The Water Relations of Leaves 51 



take what they need for transpiration. When we consider 

 that the quantity of water transpired by wheat in cultiva- 

 tion is one fifth to one eighth of the rainfall of the central 

 United States, we begin to realize how large a fraction of 

 all the water that falls on the soil is actually used by the 

 plants. In all rainfall, some water runs off the soil without 

 penetrating the surface, some evaporates from the soil surface 

 itself, and some sinks below the level of the plant roots. Con- 

 sequently, it is only when there are abundant rains, dis- 

 tributed throughout the growing season, that the amount 

 of water needed by the plants for their best development is 

 available in the upper layers of the soil. It has been shown 

 by experiment that for the production of every pound of 

 solid matter in the above-ground parts of crop plants, from 

 300 to 500 pounds of water are required in the central United 

 States, and that from 400 to 1000 pounds are needed on the 

 plains of Colorado. The amount of water used in transpi- 

 ration is, therefore, many times the amount used in the manu- 

 facture of food. 



Water supply and crop yields. Knowing these water re- 

 quirements, it is easy to understand why droughts are so dis- 

 astrous to crops. When the rainfall is slight, not only is the 

 amount of water that can be secured by the plant from the 

 soil reduced, but the sunshine is brighter and the air is 

 usually drier, so that transpiration from the plant is in- 

 creased. It is in part because of the water requirement of 

 crop plants that bottom lands lands along streams in 

 the bottoms of valleys are more valuable for growing 

 crops than are uplands. There the underground water is 

 nearer the surface, and keeps the supply for plants more 

 nearly constant. 



