PLANTS OF THE INDIAN DESERT. 285
(c) or quite below the surrounding epidermal cells, so that
the front cavity is situated at the bottom of a deep pit in
Boraginacece, and Asclepiadaceae. {cl) or a little above the plane of the surrounding cells, so that
the front cavity is on a level with the surface in the rest
of the orders.
The occurrence of the front cavity in deep pits is necessary in plants which do not possess a hairy covering, so that they can retain a moist atmosphere in the pits, thus reducing transpiration. In Acti?iiopteris dichotoma, which possesses very few stomata on the lower surface and in which the epidermal cells are sclerosed, except on a small portion of the surface where stomata occur, transpiration is a little too much reduced and it is natural that there should be some contrivance, such as elevated position of the guard-cells, to accelerate transpiration in order to get rid of an excess of water. Front cavities, situated on a level with the surface, are usually suffici- ently protected against the strong glare and sunlight reflected from the sand by a thick covering of hairs.
In species of Polygalaceae, Tamariscineae, Onagraceae and Cheno- podiaceae the front cavity is closed by the outer prominent horns of the guard-cells. It seems that in these species the front cavity is not completely closed by ventral walls of the guard-cells. The horns are therefore well developed and completely close the pore by coming into close contact with each other. The guard-cells on the axis of species of Caryophyllaceae seem incapable of closing the front cavity of the stomata ; one or more cells in addition to the subsidiary cells, therefore, clasp the guard-cells and thus assist them in closing the pore.
The stomata in Cyperaceae are characterised by the occurrence of sub-stomataed air-cavities of obconical shape which are formed by elongated obliquely placed cells.
Assimilatory Tissues. — The chlorophyll tissue is variously differ- entiated and shows a process of evolution as will be discussed later on. It consists
(a) either of a homogeneous tissue of elongated polygonal cells
in Gentianacecz and Filicinae, {b) or of a homogeneous tissue of arm-palisade cells in some
Compositae, (c) or of palisade tissue on the adaxial side and of spongy tissue on the abaxial side in some Gapparidaceae, Simarubaceae, Sapindaceae, some Papilionaceae, Lythraceae, Onagraceae*
