WATER RELATIONS OF PLANTS 



291 



influences the rate of absorption of water by the roots is 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 w^hen the pot is surrounded with ice. 

 When 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 tempwature the rate of root 

 absorption slows down so much that it is no longer able to make 

 good the loss sustained by evaporation. Such a simple method. 



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Fig. 89. — Potometer {after Molisch). 



however, is not sufficient for the exact determination of the 

 amount of water absorbed by the plant. A special apparatus, 

 called the ''potometer," is now used, which allows a quantitative 

 determination of the volume of water absorbed (Fig. 89). 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 wdthin the container ; (2) a 

 graduated capillary tube, placed horizontally, for recording the 

 amount of water absorbed. A potometer admits of an almost 

 endless number of modifications. Frequently, only branches 

 instead of whole plants are employed. It is often expedient to 

 construct a potometer that can be weighed, in order to be able 

 to compare the water absorbed with that lost by the plant in the 

 process of transpiration. 



