50 Irrigation and Drainage 



volumes of air than have been stated must be brought into close 

 contact with the growing clover in order to meet its needs. This 

 air, however, cannot come into intimate relations with the green 

 chlorophyll -bearing cells of the clover in the field without of 

 necessity permitting the evaporation of large quantities of water 

 from the plants ; and this brings us to realize how imperative is 

 the demand for water by rapidly growing crops. 



The writer has found, for example, by direct measurement, 

 that the air passing three feet above a clover field, and at a 

 moderate rate, even as early as May 30 in Wisconsin, when the 

 air temperature is only 52.48 F., may have its relative humidity 

 increased from 44 to 48 per cent by the moisture taken from the 

 field ; and this means that 3,510 pounds of w r ater are required to 

 make even the observed change of humidity in a volume of 152,- 

 600,000 cu. ft. of air, which is the amount required to carry to 

 the clover crop its carbon, supposing all the carbon which the air 

 contained to be utilized. It is quite likely, however, that the 

 volume of air which did contribute its carbon to Boussingault's 

 crop of clover not only exceeded fourfold the amount stated 

 above, but that it also had its relative humidity raised at least 

 to 94 per cent. If these suppositions are true, then the amount 

 of water borne away from the plants in question must have ex- 

 ceeded 176,100 pounds, or at the rate of about 40 pounds of water 

 for a pound of dry matter ; but it has been shown on a preceding 

 page that, as a mean of 46 trials, the clover crop did lose from its 

 tissues and from the soil in which it grew 576.6 pounds of water 

 per pound of dry matter produced, so that, large as are the 

 figures stated above, they fall far below the actual ones. 



With these estimates and considerations before us, we can 

 readily understand that one of the chief functions of water in 

 plant life is to keep the tissues moist and in a suitable condition 

 to carry on the process of breathing, whose primary object is to 

 get the plant its carbon from the air. 



In order that the plant may utilize the carbon of the car- 

 bonic acid in the air, it is necessary that this should come to 

 the chlorophyll -bearing cells when there is sunshine enough to 

 decompose it; and since the carbonic acid would be useless at 



