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 fourth 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 dry-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 
greatly modify 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 plant reared under arid conditions. 
This very interesting observation tends to confirm 
the view commonly held that plants grown under 
arid conditions will gradually adapt themselves to 
the prevailing conditions, 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 transpiration. This 
