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,
