Ch. V, 6] STRUCTURE OF SOILS 237 



passages to the places of use in the chlorenchyma ; and the 

 way in which the oxygen as released passes outward along 

 the same passages and stomata. It is also the method by 

 which sugar and proteins made in chlorenchyma cells pass 

 from cell to cell until the veins are reached, and then along 

 sieve tubes and sheath cells to places of storage or use in 

 stems or roots. It is probably also the ultimate source of 

 osmotic pressure, which is diffusion pressure (page 230). No 

 matter, however, what the details may be, the energy of 

 diffusion is in all cases the same, — heat from surroundings. 

 Two other physical processes important in plant physi- 

 ology must here receive mention. Cell walls, if of cellulose 

 or lignified but not if cutinized, absorb water forcibly by 

 imbibition, which rests fundamentally upon adhesive affinity 

 between wall and water. A familiar manifestation occurs 

 in the warping of boards, which occurs as result of access of 

 water from one side, or its removal from one side by heat. 

 Likewise certain dry cell walls can absorb water as vapor 

 from the air, even producing forcible swelling and move- 

 ments of the structures concerned ; and such hygroscopic 

 phenomena occur in connection with the dissemination of 

 seeds, and elsewhere, as will later be noted. The other pro- 

 cess is capillarity, that power by which water rises or sinks 

 in small passages according to whether it wets them or not, 

 the energy being furnished by forces of tension within the 

 liquid itself. Capillarity, however, plays but minor part 

 in the physiology of most plants, though it has an indirect 

 importance through its influence on the movements of water 

 through soils. 



6. The Composition and Structure of Soils 



Roots have most intimate connections with soils, which 

 must therefore be considered in connection with root physiol- 

 ogy. Besides, soils have high interest on their own account, 

 and because of their importance in agriculture. 



Soils are far more complex than they look, having no less 



