68 Plant Physiology 



molecular solution, or M solution), would give, according 

 to this, an osmotic pressure of 22.4 atmospheres. 1 Ordi- 

 narily the osmotic pressure of an epidermal cell of a leaf 

 or of a meristem cell is somewhat more than 4 atmos- 

 pheres, or about .20 gram-molecular (7 per cent) of sugar 

 as determined by the method subsequently discussed. 



The plant cell behaves very much as the simple os- 

 mometer above described. The root-hair, for example, is 

 a case in which the cell-sap is the strong solution, the limit- 

 ing layer or edge of cytoplasm is the plasmatic membrane 

 (the cell-wall in this instance furnishing support and pro- 

 tection), and the soil water is the weak solution. The 

 flow of water is into the cell ; in fact, under such circum- 



1 A gram-molecular solution of such substances as potassium nitrate 

 or common salt (electrolytes) yields a pressure higher than 22.4 atmos- 

 pheres. This is explainable on the ground that the number of particles 

 in solution is increased by the partial or complete dissociation of the 

 molecules of such substances into their ions. Thus KNOs, when partially 

 dissociated, yields in addition to KNOa the ions K + and NOs~. Gram- 

 molecular solutions of this salt show a pressure of about 35 atmospheres. 

 For this reason cane-sugar and potassium nitrate are not osmotically 

 equal; that is, isosmotic at the same molecular strength. Before the 

 theory of dissociation was developed De Vries determined that organic 

 substances such as sugar have about two thirds the osmotic value of 

 monovalent salts ; that is to say, that the ratio of their coefficients is aa 

 2 to 3. This ratio and those developed by De Vries for dibasic or other 

 compounds are fairly satisfactory as indications of the isosmotic relations 

 at the strengths corresponding to the plasmolysis of higher plants. 

 Nevertheless, since the per cent of dissociation varies considerably 

 among the salts of the monobasic, dibasic, or other groups, it is essential 

 in any comparative quantitative work to know accurately the per cent of 

 dissociation of any electrolyte employed. This may be obtained from 

 physical chemical tables. The discussion of the relation between elec- 

 trolytes and nonelectrolytes and the formula for comparing the latter 

 with the former is developed in the work by Livingston, cited in the 

 literature. 



