508 EDMUND NEWTON HARVEY 



INTRODUCTION 



1. Historical 



Within the past fifty years a great deal of scientific research 

 has been directed toward advancing our knowledge of the man- 

 ner in which substances may pass into living cells and of the 

 classes of substances which may or may not enter. Intimately 

 connected with the question of the permeability of cells are the 

 phenomena arising from the existence of an osmotic pressure 

 without and within the cell, and the rigidity or turgor of plant 

 parts directly dependent on the latter. This relation is most 

 evident when we recall that the existence of a continual turgor 

 of plant structures depends on the possession, by the individual 

 cells, of a surface membrane which prevents the diffusion out- 

 ward of most of the substances dissolved in their sap vacuoles. 



Questions concerning osmotic pressure must therefore always 

 go hand in hand with questions concerning the permeability 

 of the membrane whose presence is one condition for the exist- 

 ence of that pressure. Historically the two subjects have de- 

 veloped simultaneously. 



It is hardly within the scope of this paper to give a detailed 

 account of the history of this complicated subject, so numerous 

 are the papers dealing with its different phases. The funda- 

 mental facts were outlined by three observers during the latter 

 half of the last century. Their influence has been so great that 

 I shall mention briefly the contribution of each. 



Nageli ('55) first investigated the osmotic properties of the 

 cell and made clear the cause of turgor and of plasmolj^sis. The 

 word plasmolysis was introduced later by DeVries. It is to 

 Pfeffer (77, '86, '90) and DeVries, (71, 77, '84, '85) however, 

 that we owe our present conception of the important role played 

 by diffusion and osmotic pressure. Both of these authors in- 

 vestigated the magnitude of the pressures existing in plant cells 

 and the properties of the cell membranes. Pfeffer has especially 

 emphasized the conditions under which accumulation of sub- 

 stances takes place; DeVries the importance of the vacuolar 



