COMPOUNDS OF NUTRITION 33 



crops must consider if he would uniformly maintain a 

 ration up to the desired standard. 



41. Influence of soil moisture. — ^The proportion of 

 water in plants is influenced also by the lack or excess of 

 soil moisture. The soil and not the atmosphere is the 

 source of supply of vegetation water, which, taken up by 

 the roots, traverses the plant and passes into the atmos- 

 phere through the leaves. If the supply is abundant, the 

 tissues are constantly fully charged, but if, by reason of 

 drought, the soil becomes very dry, the outgo of water 

 by evaporation may exceed the income. Dm-ing a drought 

 vegetation water often falls below the normal, or below 

 what is necessary to maintain the tissues in their usual 

 condition of rigidity or to perform fully their physio- 

 logical functions. 



42. Supply of water to plants. — ^This leads to the 

 observation that the water in a growing plant is that 

 which is in transit from the soil to the air. This liquid 

 stream enters the plant with its load of building-materials, 

 takes into solution the compounds elaborated in the 

 leaves and aids in transporting them to the points of 

 rapid growth, finally passing into the air from the sur- 

 face of the foliage. Throughout the entire growing sea- 

 son the plant acts as a piunp, drawing from below through 

 the roots the water which it 'needs for various piu-poses, 

 and discharging it into the air. It was found that in 

 Wisconsin 309.8 tons of water were evaporated by the 

 plant for each ton of dry matter in the crop. Four tons of 

 dry matter to the acre is not an unusual product with 

 maize, requiring 1,239.2 tons or 10.4 inches of water for 

 its growth, the equivalent of about five-eighteenths of an 

 average annual rainfall. This is a fact of great significance 

 to the stock-feeder. His success begins with proper hus- 



