286 PLANT PHYSIOLOGY 



osmometer is the direct continuation of its cavity. The rise 

 of the hquid, therefore, is simply the result of the general increase 

 in volume of the solution filling the osmometer. In the root, 

 on the contrary, the cavities of the vessels are separated from the 

 adjoining cells through which the water is driven. Besides, it 

 is not the cell sap but a considerably more dilute solution that 

 ascends through the vessels. In order to visualize root pressure, 

 it must be accepted that at least some root cells are able simul- 

 taneously to imbibe water and to expel it into the vessels. There- 

 fore, the concept of the osmotic mechanism 

 of the cell already discussed must be some- 

 what modified in order to understand this 

 flow of water in one direction. 



For this purpose, several modified mecha- 

 nisms have been suggested. The idea that 

 has recently gained wide recognition is the 

 one chiefly put forward by Priestley, who 

 considers that the water-absorbing cell, 

 being saturated, develops no suction tension 

 Fig. 87. — Diagram of by itself. This is in perfect harmony with 

 ThrouU Zr:ll^ 'ron? t^^e experimental data of Ursprung, who has 

 direction [according to found that the suction tension of root hairs, 

 Priestley). when immersed in water, is zero. But the 



solution that is present in the cavities of the dead tracheal 

 elements as a result of excretion by the living cells adjoining the 

 vessels is not subjected to the pressure within the live cells. At 

 this point, the solution therefore possesses a suction tension equal 

 to the whole magnitude of its osmotic pressure. It must absorb 

 water from the adjoining saturated cells, which begin to show a 

 decrease in saturation and which because of suction tension absorb 

 water from the surrounding solution. This shows that it is not the 

 living cells that draw water from the soil, but rather the solution 

 present in the vessels. The whole aggregate of living cells only 

 filter the fluid and remain otherwise passive. The mechanism of 

 absorption may be represented by the foregoing simple diagram 

 (Fig. 87). One end of the cell A is immersed in a vessel of water 

 B. On the other end is poured a solution C, of a certain concen- 

 tration. Since cell A is saturated with water, solution C will 

 draw water through it, quite independently of the magnitude of 

 the osmotic pressure in cell A. 



