314 PHYSIOLOGY 
be packed into a cubic centimeter, in contrast with the area of 1,000,000 spheres 
each o.i 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 100 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 
