176 TEXTBOOK OF PLANT PHYSIOLOGY 



periods, however, water shortage commonly sets in. These plants 

 are often in the state of permanent wilting, throwing off a part 

 of their leaves and even branches. 



The thin-leaved xerophytes are commonly covered by a dense 

 layer of gray or whitish hair. This is illustrated by sagebrush and 

 many other plants of dry plains. This hair has often been regarded 

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

 ous conception, as experiments have shown that the intensity of 

 transpiration is very high in such plants. These hairs must be 

 looked upon as forming a light filter to protect the plant against 

 overheating and the chlorophyll apparatus against a 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 which the plants often possess. An important 

 physiological peculiarity of xerophytes of this type is the high con- 

 centration 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 

 which 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, some Umbelli ferae, as Eryngium, etc. 

 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 

 inwards and the stomata are hidden in a tubelike depression (Fig. 

 78). Thus, they are able to preserve for a long period the last por- 

 tion of water indispensable to life. The physiology of these plants 

 has been investigated very little. The sap concentration of the 

 cells is very high in xerophytes of this type, while in the succulents 

 it is, on the contrary, very low. 



In addition to these groups of true xerophytes, which are able 

 to withstand in one way or another the driest and hottest weather, 

 there are found in the desert also plants known as pseudo-xero- 

 phytes. These are the so-called ephemeral plants, characterized 

 by an extremely rapid development. Within the span of 3 or 4 

 weeks these plants will germinate, bloom, set fruits, produce 



