30 GENERAL M ORPHOL O G Y AND BIOL OG Y. 



menced, the bacteria are filtered off, no further production 

 of ammonium carbonate takes place, which shows that no 

 ferment has been dissolved out into the urine. If now the 

 bodies of the bacteria be extracted with absolute alcohol or 

 ether, 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. 



As has been said, some bacteria seem to be capable of building up 

 out of simple chemical compounds bodies which are more complex. 

 This function is best illustrated in a group of bacteria which probably 

 play a most important economic function in fertilising the soil by con- 

 verting ammonia compounds into nitrites and nitrates, and thus making 

 the nitrogen more available for plant nutrition. These so-called nitri- 

 fying organisms have been investigated by Professor and Mrs. Frankland, 

 by Professor Warington, and by Winogradski. Their isolation pre- 

 sented great difficulties, none of the ordinary methods being available, 

 as the organisms sought were quickly overgrown by the ordinary 

 bacteria of the soil. Winogradski, however, succeeded in getting 

 fairly pure cultures by taking advantage of the fact that they were 

 capable of growing in the entire absence of organic matter, to exclude 

 which he took most elaborate precautions. The media used contained 

 potassium phosphate, sulphate of magnesium, sulphate of ammonium, 

 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 also known 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 



