PHYSIOLOGY OF MICROORGANISMS 55 



liberation of sufficient heat to vaporize a large portion of > the 

 remainder. The solid paraformaldehyd or paraform may be 

 heated and is thereby converted into formaldehyd gas. On 

 account of its cheapness and effectiveness formaldehyd is used 

 much more commonly at present than any other of the gaseous 

 disinfectants. 



Adjustment of Organisms to Osmotic Pressure. Any crystal- 

 loid in solution behaves within the limits of the solution like a 

 gas, and the same laws of diffusion and diffusion pressures are 

 applicable. Every organism when growing is surrounded by 

 water containing substances in solution, and it also contains 

 certain salts dissolved in the " cell sap " or water in the pro- 

 toplasm. The ectoplast or limiting membrane of the protoplasm 

 lying just within the cell wall is certainly in most, probably all, 

 cases a semipermeable membrane, i. e., it will allow some sub- 

 stances to pass through readily, as water; others pass through 

 slowly, and still others, although in true solution, cannot pass at 

 all. This ectoplast, in short, serves as an osmotic membrane 

 and determines what substances may enter and leave the cell. 

 An active cell always maintains within its sap a greater con- 

 centration of solutes than the surrounding medium, the pressure 

 on the inside of the membrane is greater than on the outside, 

 and the cell is said to be in a state of turgor. When such a cell 

 is placed in water containing a greater percentage of solutes than 

 does the cell sap, water leaves the latter until the concentra- 

 tion on the inside and outside again becomes the same. This 

 means a shrinking of the protoplasm; it withdraws from the cell 

 wall and the cell is said to be plasmolyzed. After a time the 

 cell may readjust the amount of solutes in the cell sap and regain 

 its turgor. For every cell, however, there is a limit beyond 

 which the organism cannot go. Some yeast cells have been found 

 to develop slowly in a solution cbntaining 35 per cent, of cane- 

 sugar. A solution of this concentration exerts a pressure of more 

 than 350 pounds per square inch. It is apparent that such a cell 

 must be profoundly modified. The fact that concentration 

 of solutes inhibits the growth of microorganisms is utilized in 

 the preservation of many food stuffs. Such foods as syrups, 

 jellies, and candied fruits are preserved by the high osmotic 



