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TEXTBOOK OF PLANT PHYSIOLOGY 



perfect harmony with the experimental data of Ursprung, who has 

 found that the suction tension of root hairs, when immersed in 

 water, is equal to zero. But the solution which is present in the 

 cavities of the dead tracheal elements as a result of the excreting 

 activity of the living cells adjoining the vessels, is not subject to 

 the pressure of the impermeable protoplasm. At this point the 

 solution, therefore, possesses a suction tension equal to the whole 

 magnitude of its osmotic pressure. It must absorb water from 



Fig. 57. — Hofmeister's model illus- 

 trating the pumping of water by a 

 cell: A, less permeable membrane of 

 animal bladder; B, more permeable 

 membrane of parchment paper. 



Fig. 58. — Diagram of water current 



passing through the cell in one 



direction (according to Priestley). 



the adjoining saturated cells, which begin to show a decrease in 

 saturation or suction tension. This loss is compensated for at 

 the expense of the surrounding solution. It tends to prove, there- 

 fore, that it is not the living cells that draw water from the soil, 

 but rather the solution present in the vessels. The whole aggre- 

 gate of living cells only filter the fluid but remain otherwise pas- 

 sive. The mechanism of this suction may be respresented by 

 the following simple diagram (Fig. 58). One end of cell A 

 is immersed in a vessel of water B. On the other end is poured 

 a solution C of a certain concentration. Since cell A is sat- 



