GENERAL CHEMISTRY OF BACTERIA 57 



and stems possess the power of combining CO 2 , water and nitrogenous 

 salts under the influence of sunlight directly into the highly complex 

 proteins and carbohydrates essential for animal food. These products 

 of the synthetic activity of the plants are utilized by the animal 

 kingdom for food; directly by the herbivora, indirectly by the carni- 

 vora. These substances are either broken down within the digestive 

 tract of the animal body and reconstructed to form the tissues and 

 supply energy to the animal, or eliminated as excreta. The excreta 

 of animals are not sufficiently simple in composition, as a rule, to be 

 used directly by plants, and the tissues of dead animals and plants 

 are of little value in their complex state for plant foods. Further 

 cleavage, both of the excreta of animals and the dead bodies of plants 

 and animals, is necessary to make the elements contained within them 

 utilizable by plants, and this cleavage is brought about by bacterial 

 activity. Various saprophytic bacteria act successively upon these 

 complex organic compounds, changing them, chiefly by hydrolytic 

 cleavage, into stable, fully mineralized salts, which are directly utiliz- 

 able in this state by the chlorophyll-bearing plants. There is, there- 

 fore, a constant rotation of the various elements which enter into the 

 composition of animal and plant tissues between the plant and animal 

 kingdoms respectively by means of an anabolic or constructive process 

 in the one (plants), and a catabolic or destructive process in the other 

 (animals). The cycle as outlined, however, is not a continuous one, 

 for there are important gaps in the process of cleavage and in the pro- 

 cess of synthesis which if left unbridged by the bacteria would eventu- 

 ally arrest all vital activity both of plants and animals, and all life 

 would then inevitably cease on this planet. These gaps between the 

 animal and vegetable kingdoms are filled by the analytical activity 

 of bacteria. 



A small group of bacteria, on the other hand, is also important 

 from the synthetical point of view. A certain amount of nitrogen is 

 lost in the animal and vegetable kingdoms by various natural agencies, 

 and this supply of nitrogen must be made good from sources which 

 are not directly available either to plants or to animals. Approxi- 

 mately 80 per cent, of the atmosphere is made up of nitrogen, and a 

 certain group of bacteria, "the nitrogen-fixation" bacteria so-called, 

 which are found chiefly on the nodules or roots of leguminous plants, 

 are able to draw upon this great reservoir of atmospheric nitrogen 

 and synthesize it into nitrogen-containing compounds which plants 

 can utilize directly. 



