1 76 PHYSIOLOGY OF MICROORGANISMS 



with Osterhout's investigations, may be decreased in its reaction to 

 sodium chloride by alkaloids, as caffein, nicotine and cevadine, by bile 

 salts as sodium taurocholate, and by acids as hydrochloric acid; on the 

 other hand, .it is increased by alkalis, by certain isotonic combinations 

 of salts or balanced solutions and by acids following the first stimula- 

 tion. Protoplasm may vary widely from the normal in its permeability 

 and both vegetable and animal cells respond in much the same general 

 manner. 



Although these specific facts may be very limited compared with 

 the entire field of permeability possibilities to which a living organism 

 is exposed, they do, however, indicate that the membrane or protoplas- 

 mic protective surfaces have the power to act in a selective manner 

 per se or to yield to environing forces or influences which control or 

 make life possible by antagonisms, reactions, neutralizations and other 

 agencies among themselves.* 



Osmotic pressure, following the laws of gas pressure, represents 

 the pressure exerted by the particles of a given volume of a solution. 

 The particles, molecules, or ions, of the solution, as in gas are constantly 

 on an outward drive, an expansive drive, and they carry with them 

 much force which is proportional to the concentration of the solution 

 and is subject to the influence of temperature as stated previously. 

 Also the osmotic pressure of a given quantity of substance is inversely 

 proportional to the volume (p. 174). When, therefore, a solution of a 

 great concentration is separated from that of less concentration with a 

 semipermeable membrane between, the pressure exerted on each side of 

 the membrane will be proportional to the concentration of the solutions. 

 The pressure will be influenced by temperature and there will be a 

 stirring of the unequal forces to gain an equilibrium. If only the solv- 

 ent in the two solutions of different concentration, as just referred to, 

 passes the membrane, then there will be movement toward and a grad- 

 ual dilution of the more concentrated until it becomes equalized with 

 the other; if both solvent and solute pass there will be by the passage of 

 both through the not truly semipermeable membrane an effort to 

 equalize with more or less exchange from both solutions as in the case 

 of obstructed diffusion. 



* The writers call especial attention to Osterhout's work and that of his students as published 

 in the Journal of General Physiology, Journal of Biological Chemistry, Science and the Botanical 

 Gazette. 



