PLANTS OP THE INDIAN DESERT. 165
Structure of the Leaf. — The epidermis consists of thin-walled
tabular cells which are larger on the upper surface. Outer walls are
flat and lateral walls are thin and straight.
Stomata are more numerous on the lower surface and are accom- panied by subsidiary cells. Guard-cells are in the plane of subsidiary cells which are elevated. The front cavity is on a level with the surface. Stomata on the axis have the same characters as of those on the leaf.
The mesophyll is composed of palisade tissue on the upper side and of loose arm-palisade tissue on the lower with an extensive system of intercellular spaces. Internal glands occur in the form of a few cells with tanniniferous contents in the mesophyll. Oxalate of lime is found in the form of bundles of acicular raphides in lower epidermal cells of the leaf ; it does not occur in the axis.
Leaves are many -ribbed. Veins of the ribs are vertically trans- current below by sclerenchyma. Smaller veins are embedded. Veins are enclosed in green bundle-sheaths.
Hairy covering consists of a few clothing hairs. They are unise- riate and bracket-shaped and are composed of a unicellular dome- shaped pedestal seated on epidermal cells, a long neck-cell and of a terminal cell bent like a hook, the tip being sharp and solid. Hairs are absent on the axis. External glands do not occur on the leaf and axis.
Structure of the Axis. — The epidermis consists of tabular cells with outer walls thickened and convexly-arched outwards. Lateral walls are thin and straight. The cortex is composed on its outer side of subepidermal collenchyma and on its inner side of assimil- atory tissue of arm-palisade cells arranged in a lacunar manner. The pericycle forms a composite ring of stone-cells. The vascular system is composed of scattered vascular bundles usually apposed to the pericyclic stone-cell ring and protected on their inner side by thin arcs of stone-cells.
The ground tissue is formed of large thin-walled parenchymatous cells filled with starch grains. It is characterised by the occurrence of numerous isolated secretory receptacles surrounded by small-celled parenchyma.
CYPERACEAE.
Cypeurs niveus Retz.— Figs.'298, 299. Margins bluntly point- ed and supported by stereome bundles. Extensive articulation tissue on the upper side of the leaf. Stomata on the lower side only. Front cavity on a level with the surface. Veins embedded and more or less in a single layer, large ones alternating with the smaller. All
