RESISTANCE OF PLANTS 



369 



plants has been investigated very little. The sap concentration 

 of the cells is very high in xerophjrtes 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- 

 xerophytes. They are the ephemeral plants, characterized by 

 an extremely rapid development. Within the span of 3 to 4 

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

 mature seeds, and die. Their 

 whole life cycle occurs in the 

 short moist period, usually in 

 spring. During the remainder 

 of the year, they exist in the 

 form of dry seeds that cannot 

 be affected by desiccation. In 

 all their other characteristics, 

 these plants are typical 

 mesophytes. 



The groups discussed by no 

 means exhaust the various 

 xerophytes. The types sharply 

 dehmited here, moreover, are 

 connected by a series of plant ^- "^•-'^-J °J «t M."^' '"'" ^ 

 forms showing imperceptible 



transitions from one group to another. A detailed examination 

 of these types belongs to the field of ecology. 



86. Effect of Environment on the Structure of Transpiring 

 Organs. Xeromorphism and Drought Resistance of Plants. — 

 The transpiring organs of plants, which in practically all case.s 

 are the leaves, exhibit a considerable variability. Leaves of 

 different plants and even of the same plant show considerable 

 differences in structure, depending on the external environmental 

 factors under which they develop. 



A very important regularity in the structure of the leaves, 

 depending upon their position on the plant, has been established 

 by Zalensky (1904). He has found that leaves on the upper 

 half of a stem are always different from those below. The higher 

 the position of a leaf, the smaller are the dimensions of all its 

 cells, but the greater the number of stomata per surface area, 



