PHYSIOLOGY OF MICROORGANISMS 49 



rials in solution. The cell must have an internal pressure 

 i. e., it must be turgid before growth can take place. This 

 pressure is known as osmotic pressure, and is due to the 

 fact that the concentration of the liquid in the cell is 

 greater than that of the liquid in which the cell is found. 

 The protoplasmic layer that is in direct contact with the 

 cell wall functions as a semi-permeable membrane, which 

 will allow the passage of certain substances in and out of the 

 cell freely, and will not permit others to pass. Certain of 

 the cell constituents are thus prevented from leaving the 

 cell. In an effort to maintain an equilibrium of concen- 

 tration inside and outside of the cell with reference to these 

 compounds, water is drawn into the cell. The passage of 

 water into the cell creates the internal pressure. If a cell 

 is placed in a solution of some material that can not pass 

 into the cell on account of this semi-permeable membrane, 

 \\atcr will be withdrawn from the cell to assist in establish- 

 ing an equilibrium of concentration. The internal pres- 

 sure of the cell will be destroyed by this process. The cell 

 is then said to be flaccid or plasmolyzed. In this condition 

 no growth can take place. The plasmolyzed condition may 

 be a permanent one, or it may be overcome by the slow 

 passage of the substance into the cell and the reestablish- 

 ment of a turgid condition. 



The yeasts are more resistant to the effect of materials in 

 solution than are the bacteria. Many of them can grow in 

 an almost saturated solution of cane sugar, while a 15 per 

 cent, solution will inhibit the growth of most bacteria. The 

 molds are still more resistant to the action of concentrated 

 solutions. In fact, the inhibition of mold growth by in- 

 creasing the concentration of the liquid by the addition of 

 sugar or salt is so slight as to be of little importance. The 

 molds are likewise able to grow on materials from which 

 bacteria and yeasts can obtain no water. In other words, a 



