314 PHYSIOLOGY 



be packed into ;i cubic centimeter, in contrast with the area of 1,000,000 spheres 

 each 0.1 mm. in diameter occupying the same place. In the first case the area would 

 be 3141.6 sq. mm.; in the second, ten times as much, or 31,416 sq. mm. 



In coarse soils, therefore, such as sand, water largely drains away ; 

 whereas in fine soils, such as clay, it is held, and it may be so firmly 

 held as to preclude the admission of more, once the soil is saturated; 

 whence a layer of clay often forms a " hard-pan," in which water col- 

 lects as in a basin, or over which it runs. Humus soils hold much water, 

 because the particles of organic matter, besides being covered by the 

 usual films, are not only porous, thus admitting water to the interior 

 spaces, but are also able to imbibe it by their very substance. 



Capillary ascent of water. — If equilibrium were momentarily reached 

 among the water films in the soil, it would be upset the moment any water 

 evaporated from the upper grains, for the water film that clothed them 

 would thereby become thinner. This would at once cause a rearrange- 

 ment of the water in all adjacent films, because the adjacent water 

 particles are pulled more strongly to the places where the film is thin than 

 they are held where it is thick. Thus evaporation from the soil causes, 

 on the whole, an upward movement of the water from the deeper parts 

 of the soil, a disturbance which extends as far as is permitted by the 

 resistance offered by the attraction of the soil particles and by the viscosity 

 of the water. As this effect may reach the water table, the result of 

 evaporation is to lower it; its level rises after heavy rain and falls in 

 prolonged drought. Not all the water which enters the soil can leave it, 

 either by drainage or evaporation. Even if a sample of the soil be placed 

 in the air, very thin films of water remain when it is " air-dry " and seems 

 dry as dust. Only by heating above ioo° C. can all moisture be driven 

 off. 



Migration of soil water into roots. — When a root penetrates the soil 

 and root hairs develop from all sides, the entire surface becomes clothed 

 with a film of water just as is the case with the soil grains. When some 

 of this water enters a root hair or any part of a surface cell, the water 

 film becomes thinner and there takes place the same sort of readjustment 

 as is produced by evaporation of water at the surface of the soil, with the 

 same general movement of water, in this case toward the root. In both 

 cases even distant parts of the soil may thus furnish water to make good 

 the loss. All such movements of water, being mass movements and not 

 diffusion movements, involve the transfer of any solutes present ; whence 

 it comes that solutes from a distance may be brought into the vicinity 



