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V. H. Blackman 
in no way depend upon the osmotic pressure in the adjacent living 
cells E, although such a dependence is usually assumed in theories of 
osmotic pressure. The objection to the assumption of such dependence 
may be put in another way—if the osmotic pressure of E is acting 
freely it would draw water back from F, if the pressure in E is 
opposed by the tension of the cell-wall it cannot exert pressure on 
the contents of F. 
That the exudation of fluid from the open vessels of the wood 
could arise in this way seems clear, but just as in the case of Pilobolus 
it is very doubtful whether such a mechanism plays any part in 
the “bleeding” of cut stems. As with this mechanism the driving 
force depends on the concentration of the solution in the vessels 
we should expect some relation between the concentration and the 
pressure under which the fluid is excreted, but in the vine high 
pressures may be accompanied by a low concentration. In some 
cases also the fluid has almost the composition of “spring water” 
(Pfeffer, 1900, p. 262) which would mean a solution of very low 
osmotic force 1 . The marked effect of deprivation of oxygen and 
possibly of temperature suggests also, as in the case of Pilobolus, 
that root-pressure cannot be explained by means of simple osmotic 
relations but that “vital” relations of the cell involving energy 
transference are really involved. 
The value of Pfeffer’s scheme in explaining exudation and root- 
pressure seems to have been overlooked of late years. It has the 
great advantage that, although it requires a supply of energy and 
thus calls upon the “vital activities” of the cell it will explain the 
exudation of pure water from the cell. In this scheme the osmotic 
substance is supposed to exist in the cell at two different concentra¬ 
tions. It may be represented by the model in fig. 3. The curved 
glass tube is closed by two completely semi-permeable membranes 
A and B. The arm above A is filled with a strong sugar solution 
(say M 1 1), the arm above B with a weak sugar solution (say M/io), 
the intervening space being filled with water; the two arms are sup¬ 
posed to be plunged in water. On the entry of water through A and 
B pressure will develop in the tube and when the pressure reaches 
1 Priestley (1918, p. 199) has suggested that in “bleeding” the soluble 
substances which pass into the vessel below are absorbed again during their 
passage up the stem. The value of any theory is, however, seriously weakened 
when it requires the support of a subsidiary hypothesis. Furthermore if we 
are to fall back on the unknown processes of “physiological absorption” the 
theory ceases to be a physical explanation of the process. The question also 
arises why should the cells below give out substances and the cells above 
reverse the process? 
