170 DRY-FARMING 



through the stems and into the leaves. This con- 

 stant movement of the plant-foods depends upon the 

 fact that the i^lant 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 

 raj^idity of this current is often considerable. Or- 



