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 <a href=\"#Page_43">43</a>, 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.