CHAPTER VIII 



PERMEABILITY OF CELLS 



It was shown by Niigeli, Pfeffer and de Vries that concen- 

 trated solutions of neutral salts cause plant cells to shrink by 

 extracting the water from them. The shrinkage, plasmolysis, 

 of plant cells may be easily detected because, though the cell 

 wall does not shrink, the shrinkage of the protoplasm separates 

 it from the cell wall, leaving a space between the two. Solutions 

 with equal plasmolytic power de Vries (1884) called isotonic 

 solutions, and showed a relation between their percentage con- 

 centrations and the molecular weights of the solutes. This was 

 the foundation of van't Hoff's theory of osmotic pressure. The 

 discrepancies in the relation between the concentration of isotonic 

 solutions and the molecular weight of the solutes was explained 

 by Arrhenius on the assumption that electrolytic dissociation 

 increases the number of particles in solution. In this way a 

 method was developed of measuring the osmotic pressure of 

 plant cells. The osmotic pressure of solutions that just fail to 

 plasmolyze the cells is equal to the osmotic pressure of the cells. 

 If animal cells are used, some method of determining the first 

 decrease in volume must be used. When the cells are in large 

 masses, changes in volume may be determined by weighing the 

 masses. If the cells are free, as with blood corpuscles, their 

 volume may be determined by centrifuging them in graduated 

 tubes (Hamburger, 1893). 



Some substances do not plasmolyze cells no matter how great 

 the concentration, due to the fact that the cells are permeable 

 to the substances which consequently cannot exert any osmotic 

 pressure on the cells. Others cause only a temporary plasmolysis 

 because they slowly penetrate the cells. When the concentration 

 becomes the same outside and inside they cease exerting osmotic 

 pressure. 



The question arises as to whether the whole cell substance, 



