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
