Ch. V, 6] 



STRUCTURE OF SOILS 



239 



come emptied, and refill with air, though the water still 

 lingers in the smaller passages and angles in the capillary 

 condition. Such a soil is moist, and its combination of 

 water and air provides the very best conditions for roots, 

 though one that is nowhere constantly found. It is the 

 condition represented in our drawing (Fig. 169). As the 



Fig. 169. — A generalized drawing of a section, highly magnified, through 

 a good soil and a portion of a root with root hairs. 



The soil particles are cross-lined, the water is concentrically-lined, the 

 humus is black, and the air spaces, in the soil, are left white. 



water is further removed, by evaporation and root absorp- 

 tion, some moisture continues to cling tenaciously in thin 

 films around the particles of soil, from which it is removed 

 with greater and greater difficulty the thinner the films become. 

 Upon these hygroscopic films plants must depend for their 

 supply dining much of the time ; and it is apparently for 

 absorption from them that the root hairs, flattened tightly 

 against the soil particles, are especially fitted (Fig. 170). 



The hygroscopic water films have an important relation 

 with the soil particles. Not only do the films cling very 





