OSMOTIC PRESSURE 163 



We may take first an interesting fact discovered by Dale (1913). The uterus 

 of the guinea-pig is a very useful preparation for researches on the action of 

 drugs on smooth muscle tissue. When suspended in isotonic saline solution 

 (Ringer's fluid), it responds by contraction to the addition of various drugs, 

 e.g., /2-iminazolylethylamine. Suppose that the concentration of the solution in 

 sodium chloride is raised from the normal 0-9 per cent, to 1-1 per cent., the 

 response is greatly decreased and is practically abolished at T3 per cent. If 

 the osmotic pressure is raised by isotonic quantities of sodium sulphate or cane- 

 sugar, the effect is identical, so that it appears to be one of tonicity alone. 

 The reverse action may be produced by dilution, even from O9 per cent, to 0*85 

 per cent., so that the response to stimulant drugs is markedly increased. Dilution 

 with isotonic cane-sugar has no effect, but urea solution acts as pure water, 

 since the cells are permeable to it. When the action of a drug is to produce 

 relaxation of a tonic state, as in the case of adrenaline on the virgin uterus of the 

 cat, the effect of increase of osmotic pressure is to increase the inhibitory action 

 and of decrease of osmotic pressure to diminish it. The tonus itself is also 

 inhibited by rise of osmotic pressure and increased by addition of water. 



We have already discussed briefly the two typical cases of the cells of the 

 kidney and of striated muscle, as investigated respectively by Siebeck (1912) 

 and by Beutner (1913, 1 and 3). The volume of the cells was found to be in 

 exact relationship to the osmotic pressure of the solution outside them. 



Diminution in the volume of cells by loss of water must have the effect 

 of increasing the internal concentration of substances to which the membrane 

 is impermeable. By mass action, reactions of which these substances are 

 components will be accelerated, and increase in volume by absorption of water 

 will retard such reactions. 



An interesting case is that of yeast. The cells of this organism, owing to the store of 

 glycogen which they contain, undergo a process of auto-fermentation, the enzymes present 

 acting on the glycogen, first to form sugar and then to convert it to alcohol and carbon 

 dioxide. It was found by Harden and Paine (1911) that, if the cells are placed in solutions 

 which cause plasmolysis, the rate of auto-fermentation is greatly increased, no doubt by 

 increase of concentration both of enzymes and of substrate. Solutions of various substances, 

 if their osmotic pressure was the same, caused equal increase. If no plasmolysis resulted, 

 either because the solution was isotonic with the cell contents, or because the cell membrane 

 was permeable to the solute, as urea, no effect was obtained. 



Perhaps one of the most obvious phenomena in which osmotic pressure plays 

 a part is that of secretion. Let us imagine a vertical tube, closed at the lower 

 end by a semi-permeable membrane and open at the upper end. Let it be 

 filled with a solution of cane-sugar and placed with its lower end in water. 

 Water will enter the tube by osmosis and cause a continuous flow of liquid 

 over the top as long as any osmotically-active substance is present inside it. 

 It will clearly be without effect on the result if the top of the tube is closed 



a permeable membrane, or even by a membrane through which cane-sugar 

 can pass, however slowly, so long as it passes more quickly than through 

 the semi - permeable membrane at the other end. If such a tube, with a 

 permeable membrane at one end and a semi-permeable membrane at the other 

 end, be totally immersed in water, or a solution of less osmotic pressure 

 than that contained inside it, a current will flow through it, carrying out 

 the solute, until the osmotic pressure is equal inside and outside. 



A mechanism of this kind exists in certain organs of plants, in which drops of watery 

 secretion are formed at the apex of a column of cells. These organs are known as " hydathodes " 

 and the cells have been shown by plasmolytic methods to decrease in osmotic pressure as the 

 apex is approached (Lepeschkin, 1906). The aerial hyphse of the fungus Pilobolus, which 

 secrete drops at their tips, have been also investigated by Lepeschkin (1906) and a similar 

 mechanism found. 



The phenomenon of bleeding at cut ends of stems, or root pressure, receives 

 its explanation in a similar manner. The liquids in the root have a higher 

 osmotic pressure than the very dilute solution in the soil, and, since the cells 

 are provided with semi-permeable membranes, a flow of liquid takes place as 

 in our glass tube model. 



