THE METHODS OF BACTERIAL ACTION. 25 



nature of a ferment, which, when added to sterile urine, rapidly 

 causes the production of ammonium carbonate. This ferment 

 has evidently been contained within the bacterial cells. 



In considering the effects of bacteria in nature it must be recognised that 

 some species are capable of building up -complex substances out of simple 

 chemical compounds. Examples of these are found in the bacteria which 

 in the soil make nitrogen more available for plant nutrition by converting 

 ammonia into nitrites and nitrates. Winogradski, by using media containing 

 non-nitrogenous salts of magnesium, potassium, and ammonium, and free of 

 organic matter, has demonstrated the existence of forms which convert, by 

 oxidation, ammonia into nitrites and of other forms which convert these nitrites 

 into nitrates. Both can derive their necessary carbon from alkaline carbonates. 

 Other bacteria or organisms allied to the bacteria exist which can actually 

 take up and combine into new compounds the free nitrogen of the air. These 

 are found in the tubercles which develop on the rootlets of the leguminosae. 

 Without such organisms the tubercles do not develop, and without the devel- 

 opment of the tubercles the plants are poor and stunted. Bacteria thus play 

 an important part in the enrichment and fertilisation of the soil. 



The Occurrence of Variability among Bacteria. The question of the division 

 of the group of bacteria into definite species has given rise to much discussion 

 among vegetable and animal morphologists, and at one time very divergent 

 views were held. Some even thought that the same species might at one 

 time give rise to one disease, at another time to another. There is, how- 

 ever, now practical unanimity that bacteria show as distinct species as the 

 other lower plants and animals, though, of course, the difficulty of defining 

 the concept of a species is as great in them as it is in the latter. Still, we can 

 say that among the bacteria we have exhibited (to use the words of De Bary) 

 " the same periodically repeated course of development within certain empiri- 

 cally determined limits of variation " which justifies, among higher forms of 

 life, a species to be recognised. What at first raised doubts as to the occur- 

 rence of species among the bacteria was the observation in certain cases of 

 what is known as pleomorphism. By this is meant that one species may 

 assume at different times different forms, e.g. appear as a coccus, a bacillus, 

 or a leptothrix. Undoubtedly many of the cases where this was alleged to 

 have been observed occurred before the elaboration of the modern technique 

 for the obtaining of pure cultures, but at the present day there are cases where 

 evidence appears to exist of the occurrence of pleomorphism. This is espe- 

 cially the case with certain bacilli, and it may lead to such forms being classed 

 among the higher bacteria. Pleomorphism is, however, a rare condition, and 

 with regard to the bacteria as a whole we may say that each variety tends to 

 conform to a definite type of structure and function which is peculiar to it and 

 to it alone. On the other hand, slight variations from such type can occur in 

 each. The size may vary a little with the medium in which the organism is 

 growing, and under certain similar *?onditions the adhesion of bacteria to each 

 other may also vary. Thus cocci, which are ordinarily seen in short chains, 

 may grow in long chains. ' The capacity to form spores may be altered, and 



