120 



PHYSIOLOGY OF NUTRITION 



— — ; — i i 1 1 



Fig. 69. — Cell of Zygnema 

 with crystals formed by 

 methylene blue. 



succeeded in introducing useless and even injurious substances (such as aniline 

 dyes) into the living cell. He found that the following pigments penetrated: 

 methylene blue, methyl violet, bismarck brown, fuchsin, cyanin, safranin, 

 methyl green, methyl orange, tropseolin 00 and rosolic acid. The concentra- 

 tions of the solutions employed were very low (from 0.001 to 0.00001 per cent.). 

 Some of the dyes, (e.g., methylene blue) first enter the cell sap and color it, but 

 form crystals after a time; Fig. 69 shows an alga cell (Zygnema) with crystals 

 formed by methylene blue. Other dyes (e.g., methyl violet) stain the proto- 

 plasm itself. In neither case is the cell fatally injured. 



Overton 1 studied a number of different dyes and found that the permeability 



of the protoplasm to these varied according to 

 their chemical constitution. Basic aniline dyes 

 readily enter the cell, but most of their sulphuric 

 acid derivatives penetrate either not at all or very 

 slowly. Dyes that have accumulated in the cells 

 diffuse out when the cells are placed in water, this 

 outward passage being accelerated by the addition 

 of 0.01 per cent, of citric acid to the water. 2 

 Citric acid thus appears to change the osmotic properties of the protoplasm. 

 No dye accumulates in the cell if the solution contains 0.0 1 per cent, of citric 

 acid, but the dye is absorbed from the surrounding solution in the absence of 

 the acid. It is thus possible to alter at will the osmotic properties of cells. 



It is well known that plants can absorb and accumulate the essential chemical 

 elements from very dilute solutions. Some 

 non-essential elements enter the plant cell only 

 until their effective concentration becomes the 

 same within and without, but some others, as 

 well as the essential elements, continue to enter 

 and accumulate in the cell, even from a weak 

 solution, since they are converted into new com- 

 pounds after entrance and so the internal con- 

 centration never becomes equal to the external. 

 An illustration of continued absorption may 

 be found in the accumulation of iron tannate 

 in an artificial cell of collodion or animal bladder 

 filled with tannin solution and surrounded by 

 one of ferric chloride. Tannin does not escape 

 through the membrane, but ferric chloride 

 diffuses into the cell and there enters into combi- 

 nation with the tannin to form iron tannate, 



which also remains in the cell. Ferric chloride is continually consumed in the 

 formation of the iron tannate, and its concentration within the cell never be- 

 comes the same as that outside. If the tannin solution is sufficiently concen- 



1 Overton, E., Studien uber die Aufnahme der Anilinfarbe durch die lebende Zelle. Jahrb. wiss. bot. 

 34: 699-701. 1900. 



2 Pfeffer, 1886-88. [See note 1, page 121.] 



Fig. 70. — Apparatus for show- 

 ing diffusion of copper sulphate 

 through a membrane into a tube 

 containing zinc. 



