io6 



PHYSIOLOGY OF NUTRITION 



The crystalloids, saccharose and potassium nitrate, produced lower pressures 

 than did the colloids, gum arable 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. 



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,^ who used living plant cells 

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

 (or isotonic) coefl&cients 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 (lo-per cent.) solution of such substances as cane sugar, sodium 



12 3 4 



Fig. 68. — Successive stages of plasmolysis. N, nucleus; V, vacuole. (.After deVries.) 



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 

 replaced by water. Cells with colored sap are very good for plasmolytic ex- 

 periments, since the coloring matter is retained within the shrinking vacuole, 

 leaving the space between the protoplasm and the cell wall filled with colorless 

 solution. By the use of such cells plasmolysis may be readily detected, 

 even in its incipient stages. DeVries used mature cells with colored sap and 

 determined the concentration of the plasmolyzing solution when the latter was 

 just strong enough to cause separation of the protoplasm from the wall at the 

 corners of the cell (Fig. 68, 3). If no further contraction of the protoplasm 

 occurs it follows that the osmotic pressure within the vacuole just equals that 

 of the external solution. The same experiment was repeated with various 

 substances and the limiting concentration {i.e., that concentration which is 



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



