PLANTS OF THE INDIAN DESERT. 185 



aqueous tissue on the lower side, in F. cretlca to a comparatively exten- 

 sive ventilating system and to the glandular' nature of the plant and 

 in Z ygophyllum simplex; to the abundance of aqueous tissue. The 

 depressed position of the stomata in S. orientalis is perhaps clue to 

 the absence of aqueous tissue in the mesophyll. 



The mesophyll in species of Tribulus (fig. 68) is composed of a 

 layer of long palisade cells on the upper side and of a subepidermal 

 layer of polygonal aqueous cells and a layer of short palisade cells on 

 the lower ; the middle tissue is represented by the chlorenchymatous 

 cells of the bundle-sheaths. In the cylinderical leaves of Z. Simplex, 

 the assimilatory tissue forms a subepidermal composite ring of palisade 

 cells and encloses a massive aqueous tissue of large thin-walled cells, 

 which surrounds the central vascular bundles. In Scetzenia orien- 

 talis (fig. 71) there is a two-layered palisade tissue on the adaxial 

 side and a two-layered arm-palisade tissue on the abaxial side ; the 

 middle tissue consists of thin-walled polygonal cells, some of which 

 contain clustered crystals. The mesophyll in F. cretica (6g. 77) is 

 composed of palisade tissue on both the sides with the middle tissue 

 represented by the bundle-sheath cells. In F. cretica there occur 

 rounded groups of faintly green palisade-like cells, with perhaps a 

 water-storing function, between the lower epidermis and the vein of 

 the mid-rib. Internal secretory organs do not occur in any of the 

 members. 



Oxalate of lime occurs in the form of clustered crystals in the 

 neighbourhood of the veins in species of Tribulus, S. orientalis and 

 Z. simplex in which the inner cells of the sheaths of the peripheral 

 veins mostly contain clustered crystals (fig. ?5). 



The veins are embedded and are provided with green bundle- 

 sheaths. The sheath-uells are thin-walled and polygonal in F, cretica 

 they are cubical and thick-walled in the other members of the order 

 The veins are numerous in species of Tribulus, F, cretica and 

 Z. simplex. The abundance of veins corresponds with the abundance 

 of watery contents in the mesophyll. The peripheral veins in 

 Z. simplex form a supporting network of veins* for the massive 

 aqueous tissue. 



Hairy covering is present only on the leaf and axis, in species 

 of Tribulus. It consists of simple unicellular hairs which are more 

 numerous on the lower surface of the leaf (figs. 68, 69). External 

 glands are not found on any of the members. 



Structure of the Axis. — Epidermis in species of Tribulus consists 

 of small thick-walled cells. The epidermal cells in S. orientalis are 

 polygonal with outer and inner walls thickened and with lateral walls 

 thin and undulated : there are large cells, perhaps with a water- 



