TEMPERATURE. 185 



longer absorb sufficient water to replace a small loss by evap- 

 oration in a moist soil, having a temperature of from 3 to 

 5 Cent. (37 to 41 Fahr.) ; the heating of the soil to a tem- 

 perature of from 12 to 18 Cent. (53 to 64 Fahr.) sufficed 

 to raise their activity to the needful extent."* According 

 to the same investigator, the roots of the turnip and cabbage 

 continue to absorb water, even when the temperature of the 

 soil is reduced very nearly to Cent. (32 Fahr.). In the 

 winter and early spring, when the temperature of the soil is 

 low, the roots of trees and other perennials cannot absorb 

 moisture unless they extend deep enough to reach the 

 warmer strata beneath ; under such circumstances, it not in- 

 frequently happens that if the air temperature rise high 

 enough to allow evaporation, evergreen trees and shrubs are 

 killed by too great loss of moisture. 



243. Evaporation or Transpiration. In aerial plants, 

 when the temperature of the air is low, but little evaporation 

 takes place from the leaves or other living organs, while an 

 increase of temperature is followed by an increase in the 

 rapidity of evaporation. It is probable that this is due (1st) 

 to the closing of the stomata in the lower, and their opening 

 in the higher temperature, and (3d) to the fact that in all 

 ordinary cases, as the temperature of the air is lowered its 

 degree of saturation is increased, and as its temperature is 

 raised its degree of saturation is decreased. As transpiration 

 appears to be a purely physical phenomenon, we scarcely 

 need expect it to be as definitely or certainly affected by 

 changes of temperature as are the proper functions of the 

 plant. 



244. Assimilation. The lower limit of the temperature 

 in which assimilation is possible varies much in different 

 plants. The "Ked-snow Plant" (Protococcus, sp.) of the 

 Arctic regions grows rapidly upon the surface of the snow in 

 a temperature which must be little, if any, above Cent. 

 (32 Fahr.) ; in the larch, assimilation takes place at from 

 0.5 to 2.5 Cent. (33 to 36 Fahr.), and in meadow-grasses 

 at from 1.5 to 3.5 Cent. (35 to 38 Fahr.). In water- 



* " Lehrbuch," English edition, p. 652. 



