50 PLANT GROWTH SUBSTANCES 



Note, however, that when the cell is at equilibrium, the turgor pressure 

 is balanced by the wall pressure and therefore cannot cause any expansion 

 of the cell, because water cannot be absorbed without a suction difler- 

 ence. 



These relations are still under discussion, but for the given reasons I am 

 inclined to believe that the computations recently made by Frey- 

 Wyssling (7,8) of the osmotic work performed in the elongation of cells 

 are not correct. He has assumed the cell to be expanded by a force = W, 

 whereas in reality it should have been T, and these two values need 

 not be equal, certainly not during water saturation, and perhaps not 

 during the elongation. 



This connection between turgor and water absorption is not generally 

 realized. Audus (i), in his monograph on auxin actions, brings out 

 Heyn's opinion that the turgor pressure can call forth a stretching of the 

 cell, which ultimately implies a reduced hydrostatic pressure of the cell 

 contents, the consequences of which are an increased suction force and 

 water uptake in active cell stretching. Such a proposed sequence of 

 events has no purpose. The conditions are much simpler, for "stretching 

 under a turgor pressure" is but another way of saying "water absorption 

 owing to a suction difference." 



Looking for the cause of such a water uptake we must find out what 

 can cause a change from the equilibrium conditions when T = W and 

 S = E. This is possible by changing one of the three variables of the 

 equations, E, O, or W. E can, of course, be omitted from the discussion 

 leaving O and W to be considered. 



An increase in O impHes an increase in the amount of osmotic material 

 in the cell. That an actual increase in nutrients occurs from the start 

 of the elongation is easily demonstrated. The result should be an increased 

 turgor pressure, — E — W, causing absorption of water, a rise in the 

 wall pressure, and a decreasing turgor until a new equilibrium is attained, 

 with O, W, and T all higher than before. In practice there is a decrease 

 in O during the most rapid elongation, so that such a process alone 

 cannot be responsible for the cell elongation, Nevertheless it is necessary 

 to consider to what extent absorption of water in itself can contribute 

 to the cell elongation. Especially so, because such mechanisms and even 

 an auxin-induced water uptake without any change in the osmotic 

 conditions proper play a certain role in the discussion of the cell- 

 elongation process. 



