OSMOTIC PRESSURE IN ANIMALS AND 

 PLANTS 



By W. R. G. ATKINS, M.A., Sc.D., F.I.C., 



Assistant to the University Professor of Botany, Trinity College, Dublin 



A comparative study of the osmotic pressures existing in 

 animal and plant cells reveals at once the great difference in 

 the conditions under which these two great divisions of living 

 matter have developed. And as in their morphology the most 

 primitive of both groups have diverged but little, so also in their 

 relations with the external medium close similarity may be 

 observed . Again the ontogeny of each species is an abbreviated 

 recapitulation of its phylogeny as far as morphology is con- 

 cerned. The same is no less true of its physiology, of which 

 the osmotic pressure is an expression. 



The nature of osmotic pressure need not be considered here, 

 as indeed much controversy still rages concerning its manner 

 of action . It may be remarked, however, that it can be regarded 

 as a hydrostatic pressure exerted on membranes which limit 

 the diffusion of matter in the crystalloid condition when dis- 

 solved inasolvent to whichthemembrane is permeable. Further, 

 its magnitude depends upon the number of molecules of a non- 

 electrolyte or of molecules and ions of an electrolyte in a given 

 volume of solvent, or more accurately in a given weight of 

 solvent. In this and in its temperature coefficient the osmotic 

 pressure of a dilute solution obeys the gas laws with some degree 

 of accuracy. For concentrated solutions, however, a more com- 

 plicated equation has been devised, and the gas law equation 

 is a simplified form of this. 



Since, therefore, the osmotic pressure of a solution depends 

 upon the total number of ions and molecules in a definite weight 

 of solvent, it is evident that the pressure in a living cell is regu- 

 lated by the activity of the protoplasm in synthesising colloidal 

 matter from crystalloids and in splitting up colloids again to 

 form crystalloids, as well as by the entrance of external ions 



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