48 PLANT GROWTH SUBSTANCES 



water may be brought about, with the natural reservation that not every 

 kind of water uptake results in growth. This is also the usual starting 

 point for a discussion of cell elongation, but it is regrettable that there is 

 an obvious uncertainty as to the importance and nature of the osmotic 

 conditions of the cell that regulate the absorption of water. I need hardly 

 remind you of the fact that there is still no unanimously accepted 

 physical explanation of osmotic pressures. 



The fundamental formula underlying the water absorption is the 

 one given by Ursprung: S = O — W, where S denotes the suction 

 pressure of the entire cell against the external medium, is the osmotic 

 value or the diffusion pressure deficit of the vacuolar sap including 

 sweUing pressures, and W is the elastic tension of the wall, directed 

 towards the protoplast. Unfortunately the properties 5 and O are often 

 confused. 



As the driving force of the growth it is customary to refer to turgor 

 pressure, or the pressure acting from within the cell as a force striving 

 to expand it. What turgor signifies can be theoretically deduced and 

 expressed by the formula T = O - E, where T denotes the turgor 

 pressure, and E the osmotic value or diffusion pressure deficit of the 

 solution surrounding the cell, that is, the external solution. The turgor 

 pressure can thus be defined as the difference in diffusion pressure of the 

 water outside and inside the cell, or the force with which water tries 

 to enter into and expand the cell (5). 



These two formulae are, as a matter of fact, quite independent of each 

 other. They contain three independent variables: O, E, and W, the 

 last of which is in reahty not an osmotic property in the strict sense, 

 but is, as already mentioned, the elastic wall tension. 



The equations further signify that there are two existing forces or 

 pressures in the cell and not more, namely W, the elastic pressure of the 

 wall, directed inwards, and T, the diffusion pressure of water, i.e., the 

 net diffusion pressure, which normally as in turgid and expanding 

 cells causes a pressure directed outward toward the wall but in shrinking 

 cells is directed inward. These two pressures are in principle quite inde- 

 pendent of each other. If they are equally large, that is, if T = W, the 

 cell is in equilibrium with its external medium. Then also S = E, which 

 is the usual way of expressing that the suctions of the cell and of the 

 external medium are equal. We must assume that before elongation 

 starts the cell is in such an equilibrium with its surroundings. The rela- 



