452 BOTANY OF THE LIVING PLANT 



but with a special power of fixing free nitrogen. This it shares with another 

 soil-bacterium, Clostridium pasteurianum, which however is metatrophic : 

 when grown in a nutritive solution with sugar it is found to take up nitrogen 

 from the air. In either case free nitrogen of the air is brought into combination. 

 Other Bacteria are engaged in bringing about changes in nitrogenous bodies 

 already present. Various putrefactive organisms, chiefly bacteria, break 

 down more complex organic bodies into simpler. In the case of nitrogenous 

 compounds the organic nitrogen is ultimately liberated as ammonia. This 

 ammonia is probably " mineralised " before it can be used again by green 

 plants. It is oxidised in the soil, and combined with a base to form nitrate. 

 This is known as " nitrification." It is carried out by certain prototrophic 

 bacteria everywhere present in the soil. The change is effected in two steps : 

 the nitrite bacterium (Nitrosomonas) oxidises the ammonia into nitrous acid, 

 and the nitrate bacterium (Nitrobacter) converts it into nitric acid (Fig. 348). 



\ 



€t> 





e. 



Fig. 348. 



Nitrifying bacteria. (After Winogradsky.) a, Nitrosomonas europaea, from 

 Zurich : b, Nitrosomonas javanensis, from Java : c, Nitrobacter, from Queto. (From 

 Fischer, Vorl. ii. Bacterien. x 1000.) 



Both organisms are aerobic, and they are always present together in Nature, 

 so that the compounds formed by one are immediately taken up by the other, 

 and the end-product is nitric acid in the form of nitrate, which is then available 

 as plant-food. 



These examples of organisms in water and in soil will serve to suggest the 

 activity of Bacteria in Nature. They have also wide-reaching effect in 

 manufactures. For instance, acetic-acid bacteria convert alcohol into vinegar : 

 butyric bacteria cause the " retting " of Flax and Hemp : bacteria take part 

 in the preparation of Indigo ; while the flavours of cheese, butter, and 

 tobacco depend for their market-value upon the exact type and conduct of 

 the "partial decomposition of their constituents by bacterial action. 



We are most directly interested in those bacteria that affect Man, and other 

 animals. Many metatrophic forms flourish on the mucous membranes of 

 the mouth, nose, and alimentary canal, etc., and accompany the individual 

 through life without doing harm. But many other paratrophic bacteria, 

 entering the tissues, are the active causes of disease. Thus, suppuration is 

 caused by various Cocci : acute lobar pneumonia by a Diplococcus : anthrax, 

 or malignant pustule, by Bacillus anthracis : lock-jaw by Bacillus tetani : 

 tubercle by Bacillus tuberculosis : cholera by the " comma-bacillus," Vibrio 

 cholerae, etc. (Fig. 349.) The actual intrusion of the organism is the first 

 essential of disease, but the serious conseauences are due to the action of 

 poisons, or toxines, produced by the micro-organisms, and liberated into the 

 system of the host. The host defends itself by the action of the white blood- 

 corpuscles, and other cells, which take up and digest the invading bacteria : 

 these cells have accordingly been called " phagocytes." The process is physio- 



