CHAPTER LII. 

 ADAPTATION OF PLANTS TO CLIMATE. 



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



important factors in plant form and distribution is that of 

 clmate, which is modified by varying conditions, as tempera- 

 ture, humidity 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. 



Some of the plants, especially the larger ones, have very suc- 

 culent 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 propor- 

 tionately small surface area exposed, so that transpiration is 

 reduced. The chlorophyll resides here in the stems, and they 

 function as foliage leaves in many other plants do. 



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 cavities, so that the guard cells are not so 

 exposed to the drying action of currents of air at the surface. 



In still other cases the leaves and stems are covered with a 

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



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