The shoot-system.—The shoot-system varies with the duration of the life of the plant. In annual grasses stems are in most cases erect and even if they are not entirely so they become erect at the time of flowering. They are attached to the soil by a tuft of fibrous roots arising from the base of the stems. But in perennials in addition to erect branches, creeping branches, stolons and rhizomes may occur.
Fig. 8.—Prop roots of Andropogon Sorghum.
Fig. 9.—Aerial roots of Ischæmum ciliare.
The stem is either cylindrical or compressed and consists of nodes and internodes. In most grasses the internodes are usually hollow, the cavity being lined by the remains of the original pith cells. However, there are also grasses in which the stems remain solid throughout. In many grasses the basal portions of stems are more leafy and the internodes are short, but in the upper portions the internodes become longer separating the leaves one from the other.
In young shoots the leaves grow much faster than the internodes and consequently internodes remain small, and leaves become very conspicuous. The youngest portions of the shoots are by this means always well protected by the surrounding leaf-sheaths. As soon as leaves have grown fully, the internodes begin to elongate rapidly separating the leaves. At first growth in length takes place throughout its length in the internode and when it gets older this elongation ceases. But, however, the lower portion of the internode close to the node and which is enclosed by the leaf-sheath retains its power of growth for a considerable time.
Branches arise from the axils of leaves and when a considerable number of the axillary buds, especially from the lower nodes, develop into branches the plant becomes tufted in habit. In most grasses branches grow upwards through the sheath and emerge at its mouth as aerial branches. Such branches are called intravaginal branches or stems. But in some grasses axillary buds, instead of growing straight up through the sheath, pierce the leaf-sheath, come out and then they grow out as branches. This may be seen in the underground stolons of Panicum repens and in the ordinary aerial branches of Arundo Donax. Branches that pierce through the sheaths are called extravaginal branches. (See fig. 10.)