PLANTS OF THE INDIAN DESEKT. 277
of Andropogon there is a composite sub-epidermal stereome tube in which unmixed small vascular bundles are embedded. In D. bispinata there is no stereome tube and the mechanical tissue is represented by isolated sub-epidermal girders alternating with sub-epidermal I-girders.
Besides the two systems of the mechanical tissue described above, the vascular bundles in the medullary tissue are supported by arcs of stereome on the outer side in D. bispinata and in species of Eleusine, or are enclosed in rings of stereome in E. pilosa, C. villosa, E. hirsidus, L senegalensis, C. catharticus and species of Aristida. The sclerenchyma in these cases chiefly protects the vascular bundles and is therefore of a local value ; it does not add much to the strength of the mechanical tissue system.
The vascular system consists of vascular bundles of two sizes. The smaller bundles are embedded in the stereome tube, when it is developed, except in E. kirsutus (fig. 330j in which they are embedded in the assimilatory tissue. Of the larger vascular bundles, the peri- pheral ones are mostly apposed to the inner side of the stereome tube a few being embedded in it ; and the rest are placed in the medullary, tissue in which they are supported by arcs or rings of stereome in some cases as already described.
The larger vascular bundles in G. Royleana (fig. 343) are more or less arranged in a ring and are embedded in the stereome tube. The vascular bundles of the larger type in E. kirsutus are numerous and traverse the whole of the medullary tissue.
The medullary tissue consists of thin-walled cells except in E. hirsutus and L. senegalensis where it consists throughout of thick- walled cells filled with granular contents. Medullary cells of G. Roy- leana are also filled with starch grains.
General Review. — The margins of leaf-blades are curved upwards. The articulation tissue is well developed ; it is of epidermal origin and has a double function, that of an aqueous tissue and that of bringing about the curving movements of the leaf-halves. Leaf-blades are more or less furrowed on both the surfaces. The hairy covering is denser on the upper surface and usually consists of short, unicellu- lar spiny hairs arising from epidermal cells above the veins and arching over the furrows. In some members there are long unicellu- lar hairs, arising usually in the case of leaves, from articulation tissue cells. The external glands occur in 0. Tliomaeum, G. Royleana and D. bispinata ; and are composed of a stalk cell and of a club shaped head.
