110 IRRIGA T10N PRACTICE 



plicated movement of water from the soil into the plant, 

 it may be said that the concentration of the solution in 

 the root-hairs is higher than that of the soil-water solu- 

 tion. The water tends, therefore, to move from the soil 

 into the roots to make the solutions inside and outside 

 of the roots of the same concentration. If it should 

 occur that the solutions inside and outside the root-hairs 

 were of the same concentration, that is to say, if they 

 contained the same substances in the same proportional 

 amounts, there would be no further inward movement of 

 water. Moreover, if the soil moisture should become 

 stronger than the water within the root-hairs, water 

 would tend to pass from the plant into the soil. This is 

 the condition that prevails in the alkali lands of the 

 West, and is often the cause of the death of plants grow- 

 ing on such lands. 



78. Transpiration. There is a constant movement of 

 water, holding in solution the indispensable plant nutri- 

 ents, after it has entered the root-hairs, through the roots, 

 stems and into the leaves. At the leaf surfaces evapora- 

 tion occurs, and, there, much of the water taken from the 

 soil passes into the air as invisible water vapor. The 

 rapidity of this current is often considerable. Ordinarily 

 it varies from 1 to 6 feet an hour, although observations 

 on record show that the movement often reaches the rate 

 of 18 feet an hour. In an actively growing plant it does 

 not then take long for the water in the soil to find its way 

 to the uppermost parts of the plant and to be evaporated 

 from the leaf surfaces. This movement of water from the 

 soil, through the plant, into the air, is the process known 

 as transpiration. If the current of water passing through 

 the plant is stopped for any considerable length of time, 

 the plant is injured and death often results. Transpira- 



