Fig. 754.—Transverse section of exogenous wood, showing the growth of nine years.
The stem is that portion of the plant-axis which grows upwards or above ground, and may be, as we have just read, subterranean. As the great function of the root is to procure sustenance for the plant, the stem assists in carrying the nourishment through the branches and leaves. We shall find two forms of stem—the underground, or root-stock, and the stem proper. There are in these two former several varieties as under:—
1. The Bulb, which is a short globular stem surrounded by thick leaves, and producing buds—as, for example, the onion.
Fig. 755.—Section (magnified portion) of the small cut a.
2. The Tuber, similar to the foregoing in shape, having no leaves, however; the potato is an instance.
3. The Rhizome (root-stock), like a root only producing buds, which roots do not. The iris will serve as an example.
The varieties of the stem-proper are:—
(1) Filiform, or thread-like, simple, or branched, as in mosses.
