THE CONDUCTION OF WATER. I 



55 



a membrane which gives a lower osmotic pressure than the other half, then on 

 that side undoubtedly there will be a continuous outflow of water. This view 

 has been, however, demonstrated to be fundamentally erroneous. As Pfeffer 

 {1890, p. 303) has shown, and as is itself obvious from the kinetic theory of 

 osmotic pressure, the degree of pressure depends only on the number of mole- 

 cules plus free ions, and not in the least on the quality of the protoplasm ; for 

 a precipitation membrane, no matter how variable it may be, chemically and 

 physically, no matter whether it be thin or thick, must always give the same 

 pressure so long as it is impermeable. Pfeffer's (1877) explanation, on the 

 other hand, may be looked upon as the physically correct one. If at different 

 points in the cell different degrees of concentration of the cell-sap arise, the 

 inflow on the side of greater concentration must still exceed the outflow at 

 the moment when equilibrium between the two has been already established on 

 the other side ; the result is a unilateral outflow of water under a pressure 

 corresponding to the difference in concentration on the two sides of the cell. 

 Such a difference of concentration cannot obtain in physical experiments, 

 since, in consequence of diffusion, readjustment of the balance must necessarily 

 take place. If it occur and be also maintained in the plant, however, an expen- 

 diture of energy is clearly essential, such as the living cell can always furnish, 

 but which a physical apparatus (Pfeffer's osmotic cell) does not possess. 

 This is quite in accordance with what we have already learned, viz. that uni- 

 lateral secretion of water is at once inhibited by the withdrawal of oxygen or 

 by the action of chloroform, whereby the cells are transformed into non-living, 

 purely physical pieces of apparatus. 



An entirely different theory as to the cause of unilateral excretion has been 

 advanced by Godlewski (1884). He postulates rhythmic and continuous varia- 

 tions in osmotic pressure, during which the osmotically active substance is 

 always breaking down and as constantly being built up afresh. At each lower- 

 ing of the osmotic pressure there is an excretion of water, owing to the con- 

 traction of the elastically distended wall, and should these contractions follow 

 each other at longer or shorter intervals, it may be said that the cell exhibits 

 pulsations. Although there is much to be said in favour of this idea, looked 

 at by itself, its absolute correctness may be called in question since, in the 

 first place, there seems to be no good reason why the water, in these pulsations, 

 should always be forced out on one side only, and, in the second place, there 

 would seem to be every reason for the re-absorption of the water secreted 

 immediately on the re-formation of the osmotically active substance. 



A third explanation which has been advanced lays special emphasis on 

 the concentration of the sap. The assumption is that outside the cell which is 

 excreting water there arises, either in its membrane or in the membrane of the 

 vessel, an osmotically active substance which withdraws water by osmotic suc- 

 tion from the cell ; the cell itself would, according to this view, act in an entirely 

 passive manner during the process. That such a phenomenon does take place 

 in nectaries has been definitely proved, but whether it also plays a part in the 

 process of bleeding is very doubtful. Wieler has estimated that the osmotic 

 pressure of bleeding may rise in the birch to two and a half atmospheres, so that 

 it is quite legitimate to consider the pressure as due in this case to osmotic 

 action ; and it is still more feasible to make this assumption in the case of 

 the much more sugary sap of Acer, Agave, and Palmaceae. But it is quite 

 impossible to conceive that bleeding-pressures in general are in this sense 

 osmotic, since no definite relation can be observed between the amount of the 

 pressure and the concentration of the sap. In the vine, for example, high 

 pressures may be accompanied by a low concentration of sap. Moreover, 

 Wieler has conducted a series of experiments on the subject, and has found 

 that when he allowed osmotically active solutions to soak into the vessels of 

 the root-stump the bleeding was not increased at all. 



