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* 



