70 DIFFUSION AND OSMOTIC PRESSURE 



either within the protoplasm or in the vacuole. If this were 

 not so, the diffusion tension of the solute would soon become 

 as great within the cell as without, and thus there could be 

 no accumulation. But if a substance is precipitated, poly- 

 merized, or condensed within the cell through the chemical 

 action of some other body already there, which perhaps arises 

 as a secretion from the protoplasm, then the internal diffusion 

 tension of the entering substance will be kept low, and inward 

 diffusion will continue indefinitely. In this way copper salts 

 entering the cell are probably reduced to metallic copper. 

 This fact of accumulation is a very important one in under- 

 standing the process of absorption of dissolved substances by 

 the plant. 



f) Test by metabolic processes. The absorption of any 

 food substance is of course a proof of permeability to that 

 substance. The immediate effect upon the living green cell 

 of absence of carbon dioxid, or upon any living cell of 

 oxygen, shows that these gases, when in solution, enter the 

 protoplast with extreme ease. Penetration by many usually 

 solid substances may be proved in this manner ; the long 

 series of experiments upon growth, and especially the forma- 

 tion of starch by green plants in darkness, may be regarded 

 as evidence in this matter. Thus, Bouilhac J grew Nostoc in 

 the dark in a solution of glucose, where it appeared perfectly 

 healthy, and Artari 2 and Matruchot and Molliard 3 grew 

 Stichococcus in organic solutions in a similar way. The 

 absorption of organic food by many algae, and by all saporo- 

 phytes and parasites, including all of the fungi, may be 

 mentioned in this connection. In many of these cases the 



1 R. BOUILHAC, "Sur la culture de Nostoc punctiforme en presence de glucose," 

 Compt. rend., Vol. CXXV (1897), p. 880. 



2 A. ABTABI, "Zur Ernahrungsphysiologie der grunen Algen," Ber. d. deutsch. 

 bot. Ges., Vol. XIX (1901), pp. 7-10. 



3 L. MATBUCHOT ET M. MOLLIABD, " Variations de structure d'une algue verte 

 sous 1'influence du milieu nutritif," Rev. gen. bot., Vol. XL, (1902), pp. 114-30, 254-68, 

 316-32. 



