64 DIFFUSION AND OSMOTIC PRESSURE 



are soluble in water penetrate readily, glycerin being one 

 of the slowest. It appears as if the power to penetrate 

 decreased with increasing specific gravity. Overton finds 

 that the same thing is generally true of animal cells also, 

 and what is still more striking that the amount of 

 alcohols, etc., which plant and animal cells are able to bear 

 is nearly the same. 



When the solute fails to penetrate the protoplast, the 

 osmotic concentration necessary for plasmolysis is constant, 

 no matter what the solute may be. But the more readily it 

 penetrates, the higher the concentration necessary to bring 

 about plasmolysis, until at last, as in the alcohols and ethers, 

 this phenomenon does not truly occur at all. 



6) Direct test of penetrability. Another method of 

 determining the extent of permeability manifested by pro- 

 toplasm is to identify the diffusing substance after it has 

 passed the plasmic layer. De Vries ! showed that the pene- 

 tration of dilute ammonia into the cells of the red beet can 

 be demonstrated by the reaction of the colored cell sap to 

 this substance. The red sap changes to blue upon contact 

 with an alkali. By choosing other cells whose sap contains 

 red and blue dissolved pigments, Pfeffer 2 showed that not 

 only ammonia, but also the caustic alkalies and alkaline car- 

 bonates as well as acids (such as tartaric, phosphoric, and 

 carbonic) pass very rapidly through plant protoplasm. We 

 may consider that this at least proves that the H and OH 

 ions penetrate. In some cases a precipitate may be produced 

 within the vacuole by the reaction of the penetrating sub- 

 stance with the materials of the cell sap. This is the case 

 with caffein, antipyrin, and some others. 3 



!H. DE VRIES, "Sur la perm6abilit6 du protoplasme des betteraves rouges," 

 Arch. n6erl., Vol. VI (1871), p. 124; IDEM, "Sur la mort des cellules veg6tales," 

 ibid. (1871), pp. 245-95. 



2 W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 140. 



3 PFEFFER-EWART, Physiology of Plants, 1900, p. 98. 



