PLANTS OF THE INDIAN DESERT. 193 



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- 



