168 DRY-FARMING 
Owing to their minuteness, the root-hairs are in 
most cases immersed 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 
root-hair became the same concentration, that is to 
say, contained the same substances in the same pro- 
portional amounts, there would be no further inward 
movement of water. Moreover, 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 condition 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 enters 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 proper life and growth 
of plants. These plant nutrients are so indispen- 
sable that if any one of them is absent, it is absolutely 
