Bacteria and Their Relations to Life 



385 



Environmental conditions affecting bacteria. Like the higher 

 and more complex plants the bacteria have certain rather definite 



Fig. 242. Various forms of bacteria. 



% % 



water, temperature, light, and nutritive requirements for growth 

 and reproduction. The different species vary greatly in these 

 requirements ; consequently some kinds of bacteria are able to 

 live almost everywhere in nature. 



Moisture. Since water makes up about 85 per cent of the 

 bacterial cells, water is essential to their activities. Furthermore, 

 since all of their nutrient materials are absorbed by diffusion, they ' 

 must be surrounded by at least a film of water. The water or 

 solution in which bacteria live is commonly called the medium 

 (plural, media), and its properties are determined by the sub- 

 stances it contains. 



For example, sugar and salts may be dissolved in the medium, 

 thus determining its concentration. In dilute solutions the water 

 and nutrient materials diffuse readily into the cells. In con- 

 centrated solutions (15 to 40 per cent) the concentration of water 

 is less in the media than inside the cells, and water either does not 

 pass in or diffuses out of the cells and the bacteria are unable to 

 grow. They are affected in the same way as if they were dried. 

 This explains why jellies keep more readily than preserves, pre- 



