VERTICAL FARMIXG 31 



the greater part of all the starch, sugar and other similar com- 

 pnunds so valuable, in that they form one of the essential parts 

 of the foods for men and lower animals. 



Water as a Carrier of Plant Food. All of the plant foods 

 in the soil have first to be dissolved in water and then carried 

 by it to and through the roots and up to the above ground parts 

 of the plant. The tiny feeding roots must be immersed in the 

 thin film of water that clings to the soil particles. The soil 

 solution is absorbed into the rootlets through their walls and 

 forced upward with a considerable pressure. This can easily 

 be noticed by cutting off a rank growing weed and watching 

 how quickly the sap is thrown up over the newly cut stump. 

 This pressure is often several pounds per square inch. All the 

 water that is not combined in the plant as a part of it is thrown 

 out through the leaves, this process being called " transpira- 

 tion." 



If the moisture conditions of the soil are good and water is 

 abundant, transpiration is encouraged and there is an attending 

 >atisfactory growth of crop; but if the conditions are reversed, 

 the plant growth is immediately stunted by the deficiency in the 

 amount of the essential water. 



Amounts Necessary for Crops. Taking crops altogether, 

 from rice to date palms, the amount of water required varies 

 from complete submergence to almost perpetual drouth, and 

 within this range crops vary widely as to the amount necessary 

 for living. As a rule the amount is large, as it is only the desert 

 plants that thrive on a scant supply. Take a small potted plant 

 and place it under a glass jar and notice how soon the jar is 

 clouded by the water vapor taken up from the soil and given 

 off through the leaves. From 200 to 600 pounds of water are 

 transpired by ordinary crops for every pound of dry matter 

 produced, but this varies with each crop according to the 

 climate and other factors affecting it. 



A good yield of pea vine hay will draw 1200 to 1500 tons of 

 water through its roots and stems and liberate it in the air. Corn 

 and cane drew equally heavily upon the soil supply of moisture. 



