In most of the spongy parts of plants, as in the pulp of fruits and pith of elder, the cells preserve the globular or oval shape represented in fig. 739 A. But the cells, in consequence of that mutual pressure, more frequently assume the form of a polygon (fig. 739 B), the section of which is generally hexagonal. The cellular tissue may generally be compared to the bubbles produced by blowing through a straw or tobacco-pipe into soap and water; or it may be illustrated by placing balls of moist clay together, and then pressing them more or less strongly. In this manner every individual ball assumes a polygonal shape corresponding to the form of the cells represented in fig. 739 C, and which disposition is, in many plants, preserved with the utmost regularity. Such cells as are, with tolerable equality, extended in all directions, are named parenchyma, and of these are composed the tuberous parts of plants, as the potato, dahlia-roots, etc., and especially the soft, spongy parts of the pith, bark, leaves, etc. We frequently, however, meet with cells which are extended longitudinally, and pointed at both extremities, as in fig. 740. The sections of these cells, which are compactly arranged, have the appearance of a hexagon. They are termed woody cells, or woody tissue (prosenchyma), and constitute the chief portions of the more solid parts of plants, as the ligneous parts of trees, shrubs, etc. Very long, flexible cells, as those which constitute the fibres of flax and hemp, are called bast-cells, and appear under the microscope as round threads of uniform thickness, whereas the fibres of cotton wool, which rarely exceed one or two inches in length, when magnified, present the appearance of flattish bands with somewhat rounded margins. By these marks, the union of flax and cotton in the same web or piece of cloth may be detected.
Occasionally the cells assume very abnormal shapes, as the stellate or star-formed cells. These are described as irregular cells.
Fig. 741.—Vascular tissue.
As every plant, whether small or great, is only an aggregate of a great number of cells, so, also, the life of a plant is nothing else but the sum of the activities of all the cells of which it is composed. The special province of the cells is to receive from the soil or atmosphere the water necessary for the various vegetative purposes, together with the nutritious materials dissolved in the watery and aerial fluids, and to circulate them through the whole body of the plant. The circulation within a plant is not carried on through the agency of tubular channels, but only by the passage of sap in all directions from one cell to another.
Since the cells have no openings, it is somewhat difficult to understand in what manner the fluid can enter into the plant from without, and by what means it can inwardly pass from cell to cell. This phenomenon, however, is dependent on the peculiar quality both of vegetable and animal membranes and fibres—viz., that they are permeable by many fluids, without being dissolved by them. Experiments show that this permeative action is carried on in accordance with definite laws. When two fluids of unequal densities—as, for example, an aqueous solution of sugar and mere water—are separated from each other by a diaphragm of pig’s bladder, we perceive a constant tendency on both sides to restore the equilibrium in the density of the two fluids. A portion of the water penetrates the bladder, mixing with the solution, and a portion of the latter finds its way to the former by the same medium. In this experiment one important fact is to be observed—viz., that the lighter fluid always passes through the separating medium more rapidly to the denser than vice versâ; consequently, in this experiment more of the water passes through the bladder to the saccharine solution than of the latter to the water. This permeative capability of the tissue of vegetables and animals is called endosmose.
The cells both circulate the sap and alter its condition, so we find differing substances in the same plant. The cell as described creates new cells, and the force with which the sap rises is rather greater than the pressure of the atmosphere.
The vascular, or fibrous tissues, are illustrated in the margin (fig. 741). They usually contain air. Some plants have no vascular tissue, and are termed cellular plants—such as mushrooms, fungi, mosses, and seaweeds. Many contain both tissues, and these are the more highly developed kinds.
