TUEGIDITY 81 



trating substances are readily soluble, while those which fail 

 to penetrate are not. Thus, in the case of an osmotic cell 

 containing a sugar solution and surrounded by pure water, 

 it may be supposed that the membrane dissolves water 

 readily, but cannot dissolve sugar. Thus, water would go 

 into solution in the membrane on the side of higher diffusion 

 tension of water (the side of the solvent), and, after diffusing 

 through the membrane, would be given off on the side of 

 lower diffusion tension of that substance (the side of the 

 solution). The solute, not being able to go into solution in 

 the membrane, merely exerts its expansive force upon it, and 

 does not diffuse. 



From the researches of Overton' and others it appears 

 that this is probably the true explanation of many cases of 

 the development of osmotic pressure in plant and animal 

 cells. By comparing the numerous substances which he 

 found able to penetrate the protoplasm, Overton observed 

 that penetrating power seemed to vary in proportion to solu- 

 bility in aliphatic oils and ethers. He went still further: A 

 given substance, like glycerin, may be only slightly soluble 

 in aliphatic oils, and may penetrate the protoplast slowly; 

 but if its substitution products (as mono- and dichlor-hydrin, 

 or mono- or dimethyl-glycerin) are more soluble in this 

 class of substances, their power of penetrating the plant 

 protoplast is also found to be greater, and this in proportion 

 as solubility has been increased by the substitution. From 

 these and many other observations Overton is led to con- 

 clude that the peculiar property of the protoplast of being 

 permeable to certain substances and not to others probably 

 depends upon the presence within it of some aliphatic oil 

 or ester, or a mixture of these substances. He goes so far 

 as to point out that cholesterin and the lecithins, substances 



IE. OvBETON, "Ueber die allgemeinen osmotisohen Eigensohaften der Zelle, 

 ihre vermuthlichen tJrsaclien und ihre Bedeutung fflr die Physiologie," Vierteljahr- 

 schr. d.Naturf.-Ges. in Zurich, Vol. XLIV (1899), pp. 88-135. 



