148 PHYSICAL INFLUENCES. 



The pressure increases in direct ratio with the number of molecules in 

 solution. Therefore, the pressure of solutions of equal concentration by 

 weight will be the smaller the larger the molecule, because the larger the 

 molecules the smaller the number required to make a certain weight. 

 The osmotic pressure of protein, starch and peptone solution can be 

 measured only with the finest instruments, while the pressure of a 30 per 

 cent dextrose solution is 22 atmospheres.* With acids, alkalies and 

 salts, the pressure is higher than would follow from the concentration, 

 because the molecules of these electrolytes are dissociated, thus increasing 

 the number of unit-particles in solution. 



PLASMOLYSIS. If a cell is brought into a strong solution of a substance 

 which cannot pass the plasma-membrane, this substance will cause an 

 osmotic pressure and the concentration in the cell being lower than in the 

 medium, the water will pass out from the cell until the pressure inside and 

 outside is the same. This causes a shrinking of the protoplasm, while 

 the rigid cell-wall keeps its shape. Such plasmolyzed organisms are 

 illustrated in figure 30, p. 50. 



While plasmolysis is easily demonstrated with the cells of higher 

 plants, microorganisms do not show it so readily. " In fact, many bacteria, 

 like B. subtilis, Bact. anthracis, cannot be plasmolyzed by any concentra- 

 tion of salt in solution. Others, as B. coli, B.fluorescens. react promptly. 

 But even though many are killed, the rest recover from plasmolysis after a 

 few hours, and appear normal. This indicates that the salt passes slowly 

 through the plasma membrane and thus increases the pressure inside the 

 cell until finally the inside and outside pressure are the same again. 



The fact that many microorganisms show no plasmolysis whatever is 

 explained in the same way. These organisms probably have plasma- 

 membranes so constructed that the salts diffuse through nearly as fast as 

 the water. An absolute exclusion of all soluble substances by the mem- 

 brane is impossible since the food can get into the cell only by diffusion 

 through the membrane. 



The resistance of various microorganisms against concentrated 

 solutions depends upon the organism as well as upon the dissolved' sub- 

 stance. The sodium and potassium salts of the common mineral acids 

 act upon a culture nearly in proportion to their osmotic pressure, but the 

 potassium salts always retard growth a little less than the sodium salts. 



* One atmosphere equals the pressure of one kilogram per square centimeter or about 15 

 pounds per square inch. 



