CHAPTER LVI. 

 ADAPTATION OF PLANTS TO CLIMATE. 



731. Some characteristics of desert vegetation. — One of the 



important factors in plant form and distribution is that of climate, 

 which is modified by varying conditions, as temperature, hu- 

 midity of the air, dryness, etc. In desert regions where the air 

 and soil are very dry, and plants are subject to long periods of 

 drought, there is a very characteristic vegetation, and a variety of 

 forms have become adapted to resist the drying action of the 

 climate. 



732. Some of the plants, especially the larger ones, have very 

 succulent stems or trunks, or they are more or less expanded but 

 thickened, while the leaves are reduced to mere spines or hairs, 

 as in the cacti. If plants in desert regions had thin and broadly 

 expanded leaves, transpiration would be so rapid, and so great, 

 as to kill them. In these succulent stems there is a proportion- 

 ately small surface area exposed, so that transpiration is reduced. 

 The chlorophyll resides here in the steins, and they function as 

 foliage leaves in many other plants do:" 



733. Other plants of the desert, which do not have succulent 

 stems, are provided with closely appressed and small, thick, 

 scale-like leaves. The leaves in many of these plants have an 

 epidermis of several layers of cells, so that transpiration does not 

 take place so rapidly. In addition to this the stomata are sunk 

 in pits, or ca\'ities, so that the guard cells are not so exposed to 

 the drying action of currents of air at the surface. 



734. In still other cases the leaves and stems are covered with 

 a dense felt of hairs which serves as a cushion to protect them 



419 



