Page:The Journal of Indian Botany.djvu/231

 horizontally tabular (fig. 88.) The outer walls are thickened and the lateral walls are thin and straight. The inner walls are thin and convexly arched inwards, so as to come into close contact with the assimilatory cells. Some of the upper epidermal cells are filled with yellowish brown contents of the nature of cellulose slime. The epidermal cells of bhe lower yurface are smaller and are horizontally tabular, the outer walls being thickened and the inner and lateral walls thin.

Stomata occur only on the lower surface and are mostly found on the ridges. Guard-cells are situated in the plane of surrounding cells and the front cavity is on a level with the surface.

The mesophyll is composed wholly of palisade cells which are more compact towards the upper surface and are somewhat loosely arranged towards the lower, especially so below the ridged where the stomata are mostly situated (fig. 93.) There are groups of palisade- like cells with faintly yellowish contents, usually above the veins. These perhaps function as water reservoirs. Cells with clustered crystals are numerous near the veins.

Internal glands are represented by groups of palisade-like cells on the upper side of the veins and by rounded cells near the veins, with faintly yellowish contents and functioning perhaps as water- storing tissue. Some of the upper epidermal cells hold faintly yellowish contents of the nature of the cellulose slime.

Secretory receptacles of lysigenous origin occur in the pith of all members and in the inner portion of primary cortex of Z. trinervia and Z. truncata. The secretory receptacles, as presented in T. S., are numerous and elongated in the former and rounded and few in the latter. The contents seem to be mucilaginous. The secretory receptacles in the pith are large and are lined by a layer of flattened cells resembling an epithelium. Some of the pith cells contain tannin.

Oxalate of lime occurs in the form of numerous clustered crystals in the neighbourhood of the veins.

The veins are embedded and have no distinct bundle-sheaths. There are numerous groups of water-storing tracheids at the termina- tions of the veins (fig. 93.) The veins of the mid-rib are vertically transcurrent above and below by collenchyma. The mid-rib is prominent below in all species ; it is grooved on the upper surface in Z.jujuba and Z. rotundifolia ; in the other species it is not grooved, the upper epidermis being composed of vertically elongated cells with inner and lateral walls thickened.

The epidermal cells of the lower side of the mid-rib and of the upper side, when it is not grooved, are small polygonal cells slight-