Water 



97 



With the wax is often associated resin, which acts in 

 a siiijiiarly protecting way apparently. No explanation 

 indeed has hitherto been given of the use to the plant 

 of gums, resins, caoutchouc, and the strong-smelling 

 oils frequently found in leaves ; but, as water in which 

 gum or any other substance is dissolved evaporates 

 more slowly than pure water, it seems not unlikely 

 that one at least of the uses of these substances is to 

 check the escape of water. And this seems the more 

 probable when we consider that aromatic, as well as 

 gum and resin-bearing, plants are especially charac- 

 teristic of deserts and dry regions, hot or cold. Thus 

 the pine-tree of the north has its turpentine, the 

 eucalyptus of hot, dry Australia its oil, and the acacias 

 of Africa their gums. 



Many trees and shrubs in hot, dry countries are 

 protected also by having either small or very few 

 leaves, or even none at all. 



Where the air is constantly damp, as it is in many 

 parts of the tropics, there the trees may boldly venture, 

 as the plantain does, to spread broad leaves many feet 

 square to the sun, for the water-supply never fails, and 

 the air is not outrageously thirsty, as it is in the desert. 

 But in those parts of Australia where rain is scanty and 

 droughts are frequent, there the leaves are not only 

 small, as we have said, but they, most of them, also 

 protect themselves by turning only their edges, not 

 their broad sides, to the sun ; for they have to economize 

 their resources as much as possible. This is par- 

 ticularly the case with many species of Eucalyptus, 

 some of which turn one leaf-edge to the earth and the 

 other to the sky, or stand erect, turning one edge 

 towards the stem and the other away from it, in each 



7 



