46 BIOLOGY AND TECHNIQUE 



only, made up of substances simple enough to be directly utilized 

 by plants. The dead bodies, moreover, of both animals and plants 

 would be of little" further value as stores of matter unless new factors 

 intervened 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 legu- 

 minous plants bacteria which possess the power of assimilating or 

 aiding in the assimilation of atmospheric nitrogen and its prepara- 

 tion 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 decomposition, 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 

 destructive process in the other, rendering them mutually interde- 

 pendent 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 

 compounds. 



KATABOLIC ACTIVITIES OF BACTERIA 



The katabolic activities of bacteria, then, consist in the fermenta- 

 tion of carbohydrates and in the cleavage of proteins and fats. 



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

 also to no inconsiderable degree by bacteria. Protein decomposition 

 and the cleavage of fats are carried out almost exclusively by 

 bacteria. 



