88 PLANT GROWTH 



The amount of water transpired by growing plants is 

 large. A single tomato, sunflower, or corn plant has been 

 shown to absorb about four hundred fifty pounds of water 

 (the equivalent of a large barrel), but all of this large 

 amount is transpired during its life except about 1 per cent 

 which remains in the plant at maturity. This is equivalent 

 to three or four quarts per plant per day, during periods of 

 high transpiration. An acre of corn or grass must frequently 

 give off water in transpiration in excess of two tons per hour 

 during the warm part of the day. Corn has been shown to 

 use from eleven to fifteen inches of water or rainfall to grow 

 a crop. 



Some measurements have shown the transpiration rate 

 to vary by several hundred per cent on succeeding days de- 

 pending on the conditions of the plant and the environment 

 of the roots and the leaves. Many of the natural laws of 

 physics and chemistry have an important influence on the 

 transpiration rate. A rise in temperature increases the rate 

 of evaporation of water; the speed of the diffusion of water 

 molecules through the protoplasm, the cell walls, and the air 

 spaces in the leaf; and the water-holding capacity of the air. 



Light increases the transpiration rate in several ways, 

 perhaps chiefly by increasing the temperature, but also by 

 causing the stomata to remain open, and by supplying the 

 energy actually used in the evaporation. Ninety per cent 

 or more of the water lost in transpiration is lost during the 

 day. The rate increases rapidly during the morning hours 

 and usually reaches a maximum shortly after one o'clock, 

 but the afternoon rate is usually lower than the morning 

 rate. During the night absorption is faster than the loss 

 and the cells regain their turgidity. In the morning the rate 

 of water loss is faster than the rate of absorption and the 

 amount in the plant cells is reduced causing a greater con- 

 centration of the cell sap, which retards the rate of evapora- 

 tion and the loss from the leaves. 



