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.