CHAPTER VI. 



ROOTS AND THE SOIL. 



The surface soil, in which the roots find water and 

 substances in solution, constitutes but a very small pro- 

 portion of the earth's crust. Generally it is exceedingly 

 shallow ; often only a few inches in depth ; though, 

 sometimes, roots do penetrate 20 feet or more. Yet 

 the entire vegetable and animal world is supported by 

 this mere film of altered and crumbled rock. 



The supply of water to the root is of the greatest 

 possible importance ; if it is insufficient the whole 

 character of the vegetation alters. Thus upon the Nile, 

 above the first Cataract, the " Seyal " and other Acacias, 

 with roots capable of penetrating to 20 or 30 feet, are 

 almost the only plants able to retain their foliage. 

 This and similar floras in South and East Africa are 

 produced by the permanent water being from 16-20 

 feet below the surface, and too deep, therefore, for 

 the roots of ordinary plants. Where subsoil water is 

 absent, or lies at extreme depths, the result is such 

 a desert as the Sahara. 



It must be remembered that the water in the atmo- 

 sphere, in the soil, and in the ocean, is almost always in 

 motion. That which evaporates from the sea, or the 

 earth, or plants, is promptly carried off by winds. 

 It may be carried to a very great distance, before it 

 meets a mountain range high enough to force the 



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