45 o BOTANY OF THE LIVING PLANT 



intervene : and by-products may be produced which are sometimes 

 useful, though often harmful to other organisms, and to Man. 



Where suitable food-material is available Bacteria may multiply 

 indefinitely. But there are important external checks which control 

 them. Many Bacteria are susceptible to injury by light. This has 

 been shown for the Anthrax-Bacillus, by growing it on culture- 

 plates, partly exposed to light and partly shaded ; and the results 

 have been verified in various others. The destructive effect lies 

 in the blue-violet end of the spectrum. After incubation in the dark 

 for three or four days, the area of a plate exposed for a time to such 

 rays at the beginning of the experiment will remain clear, while the 

 shaded portion will be covered by bacterial colonies : this shows that the 

 bacteria exposed to light had been killed. Such facts are of prime im- 

 portance in relation to general health : for sunlight thus offers a natural 

 and wide-reaching check upon the spread of many harmful germs. 



The relation of Bacteria to the free oxygen of the air is also a matter 

 of importance. Like other organisms they may be distinguished 

 as aerobic and anaerobic, according to their dependence upon the 

 presence of free oxygen or their independence of it (p. 136). But 

 no sharp line can be drawn between these two modes of life. Many 

 Bacteria carry on their life with absorption of oxygen, like other 

 plants. If they have a power of movement they are attracted by 

 oxygen, crowding around air-bubbles. But some of the most dele- 

 terious, such as the Bacillus of Tetanus, flourish only in the absence 

 of free oxygen, obtaining their supply of energy at the expense of the 

 organic material which they destroy. Some Bacteria that cause 

 butyric fermentation behave in this way. Such activity may be 

 compared with that of the bottom-yeast of beer-vats ; in both cases 

 the activity is anaerobic. It is this mode of life, together with the 

 toxines which result from it, that makes the Tetanus-Bacillus specially 

 dangerous in wounds. 



Such questions as these are, however, the material for more special 

 treatises than this. It must suffice here to have pointed out that the 

 partial decompositions due to bacterial action are of most varied 

 importance, economically and socially. Physiologically they may be 

 referred for the most part to that degradation of organic material 

 which supports parasitic and saprophytic Life. 



On the basis of nutrition Bacteria have been classified into three 

 groups : (i) Prototrophic or autotrophic, those which require no organic 

 compounds at all for their nutrition. These are represented by the 

 nitrifying Bacteria which live in open nature, in the soil, and are never 



