[§ 2.] CELL-CONTENTS.
414. The living contents of young and active cells are mainly protoplasm with water or watery sap which this has imbibed. Old and effete cells are often empty of solid matter, containing only water with whatever may be dissolved in it, or air, according to the time and circumstances. All the various products which plants in general elaborate, or which particular plants specially elaborate, out of the common food which they derive from the soil and the air, are contained in the cells, and in the cells they are produced.
[415.] Sap is a general name for the principal liquid contents,—Crude sap, for that which the plant takes in, Elaborated sap for what it has digested or assimilated. They must be undistinguishably mixed in the cells.
416. Among the solid matters into which cells convert some of their elaborated sap two are general and most important. These are Chlorophyll and Starch.
[417.] Chlorophyll (meaning leaf-green) is what gives the green color to herbage. It consists of soft grains of rather complex nature, partly wax-like, partly protoplasmic. These abound in the cells of all common leaves and the green rind of plants, wherever exposed to the light. The green color is seen through the transparent skin of the leaf and the walls of the containing cells. Chlorophyll is essential to ordinary assimilation in plants: by its means, under the influence of sunlight, the plant converts crude sap into vegetable matter.
418. Far the largest part of all vegetable matter produced is that which goes to build up the plant's fabric or cellular structure, either directly or indirectly. There is no one good name for this most important product of vegetation. In its final state of cell-walls, the permanent fabric of herb and shrub and tree, it is called Cellulose ([408]): in its most soluble form it is Sugar of one or another kind; in a less soluble form it is Dextrine, a kind of liquefied starch: in the form of solid grains stored up in the cells it is Starch. By a series of slight chemical changes (mainly a variation in the water entering into the composition), one of these forms is converted into another.
[419.] Starch (Farina or Fecula) is the form in which this common plant material is, as it were, laid by for future use. It consists of solid grains, somewhat different in form in different plants, in size varying from 1/300 to 1/4000 of an inch, partly translucent when wet, and of a pearly lustre. From the concentric lines, which commonly appear under the microscope, the grains seem to be made up of layer over layer. When loose they are commonly oval, as in potato-starch (Fig. [462]): when much compacted the grains may become angular (Fig. [463]).
Fig. 462. Some magnified starch-grains, in two cells of a potato. 463. Some cells of the albumen or floury part of Indian Corn, filled with starch-grains.