94 General Botany 



rate of transpiration. There are plants like the dandehon that 

 become adjusted to both these conditions. Most plants, how- 

 ever, are not so readily modified, and those with a high transpira- 

 tion rate die off on a dry hillside, while those with a low transpira- 

 tion rate survive. This indicates only one of the factors which 

 must be taken into account in attempting to explain the distri- 

 bution of plants in nature and in the selection of plants for par- 

 ticular habitats. Other habitat factors will be considered in 

 later chapters. 



Water balance and crop yields. In view of their large water 

 requirements, 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 increased. 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. 



Irrigation is a method of artificially maintaining a constant 

 supply of soil water for crop plants in dry regions. It prevents 

 the slowing down of the plant processes during the growing sea- 

 son, thus enabhng the plant to work at its highest efficiency and 

 produce its greatest yield. For example, at the Utah Experi- 

 ment Station an acre of corn without irrigation produced 26 

 bushels; with 15 inches of irrigation water added, 52 bushels; 

 with 7,S inches, 82 bushels. An acre of wheat produced 4J 

 bushels without irrigation and 26 bushels when 30 inches of irriga- 

 tion water was added. One of the highest recorded yields of 

 corn on a small plot (225 bushels an acre) was obtained in 

 Colorado with irrigation. 



Plants classified according to their water relations. In pre- 

 ceding chapters we have pointed out the importance of water in 



