y2 The Great World's Farm 



may be, in most cases, and especially in dry climates, to 

 the legitimate openings, the pores made for this purpose. 



Soft, thin leaves lose water by evaporation from the 

 whole surface, and have a large number of pores as well, 

 but they grow in situations where they can easily make up 

 the loss. All leaves, however, have some protection 

 more or less in the skin which covers them, this skin 

 being, moreover, as we have said, impregnated with wax, 

 which, though commonly invisible, often appears as a 

 shiny coating, or as "bloom." 



A cabbage, for instance, has a mealy look about it, as 

 if it had been dusted with flour; many grasses, acacias, 

 and now well-known Austrahan gum-tree or eucalyptus, 

 have a similar appearance, and when this "bloom" is 

 examined it is found to consist of minute rods, or needles, 

 of wax. The substance forms a regular incrustation on 

 the stem of the Peruvian wax-palm, whose native land is 

 one of the most rainless regions of the earth; and there is 

 nothing more effectual than wax for excluding air and 

 preventing evaporation. Honey stored in wax-cells is, as 

 it were, hermetically sealed up and preserved. 



With the wax is often associated resin, which acts in a 

 similarly 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 con- 

 sider that aromatic as well as gum and resin-bearing 

 plants are especially characteristic of deserts and dry 



