ABSORPTION or MATERIALS IN GENERAL 



113 



pletely withdrawn from the cells after it has once entered. When the alga is 

 placed in an isosmotic solution (4 per cent.) of sodium nitrate the chloride con- 

 tent of the cell sap rapidly decreases at first, but the outward diffusion of chloride 

 ceases after a time, as is clear from the following table. The figures denote 

 chlorine content, calculated as per cent, of HCl. 



Plasmolysis of cells has already been described (Fig. 68). DeVries^ plas- 

 molyzed whole plant organs as well as cells, and showed that growing parts 

 (such as stems, roots and flower stalks) are noticeably shortened after immer- 

 sion in a plasmolyzing solution, but regain their original stiffness and elas- 

 ticity when returned to pure water. This rigidity, which is a result of 

 osmostic pressure, is called turgidity. 



The rate at which water and dissolved substances penetrate the protoplasm 

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

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

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

 ferred to 2S-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 jpith, 

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

 following table. 



• No further shrinkage. 



* Vries, Hugo de, Untersuchungen uber die mechanischen Ursachen der Zellstreckung, ausgehend von 

 der Einwirkung von Salzldsungen auf den Turgor wachsender Pflanzenzellen. Leipzig, 1877. Idem, 

 Untersuchungen uber die mechanischen Ursachen der Zellstreckung. Halle, 1877. 



" Van Rysselbergbe, Fr., Influence de la temperature sur la permeability du protoplasme vivant pour 

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

 osmotique des cellules vfigfitales & la concentration du milieu. M6m. cour. Acad. Roy. Belgique 58: i -loi. 

 1898.J 



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