518 TEXTBOOK OF BOTANY 



supposed to contain much less water than the vegetative cells. Diffusion 

 of raw materials and foods into the bacterial cell is impossible unless 

 the cell is surrounded by at least a film of water. Current statements 

 that living bacteria have persisted inside rocks since the time of their 

 formation a million years ago have not been generally accepted, because 

 of the almost insurmountable difficulties in proving that these micro- 

 organisms may not have been carried into the minute pores of the rocks 

 with the movements of underground water and gases. 



The solution or substrate in which bacteria grow is commonly called 

 the medium (pi., media). The properties of a medium are determined 

 by the substances it contains. For example, sugar and salts may be in 

 solution in the medium, thus determining its concentration. When the 

 concentration of water in the medium is less than in the bacterial cells, 

 water diffuses out of the cells and the bacteria may become inactive. 

 Consequently jellies keep more readily than preserves, preserves more 

 readily than canned fruits, and canned fruits more readily than fruit 

 juices to which no sugar has been added. Jellies have a high concentra- 

 tion of sugar and a comparatively low concentration of water. In the 

 fruit juices the converse is true. 



In the laboratory bacteria are often placed in gelatin or agar media. 

 When the water content of gelatin falls below 50 per cent, bacteria de- 

 velop very slowly. When the gelatin dries out, the vegetative cells be- 

 come inactive and eventually die. As noted above, however, many 

 bacteria, especially those found in the soil, may live — but not grow — for 

 months or years in a desiccated state. Since most of the bacteria that 

 are causes of disease cannot survive desiccation, there is little danger 

 of living ones being carried long distances by air currents. 



Oxygen. Molecular oxygen is essential to nearly all living organisms. 

 A few kinds of bacteria, however, can live without it. Most organisms 

 use free oxvgen in the respiratory processes and are known as aerobes. 

 Some bacteria, such as those that are the causative agents of lockjaw 

 and those that cause butter to become rancid, grow only when the free 

 oxygen content of the medium is extremely low, and when organic 

 substances containing combined oxygen are available. These bacteria 

 are known as anaerobes. Anaerobic bacteria occur especially in poorly 

 drained soils, in deep waters of lakes and seas, and in all sorts of media 

 from which oxygen has been removed or excluded. 



Bacteria are exposed only to the oxygen that is dissolved in the medium 

 surrounding them. At room temperature this is ordinarily a very dilute 



