n8 PRINCIPLES OF GENERAL PHYSIOLOGY 



and we have already seen that imbibition is incapable of explaining the phenomena 

 met with. From the molecular concentration of the cell sap, as determined by the 

 depression of the freezing point, in the way explained in the next chapter, or in 

 other ways, the maximum possible pressure that could be developed if the 

 membrane were completely semi-permeable can be known. Although it is 

 naturally a matter of difficulty to obtain the juice of one kind of coll alone, it 

 appears from results obtained that, on the whole, the concentration of the cell sap 

 is not greater than is necessary to give the turgor pressure known to exist. 



A large number of measurements of depressions of freezing point will be found collected in 

 the article by Bottazzi (1908); the usual figures correspond to pressures of about 11-15 

 atmospheres and would be given by a solution of potassium nitrate of nearly half molar 

 strength (5 '05 per cent.). 



The shrinking of a cell placed in hypertonic solutions shows itself in plant 

 cells by the protoplasmic layer retreating from the rigid cell wall, leaving a gap. 

 between the two. This phenomenon is known as " plasmolysis," which was 

 worked out mainly by de Vries (1884), and has played a large part in the 

 investigation of the permeability of cells. 



To interpret the facts observed when cells are exposed to solutions differing in 

 osmotic pressure from that of the cell contents, let us return to the schema of the 

 cell, viz., a solution of some substance contained within a membrane forming a 

 vesicle, which can be immersed in water or solutions of various osmotic pressures. 

 Suppose, first, that the membrane is impermeable to the solute, and that the 

 vesicle is immersed in a slightly hypotonic solution of the same substance. The 

 vesicle will at first absorb water, becoming distended, until its contents are diluted 

 to such a degree that their concentration is equal to that of the outer solution. 

 Nothing further will happen, but the cell remains permanently distended. 



Next, let us imagine that the membrane is easily permeable both to water 

 and to the solute, and that it is elastic as before. It is clear that, in this case 

 also, the cell will be distended to begin with, because the osmotic pressure is 

 greater inside than outside, while the solute cannot escape instantaneously. But 

 subsequently, and contrary to the previous case, the original volume will be 

 regained. As the solute gradually escapes, the internal osmotic pressure becomes 

 equal to the external by free diffusion, and there can be no permanent force to 

 keep the membrane stretched. In the previous case, the cell could return to 

 its original volume only by escape of water ; but, since the solute could not 

 escape, the original concentration would by this means be arrived at and 

 equilibrium would no longer exist. Now, there may be numerous degrees of 

 permeability between the two cases given, such that the solute may be able 

 to escape at different rates. The result is that a longer or shorter time would 

 elapse before the cell returned to its original size. In both cases, however, if 

 no change of volume occurs at all, the conclusion may be drawn that tin- 

 outer solution is isotonic with the contents. If the change of volume is only 

 temporary, while the membrane is elastic, it is to be concluded that this 

 membrane is more or less permeable to the solute. 



Another case to be considered is one that is met with in certain experiments 

 on living cells or blood corpuscles, viz., when the membrane is permeable for 

 the solute of the outer liquid, but impermeable for those of the cell contents. 

 Suppose that the two solutions are isotonic. No immediate change will take 

 place. But, presently, the cell will begin to swell. Why ? Because the solute 

 of the outer solution passes into the cell, so that the osmotic pressure therein 

 is now the original one plus that of the substance which has diffused in; while 

 the outer solution remains the same as before, always assuming, as in all the 

 cases discussed, that the volume of this solution is large compared with that of 

 the cell. Ultimately, the state of affairs will be the same as if the outer liquid 

 had been water only, since the concentration of the diffusible solute is equal on 

 both sides of the membrane of the cell, while the latter retains the whole of 

 the indiffusihle substance with its osmotic pressure. 



It appears that, unless we know that the cell membrane is elastic, some uncertainty may 

 arise aa to the conclusions to be drawn from the effect of a solution which is not isotonic 



