VEGETABLE ELEMENTS AND TISSUES.
We may now enter upon the consideration of the microscopic structure of
objects, beginning with those which are derived from the vegetable
kingdom, as they are more easily procured and prepared for examination
than those belonging to the animal kingdom; moreover they are not so
transparent, and hence are more readily distinguished under the
microscope, which is of importance in the case of an unpractised
observer.
_Cells._--The elements of which all plants consist are cells. Cells, in
their simplest condition, are microscopic, rounded, colourless, closed
sacs or vesicles, resembling small bladders (Plate I. fig. 2), and
consist of a thin, transparent, colourless, vegetable skin or membrane
(_a_) called the cell-wall. The cells are well seen in a little of the
pulp of an apple (fig. 2), or in a section of almost any soft part of a
plant. A high power is usually required to show them distinctly, on
account of their minute size. The outline of the cells is seen to be
double, one line indicating the inner, the other the outer, surface of
the cell-wall, the space between the two lines corresponding to the
thickness of the cell-wall.
In the pulp of the apple, the cells are loosely connected, and so retain
their rounded form; but in most parts of plants, the cells become
crowded and squeezed together, from their ordinary or normal expansion
being limited in certain directions, so as mutually to alter each
other’s shapes. The sides then lose their originally rounded form and
outline, becoming more or less straight (Pl. I. figs. 1 & 4),--the
cells at the same time mostly adhering to each other, so as to be
separated with difficulty.
The forms thus produced are various and interesting, and have all
received names by which they are distinguished. They are described in
works on botany in two ways--according to the outline (which is the most
common, as this expresses the appearance usually presented in sections
and on the surfaces of vegetable structures), or according to the entire
or solid form, which it is often a difficult matter to determine.
_Cellular tissue._--Cells aggregated thus form a tissue, which is called
cellular tissue or _paren´chyma_ (παρά, among, and ἕγχυμα, poured
substance), because it fills up the interstices of the other tissues of
plants.
In technical descriptions, the cell-structure is often left out of
consideration; and bodies composed of parenchymatous tissue are
described as being reticulated or netted, because the united sides of
the cell-walls appear as a network covering the surface.
It must be understood that parenchymatous cells are such only as have
the three dimensions of solidity (viz. the length, breadth, and depth)
nearly equal.
_Intercellular passages._--The observer will not have examined many
sections of cellular tissue, without noticing certain irregular black
lines running between the cells, as in a piece of a Geranium-(_Pelargo´nium_-)
leaf (Pl. I. fig. 1). These lines arise from the existence of passages
between the cells, containing air; and they are called intercellular
passages. By gently warming a section containing them in water over a
spirit-lamp, or by moistening the section with a drop of spirit, the
passages will be filled up with the liquid, so as to become transparent.
When the intervals between the cells are larger and broader, they are
called _intercellular spaces_.
So far, cells have been considered simply in regard to their form, as
vesicles, either rounded or altered in shape by mutual pressure. We have
now to notice the matters contained within the cells, or the
cell-contents.
_Cell-contents._--In most cells, especially when young, a minute,
rounded, colourless body may be seen, either in the middle or on one
side, called the _nucleus_; this is very distinct in a cell of the pulp
of an apple (Pl. I. fig. 2 _b_). And within this nucleus is often to be
seen another smaller body, frequently appearing as a mere dot, called
the _nucle´olus_.
The nucleus is imbedded in a soft substance, which fills up the entire
cell (Pl. I. fig. 2 _c_); this is the pro´toplasm (πρῶτς, first, πλἁσμα,
formative substance). As it is very transparent, it is readily
overlooked; but it may usually be shown distinctly by adding a little
glycerine to the edge of the cover with a glass rod, when it contracts
and separates from the cell-walls, as in the lower cell of fig. 2. The
protoplasm in some cells is semisolid and of uniform consistence, while
in others it is liquid in the centre, the outer portion being somewhat
firmer and immediately in contact with the cell-wall. In the latter
case, it forms an inner cell to the cell-wall, and is called the
_primordial utricle_. The terms “protoplasm” and “primordial utricle”
are, however, used by some authors synonymously.
The protoplasm is the essential part of the cell, and it forms or
secretes the cell-wall upon its outer surface in the process of
formation of the cell considered as a whole. It is also of different
chemical composition from the cell-wall, being allied in this respect to
animal matter.
_Chlor´ophyll_ (χλωρὀς, green; φὐλλον, leaf).--On examining a section
of any green part of a plant, as the green substance of a
Geranium-(_Pelargonium_-) leaf, it will be seen that the green colour
does not arise from the whole substance being coloured, as appears to
be the case to the naked eye, but from the presence of little grains or
granules of a green colouring-matter in the protoplasm of the cells.
This green matter is called chlorophyll. If the cells be crushed, the
granules will escape, and can be examined in the separate state.
Chlorophyll is most abundant in those parts of plants which are exposed
to the light.
_Starch._--In many cells of plants, particularly those which have
attained their full growth, other granules, larger than those of
chlorophyll, and colourless, are met with; these are the
_starch-granules_ (Pl. I. fig. 3). They are usually rounded or oblong,
and exhibit on the surface a number of rings, one within the other, or
concentric, as it is called. In the centre of the innermost ring is a
black dot or streak, arising from the presence of a little pit or
furrow, and called the _hilum_.
The starch-grains may be readily seen within cells in a thin section of
a potato (Pl. I. fig. 4); here they are very numerous, and larger than
in most other plants. A separate grain is represented in fig. 3.
The appearance of rings in the separate grains arises from the
starch-granules being composed of numerous concentric coats or layers,
like those of an onion.
A very simple and striking method of determining whether any granule is
composed of starch or not, consists in adding to it, when placed in
water on a slide, a drop of solution of iodine. As soon as this touches
the granule, it assumes a beautiful purple colour, the depth of tint
depending upon the quantity of the iodine-solution; if this be very
considerable, the granule appears almost black. The section of potato
forms a very interesting object when moistened with the iodine-solution,
the starch-granules becoming beautifully coloured, whilst the cell-wall
remains colourless, and the protoplasm becomes yellow.
The form of the starch-granules differs in different plants, so that
the kind of plant from which starch has been derived may be
distinguished by attention to the size, form, and structure of its
starch-granules. Thus, the granules represented in Pl. I. fig. 3, which
it will be noticed are all drawn under the same power, are derived from
different plants,--_a_ being those of wheat-flour, in which the hilum is
obscure, and the rings faint; _b_ is a granule of West Indian arrowroot,
in which the hilum forms a transverse crack; _c_ is a granule of
potato-starch, in which the hilum is a dot, and the rings are very
distinct; _d_ represents the compound granules of the oat, the separate
granules being figured below; _e_ is a granule of lentil-starch, with
its long dark hilum and elegant oval concentric rings; and _f_
represents a compound and separate granule of rice-starch. It will be
noticed that the granules of oat-and rice-starch are angular, as it is
called.
The knowledge of the peculiar forms of the starch-granules is important
in a practical point of view, for it enables us to recognize them when
mixed as an adulteration with other substances, and also to distinguish
the different kinds of starch from each other. Thus table-mustard, as it
is called, is principally composed of the cheaper wheat-or pea-flour,
which is easily recognized by the structure of the starch-grains.
Arrowroot is considerably dearer than potato-starch; hence in trade the
latter is fraudulently sold for the former, the adulteration being
detected with difficulty by the eye, but easily under the microscope.
Again, rice is largely mixed with wheat-flour, as it makes inferior
flour into very white bread; and this may also be readily detected under
the microscope. The reader can now understand how valuable the
microscope is in detecting adulterations, with a knowledge of the
various forms and structures of substances, especially with the aid of a
few chemical tests.
Starch-grains are altered by boiling in water, becoming swollen and
often changed into curious forms, the rings becoming faint or
disappearing. If a piece of boiled potato be examined, the
starch-granules will seem to have vanished from the cells, which are
swollen and covered with an irregular kind of network. The network
consists of parts of the protoplasm situated in the interstices of the
starch-granules, and solidified or coagulated by the heat. On crushing
the cells by pressing upon the cover, the starch-granules will escape,
swollen and partly fused together; but they may easily be recognized as
consisting of starch by the iodine test.
The granules of “tous les mois” starch are particularly well adapted for
showing the concentric rings, the granules being about twice as large as
those of the potato.
Starch-granules are best examined in water; and a small quantity only of
the starch must be placed on the slide, if the structure of the granules
is to be seen clearly. They may be mounted in glycerine, although this
makes them very transparent.
To those who possess a polariscope, starch-granules are particularly
interesting, as they exhibit a black cross, and, with a plate of
selenite laid beneath the slide, a beautiful play of colours.
In addition to the starch and chlorophyll, the cells of plants contain
other matters, as gum, sugar, &c.; but as they are dissolved in the
cell-liquid, they are not visible. In the cells of certain plants,
however, spherical globules, with light centres and black outlines, will
be met with: these consist of oil.
_Raph´ides._--Lastly, occurring in the cells of plants, especially such
as are soft and juicy (succulent), will be found minute, hard,
colourless crystals, called _raphides_ (ῥαφὶς, a needle). These are most
frequently needle-like or acicular (_acus_, a needle), but sometimes
prismatic or rod-like with flat sides; they are also not unfrequently
grouped into little tufts. They may be readily found in a piece of the
stem of garden-rhubarb (Pl. I. fig. 5 _a_), or of the common balsam.
_Porous and spiral cells._--The walls of the cells of cellular tissue
are sometimes covered with little dots (Pl. I. fig. 11 _a_), or
slit-like markings; the cells are then called porous cells. A specimen
of them may be obtained from a section of the pith of the elder
(_Sambúcus nígra_).
Sometimes cells exhibit the appearance of a spiral line marking their
walls, as if a little bell-spring were coiled up in them (Pl. III. fig.
2 _a_). These are called spiral cells, or spiral fibrous cells, and the
tissue formed by them is called fibro-cellular tissue.
We now leave the cells of ordinary cellular tissue, to examine those in
which the dimension of length predominates, so that they form tubular
cells; and first of those required to possess strength and firmness,
combined with flexibility. These qualities are met with in the cells
constituting
_Woody tissue._--Of this there are two forms, called respectively
wood-cells and woody fibres.
The _wood-cells_ are moderately long, more or less tapering and
overlapping at the ends; and the cell-walls are thickened, so as to
possess considerable firmness. These cells are found in the wood of
stems, as in the white woody portion of an ash stick, that of a
lime-tree, the stem of a Chrysanthemum, &c. (Pl. I. fig. 6). They are
closely packed, and the tissue formed by their union is called
_prosen´chyma_ (πρὸς, close, ἔλχυμα, tissue).
In the other kind of woody tissue the cells are very long and slender,
strong, yet flexible, gradually tapering at the ends, where they overlap
each other; and they have thick walls, so that, when divided
transversely, the cavity appears almost filled up (Pl. I. figs. 5 _d_,
9, & 7 _b_). This tissue is called _woody fibre_ or _pleuren´chyma_
(πλευρἀ, rib, ἔγχυμα), from its strength.
The walls of the cells of woody tissue are often covered with dots,
either simple or with an inner dot (Pl. I. fig. 6 _b_, fig. 11 _b_), or
with streaks (Pl. I. fig. 6 _a_) or with a spiral fibre (fig. 11 _b_,
_c_), either alone or with dots also.
This tissue is of great importance in plants, from its strength and
flexibility; it forms a considerable part of the veins of leaves, the
inner bark (_liber_), and of the wood of the stems of trees. It is also
very useful to man: for it constitutes hemp, of which rope and string
are made; flax, of which linen is made; cocoanut fibre; bast, used by
gardeners for tying up plants, which is the inner bark of the lime; and
jute, which is the inner bark of an Indian lime-tree.
In the white woody part of the stems of trees belonging to the fir-order
(Conif´eræ), as a piece of deal or pine, which is mainly composed of
wood-(prosenchymatous) cells, the cells exhibit rows of minute circular
markings (Pl. I. fig. 10). These were formerly supposed to be solid
bodies or glands; hence the tissue is still sometimes called glandular.
Within the outer ring of each marking is an inner central dot, or
sometimes an oblique streak. The side view of the cells (Pl. I. fig. 8
_a_), which is seen in a tangential section, shows that the markings are
minute pits, each being opposite to one of an adjacent cell, and sunk
inwards towards the centre of the cell, the inner dot or streak being a
thinner portion of the cell-wall. This glandular tissue of the Coniferæ
is interesting as forming a test-object for the defining power of the
microscope, which should show the two rings sharply and free from
colour; the section of the wood should be examined as a dry transparent
object.
The difference between the woody fibre and the wood-cells of coniferous
wood may also be seen well in a piece of deal, as cut up for fire-wood.
If the end of a stick of this be examined with the naked eye, parts of
brown rings will be seen traversing the whiter portion of the wood.
These brown rings consist of woody fibre; the white portion of
wood-cells. On making a very thin transverse section, the interior of
the woody fibres is seen to be almost entirely filled up (Pl. I. fig. 7
_b_), while the cavity of the wood-cells is much more open (Pl. I. fig.
7 _a_); the former also contain globules of turpentine.
It must be remarked here that some botanical authors include both forms
of woody tissue under the term prosenchyma. But, as we shall see
hereafter, the form of the prosenchymatous cells being sometimes used as
a character for distinguishing the cells of leaves, to which the term
pleurenchymatous cells would be inapplicable, the above distinction will
be found important.
_Vessels, vascular tissue._--In the next form of tubular cells, these
are broader and softer than the cells of woody tissue, thin-walled, and
the ends pointed; and their walls exhibit spiral or ring-like markings,
or rows of dots (Pl. I. fig. 5 _c_, _e_, _b_), indicating the existence
of one or more spiral fibres or rings. When the vessels contain spiral
fibres, they are called spiral vessels (Pl. I. fig. 5 _c_); when they
contain ring-shaped portions of fibre, they are called annular
(_an´nulus_, a ring) vessels (Pl. I. fig. 5 _e_); and when the spaces
between the fibres are partly filled up, leaving only dots, the deposit
forming a kind of network, we have a reticulated (_réte_, a net) vessel
(Pl. I. fig. 5 _b_). This tissue can easily be obtained from a piece of
cooked rhubarb, the stem of a balsam, or from any soft-stemmed plant.
Vessels very frequently contain air.
_Ducts._--The tubular cells forming ducts (Pl. I. figs. 5 _b_, 11 _c_)
are large, more or less flattened or blunt at the ends (truncated); and
the cell-membrane at first closing the ends is often removed or
absorbed, so that the ducts communicate with each other, to allow of
the free passage of the sap through them. Their walls are invariably
covered with markings, consisting of either simple or bordered dots,
resembling those met with in the preceding forms of tissue. The ducts
are often easily recognizable with the naked eye, in transverse sections
of stems, by the large pores which they form in the wood. These may be
well seen in a section of a piece of cane. The tissue composed of dotted
ducts is called bothren´chyma (βὁθρος, pit); but the term is principally
applied to those ducts in which the dots are simple, _i. e._ have no
inner dot.
The structure of the above forms of tissue may be best understood in
relation to their development. It has been stated that the essential
part of the cell is the protoplasm. As cells grow older, new matter is
deposited by the protoplasm upon the inner surface of the cell-wall,
either to a small extent, evenly and uniformly, as in ordinary
parenchyma, or unevenly, in the form of spiral layers, forming fibres or
bands, leaving bare spaces, where the original cell-wall exists alone.
The matter thus deposited is called secondary deposit, the original
cell-wall being the primary deposit. When the secondary deposit covers
the interior of the cells except at certain slit-like spaces, we have
the appearance figured in Pl. I. fig. 6 _a_. When the deposit forms a
spiral fibre, or a series of rings, we have the spiral or annular vessel
or duct. And when the interspaces between the coils of a close spiral
fibre are filled up except at certain spots, we have the dotted or
reticulated vessel or duct.
In many instances, these deposits are present together: thus, sometimes
the outermost deposit leaves rounded pits or dots, while an inner
portion forms a spiral fibre (Pl. I. fig. 11 _b_); or one layer leaves
simple rounded pits, while the other leaves smaller slits or dots placed
opposite the former (Pl. I. fig. 6 _b_).
In some cells the cavity is almost entirely filled up by secondary
deposit, which leaves minute canals radiating from a small cavity in the
centre to the circumference, as seen in the transverse section of a
plum-stone (Pl. I. fig. 56); here the canals appear as dark lines. In
others, again, the secondary deposit forms several distinct layers,
leaving channels very similar to those of the last; an example is met
with in the gritty tissue of the pulp of a pear.
The obvious use of the pits and channels in the above tissues is to
preserve the permeability of the walls of the elements, which would be
destroyed if the walls were equally thickened all over.
_Cell-formation._--New cells are formed by the division of old or parent
cells. The actual process of division is difficult to observe, as it
requires prolonged observation; but cells are often met with in all
stages of division, of which some instances will be pointed out
hereafter. The cell-division takes place in two ways, either according
to the endogenous (ἔνδον, within, γεννἁω, to produce), or the exogenous
(ἔξω, outside, γεννἁω) method. The manner in which the division takes
place in the former is this:--At first a slight indentation or
constriction of the protoplasm occurs at the line of division; this
deepens until the protoplasm is completely divided. The freshly divided
surfaces then become coated with a new portion of cell-wall, so as to
make two or more new cells, which either remain in contact or separate
from each other. In some cases, the divided portions of protoplasm
become coated all over with new cell-walls.
In the exogenous process, a portion of the protoplasm protrudes from the
surface of the cell, carrying the cell-wall before it, so as to form a
little bud-like body; this is next cut off at its point of junction with
the parent-cell, and coated, as in the first case, with a new cell-wall,
so as to form a new cell.
_Preparation._--In examining the vegetable elements and tissues, very
thin sections must be made with a razor or thin sharp knife; these are
then to be placed in a little water on a slide. As the structures are
all minute, the distinctness with which they are seen will mainly depend
upon the proper thinness of the sections. When sections of dry stems are
to be examined, the black margins of the air-bubbles contained in the
cells often render the structure indistinct; these must therefore be
displaced by first wetting the tissue with methylated alcohol, and then
adding water to it in a watch-glass or on a slide; or the tissue may be
soaked in warm water for some hours: and this is mostly requisite in
preparing thin sections of dry tissues.
Attention must also be paid to the manner in which the section is made,
or the direction in which the portion of the plant is cut. There are
three important directions which must be distinguished, producing
transverse, longitudinal, and tangential sections. If the cuts be made
across the length of a stem, for instance, the section is called
transverse. If the cuts be made in the direction of the length, through
the centre, the section is longitudinal; and if the cuts are made in a
direction parallel to a line running down the centre of the stem, but
nearer its margin, it is a tangential section. It is scarcely necessary
to mention that an oblique section is intermediate between a transverse
and a longitudinal section.