrated. Perhaps the best plan is
to substitute the carmine solution for the 50 per cent. alcohol, and
thus to make the staining a stage in the hardening process. From the
carmine solution the tissue is transferred to 70 per cent. alcohol, to
each ounce of which two drops of hydrochloric acid have been added, and
after remaining in it for a day (with a piece of the usual size) is
placed in 70 per cent. alcohol, in two successive quantities. Sections
from this material now only require treatment with the carbolic acid
and turpentine above mentioned to be fit for mounting and covering in
balsam. We now proceed to indicate how the sections may be cut.
A mixture of wax and almond oil, in proportions varying with the
heat of the weather, usually about equal ones, is prepared. The
piece of tissue is freed from superfluous spirit by being placed on
a bit of blotting-paper for a minute or two, and is then immersed in
a quantity of the wax-and-oil mixture contained in a little box of
paper or lead-foil. The tissue is held on the point of a needle, and
lifted up and down until it is coated with the mixture, and, before
solidification of the mass sets in, is lowered into the box and left
to cool. The block now furnishes a handle, and this should be wrapped
round with paper, the sections cut with the keenest possible razor,
and as thin as possible, and placed in spirit as cut. From the spirit,
which must be the strongest obtainable, they are placed in the clearing
liquid, carbolic and turpentine, and then slid on to the slide, a drop
of balsam placed on the section, and the cover over all. Of late years
all sections of ordinary soft tissues, animal or vegetable, have been
cut by one of the infiltration methods, in which the interstices of the
tissue are filled up by some material which prevents the relations of
the cells from being altered during the process of cutting. To enter
fully into this matter would occupy too much space, and would serve no
useful purpose, for the worker who requires to make use of such means
will find it indispensable to obtain Bolles Lee’s _Microtomist’s Vade
Mecum_, in which the whole matter is exhaustively treated.
The simple method above detailed will answer most ordinary purposes,
provided that a few precautions be attended to. The chief are as
follows. The outside of the block of tissue must be sufficiently dry
for the wax-and-oil to adhere to it. The razor must be extremely
sharp, and must be kept so by application to a Turkey stone during
the section-cutting. The blade must be drawn across the tissue from
heel to point, and kept wetted with spirit the whole time, so as to
prevent any dragging of the section. The transference of the section to
the slide must be effected by means of a section-lifter, which may be
made by beating out a piece of stout copper wire to a thin flat blade;
or a small palette-knife, or German-silver lifter, may be purchased
for a few pence. The carbolic turpentine is best used by placing a
little in a watch-glass, and floating the sections on to it by lifting
them singly with the lifter, freeing them from superfluous spirit
by draining on to blotting-paper, and allowing them to float on to
the surface of the liquid in the watch-glass, so that the spirit may
evaporate from above, and be replaced by the clearing agent from below
The balsam solution should be thin, and the cover-glass must be allowed
to settle down into place without pressure.
The question of staining sections is a very large one, and is becoming
of daily increasing complexity.
We cannot go into it here, further than to say that most sections cut
from unstained tissue will yield excellent results if stained first
with Delafield’s logwood solution (to be obtained at Baker’s) to a
very slight extent, and then with a solution of safranin. The sections
should be washed with tap-water after the logwood stain, and should be
of a pale violet colour. If over-stained, the colour may to a great
extent be removed by washing with a very weak solution of hydrochloric
acid, about two drops of acid to each ounce of water, and repeated
washing in tap-water to remove the acid, and restore the violet. The
safranin stain should be weak, and should be allowed to act for some
time. From this last the sections are transferred to strong spirit,
the latter being renewed until the sections cease to give up the red
dye; and they may then be mounted as described. The results with most
tissues are superb, every detail of the structure being splendidly
brought out. Safranin alone is also an admirable stain for many
purposes.
Further information must be sought in the book already mentioned.
Let us, in closing, warn the beginner of two things which are of
general application in practical microscopy. The first is, not to be
discouraged by failures. The manipulations are in many cases very
delicate, and premiums must be paid to experience for insurance against
failure in every one of the processes.
The second is, that the most scrupulous cleanliness will hardly suffice
to prevent contamination of preparation by the all-pervasive dust
which, invisible to the eye, assumes colossal proportions under the
microscope, and the particles of which have an unpleasant habit of
collecting on the most interesting or most beautiful portion of the
preparation. This can only be guarded against by careful filtration of
all fluids, and constant watchfulness.
A preparation properly made is a thing of beauty, and a joy for
ever,--or if not for ever, at any rate for many years; and one such
will repay an infinitude of pains taken in its production.
INDEX
PAGE
Air-pump, 174
Algæ, 78
" marine, 92
Anemones, sea, 159
Antennæ, 96
Ants, 97
Bacillaria, 87
Balancers of Fly, 112
Bark, 61
Blights, 89
Blood, circulation of, 129
" corpuscles of, 128
Bone, 123
Breathing-tubes, 109
Bull’s-eye, use of, 32
Camera lucida, 25
Canada Balsam, 170
Cartilage, 124
Cells, animal, 122
" circulation in, 40
" mounting dry in, 173
" pigment, 121, 165
" spiral, 46
" vegetable, 37
Ceramidia, 93
Chlorophyll, 40
Compressorium, Beck’s, 18
Condenser, bull’s-eye, 19
" substage, 19
Confervæ, 84
Conjugation, 82, 84
Convex lenses, 7
" foci of, 7, 8
" image formed by, 10
" virtual image, 11
Corallines, 164
Corrosive sublimate, 170
Cover-glasses, 18
Desmids, 81
Diatoms, 85
" preparation of, 175
Dipping-tubes, 22
Dissection, 20
" instruments, 21
" under microscope, 24
Drawing, 25
" squares, 26
Echinoderms, 162
Entomostraca, 152
Epidermis, animal, 122
" vegetable, 68
Extemporised apparatus, 5
Feathers, 119
Fish, scales of, 118
" parasites of, 153
Fixation of cell-forms, 171
Focus of mirror, 29
Foraminifera, 156
Formalin, 164
Frog-plate, 129
Gills of mussel, 122
Gizzard of insects, 109
Glycerine-gelatine, 172
" jelly, 171
Hairs, animal, 116
" vegetable, 53
Heads of Insects, 104
Illumination, correct, 31, 32
" dark-field, 34
" for opaque objects, 33
Imbedding, 180
" by infiltration, 182
Infusoria, 135
Injection, 164
Insects, 97
" mounting of, 177
Jelly-fish, 158
Larva of _Chironomus_, 152
Light, arrangement of, 29
Live-box, 17
Logwood solution, 183
Magnification, to measure, 27
Mare’s tail, 91
Marine life, 155
Microscope, Baker’s, 14
" " “portable”, 15
" primitive, 5
" simple, 12, 13
Mildew, 89
Mirror, concave, 29
Mollusca, 161
Mounting, 168
" dry, 173
" foraminifera, 157
Mosses, 96
Muscle, 127
Nails, 124
Needles, 22
Nerve, 127
Net, 133
Nucleus, 40
Objectives, 16
Objects, drawing of, 24
" photography of, 36
Oil-cells, 58, 61
Oscillatoriæ, 84
Osmic acid, 170
Parasites, 153
Petals, 69
Picric acid, 170
Pigment, 121
Pocket magnifiers, 13
Polariscope, 166
Pollen, 71
Polyzoa, 147
Pond-hunting, 132
Preservatives, 169
Radiolaria, 157
Rotifers, 147
Safranin stain, 183
Sap, 128
Scent-glands, 57
Sea-weeds, 92
Section-cutting, 178
Seeds, 75
Skin, 120
Spiracles, 102
Sponge, fresh-water, 135
" spicules, 155
Sporangia, 92
Stage-forceps, 116
Starch, 63
" mounting, 172
Stomata, 49
Suckers, 108
Teeth, 125
Troughs, glass, 18
Water-bears, 152
Wings, 110
Wool, 116
Worms, fresh-water, 14
" marine, 160
Yeast, 89
Zoœa, 161
Zoophytes, 157
Zygnemaceæ, 85
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