170 DRY-FARMING 
through the stems and into the leaves. This con- 
stant movement of the plant-foods depends upon the 
fact that the plant consumes in its growth consider- 
able quantities of these substances, and as the plant 
juices are diminished in their content of particular 
plant-foods, more enters from the soil solution. The 
necessary plant-foods do not alone enter the plant, 
but whatever may be in solution in the soil-water 
enters the plant in variable quantities. Nevertheless, 
since the plant uses only a few definite substances and 
leaves the unnecessary ones in solution, there is soon 
a cessation of the inward movement of the unimpor- 
tant constituents of the soil solution. This process 
is often spoken of as selective absorption; that. is, 
the plant, because of its vital activity, appears to 
have the power of selecting from the soil certain 
substances and rejecting others. 
Movement of water through the plant — 
The soil-water, holding in solution a great variety 
of plant nutrients, passes from the root-hairs into 
the adjoining cells and gradually moves from cell to 
cell throughout the whole plant. In many plants 
this stream of water does not simply pass from cell 
to cell, but moves through tubes that apparently 
have been formed for the specific purpose of aiding 
the movement of water through the plant. The 
rapidity of this current is often considerable. Or- 
