To understand the general structure of grasses and to become familiar with them it is necessary to study closely some common grasses. We shall begin our study by selecting as a type one of the species of the genus Panicum.
Panicum javanicum is an annual herb with stems radiating in all directions from a centre. The plant is fixed to the soil by a tuft of fibrous roots all springing from the bases of the stems. In addition to this crown of fibrous roots, there may be roots at the nodes of some of the prostrate branches. The stems and branches are short at first, and leaves arise on them one after the other in rapid succession. After the appearance of a fair number of leaves the stem elongates gradually and it finally ends in an inflorescence.
Fig. 1.—Panicum javanicum. (Full plant.)
The stem consists of nodes and internodes. The internodes are cylindrical and somewhat flattened on the side towards the axillary bud. When young they are completely covered by the leaves and the older ones have only their lower portions covered by the leaf-sheaths. Usually they complete their growth in length very soon, but the lower portion of the internode, just above the node and enclosed by the sheath, retains its power of growth for some time.
The leaf consists of the two parts, the leaf-sheath and the leaf-blade. At the junction of these two parts there is a very thin narrow membrane with fine hairs on its free margin. This is called the ligule. (See fig. 2.)
The leaf-sheath is attached at its base to the node and it is slightly swollen just above the place of insertion. It covers the internode, one margin being inside and the other outside. The surface of the sheath is sparsely covered with long hairs springing from small tubercles. The outer margin of the sheath bears fine hairs all along its length. (See fig. 2.)
The leaf-blade is broadly lanceolate, with a tip finely drawn out. Its base is rounded and the margin wavy, especially so towards the base. On the margin towards the base long hairs are seen, and some of these arise from small tubercles. The margin has a hyaline border which is very minutely serrate. There is a distinct midrib and, on holding the leaf against the light, four or five small veins come in to view. In the spaces between these veins lie many fine veins. All the veins run parallel from the base to the apex. At the base of the blade the veins get into the leaf-sheath and therefore the sheath becomes striated. Just above the ligule and at the base of the leaf-blade there is a colourless narrow zone. This is called the collar.