ii4 



PHYSIOLOGY OF NUTRITION 



The crystalloids, saccharose and potassium nitrate, produced lower pressures 

 than did the colloids, gum arabic and gelatine, when plant or animal membranes 

 were used. This seems to be in disagreement with statement 3, above, but it 

 is explained by the fact that these two crystalloids readily pass through such 

 membranes, while the precipitation membranes are almost impermeable to them. 



1 2 



Fig. 68. — Successive stages of plasmolysis. 



3 4 



N, nucleus; V, vacuole. (After deVries.) 



Pfeffer's experiments indicated that, other conditions remaining the same, 

 the magnitude of the osmotic pressure differed according to the nature of the 

 dissolved substance, and the question arose whether this phenomenon obeyed 

 any law. This question was answered by deVries, 1 who used living plant cells 

 instead of the artificial cells employed by Pfeffer. He determined the isosmotic 

 (or isotonic) coefficients of various substances by means of the plasmolytic 

 method. 



As is well known, plasmolysis occurs when a living plant cell is placed in a 

 sufficiently strong (10-per cent.) solution of such substances as cane sugar, so- 

 dium chloride, etc. At first there is a decrease in cell volume, to a certain point, 

 after which the protoplasm separates from the cell wall and withdraws inward 

 (Fig. 68). The cell gradually regains its earlier form if the salt solution is 



1 Vries, Hugo de, Eine Methode zur Analyze der Turgorkraft. Jahrb. wiss. Bot. 14: 427-601. 1884. 



