58 Irrigation and Drainage 



Just as soon as this happens, however, the balance between 

 the motion inside of the root- hairs and that outside of them will 

 be destroyed, and then more water will enter the root-hair from 

 the soil than has been escaping from it into the soil in a unit of 

 time. This will thin out the film of water which surrounds the 

 root-hairs, and then water which has been surrounding the soil 

 grains, impelled by surface tension, must advance upon the root- 

 hairs to make good that which has been lost ; and just so long 

 as the water continues to enter the roots from the root- hairs 

 faster than osmotic pressure can restore it, just so long will 

 surface tension force the water from the soil grains upon the 

 walls of the root-hairs. 



Not only will the water which surrounds the soil grains move 

 toward and upon the root-hairs so long as evaporation is going on 

 from the plant and assimilation is taking place in its cells, but 

 with it will go the salts containing potash, nitrogen, phosphorus, 

 and other ash ingredients of plants, which have been dissolved 

 by the moisture surrounding the grains. 



In the figure the root-hair, h, h, leading out from the main 

 root, e, is represented, for the sake of clearness, nearly full width 

 throughout its course, and, as if it had either found or had made 

 for itself, by setting the soil grains aside, an unobstructed path 

 in which to develop. As a matter of fact, these root-hairs are 

 obliged to work their way as best they can between the angles 

 formed by the meeting of the soil grains, changing both their 

 direction and their form in order to do so, and sometimes the 

 spaces are so narrow or the turns so abrupt that the root-hair 

 seems to have applied itself to the soil, and to have adapted its 

 shape so as to more completely come in contact with the surface 

 of the grain itself. 



As the water surrounding the soil grains, and which is also 

 drawn out upon the root-hairs, becomes more and more ex- 

 hausted, the film finally becomes so thin that the rate at which 

 the water can be moved out upon the root -hairs is so slow that it 

 is no longer able to meet the needs of the plant, and it wilts, 

 and finally ceases to grow altogether. 



Attention should be called to the fact that fresh growing 



