Page:The Journal of Indian Botany.djvu/781

PLANTS OF THE INDIAN DESEKT. 273 Structure of the Leaf. — The epidermal cells differ much in size and shape in different species as well as on two surfaces in the same species. They are either polygonal as in the greater number of species ; or vertically elongated either on both the surfaces as in G. catharticus, E. flagellifera and species of Andropogon or only on the upper surface as in D. sanguinalis, E. Royleamis and E. aristata. They are extremely small in D. bispinata and in species of Aristida. The cells near the stereome bundles are very small. The outer walls are thickened and silicified. The lateral walls are thin and straight. The former are papillose in E. aristata (fig. 349), G. villosa and 0. Tho- maeum, the papilloso differentiation being quite conspicuous on the upper surface in the former. The upper epidermis is usually char- acterised by furrows of various depths and the epidermal cells of the furrows are quite characteristic. They are vertically elongated, thin- walled, colourless and narrowed above, and they form the articulation tissue. The articulation tissue either occupies the upper half of the mesophyll, or it extends in the form of vertical strands almost to the lower surface, the lower half of the strand being formed of horizontally elongated thin-walled cells as in C. villosa (fig. 345), P. elegans (fig. 351) species of Eleusine (figs. 346, 349), E. Royleanus (fig. 323), L. Senegal- ensis (fig. 325), and D. bispinata (fig. 357j. In E. Royleanus (fig. 326) epidermal cells on the lower surface are characterised by distinct angular thickenings of the outer walls. The cuticle is toothed between the cells in species of Aristida.

The margins are curved upwards and are strengthened by stereome bundles. The margins are sharp or bluntly pointed in T. S. of the leaf-blade. The stereome bundles are larger in the latter.

Guard-cells are accompanied by subsidiary cells ; they are either in the plane of the surrounding cells or elevated above that plane (figs. 347, 354). The front cavity is placed in a depression formed by outer thickened epidermal wall, the depression being very great in case of papillose outer walls of E. aristata (fig. 349). The stomata either occur only on the upper surface as in E. hirsutus or only on the lower as in E. pilosa, or more numerous on the upper surface as in A. funiculata, P. turgidum, D. bispinata, E. interrupta and L. senegalensis, or more numerous on the lower as in other members.

The palisade tissue forms arcs on sides of vertically transcurrenfc veins and complete girders round non-transcurrent veins. Complete girders of palisade cells are found round all the veins, whether they are vertically transcurrent or not in D. sanguinalis and P. turgidum. The palisade cells of the adjacent veins are in contact when the strands of the articulation tissue extending between the two surfaces