ABSORPTION OF WATER 



147 



somewhat greater, almost three times the hygroscopic capacity of 

 the soil. 



46. Influence of Environmental Conditions on the Absorption 

 of Water. Methods of Studying the Absorption of Water by 

 Roots. — One of the environmental factors which strongly influ- 

 ences the rate of absorption of water by the roots is undoubtedly 

 the temperature of the soil. This was detected long ago by Sachs, 

 by means of very simple and illustrative experiments. He noticed 

 that such potted plants as tobacco, beans, or pumpkin, rapidly 

 show signs of wilting when the pot is surrounded with ice. When 



Fig. 60. — Potometer (after Molisch). 



this is removed and the soil is heated, the plants recover without 

 additional moisture. From this, he drew the conclusion that under 

 the influence of low temperature the rate of root absorption slows 

 down, so much that it is no longer able to make good the loss sus- 

 tained by evaporation. Such a primitive method, however, is 

 not sufficient for the exact determination of the amount of water 

 absorbed by the plant. Special apparatus, called "potometers," 

 are now used, which allow a quantitative determination of the 

 volume of water absorbed (Fig. 60). The essential parts of a 

 potometer are: (1) a container through the upper end of which a 

 plant, inserted in a stopper, is introduced in such a way that the 

 root system is within the container; (2) a graduated capillary tube 

 placed horizontally for observing the amount of water absorbed. 

 Otherwise, a potometer admits of an almost endless number of 



