ABSORPTION OF MATERIALS IN GENERAL 



123 



The cell sap frequently exhibits high osmotic values." DeVries found that 

 sap expressed from young plant organs showed the osmotic values given in 

 the table below. 



Source of Expressed Sap 



Gunnera scabra (petioles) . . . 

 Solatium tuberosum (leaves) 

 Sorbus arcuparia (berries) . . 

 Beta vulgaris (roots) 



Osmotic Value 



atmospheres 



3-5 



5-5 



9.0 



21 .0 



The moulds Aspergillus niger and Penicillium may develop osmotic pres- 

 sures as great as 157 atmospheres, when they are grown in concentrated sugar 

 or salt solutions. 



n A solution alone has no osmotic pressure, this being produced by two solutions (or a solu- 

 tion and the pure solvent) and a membrane, all acting together. When the "osmotic pres- 

 sure" of a solution is spoken of, the maximum osmotic pressure that might be obtained with 

 that solution, at the given temperature, is meant. To obtain this maximum the membrane em- 

 ployed must be quite impermeable to all the solutes (dissolved substances) of the solution, and 

 the membrane must be in contact with the solution on one side and with the pure solvent 

 (water) on the other. These conditions are probably never actually fulfilled in the case of plant 

 cells. If we employ the term osmotic value for the maximum pressure, then the actual pressure 

 developed in any cell is usually of somewhat lower magnitude than is the osmotic value 

 of the cell sap. Diffusion tension of the solutes is another term that may be employed for 

 the osmotic value, with reference to the solution itself, but this is not without objection. 

 These measurements of deVries were made by means of cell membranes (plasmolytic method), 

 so that the nature and condition of the cells used as indicators enter into the argument 

 here, and he was not really measuring the osmotic values of these expressed solutions. — Ed. 



Fitting has studied the osmotic pressures of the cells of plant leaves, by the plasmolytic 

 method, using potassium nitrate solutions, in a very thorough way. He dealt especially with 

 desert plants. See: Fitting, Hans, Die Wasserversorgung und die osmotischen Druckver- 

 haltnisse der Wustenpflanzen. Zeitsch. Bot. 3: 209-275. 1911. Livingston, B. E., The rela- 

 tion of the osmotic pressure of the cell sap in plants to arid habitats. Plant world 14 : 153-164. 

 191 1. (This is a somewhat critical review of Fitting's paper.) While plant cells in general 

 have osmotic pressures of from 5 to 1 1 atmospheres, Fitting found pressures much exceeding 

 100 atmospheres in the leaves of some desert plants. This value is greater for plants growing in 



