XEROPHYTES 
165 
cuticle upon the epidermis of the leaf. The leathery texture 
of the leaves of many xerophytes is partly due to the thick- 
ness of their cuticle. The number of the stomata is 
commonly much reduced, and also the size of the inter- 
cellular air-spaces in the leaf ; therefore evaporation is 
lessened. Instead of being flush with the surface the 
stomata are commonly sunk at the base of small pits in the 
leaf, into which the wind cannot enter, and which therefore 
become filled with saturated air, thus reducing transpiration. 
In Pinus, Aloe, and many xerophytes there is a separate 
small pit for each stoma, but in Nerium and others the pits 
are large and contain several stomata. A somewhat similar 
arrangement on the large scale is seen in the grooves of 
such leaves as those of Empetrum, Vaccinium sp., Phylica, 
Cassiope, Rosmarinus, or such stems as those of Casuarina, 
Spartium, Cacti, &c., the stomata being at the bottom of 
the furrows. The grooves are more or less covered in in 
the leaves mentioned, especially in Empetrum ; in many of 
the grasses ( e.g . Stipa) the leaf rolls up in dry air, completely 
closing in the stomata, and exposing only the impervious 
surface to the outer air. 
The surface of xerophytes is often closed with a thick 
covering of hairs, w r hich retard transpiration by preventing 
the wind from reaching the stomata. This is well seen 
in the Edelweiss, in many alpine willows, in Stachys sp., 
Alchemilla alpina , Helichrysum, &c. A covering of wax 
occurs on the epidermis of many Liliaceae {e.g. Aloe), 
Crassulaceae, &c., a covering of silica on that of Crassula 
( Rochea ) falcata. The thick sap of such plants as Aloe is 
another preventive against excessive transpiration, for it only 
evaporates very slowly. 
Instead of being spread out to the utmost extent the 
leaves of many xerophytes are closely placed and overlap 
one another very much, e.g. in Gasteria, Aloe, many Crassu- 
laceae, Calluna, &c. The plants often grow in dense tufts, 
closely crowded together, and thus further reduce the free 
access of air and check evaporation, e.g. in Raoulia and 
Azorella ; this is especially frequent in alpine plants. 
Further arrangements for reduction of transpiration 
usually occur ; we shall deal first with evergreens, to which 
group most xerophytes belong. 
