122 PHYSIOLOGY OF NUTRITION 



is influenced by external conditions. Van Rysselberghe 1 studied the effect of 

 temperature upon this rate. In one series of experiments pieces of pith from 

 young twigs of Sambucus nigra (elder) were placed in water and then trans- 

 ferred to 26-per cent, solutions of cane sugar at different temperatures. Each 

 piece was 114 mm. in length at the outset, and their lengths were redetermined 

 at stated intervals. The lower the temperature, the more slowly did plasmolysis 

 occur. The amounts of shrinkage observed for such pieces of Sambucus pith, 

 with different temperatures and after different periods of time, are shown in the 

 following table. 



*No further shrinkage. 



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.) 



~~^'"' 



Z ZZZÜZZZZZZZZZ 



- — 5 <*■ 



Fig; 71. 



O" 6" 72° f6° 20° 2i° 30° 



-Graph representing relation of temperature to velocity of penetration of water through 

 the protoplasmic membrane. 



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. 



!Van Rysselberghe, Fr., Influence de la temperature sur la permeabilite du protoplasme vivant pour 

 l'eau et les substances dissoutes. Recueil Inst. Bot. Bruxelles 5: 209—249. 1902. [Idem, Reaction 

 osmotique des cellules vegetales ä la concentration du milieu. Mem. cour. Acad. Roy. Belgique 58: 1-101. 



