148 Physics of the Soil. 



them in such a way that in form and position they make 

 up a part of the soil mass. In this relation the force of 

 adhesion draws the capillary water out over their walls so 



-1 



FIG. 46. Distribution of water on the surfaces of soil grains and of 

 root-hairs, e, main root: 1, air space; 2, soil grain; 3, film of water; 

 h h, root-hairs. (After Sachs.) 



as to leave them and the soil granules surrounded by the 

 water film. Each root hair is or should be in a sense under 

 water, that is invested in a film of greater or less thickness. 

 When a portion of this water enters the root hair and 

 passes on into the root and up to the leaves, the water layer 

 surrounding the root hair is left thinner ; but no sooner 

 does this thinning out occur than the equilibrium is de- 

 stroyed and surface tension at once squeezes more water 

 onto the surface from the surrounding soil. In this way 

 capillarity keeps the water moving to the root hairs as they 

 pass it on to the plant. 



182: Advance of Roots through the Soil. Until the 

 method by which roots advance through the soil is under- 

 stood it is difficult to realize how it is possible for such deli- 

 cate structures to set the heavy soil aside sufficiently to 

 reach the great depths they do. Nature's method of over- 

 coming the difficulty is simple enough and it is as effective 

 as it is simple. The large amount of open space there is in 

 the surface four to six feet of soil makes it easier to set the 



