Page:The Journal of Indian Botany.djvu/790

282 THE JOUENAL OF INDIAN BOTANY.

Epidermis. — As a general rule, the epidermal cells on the upper surface of the leaves are larger than those on the lower and epidermis of the axis in the same species consists of much smaller cells than those of the leaves. The epidermal cells are generally tabular with outer walls convexly arched outwards. The outer walls are flat in Menisper- maceae, Commelinaceae and Cyperaceae. The outer walls are thickened except in Tamariscineae and Commelinaceae, the thickening being considerable in Capparidaceae, Tiliaceae, Gaagraceac, Bubiaceae, Gentianaceae, Scrophulariaceae, Acanthaceae, Verbcnaceae, Labiatae. Amarantaceae, Polygonaceae and Euphorbiaceae. The outer walls when flat are generally smooth ; when they are convexly arched, outwards, as is generally the case, they are {a) either superficially granulated in Portulaceae, Zygophyllaceae, Ficoideae, Asclepiadaceae and Chenopodiaceae, (b) or with cuticle striated in Simarubaceae, Lythraceae, Gentianaceae, Convolvulaceae, Scrophulariaceae and Polygonaceae, (c) or with cuticle muriculate in Violaceae, Rubtaceae, Boraginaceae and Solanaceae, {d) or with cuticle toothed in certain Lcguminosae, Ficoideae, Compositae, (/) or with papillose differ- entiation in Capparidaceae, Caryophyllaceae, Malvaceae, Geraniaceae, Moringaceae, Caesalpinieae, Euphorbiaceae and Gramineac.

It should be noted that in succulent plants the outer walls are flat and that in other plants they undergo various modification. Convexity and papillose differentiation of the outer walls are chiefly useful for reflecting back the rays of light ; and in the case of papillose differentiation the papillae can also retain a comparatively moist atmosphere in the interspaces between them. The thickening of the outer walls reduces the loss of water by way of transpiration which is accelerated by various climatic factors in the desert. Cuticle, roughened in various ways as described above, can absorb less light and can thus reduce the loss of water which is accelerated by the strong glare and sun light reflected from the sand. The hard excres- cences on the surface of the leaf and axis are produced by the defi- ciency of water.

The lateral walls are either straight or undulate. The latter character is common among the desert plants and givos rigidity to the epidermis.

The inner walls are usually thin and convexly arched inwards, much more so in Elatineae and Geraniaceae, thus coming into close contact with the assimilatory tissue. They, however, undergo various modifications which are all instances of xerophytic characters. They