PLANTS OF THE INDIAN DESERT. 283 



the epidermal cells large and having water-storing function. Mesophyll 

 isotailateral. Clustered crystals found near the veins and in cortex. 

 Hair-like structures, with walls thickened and superficially rugose, 

 found on the leaf. Perioycle formed of rhomboidal groups of stone- 

 cells, Pith formed of thick-walled cells. 



Structure of the Leaf. — The epidermis consists of polygonal cells 

 with outer walls thickened, papillose and superficially granulated, 

 except in species of Mollugo in which the cells are very thin-walled. 

 The thickening of the outer walls is considerable in L. indicum 

 (fig. 157). Some of the epidermal cells in T. pentandra (fig. 114) are 

 bladder-like and are attenuated at their apices into hair-like structures ; 

 they are numerous on the lower side of the mid-rib. There are also, 

 in M, Gerviana and L. indicum, large cells intercalated amongst the 

 epidermal cells of ordinary dimensions. All these specialised cells 

 seem to have the function of absorbing and storing water. The lateral 

 walls are thin and undulated. 



The thinness of the outer walls, specially in M. hirta and M 

 Gerviana, can be accounted for by the presence of a protective covering 

 of dense stellate hairs and by the occurrence of large water-storing 

 epidermal cells. 



The stomata are usually numerous on both the surfaces of the 

 leaf and are surrounded by ordinary epidermal cells. The front 

 oavity is placed in a depression formed by outer thickened and papil- 

 lose epidermal walls (figs. 145, 158). The stomata in T, triquetra 

 (fig. 139) are elevated and occur only on the lower surface, owing to 

 the occurrence of an extensive aqueous tissue on the upper surface. 

 The guard-cells are usually elevated, though in T. triquetra, T. pent- 

 andra and in M. Gerviana elevated guard-cells occur side by side 

 with guard-cells situated in the plane of the surrounding cells. The 

 stomata are, as a rule, numerous and can be accounted for by the 

 fleshy character of the leaf. 



The mesophyll is isobilateral'in M. hirta (fig. 150) and M. Cerviana 

 (fig. 154), in other members it is bifacial. The mesophyll, in T. Pen- 

 tandra (fig. 141), M. nudicaulis (fig. 151) and G. pharnaceoides, con- 

 sists of palisade tissue on the upper side and of spongy tissue on the 

 lower. The structure of the mesophyll in T. triquetra is characteristic ; 

 it consists on the abaxial side of a subepidermal aqueous layer and of 

 arcs of palisade cells on the lower side of the veins and on the 

 adaxial side of an extensive aqueous tissue of large polygonal cells. 

 In 0. dccumbens and L. indicum (fig. 157) there is palisade tissue on 

 the upper side and arm-palisade tissue on the lower. 



Internal secretory organs are not found in the leaf and axis. 

 Oxalate of lime occurs in the form of numerous large clustered cry- 



