142 XYROPHYTIC PLANTS [CH. 



therefore not exposed to the dangers of the Alpine 

 winter. 



The danger of breakage of the ordinary Conifers by 

 the accumulation of snow is also met by the ease with 

 which the glossy narrow leaves shoot it off at steep angles 

 when the weight becomes critical. 



Moreover the narrow form has other advantages, e.g. 

 against chilling by radiation, the shearing action of high 

 winds, and so forth. 



We have seen that the typical leaf is a thin and flat 

 organ exquisitely adapted for the performance of certain 

 primary functions transpiration, assimilation and respira- 

 tion which depend on its exposure to light and air, and 

 the accessibility of gases to its interior through the 

 stomata and intercellular spaces. 



Many plants, however, are in such a position that 

 they must either modify the structure of their leaves in 

 accordance with peculiarities of climate, soil, intensity of 

 light, &c, or give up forming thin aerial leaves of the 

 typical form altogether; and we may safely assume that 

 only such plants as have varied in the necessary directions, 

 and have been selected out to fit such conditions, have 

 been able to persist in the positions referred to. Such 

 plants are known as Xerophytes. In other words, plants 

 innumerable during the ages gone by have in vain been 

 carried by wind, water, or animal agencies into such 

 situations as deserts, salt-plains, marshes, mountain tops, 

 &c, because their leaves are too thin and delicate to 

 withstand such exigencies as intense cooling by radiation, 

 brilliant sunshine, flooding with water, persistent drought, 

 &c; and only such as varied in some direction which 

 proved of advantage to meet the new conditions could 

 survive and leave progeny more and more fitted to the 

 altered environment. 



