122 BACTERIA IN THE SOIL 



Plant life seizes upon its required constituents, and by means of 

 the energy furnished by the sun's rays builds these materials up into 

 its own complex forms. Its many and varied forms fulfil a place in 

 beautifying the world. But their contribution to the economy of 

 nature is, by means of their products, to supply food for animal life. 

 These products of plant life are chiefly sugar, starch, fat, and proteids. 

 Animal life is not capable of extracting its nutriment from soil, but 

 it must take the more complex foods which have already been built 

 up by vegetable life. Again, the complementary functions of animal 

 and vegetable life are seen in the absorption by plants of one of the 

 waste materials of animals, viz., carbonic acid gas. Plants abstract 

 from this gas carbon for their own use, and return the oxygen to the 

 air, which in its turn is of service to animal life. 



By animal activity some of these foods supplied by the vegetable 

 kingdom are at once decomposed into carbonic acid gas and water, 

 which goes back to nature. Much, however, is built up still further 

 into higher and higher compounds. The proteids are converted by 

 digestion into more soluble forms, such as albumoses and peptones ; 

 these in their turn are reconverted into less soluble proteids, and 

 become assimilated as part of the living organism. In time they 

 become further changed into carbonic acid, sulphuric acid, water, and 

 certain not fully oxidised products,* which contain the nitrogen of 

 the original proteid. In the table these bodies have been represented 

 by one of their chief members, viz., urea. 



It is clear that there is in all animal life a double process 

 continually going on ; there is a building up (anabolism, assimilation), 

 and there is a breaking down (katabolism). These processes will not 

 balance each other throughout the whole period of animal life. We 

 have, as possibilities, elaboration, balance, degeneration; and the 

 products of animal life will differ in degree and in substance accord- 

 ing to which period is in the predominance. These products we 

 may subdivide simply into excretions during life and final materials 

 of dissolution after death, both of which may be used more or less 

 immediately by other forms of animal or vegetable life, or immediately 

 after having passed to the soil. We may shortly summarise the 

 final products of animal life as carbonic acid, water, and nitrogenous 

 remnants. These latter will occur as urea, new albumens, compounds 

 of ammonia, and nitrogen compounds of great complexity stored up 

 in the tissues and body of the animal. The carbonic acid, water, and 

 other simple substances like them will return to nature and be of 

 immediate use to vegetable life. But otherwise the cycle cannot be 

 completed, for the more complex bodies are of no service as such 

 to plants or animals. 



* E. A. Sehafer, Text-hook of Physiology, vol. i., p. 25 (W. D. Halliburton). 



