368 PLANT PHYSIOLOGY 



lower considerably the temperature of their leaves. In the hot air 

 and soil of the desert, this protection against overheating is of 

 great importance. The considerable loss of water is replaced from 

 the deeper soil layers, as xerophytes of this type possess an 

 extremely well-developed root system. Where the water table 

 is not too deep, the plants are able to reach it with their long 

 roots and can stand, therefore, the greatest heat. Where water 

 cannot be reached, the root system spreads through a very great 

 volume of soil. During the driest periods, however, water 

 shortage commonly sets in, and these plants remain in a state 

 of permanent wilting, throwing off a part of their leaves and even 

 branches. 



The thin-leaved xerophytes are often covered with a dense 

 layer of gray or whitish hair. Sagebrush and many other plants 

 of dry plains display this feature, which has often been regarded 

 as a means of checking transpiration. This, however, is an 

 erroneous conception, as experiments have shown that the inten- 

 sity of transpiration is very high in such plants. These hairs 

 must be looked upon as forming a light screen to protect the 

 plant against overheating and the chlorophyll apparatus against 

 too bright light. They also serve as a protective covering against 

 the attacks of animals, as do the spines and the bitter, astringent, 

 and pungent substances that the plants often possess. An 

 important physiological peculiarity of xerophytes of this type 

 is the high concentration of the cell sap, which enables the wilting 

 plant to develop an enormous suction tension and to draw 

 water from the soil very rapidly. Moreover, it is possible that 

 the soluble substances that accumulate in large amounts in these 

 plants make them more resistant to permanent wilting. 



The third type of xerophytes are sclerophyllous plants with 

 rigid leathery leaves. To this group belong many prairie grasses, 

 such as Stipa and Festuca and some Umhelliferae such as Eryn- 

 gium. The sclerophyllous xerophytes are especially widely 

 distributed in the dry districts of the Cape Province, Africa, 

 where may be found many representatives of the family Ericaceae. 

 These plants are characterized by an unusual capacity to resist 

 permanent wilting. The margins of the leaves commonly curl 

 inw^ard, and the stomata are hidden in a tubelike depression 

 (Fig. 117). They are able to preserve for a long period the last 

 portion of water indispensable to life. The physiology of these 



