170 



GROWTH HABITS OF PLANTS 



The most typical of the true xerie habitats are found in desert 

 regions where such plants as sage brush and the various kinds of 

 cacti are characteristic (Fig. 7G). The roots frequently are strongly 

 developed, possessing either considerable length or great size, though 

 this is generally not true of cacti. Roots that have great size serve to 

 accumulate large amounts of water. Root hairs are abundant, often 

 extending far back from the root tip, and in some cases having thick, 

 rigid cell walls. Palisade tissue is strongly developed and the chlor- 

 enchyma in both leaves and stems is often deep-seated so that these 

 organs have a pale green color. There is a marked reduction in the 

 size of all cells including the guard cells of the stomata. 



Fig. 7Q.^Lemaireocereus thurheri, a desert xerophyte. 



Protective features are remarkably well developed and of course 

 are needed because of the great exposure of xerophytes to transpira- 

 tion. The transpiring surface is usually relatively reduced, the 

 leaves, if present, being small and thick. Many species are leafless, 

 their cylindrical stems exposing a relatively small surface to tran- 

 spiration while their vertical orientation affords some protection 

 from the intense rays of sunlight during the middle of the day. In 

 many cases there is a temporary reduction of transpiring surface, as 

 in the leaves of grasses which roll up and those of legumes which fold 

 during the hottest and dryest parts of the day. Temporary reduc- 

 tion is exhibited also by those deciduous plants which shed their 

 leaves at the beginning of each period of dry or cold weather. 



