On Molecular and Microscopic Science, Volume 1 (of 2)

by Mary Somerville

Fig. 3. Development of Ulva:—A, isolated cells; B, C, clustered subdivided cells; D, E, confervoid filaments; F, G, frond-like expansions.

Plants which spread in two directions are formed and increased by the successive division of the cells into four equal parts, as in some of the fuci, and the solid vegetable mass is formed and augmented upon the same principle, so that it consists of a congeries of primordial cells or globules coated with cellulose, which by mutual pressure take a many-sided cellular form. Six or eight sides are most common; when six-sided, a section of the solid is like honeycomb, but it frequently resembles a very irregular fine lace or network. The form of the cells, which not only depends upon the number of sides but on the direction of the pressure, varies exceedingly in different plants, and in different parts of the same plant. The size of the cells averages from the three to the five hundredth part of an inch in diameter. Some are very large, as in the pulp of the orange and lemon; but in the pollen of flowering plants, and other cases, they are not more than the thousandth part of an inch in diameter, consequently invisible to the naked eye. Occasionally the cells are elongated in the direction of least pressure, as in the stems and hairs of plants, or sometimes they have a stellar form. In the looser and fleshy parts they retain their globular form and only touch one another, leaving triangular spaces between them filled with air in water plants; but in general the cells are held together by a viscid liquid. When these intercellular spaces, whether left by globular or polyangular cells, are placed the one over the other for some distance, they constitute intercellular passages or channels, and sometimes they form lacunæ or large empty spaces.

Notwithstanding its great variety of forms, this solid congeries of cells, called cellular tissue, is the universal basis of vegetable structure; it forms the principal part of all plants, and the entire mass of many. Though often highly coloured, as in flowers, green leaves and young shoots, it is frequently hyaline and colourless. The dark cells in [fig. 4] represent the green part of a leaf, the white ones are those of the colourless skin. Since the primordial cell is the medium in which light and heat act, cellular tissue is present wherever growth is in progress, for all the vital operations take place within its cells. All the organs of plants in their earliest stage consist entirely of cellular tissue, and even in full grown trees the bark and pith of the stem, as well as the soft parts of leaves and flowers, are generally composed of the cells of this tissue, which though assuming a great variety of forms never deviates far from the original type. Every important change in the structure of the cell diminishes or destroys its power of contributing to the nourishment of the plant, as appears in all the tissues derived from it, and which, according to M. von Mohl, is a necessary consequence of the disappearance of the vital part of the primordial cell from those parts of the cellular tissue destined to undergo the change.

Fig. 4. Vertical section of the cuticle of Iris germanica:—a, cells of the cuticle; b, cells at the sides of the stomata; c, small green cells placed within these; d, openings of the stomata; e, lacunæ of the parenchyma; f, cells of the parenchyma.

The fibro-vascular bundles which constitute the wood of trees, and form consecutive cylinders round the stem between the pith and the bark, consist of vascular ducts and woody fibre. The vascular ducts are formed of large wide cells each standing upon the other’s flattened end, their cavities being thus separated from each other by septa or partitions directed at right angles to their longitudinal axis. This vascular tissue when young conveys the sap from the roots through the stem and branches to the leaves. It forms part of the stems of all climbing and quick-growing plants, in which the circulation of the sap is rapid, and the perspiration great. The sap in this crude state passes freely through the partitions, being probably a dialysable liquid; but in the autumn, when the sap ceases to rise, the septa are either absorbed or destroyed, as appears from the fragments of them that sometimes remain, and then the ducts become filled with air, which they convey to mature the sap in the leaves and all parts of the plant. Some of the vascular ducts have very narrow parallel fibres of a bluish colour twisted in a more or less elastic spiral from end to end of their internal surface, which in by far the greater number of cases turns in the same direction as a left-handed screw. In some ducts they merely cross the inner wall of the cell at regular distances as circles. The reticulated form from the crossing of right and left-handed spirals is still more frequent than the simple spiral; there is scarcely a plant, from the mosses upwards, in which that structure cannot be found.

In a vast majority of cases the secondary internal membrane of some of the ducts is perforated by orifices of numerous forms, sometimes irregularly, and sometimes in a regular pattern like a sieve, and on that account they are called the pitted tissue.

In the stem of a tree the vascular cylinders alternate with cylinders of woody fibre, as may be seen in a section perpendicular to the axis, in which the two tissues form a series of alternate rings. The woody or ligneous tissue, which gives strength and solidity to all vegetable structures, consists of bundles of nearly parallel spindle-shaped tubular fibres, having their attenuated extremities applied end to end to the extremities of those above and below them, so that they form groups of nearly straight lines; but although the ends of these tubular fibres overlap each other they do not prevent a free circulation of the sap. The different layers of these combined tissues which form the wood do not convey the rising sap in equal quantity. The outermost layers that are nearest the bark, which are always the last formed or youngest, convey it in greatest quantity, and on that account are called the sap-wood: the older the layers the less they convey, because the interior walls of the cells of both tissues are coated with successive layers of a mucilaginous substance which is the colouring matter of the wood (lignin), and is called sclerogen, which becomes hard, is ultimately united to the cell walls, and fills or nearly fills the tubular fibres and vascular ducts, so that those nearest the centre of the tree lose or nearly lose the power of conducting the sap, as in hard wood like the oak, though in softer wood, as the lime tree, it is not entirely lost. Ligneous tissue forms the chief part of the stems, branches, and shoots of trees and shrubs; it gives firmness to leaves, flowers, and all their parts, and strength to the stems and skins of herbaceous plants; it is found in the bark of all trees, and constitutes the strong fibre of hemp, flax, the agave, and many other plants, whence linen, canvas, and cordage are made. Cells lined with sclerogen form the shells of nuts, cocoa-nuts, and walnuts, as well as cherry, peach, and plum stones, the brown coat of apple and pear seeds, the gritty particles in the heart of the pear, the white coats of the pips of the orange and lemon, the husks of peas, &c.

All the tissues are represented in [fig. 5], which is a longitudinal section of the Italian reed, much magnified. It consists of three parts: at a the cellular tissue of the pith is represented; b is a fibro-vascular bundle containing annular ducts (1), spiral ducts (2), a pitted duct (3), besides the long spindle-shaped threads of woody fibre; c is the exterior part of the reed, which consists of cellular tissue, the two surface rows being rather compressed and filled with coloured particles.