114 



PHYSIOLOGY OF NUTRITION 



In another series of experiments plasmolyzed pieces of Sambucus pith were 

 placed in water at various temperatures, with the same result; the return of 

 turgidity was more rapid as the temperature of the water was higher. These 

 results are shown graphically in the curve of Fig. 71, where the abscissas are 

 the temperatures and the ordinates are the velocities of the movement of 

 water through the protoplasmic membrane (both inward and outward). 



The rates at which dissolved substances diffuse through the protoplasm 

 also depend on temperature. If the velocity of movement at o°C. be taken as 

 unity, then the following relative velocities are obtained for potassium nitrate, 

 glycerine and urea, for various higher temperatures. 



O' 6° /?" 



Fig. 71. — Graph representing relation of temperature to velocity of penetration of water 

 through the protoplasmic menibrane. 



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 oe Expressed Sap 



Osmotic Value 



Gunnera. scabra (petioles) . . . 

 Solahum tuberosum (leaves) . 

 Sorbus arcuparia (berries) . . . 

 Beta vulgaris (roots) 



atmospheres 



■ 35 



SS 



9.0 



21.0 



" 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 majymum pressure, then the actual 

 pressure developed in any cell is usually of somewhat lower magnitude thar is the osmotic 

 value of the cell sap. Diffusion tension of the solute 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. 



