80 Diffusion and Osmotic Pressure 



been deprived of CO2 for many days still exhibited this 

 response seems to show that the phenomenon stands in no 

 direct relation to the process of photosynthesis. 



IV. ACTION OF THE PROTOPLASMIC MEMBRANE 



If ignorance still prevails regarding the manner in which 

 a purely physical osmotic membrane acts, it is even more 

 prevalent regarding the action of the protoplasmic mem- 

 brane of the living cell. That the tonoplast and ectoplast 

 are the main factors in the production of semi-permeability 

 there seems little reason to doubt. There are three possible 

 ways in which they may act, and of course the same mem- 

 brane may act in different ways at the same time. These 

 three possible explanations of semi-permeability may be briefly 

 stated as follows: 



a) The filter theory. — The simplest explanation of 

 osmotic pressure, whether within a living cell or not, is this: 

 That the semi-permeable membrane acts merely as a sieve 

 or filter, and that the larger solute particles are prevented 

 from passing by the smallness of the opening, while the 

 smaller solvent particles pass with only slightly increased 

 friction. Since we cannot be sure of the relative sizes 

 of the different molecules and ions, there is no way of 

 directly testing this hypothesis. But there are many facts, 

 both in chemistry and physiology, which make the filter 

 theory at least very improbable in many instances. Among 

 the physiological facts which tend in this direction may be 

 cited the effects of chemicals in varying the permeability of 

 the protoplasm, especially the observations of Maquenne, 

 De Vries, and Loeb, already discussed' and the work of 

 Overton, to be taken up under h). 



b) The solution theory. — According to this theory, the 

 membrane is to be considered as a solvent in which the pene- 



1 See above, p. 74. 



