16 



PLANT PHYSIOLOGY AND ECOLOGY 



and reduces the chresard. while an excess of acids has the oppo- 

 site effect. The structure of the soil, on the contrary, has an 

 almost absolute control upon the fate of the water that enters 

 the ground, in addition to its influence upon the water that runs 

 off. It determines the amount of water drained away in response 

 to gravity, and also the amount that can be raised from the lower 

 layers by means of capillary action. The total water content is 

 dependent in the first place upon the amount of water that soaks 

 into the soil. Of this, the holard is that part which the soil holds 

 in spite of the action of gravity, together with that which may 

 be raised from time to time by capillarity. The chresard, how- 

 ever, is only that part which the root-hairs are able to take up 

 in opposition to the pull of the soil particles. 



21. Origin and structure. Soils are formed from rock by 

 the action of weathering. The latter is due to the influence of 







Fig. 3. Decomposition of a granite boulder into gravel and sand, and 

 the further breaking down of these by the roots of herbaceous plants. 



both physical and biological factors, acting separately or together. 

 Weathering consists of two processes. The one is disintegration, 

 by which the rock is broken into fragments of various sizes; the 



