PLANTS OF THE INDIAN DESERT. 293 



The articulation tissue, which is composed of aqueous cells 

 of epidermal origin, is abundantly developed in Cyperaceae, Gramineae. 

 It regulates the inrolling of the margins, to such an extent sometimes 

 in Gramineae that the leaf blades are almost cylindrical, thus protect- 

 ing the stomata on the upper surface. It may, besides, supply water 

 to the tissues of plants in dry seasons. 



A tissue composed of thin-walled colourless parenchymatous 

 cells, is clearly differentiated in the middle of the mesophyll or below 

 the epidermis and may form an aqueous tissue. A clearly differen- 

 tiated aqueous tissue in the middle of the mesophyll is found in some 

 of the species of Gruciferae, Capparidaceae, Portulaceae, Salvador aceae, 

 Asclepiadaceae, and Boraginaceae. A sub-epidermal aqueous tissue 

 occurs in some species of Ficoideae, Salvador aceae, Acanthaceae and 

 Nyctaginaceae. In the centric leaves of Zygophyllum and of species 

 of Chenopodiaceae an extensive aqueous tissue is found in the central 

 portion of the mesophyll. 



Groups of palisade cells, faintly green in colour, are found in 

 some of the species of Bhamneae and Gucurbitaceae, and may serve 

 occasionally as groups of water-storing cells. Lysigenously formed 

 cavities occur frequently in the mesophyll of some of the species of 

 Polygalaceae, Bubiaceae and Cyperaceae and may serve as reservoirs of 

 water. 



Water-storing tracheids found either at the terminations of the 

 veins or independently developed at intervals between the veins are 

 the means of supplementing the water supply in some of the species 

 of Capparidaceae, Tiliaceae, Simarubaceae, Bhamneae and Salvador- 

 aceae. The occurrence of water-storing tracheids in these species, 

 which possess otherwise no aqueous tissue, is necessary on account 

 of the fact that they are usually found on the driest portions of the 

 desert. 



Gelatinisation of the inner walls of epidermal cells in species of 

 Violaceae, Burseraceae and Sapindaceae is a means of absorbing and 

 retaining moisture. Mucilaginous tissues also occur in the leaf and 

 axis of some of the species of Malvaceae, Sterculiaceae, Tiliaceae, 

 Papilionaceac and Bosaceae ; and they have the same function. 



An aqueous tissue is developed in the cortical parenchyma of 

 the axis of soma of the species of Bosaceae, Salvadoraceae, Asclepi- 

 adaceae, Boraginaceae, Acanthaceae and Chenopodiaceae. The pith 

 also may sometimes form an aqueous tissue. 



The development of an aqueous tissue in the desert plants is the 

 direct result of the arrest of transpiration, which is brought about 

 by thickened and cuticularised or silicified outer epidermal walls, 



