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 (pa??, among, and ????a, 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 (p??t?, first, p??sa, 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 (??????, green; f?????, 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 (?af??, 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 (p???, close, ????a, 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 (p?e???, rib, ????a), 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 (?????, 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 (??d??, within, ?e????, to produce), or the exogenous (???, outside, ?e????) 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.