30 GENERAL MORPHOLOGY AND BIOLOGY. 



basic carbonate of magnesium, and water. An inorganic gelatinous 

 substance was sometimes added, namely, hydrate of silica. On such a 

 medium ordinary bacteria could not develop to any extent. The 

 nitrifying organisms flourished, and there was evidence of abundant 

 oxidation of ammonia and the formation of nitrites, and to a less extent 

 of nitrates. Not only so, but these organisms could derive their 

 carbon from the carbonates present. There is evidence that the 

 nitrifying organisms consist of two groups, one of which forms nitrites 

 from ammonia compounds, the other forming nitrates from these 

 nitrites. It is probable that other organisms exist which are capable 

 of forming compounds by taking up the free nitrogen of the air. On 

 the roots of all leguminous plants small nodules, usually called tuber- 

 cles, are found. These are not developed if the plant is growing in 

 soil free from bacteria, and plants thus grown are not so vigorous as 

 those which grow in ordinary soil. Further, in the interior of these 

 tubercles bacteria-like bodies are observed. There is a good deal of 

 evidence that these are either bacteria or allied organisms, that they 

 take up free nitrogen from the air, and make it available for the 

 nutrition of the plant. If this be the case, a reason is found for the 

 idea long held by agriculturists that the growth of a crop of beans or 

 peas fertilises the soil and improves subsequent crops. 



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. 



In 1872 Cohn stated the opinion that bacteria showed as distinct 

 species as the other lower plants and animals. He recognised the 

 great divisions into, which bacteria naturally fall when the forms under 

 which they appear are considered, but he carefully guarded himself 

 against the error that mere considerations of form are sufficient for a 

 proper natural classification of such a group. In such a classification 

 the history of the whole life cycle of an organism, and especially the 

 course of its development, must be taken into account. Variations in 

 form occurring in particular cases have, however, been made the basis 

 for criticism of the statement that the large numbers of bacteria which 

 have been identified are really to be looked upon as distinct species. 

 The extreme case for the existence of variability was put by Nageli, 

 who held that "the same species in the course of generations might 

 present different morphological and physiological forms which might 

 give rise at one time to the souring of milk, at another to butyric acid 

 fermentation, at another to the putrefaction of albuminous matter, at 

 another to diphtheria, at another to typhoid, at another to cholera." 

 Such an extreme view was advanced before the elaboration by Koch 



