THE WATER OF THE HABITAT 



17 



other is decomposition, by means of which the original rock, or 

 its fragments, is broken clown into minute particles. These two 

 processes usually go hand in hand, although as a rule one is more 

 marked than the other. Their relative importance is deter- 

 mined by the character of the rock, and by the forces that act 

 upon it. Hard rocks, e.g. granite, as a rule, disintegrate more 

 rapidly than they decompose, while sedimentary rocks, such as 

 sandstone, limestone, etc., tend to decompose more rapidly than 

 they disintegrate. In many cases of weathering, the two pro- 

 cesses are of equal importance. 



22. Water capacity. A soil owes its capacity for water to 

 the fineness of its particles. Since the water is in the form of 



Fig. 4. Diagrammatic sketch showing the structure of a fine soil and the 

 position of root-hairs in it. The root epidermis (e) gives rise to root- 

 hairs (/i) which push their way between the angular soil particles sur- 

 rounded by thin water films. The air spaces are white. (After Sachs.) 



thin films upon the soil particles, the amount necessarily increases 

 with an increase in the water-holding surface. The latter is in- 

 creased as the particles become finer and more niunerous, and 

 thus produce a greater aggregate surface. The movement of 

 water upward and downward in the soil is likewise dependent 

 upon the size of the particles. As the latter become finer, the 



