EFFECT OF PHYSICAL ENVIRONMENT 125 



that the tip is forced open and the protoplasm is 

 squeezed out much as tooth paste may be forced out from 

 a collapsible tube. 



By suitably increasing the concentration of 'solutes the 

 point may be reached beyond which an organism cannot 

 adapt itself. Such a concentrated solution then acts as a 

 preservative. This is the important factor in preserving 

 such foods as syrups, jellies, and jams. 



Solutions which have equal osmotic pressures are said 

 to be isotonic. Solutions are isotonic for a cell when they 

 will cause neither shrinking nor swelling of the cell con- 

 tents. The most common of the isotonic solutions used in 

 the laboratory is the so-called physiological salt solution, 

 containing usually 0.85 per cent sodium chloride. This 

 concentration of sodium chloride is such that it is isotonic 

 with the red blood cells of the blood. It is also isotonic 

 with the blood serum, consequently red blood corpuscles 

 dropped into such solution neither shrink nor swell. 



Effect of Desiccation 



Most yeasts and bacteria grow best in the presence of an 

 abundance of moisture. Complete desiccation, that is, 

 drying, will cause many kinds of bacteria to die ofE rela- 

 tively rapidly. Other species, particularly the spore 

 formers, are more resistant to the drying and they die off 

 much more slowly. Some bacteria that do not produce 

 spores and many of the yeasts are also relatively resistant 

 to drying. 



The reasons why drying should be so destructive to cer- 

 tain kinds of bacteria are not easily determined. Undoubt- 

 edly in some cases the death of the cell is due to the 

 unusual concentration of solutes about the organisms when 

 it is drying, and to the consequent increase in the osmotic 

 pressure. It has been found that certain microorganisms 

 if frozen in a vacuum and dried quickly, are not killed. 



