OSMOSIS. 277 



morpJia, growing in sea water, is practically isosmotic with that of 

 Spirogyra, growing in fresh water, though the osmotic pressure of sea 

 water is some 240 times that of fresh. Thus the osmotic pressure of the 

 cell sap of Chcetomorpha is far below that of the water in which it lives, 

 while that of the sap of Spirogyra is far above that of fresh water. 



One can investigate the permeability of the living protoplast by the 

 plasmolytic method already alluded to above. The cell is plasmolysed 

 with a solution of some substance indifferent to the protoplasm and known 

 not to penetrate (sugar). A solution of the substance to be tested is now 

 prepared of the same osmotic pressure as the solution of indifferent 

 substance which just causes plasmolysis. If the solution so prepared has 

 exactly the same effect as the standard, it cannot pass through the proto- 

 plast, for, if it did, there would no longer be equality of osmotic pressure 

 on the two sides thereof. If the substance to be tested is only slightly 

 soluble in water, or is poisonous to the protoplasm, a small amount of it 

 is added to the standard indifferent solution, and the effect of the 

 addition on the plasmolysis noted. If it does not pass the membrane, 

 then, by virtue of the higher osmotic pressure due to its addition, the 

 mixture will produce more plasmolysis than did the standard solution, 

 and the effect will be lasting. If no effect results from the addition, it 

 must pass quickly through the membrane ; if a passing effect, with 

 subsequent recovery, it must pass slowly. 



In this way Overton 1 has investigated the permeability of the 

 protoplast by a number of chemical substances, and finds that salts 

 much dissociated in solution hardly pass the membrane, while many 

 complex organic bodies rapidly penetrate, and that the presence of 

 certain radicles in these markedly affects the result. 



In animal cells investigations are rather limited (by the fact that 

 there is no plasmolysis) to shrinkage and swelling and escape of 

 haemoglobin (in red corpuscles), as indices of permeability, under 

 conditions of variation of osmotic pressure of surrounding solutions. 

 More, therefore, is known about the permeability of the red corpuscle 

 than any other cell. A table of substances is given by Gryns, 2 and we 

 here confine ourselves to stating that red corpuscles are permeable 

 to urea, 3 glycerin, ethyl- and methyl-alcohol, and most ammonium salts 

 (not to sulphate, phosphate, and thiocyanate), impermeable to sugars, 

 sodium and potassium salts, barium and calcium chlorides, glycin and 

 asparagin. 



Thus, as regards action on red blood corpuscles, dilution of an isosmotic 

 sodic chloride solution with urea solution produces the same effect as 

 dilution with water, because the urea diffuses at once into the interior of 

 the corpuscle, while, on the other hand, addition of sugar at once causes 

 contraction of the cell. 



Obviously, therefore, a so-called " hyperisotonic " solution does not 

 necessarily extract water from a cell, and absorption of water from such 

 a solution by the blood may be a purely physical action, if the substance 

 in solution can permeate the wall separating it from the blood. Again, 

 a drug, by making the wall of a cell less permeable by virtue of its 

 chemical action on the protoplasm, may markedly affect the "water 

 extracting power " of a salt solution. Possibly the fact that some salts 



1 Ztschr. f. physical. Chem., Leipzig, 1897, Bd. xxii. S. 189. 



2 Loc. cit., p. 102. 



3 See also Schondorff, Arch. /. d. cjes. PhysioL, Bonn, 1896, Bd. Ixiii. S. 192. 



