Stiles and j0rgensen. — Studies in Permeability. V. 431 
This complexity of the system, as we have already said, makes it 
difficult to draw conclusions as to the causes of the observed phenomena. 
With quantitative data and proper methods, however, mathematical treat- 
ment becomes possible and will in the future no doubt be a great aid in the 
analysis required for an understanding of the processes of plant physiology. 
However, although mathematical treatment is possible in regard to the 
subject dealt with in this paper, we have avoided this, as it seems to us con- 
siderably more data are required before this can be done profitably in this 
connexion. 
The explanations so far put forward to account for swelling and 
plasmolysis probably do not take enough account of the complexity of the 
system. Of the two chief theories, one, that of de Vries and Pfefifer, 
explains the absorption of water on the basis of semi-permeable membranes 
and osmosis. 1 The protoplast is supposed to be surrounded by mem- 
branes which are permeable to water and impermeable to certain substances. 
The argument advanced against this theory, that salts get into the 
plant, and therefore the plasma membrane cannot be impermeable to salts, 
is frivolous, for in the first place the plasma membrane may undergo 
reversible changes in regard to its permeability, 2 and, secondly, plasmolysis 
would still take place if the membrane were not absolutely semi-permeable 
(as no membrane is) but let through the salt more or less slowly. 
Although we do not think the theory of de Vries is likely to be a com- 
plete expression of the facts, yet we may point out that there is not a fact 
recorded in this paper which cannot be explained on its basis. The 
swelling of tissue in distilled water, the plasmolysis in sodium chloride and 
sucrose, the behaviour in solutions of substances with poisonous properties 
and which are known to enter the cell, such as acids and the primary 
alcohols, are all phenomena which follow directly from the semi-permeable 
membrane theory. On the other hand, unpublished work of Stiles and Kidd 
indicates that sodium chloride passes rapidly into the cells of potato and 
carrot, while Brooks (2), using de Vries’s method of tissue tension, concludes 
that sucrose as well as sodium chloride passes readily into the scapes of the 
dandelion. 3 
Such difficulties as this have caused some workers on animal cells, e.g. 
Moore, Roaf, and Webster (9, 10) to reject the osmotic theory altogether 
and to explain all permeability phenomena as due to the presence of 
1 Pfeffer was well aware of the capacity of protoplasm to absorb water on account of its 
colloidal nature, but apparently did not regard it of importance in such observations as are recorded 
here. Cf. Pfeffer (14), p. 136 . 
2 ‘ Apparently it is by temporary or permanent alterations in the specific nature of the 
plasmatic membrane that an absorption (or excretion) of a particular substance may be temporarily 
or permanently permitted or prevented ’ (Pfeffer, 14, p. 102 ). 
3 Pfeffer points out (14) that the formation of starch in the plastids, or the accumulation of other 
carbohydrates, when plants are supplied externally with sugar, indicates that sugar can be absorbed 
under certain circumstances. 
