24 GENERAL MORPHOLOGY AND BIOLOGY 



which of course destroy their vitality, a substance is obtained 

 of the 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 the investigation of the phenomena of the ferment action of 

 bacteria, it has been noted in certain cases that the ferments 

 formed depend on the food supply offered to the bacterium. 

 Thus in one case a bacterium growing in starch forms diastase, 

 which it does not do when grown on sugar. 



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 leguminosre. Without 

 such organisms the tubercles do not develop, and without the development 

 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, however, 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 AVC 

 see exhibited (to use the words of De Bary) "the same periodically 

 repeated course of development within certain empirically determined 

 limits of variation " which justifies, among higher forms of life, a species 

 to be recognised. What at first raised doubts as to the occurrence of 

 species among the bacteria was the observation in certain cases of what is 

 known as pleomorpliism. 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 especially 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 



