I 4 PLANT PHYSIOLOGY 



was previously assumed that the plasmatic membrane possessed different 

 qualities, for it was customary to hold that the height of osmotic pressure 

 was dependent on the character of the plasmatic membrane. Should one half 

 of a cell be composed of a membrane which gives a lower osmotic pressure 

 than the other half, then water must of necessity at once flow out on that 

 side. This assumption has, however, been shown to be erroneous ; the height 

 of osmotic pressure, as PFEFFER (1890, p. 303) has shown and as is obvious 

 from the kinetic theory of osmotic pressure, depends only on the number of 

 dissolved molecules and ions and not on the character of the plasma ; any 

 precipitation membrane, whatever its chemical and physical characters may 

 be, more especially whether it be thin or thick, gives the same pressure so 

 long as it is impermeable. The following conception which we owe to PFEFFER 

 (1877) is, on the other hand, physically correct. If the cell-sap at different points 

 in the cell has different concentrations, then at the region of higher concentra- 

 tion the inflow must exceed the outflow until an equality is reached between 

 the two sides ; hence there results a unilateral exudation of water, and this is 

 effected with a pressure corresponding to the difference in the concentration 

 on the two sides of the cell. Such a difference in concentration could not 

 occur in a physical experiment, for an equilibrium must necessarily arise on 

 account of diffusion ; should a similar equalization be effected and maintained 

 in the cell, one must clearly realize that an expenditure of energy is necessary 

 for that purpose an expenditure which the living cell can certainly furnish at 

 any time, but which is impossible in a physical apparatus such as PFEFFER'S 

 osmotic cell. This explanation is also in accord with the fact that unilateral 

 expression of water is at once inhibited if we, by withdrawal of oxygen or by 

 subjecting the cells to the influence of chloroform, reduce them to the condition 

 of non-living purely physical pieces of apparatus. 



GODLEWSKI (1854) nas arrived at an entirely different conception of the 

 cause of unilateral exudation of water. He postulates continuous variations 

 in the height of osmotic pressure, resulting from the fact that osmotically 

 active material is used up and once more reconstructed. At each reduction 

 in the osmotic pressure exudation of water is brought about by contraction 

 of the elastically distended cell-wall, and if such contractions succeed each 

 other at longer or shorter intervals the cells at once exhibit pulsations. Although 

 there is much to be said for this idea it nevertheless appears to us not in all 

 respects satisfactory ; first, because it does not explain why in these pulsa- 

 tions the water should be excreted on one side only, and, secondly, because 

 when the osmotically active material again increases the water previously 

 expressed must be again reabsorbed. A third interpretation is one especially 

 applicable to such cells as give off sap rich in dissolved substances, such as 

 nectar and the sacchariferous bleeding-sap of many plants. If we assume that 

 the sugar arises locally out of or on the cell-membranes, it must withdraw 

 water osmotically from the cell. Since WILSON (1881) succeeded apparently 

 in stopping the secretion by thoroughly washing off this extracellular sugar 

 in the case of many nectaries and also in Pilobolus, this explanation is often 

 accepted as correct in cases like those referred to, in spite of individual argu- 

 ments to the contrary. Since LEPESCHKIN (1906) has, however, shown con- 

 clusively (as it appears to us) that in Pilobolus no such osmotic suction takes 

 place, it will be necessary to re-investigate the phenomenon in the case of 

 nectaries. Individual cases of bleeding-pressure might also be referred to 

 osmotic suction, due to materials which are produced from the membranes 

 of the cells or of the vessels. WIELER has estimated that the osmotic pressure 

 of the bleeding-sap of the birch can reach two and a half atmospheres, so that 

 one might very well imagine that the bleeding-pressure actually observed in 

 this tree should be regarded as osmotic. Indeed one might claim this to be 



