THE ORIGIN OF ORGANIC MATTER 51 



The quantity of carbon dioxide consumed in the weathering of rocks 

 can also not be given any precise valuation though undoubtedly considerable 

 amounts are thus consumed. The carbon in the coal and petroleum de- 

 posits and probably also that in the sedimentary rocks was drawn from 

 the atmosphere ages ago. 



It has already been stated that photosynthesis constitutes the chief 

 means of counteracting the "running down" in energy of our planet. 

 Photosynthesis is essentially a process of reduction, in which a carbon 

 compound containing the maximum amount of oxygen is reduced to com- 

 pounds of carbon containing hydrogen and oxygen in the proportion 

 in which the latter two elements are found in water, i.e. carbohydrates. 

 This is an endothermal reaction and the energy for it is obtained from 

 the radiations of the sun. Similarly, nitrates are reduced to amino com- 

 pounds. Further reductions are carried on through the metabolic activity 

 of the plant resulting in alcohols, fats and hydrocarbons. For the latter 

 reactions, so far as we know, solar energy is not essential, the plant appar- 

 ently being able to utilize the energy derived from the oxidation of a por- 

 tion of its carbohydrate supply for these reductions. The chemical 

 kinetics of the metabolic reduction reactions taking place in plants is still 

 an unsolved problem. Largely by means of these reactions there are 

 produced the enormous number of organic compounds found in plants 

 and for which these have become valuable to man. The reducing power 

 of the metabolic activity of plants is illustrated by the fact that a wide 

 variety of compounds can be reduced, many of which never enter into 

 the normal metabolism of plants. This is clearly demonstrated by experi- 

 ments with the yeast plant.^' If this plant has an ample supply of sugar 

 it is capable of reducing many different compounds including aliphatic, 

 aromatic and cyclic aldehydes, ketones, nitro-compounds, sodium thio- 

 sulfate and others. 



Thus the plant in its photosynthetic and metabolic activities may be 

 considered as a reducing machine, providing materials which are capable 

 of combining with oxygen and thus serve man as food and fuel. The 

 object of agriculture is, of course, the production of food materials for 

 man by means of the photosynthetic process. Our present chief sources 

 of energy in the form of coal and petroleum are likewise the product of 

 photosynthetic activity, produced ages ago. It is only comparatively 

 recently that man is realizing that he is using energy at a greater rate than 

 it is being placed at his disposal, that his industries depend upon the accumu- 

 lation of centuries. Especially in the case of petroleum is it being realized 

 that, while depletion of the supply may not be imminent, nevertheless this 

 source of energy is not in the form of a continuous flow, and that the 

 question of its exhaustion should be given timely consideration.^^ While 

 opinions are still divided as to the extent of the world's petroleum reserves, 



"Neuberg and Ehrlich, Biocliem. Zcit., 101, 276 (1919-1920). 

 ''White, D., Sihlcv J own. Eng., 1920, p. 156. Burrell, G. A., Oil Gas Journ., 

 1920, p. 84. Smith, G. O., Amer. Petroleum Institute, Bull. 132 (1920). 



