ABSORPTION OF MATERIALS IN GENERAL 121 



all of the ferric chloride will pass from the outer solution into the cell. In 

 a similar way plant roots appear to absorb the essential elements, as well as 

 other substances, from the surrounding solution. 



The following experiment also illustrates this phenomenon of continued 

 absorption (Fig. 70). A roll of sheet zinc is placed in a short glass tube of 

 large diameter, the tube being filled with water and having both ends closed 

 with animal bladder or parchment paper. The tube is placed in a dilute 

 solution of copper sulphate, which passes through the membranes into the 

 tube. Here the copper of the salt is replaced by zinc, and the zinc sulphate 

 thus formed diffuses into the outer solution. Copper sulphate continues to enter 

 until all of it, or all of the zinc, has been used up. The same phenomenon 

 occurs in the growth of bacteria and moulds on various organic compounds. 

 Of two substances having different nutritive values, the cells take up mostly 

 the one with the higher value, frequently leaving the other entirely untouched. 

 For instance, Aspergillus niger absorbs only glucose from a mixture of this 

 substance and glycerine, so long as the former is present in the solution. 1 



Outward diffusion through the cell membranes is also subject to regulation. 

 Nathansohn's experiments 2 indicate that sodium chloride easily penetrates the 

 cells of Codium tomentosum (a marine alga) but that this salt cannot be com- 

 pletely withdrawn from the cells after it has once entered.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 3 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 



1 Pfeffer, W., Ueber Election organischer Nährstoffe. Jahrb. wiss. Bot. 28: 206-268. 1895- 

 - Nathansohn, Alexander, Zur Lehre vom Stoffaustausch. Ber. Deutsch, Bot Ges. 19: 509-513. 1901. 

 3 Vries, Hugo de, Untersuchungen über die mechanischen Ursachen der Zellstreckung, ausgehend von 

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

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



