PLANTS OF THE INDIAN DESERT. 187 



The prostrate habit even affects the symmetry of the whole axis in 

 F. cretica and Z. simplex where the axis is semi-terate and presents 

 a kidney-shaped appearance in T. S., the grooved surface represent- 

 ing the side in contact with the substratum. 



Soft bast forms a continuous ring and follows the outline of 

 the wood cylinder. Oxalate of lime is found in the form of clustered 

 crystals in the cortex and pith in the species of Tribulns and 

 T. oriontalis. Pith consists of thick-walled cells in species of 

 Tribulns and F. cretica ; in others it is composed of thin- walled cells, 

 The pith in S. orientalis and Z. simplex may occasionally form an 

 aqueous tissue. 



General Review, — The plants are fleshy and characterised by the 

 plasticity of the tissues as was seen in the different modifications intro- 

 duced in the prostrate axis. Outer walls of the epidermal cells are not 

 much thickened and are superficially granulated. There is abundance 

 of aqueous tissue in the leaf and axis. Mesophyll is truly isobilateral 

 and shows a distinct tendency to isobilateral symmetry. Hairs, 

 when present, are unicellular. Guard-cells are elevated, so that the 

 frflnt cavity is on a level with the surface, with the exception of S. 

 orientalis, where the stomata are depressed. The veins, are em- 

 bedded and provided in some members with distinct bundle-sheaths. 

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

 and axis. 



The assimilatory tissue is composed either of palisade cells or of 

 chlorenchyma or is absent, in which latter case the primary cortex 

 functions as an aqueous tissue. The pericycle is composed of groups 

 of stone-cells and presents an isobilateral structure. Wood forms a 

 composite hollow cylinder. Medullary rays occur only in F. cretica 

 and are uniseriate. The size of the xylem bundles and the size and 

 abundance of vessels vary on the upper and lower surface of the 

 prostrate axis. Soft bast forms a continuous ring and follows the 

 outline of the wood cylinder. 



The pith is composed of thick-walled or thin-walled cells. In 

 the latter case the pith may occasionally form an aqueous tissue. 



GERANIACEAE. 



Monsonia heliotropioides Boiss.— Plate X figs. 79, 80, 81. 

 Leaf only. Epidermal cells polygonal with outer walls papillose 

 especially so on the lower surface. Inner and outer walls equally 

 thickened. Stomata occurring on both the surfaces and surrounded by 

 ordinary epidermal cells. Guard-cells elevated. Mesophyll formed 

 of greatly elongated palisade cells on the upper side and of arm-palisade 

 tissue on the lower. Internal glands absent. Clothing hairs unicel- 



