THE BIOLOGICAL ACTIVITIES OF BACTERIA 41 



be of little further value as stores of matter unless new factors inter- 

 vened to reduce them to that simple form in which they may again 

 enter into the synthetic laboratory of the green plant. Agents for 

 further cleavage of these compounds are required, and these are supplied 

 by the varied activities of the bacteria. 



On the other hand, bacteria are also important in the process of 

 synthesis. The main supply of nitrogen available for plant life is found 

 in the elementary state in the atmosphere a condition in which it 

 can not be utilized as a raw product by the plant. This gap again is 

 bridged by the bacteria found in the root bulbs of the leguminous plants 

 bacteria which possess the power of assimilating or aiding in the as- 

 similation of atmospheric nitrogen and its preparation for further use by 

 the plant itself. Another bacterial activity which may be classified as an 

 anabolic process is the oxidation of the ammonia, released by decomposi- 

 tion, into nitrites and nitrates. This is carried on by certain bacteria of 

 the soil. These are to be treated of in greater detail in another section. 



There is a constant circulation, therefore, of nitrogen and carbon 

 compounds, between the plant and the animal kingdoms, by virtue of an 

 anabolic or constructive process in the one, and a katabolic or destruc- 

 tive process in the other, rendering them mutually interdependent and 

 indispensable. The circuit, however, is not by any means a closed one; 

 there are important gaps, both in the process of cleavage and in that of 

 synthesis, which, if left unbridged by the bacteria, would effectually 

 arrest all life-activity of plants and eventually of animals. 



Far from being scourges, therefore, these minute microorganisms 

 are paramount factors in the. great cycle of living matter, supplying 

 necessary links in the circulation of both nitrogenous and carbon com- 

 pounds. 



KATABOLIC ACTIVITIES OF BACTERIA 



The katabolic activities of bacteria, then, consist in the fermentation 

 of carbohydrates and in the cleavage of proteids and fats. 



Fermentation is carried out to a large extent 'by the yeasts, but also 

 to no inconsiderable degree by bacteria. Proteid decomposition and the 

 cleavage of fats are carried out almost exclusively by bacteria. 



For our knowledge of the fundamental laws underlying these phe- 

 nomena of fermentation and proteid decomposition, we are indebted 

 to the genius of Pasteur, 1 who was the first to prove experimentally the 



' Pasteur, " fitude sur la bi;re," Paris, 1876. 



