168 DRY-FARMING 



Owing to their minuteness, the root-hairs are in 

 most cases inmiersed in the water film that surrounds 

 the soil particles, and the soil-water is taken directly 

 into the roots from the soil-water film by the process 

 known as osmosis. The explanation of this inward 

 movement is complicated and need not be discussed 

 here. It is sufficient to say that the concentration or 

 strength of the solution within the root-hair is of dif- 

 ferent degree from the soil-water solution. The water 

 tends, therefore, to move from the soil into the root, 

 in order to make the solutions inside and outside of 

 the root of the same concentration. If it should ever 

 occur that the soil-water and the water within the 

 loot-hair l^ecame the same concentration, that is to 

 say, contained the same suljstances in the same pro- 

 portional amounts, there would be no further inward 

 movement of water, ^loreovcr, if it should happen 

 that the soil-water is stronger than the water within 

 the root-hair, the water would tend to pass from the 

 plant into the soil. This is the conchtion that pre- 

 vails in many alkali lands of the West, and is the 

 cause of the death of plants growing on such lands. 



It is clear that under these circumstances not only 

 water entei's the root-hairs, but many of the sub- 

 stances found in solution in the soil-water enter the 

 plant also. Among these are the mineral substances 

 which are indispensable for the iirojxn' life and growth 

 of plants. These plant nutrients are so indispen- 

 saljle that if any one of them is absent, it is absolutely 



