56 DIFFUSION AND OSMOTIC PRESSURE 



of potassium nitrate, plasmolysis will occur. If tissues with 

 colored cell sap, such as portions of the lower epidermis of 

 the leaves of Tradescantia, are used, contraction of the 

 vacuole may be seen very readily under the microscope. The 

 coloring matter of the sap fails to pass the protoplasmic 

 layer, and thus plasmolysis is accompanied by a deepening 

 of the color of the sap. If the experiment be repeated on 

 fresh bits of tissue, continually weaker and weaker solutions 

 of potassium nitrate being used, a concentration of the latter 

 will at length be reached, such that no plasmolysis will 

 occur. But plasmolysis indicates that the external solution 

 is more concentrated than that within the vacuole, and its 

 failure to appear indicates that the cell sap is more concen- 

 trated than the external solution. Therefore, it may be 

 considered that the maximum concentration of potassium 

 nitrate which does not cause plasmolysis is isosmotic (i.e., 

 has the same osmotic pressure) with the cell sap. If we can 

 choose a plasmolyzing substance to which the protoplasmic 

 membrane is very nearly or quite impermeable 1 (see the fol- 

 lowing section), this will give a very exact method for 

 measuring turgor pressure. In this way De Vries was able 

 to show that, in general, the concentration causing plas- 

 molysis was always the same, no matter what substance was 

 used to produce it. There were some exceptions, however, 

 glycerin being the most notable of those used by him. He 

 found, too, that certain electrolytes gave extraordinarily high 

 osmotic pressures. The last is now known to be due to 

 ionization. The " isotonic coefficients " given by this author 

 express approximately the amount of ionization for the con- 

 centrations which he used. The results are exceedingly 

 valuable, for they have led to great advance, not only in 

 physiology, but also in physical chemistry ; but since these 



i DE VKIES has given some very pointed directions for the critical use of his 

 method in "Zur plasmolytischen Methodik," Bot. Zeitg., Vol. XLII (1884), pp. 



