PLANTS OP THE INDIAN DESEKT. 283
are thickened in Capparidaceac and Euphorbiaceae, angular in Cyperaceae and gelatinised in Violaceae, Rhamneae and Sapindaceae. The thickening of inner walls reduces the loss of water. Mucilaginous modifications of the inner walls have the property of absorbing and retaining water ; and the epidermis, thus, forms a water-storing tissue. The epidermal cells in the Cy per aceae, which form an articulation tissue, have inner-walls angular, so that they come into close contact with the inner cells of the articulation tissue.
The articulation tissue is of epidermal origin and is extensively developed in Cy per aceae and Gramineae. It usually forms strands of fchin-walled cells running between two surfaces of the leaf. In the Cyperaceae it forms usually a many-layered tissue in the upper half of the leaf. The margins curve upwards as on a hinge and protect sfcomata on the upper surface of the leaf in Gramineae as well as watery contents in the aqueous cells of the articulation tissue in both the orders.
The epidermis in Violaceae, Celastraceae and Gramineae is char- acterised by palisade-like elongation of epidermal cells. Large cells, intercalated among ordinary epidermal cells, distinguish Cappari- daceac, Elatineae, Malvaceae, Moring aceae, Lcguminosae, Ficoideae, Salvadoraceae and Polygonaceae. Both of these characters are con- trivances for storage of water.
The epidermis is sometimes locally two-layered by means of cross walls parallel to the surface, appearing in the epidermal cells of Violaceae, Tamariscineae, Burseraceae, Celastraceae, and Salvadoraceae. In Violaceae and Burseraceae the inner walls are gelatinised and a mucilaginous mass is found beneath the division walls. It is usually two-layered in the axis of some of the species of Polygalaceae, Elatineae, Geraniaceae, Papilionaceae, Salvadoraceae, Asclepiadaceae and Euphor- biaceae. The occurrence of a two- layered epidermis either locally or throughout intensifies the protective function of the epidermis, name- ly that of reducing the loss of water by transpiration.
Yellowish contents in the epidermal cells of Rhamneae are of the nature of cellulose slime which reduces loss of water by transpiration by thickening the watery contents in the cells. Cystolith-like struc- tures occur in the basal cells of clothing hairs of Cucurbitaceae and Boraginaceae and also in large cells intercalated amongst ordinary epidermal cells in Acauthaceae. Calcareous cystolith-like structures give an acrid taste to juices of foliage and prevent the plants from being easily devoured by animals. The epidermis, in Actinioptcris dichotoma which is the only Eern found in the Indian Desert, is mostly sclerosed. This sort of modification of the epidermis is
