178 DRY-FARMING 



the plant. As the plant grows older transpiration 

 diminishes, and finally at the ripening stage it almost 

 ceases. 



Eighth, transpiration varies greatly with the crop. 

 Not all plants take water from the soil at the same 

 rate. Very little is as yet known about the relative 

 water requirements of crops on the basis of transpira- 

 tion. As an illustration, MacDougall has reported 

 that sagebrush uses about one foiu'th as much water 

 as a tomato plant. Even greater differences exist 

 between other plants. This is one of the interesting 

 subjects yet to be investigated by those who are en- 

 gaged in the reclamation of drj^-farm districts. More- 

 over, the same crop grown under different conditions 

 varies in its rate of transpiration. For instance, 

 plants grown for some time under arid conditions 

 greatl}' mocUf}' their rate of transpiration, as shown 

 by Spalding, who reports that a plant reared under 

 humid conditions gave off 3.7 times as much water 

 as the same ])lant reared under arid conditions. 

 This very interesting observation tends to confirm 

 the view commonly held that j^lants grown under 

 arid conditions will gradual!}' adapt themselves to 

 the prevailing concUtions, and in spite of the greater 

 water dissipating conditions will live with the ex- 

 penditure of less water than would be the case under 

 humid conditions. Further, Sorauer found, many 

 years ago, that different varieties of the same crop 

 possess very different rates of trans])iration. This 



