460 BOTANY OF THE LIVING PLANT 



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

 the shaded portion will be covered by bacterial colonies : showing that 

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

 importance 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 pres- 

 ence of free oxygen, or their independence of it (p. 115). 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. But some of the most deleterious, 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. Such activity may be compared with that of the bottom- 

 yeast of beer-vats, both being anaerobic. It is this mode of life, 

 together with the toxins 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, 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 parasitic, (ii) Metatrophic, those which 

 cannot live unless they have organic substances at their disposal, both nitro- 

 genous and carbonaceous. They occur in the open, and are saprogenic and 

 sometimes parasitic (facultative parasites), (iii) Paratrophic, those which 

 develop normally only within the living tissues of other organisms, and are 

 true, and obligatory parasites, such as the germs of Tubercle or Diphtheria. 



This classification may be extended, however, to all other organisms. All 

 green autophytes are prototrophic in the same sense as the first group of 

 Bacteria. All fungi and animals are metatrophic, except the parasitic forms, 

 which are paratrophic. Thus the Bacteria exemplify types of nutrition 

 which run parallel with those seen in larger organisms. 



