THE ORIGIN OF ORGANIC MATTER 23 



process is called Photosynthesis.- The plant absorbs through its leaves 

 the carbon dioxide which is universally present in the atmosphere and 

 which is formed by the burning of coal, fuel oil and so forth, and is also 

 exhaled by animals. By means of the light from the sun the carbon dioxide 

 thus absorbed by the leaves is changed into material such as sugar or wood, 

 which can again be used as food for animals or as fuel. Van Helmont, 

 whose classical experiment was described in the previous section, knew 

 nothing of the composition of the air and never suspected that the 

 plant absorbed from the atmosphere the major portion of the material 

 which went to make up the 164 pounds which his willow twig had gained in 

 weight in five years' time. As far as the changes of materials are con- 

 cerned in this interrelation of plants and animals, the net result is nil. 

 Thus, in brief, a plant yields a certain amount of substance which can be 

 used as food. The food is consumed by man and thus enables him to do 

 some work. Thereby the food material is burned in the body and is ex- 

 haled as the gas, carbon dioxide. Or the fuel is burned and the products 

 of combustion, carbon dioxide, escai>e into the atmosphere. 



The fundamentally important point is in relation to the energy changes. 

 The energy expended by the man has been permanently lost to a large ex- 

 tent. Similarly that obtained from fuel. The reconversion of the carbon 

 dioxide into food or fuel material can be accomplished only by the use 

 of a great deal of energy. The cycle is made possible only by the intro- 

 duction of energy from without. This energy is derived from the sunlight 

 which the plant, unlike the animal, is able to utilize and so convert the 

 waste carbon dioxide again into food or fuel material. 



If our earth were an isolated system in which there were no imports 

 and no exports, our state of affairs would be very different from that which 

 now presents itself to us. According to our experience formulated in the 

 laws of thermodynamics, in all naturally occurring transformations the 

 tendency is to arrive at a condition of stable equilibrium. Thus the sub- 

 stances on the earth are constantly tending to arrive at a condition of 

 greatest entropy, meaning "rundownness." Most of the metals, for in- 



" As yet no term has been proposed for this process which is entirely satis- 

 factory. This is due largely to the difficulty of expressing adequately in a suc- 

 cinct term a process which appears to be highly complex and about which we 

 have as yet incomplete information. Carbon assimilation used very generally by 

 British writers, describes the process but very incompletely, especially as the 

 word assimilation has been employed in so many different connotations. Further- 

 more the light factor is in no way suggested, nor that of chlorophyll. The same 

 criticism applies to the German Kohlensacureassimilation. On the other hand the 

 French assimilation chlorophyllienne is cumbersome and not very much more ex- 

 pressive. The term photosyntax suggested by Barnes, Bot. Gaz. 18, 403-411 

 (1893), has found little favor. While photosynthesis is not an altogether adequate 

 expression, it is not too narrow and has come into very general use to mean the 

 synthesis of complex carbon compounds out of carbon dioxide and water, in the 

 presence of chlorophyll, through the action of light. The following have been sug- 

 gested as more definite phrases : "Photosynthesis of carbon compounds," "Chloro- 

 phyll-photosynthesis of carbohydrates," "Photosynthetic utilization of carbon 

 dioxide," "Photochemical synthesis of carbo-hydrates," "Photosynthetic assimilation 

 of carbon," "Photosynthetic appropriation of carbon." 



