272 PROTOPLASM 



ability is a normal feature of cellular activity. It may also 

 be brought on by a change in environment. 



Experimental results on the effect of salts on cell permeability 

 are contradictory, but one fact seems to be rather generally true, 

 viz., that sodium increases the permeability of the protoplasmic 

 membrane, and calcium decreases it. Even this conclusion has 

 been found not to hold at all times; thus, the American physi- 

 ologist S. C. Brooks found that sodium chloride, potassium 

 chloride, calcium chloride, and magnesium chloride all decrease 

 the rate of penetration of a dye into the living cell; i.e., they 

 lessen the permeability of the protoplasmic membrane. That 

 sodium increases and calcium decreases permeability agrees well 

 with the disintegrating (solation) effect of sodium and the aggre- 

 gating (gelation) effect of calcium in regard to such protoplasmic 

 properties as elasticity and membrane repair (page 444). 



Certain workers have been inclined to group the elements to 

 which protoplasm is permeable on the basis of valence and to 

 predict that if one monovalent cation (sodium) affects proto- 

 plasm in one way, another monovalent cation (potassium or 

 lithium) will do likewise. It is also assumed that bivalent ions 

 (calcium, magnesium, barium) will affect protoplasm alike. 

 There appears to be some justification for this in the behavior 

 of nonliving systems (gelatin), but among living things the law 

 does not hold (see also page 440). Gellhorn finds that sodium 

 (monovalent) and magnesium (bivalent) both increase the perme- 

 ability of sea-urchin eggs to dyestuffs, while calcium (bivalent) 

 decreases it. Brooks and Gellhorn thus both find that mono- 

 valent sodium and bivalent magnesium have identical effects 

 upon protoplasm; but Gellhorn finds that these metals increase 

 the permeability of animal cells, while Brooks finds that they 

 decrease the permeability of jplant cells. Again do experimental 

 facts indicate the danger of drawing all-inclusive deductions; 

 and, after all, why should an animal egg react to a salt in the 

 same way as a plant cell reacts to it? The reactions of the two 

 may be the same, for cells wherever found have certain charac- 

 teristics in common, but they may also differ as widely as do the 

 plants and animals of which they are a part. 



Another interesting problem is the accumulation of substances 

 within the cell at a concentration greatly in excess of that in the 

 surrounding solution. Here, again, we may be dealing simply 



