Osmotic Pressure , Root Pressure , and Exudation 107 
is an unequal distribution of osmotic material in different parts of 
the cell; (3) that osmotic material is present in the cell wall outside 
the membrane so that water is sucked out of the cell. The first 
hypothesis is obviously unsatisfactory, since it makes osmotic pres¬ 
sure a function of the membrane instead of the concentration of 
the solution. At the time, however, that Pfeffer was writing osmotic 
pressure was very vaguely conceived, so that the presentation of 
such a view by a worker of the competence of Pfeffer is hardly 
surprising. In a later publication Pfeffer (1890) recognised the physical 
fallacy involved in this view, and characterised his first hypothesis 
as “irrig.” 
The value of Lepeschkin’s work on Pilobolus and Phaseolus lies 
in the very careful measurements which he made of the effect of 
various factors on the rate of exudation of water. When, however, 
he deals with the interpretation of his results he passes in careful 
review the three hypotheses of Pfeffer outlined above and makes a 
surprising decision in favour of the first scheme—that of a cell with 
a plasma-membrane of different osmotic pressures in different parts, 
no reference being made to the fact that Pfeffer had himself rejected 
this scheme fifteen years earlier. 
It is clear that the permeability of the membrane can only affect 
indirectly the osmotic pressure of the solution which it encloses by 
controlling through exosmosis the concentration of the solution. The 
osmotic pressure developed with such a leaky membrane will thus 
depend only in part on the specific permeability of the membrane. 
Other important factors will be the original concentration and mass 
of the solute and the time during which exosmosis has continued. If 
the membrane is rigid the pressure first developed will be practically 
the same as if the membranes were truly semi-permeable. Lepesch- 
kin further holds that not only has a membrane a specific osmotic 
pressure, but that directly that pressure is exceeded water begins 
to pass out through the membrane {loci cit. p. 425). The same solution 
can thus at the same time be both in equilibrium with and not in 
equilibrium with water; or rather it can be on both sides of the 
equilibrium point at the same time! 
Enough has probably been said to indicate that Lepeschkin’s 
view, that the exudation of fluid by Pilobolus and Phaseolus is due 
to the osmotic pressure of the excreting cell together with the dif¬ 
ferential permeability of the two sides of the cell, is supported by 
arguments of such doubtful validity that it cannot be considered 
acceptable. It is perhaps the barricade of mathematical equations 
