Chemical Processes in Plants. 329 



teurianum (Winogradsky) and of Azobacter chroococcum 

 (Beyerinck) ; the former is anaerobic (that is, grows in the 

 absence of oxygen), whereas the latter is aerobic. These 

 results are remarkable, as it is difficult to produce complex 

 organic nitrogenous compounds directly from the nitrogen 

 of the air by any simple chemical laboratory process. . 



THE R6LE OF THE GREEN PLANTS IN THE GENEBAL 

 ECONOMY OF NATURE. 



From the foregoing remarks it will be evident that the 

 plants play an important part in the synthesis of the more 

 complex organic substances which are necessary for the 

 nutrition of animals. The plants by means of the energy 

 derived from the sun's rays utilize the carbon dioxide of 

 the air and the nitrogen of the nitrates, and build up the 

 carbohydrates, fats, and proteins. The animals, on the 

 other hand, degrade these complex substances, oxidizing 

 them and utilizing the energy thus obtainable for the 

 maintenance of a body temperature higher than the sur- 

 rounding space, and for the performance of muscular work. 

 The chief end products of animal metabolism are carbon 

 dioxide and urea, and the undigested or partially digested 

 substances voided in the faeces. These end products can 

 serve as the chief sources of food for the plants. The 

 carbon dioxide is reduced by them, and oxygen is a pro- 

 duct of this action, and serves to regenerate an atmosphere 

 vitiated by the products of animal expiration. The nitro- 

 genous excretory products from the animal, furthermore, can 

 be converted into nitrates by the action of various species 

 of bacteria, and which serve directly as the nitrogenous 

 food of plants. There is, therefore, a continuous cycle, 

 the animals supplying the food of plants from their 

 excreta, the plants building up therefrom the complex 

 materials which by oxidation form the source of energy 



