ABSORPTION OF CARBON 25 



8. The Importance of Light for Carbon Assimilation. Photo- 

 synthesis and Chemosynthesis. — The mere presence of green 

 plastids does not assure carbon assimilation. They alone cannot 

 separate oxygen from carbon dioxide. Light is an essential 

 condition for this process. As early as 1779, Ingen-Housz defi- 

 nitely established that only in the presence of light do plants 

 "improve the air." The importance of light consists in the fact 

 that it is a source of energy and is necessary for breaking the 

 strong linkage between oxygen and carbon. Light is also required 

 for the production from the highly oxidized carbon-dioxide sub- 

 stances of the type of carbohydrates, which on combustion can 

 liberate a considerable amount of energy. Thus 1 g. of glu- 

 cose produces on combustion 3.76 Cal., 1 g. of starch, 4.1, and 

 1 g. of protein, 5.7. Approximately, the same value — 4.4 to 5.2 

 Cal. — is produced by the combustion of 1 g. of dry plant substance. 

 In Part III it will be shown that this process of the combustion of 

 carbohydrates and other substances of the plant body is the source 

 of energy for the plant. If these processes were used as the 

 source of energy for assimilation, then the plant, in order to create 

 a unit weight of starch, would have to spend at least more than a 

 unit weight of carbohydrates and would thus work not with a 

 profit but with a loss, and, instead of increasing, would lose in 

 weight. To accumulate organic compounds through the union of 

 carbon dioxide and water, therefore, the plant requires a supply of 

 energy from outside and receives it in the form of light, by means 

 of which the synthesis of carbohydrates takes place. This process 

 has been called "photosynthesis," that is, synthesis using the 

 energy of light. 



Photosynthesis is not the only way of creating organic com- 

 pounds. There are plants belonging to the group of bacteria 

 which use oxidation processes as the source of energy for the syn- 

 thesis of organic compounds. In these instances, not the sub- 

 stances composing the body of the bacteria, but substances from 

 outside serve as fuel. Examples of such microorganisms are the 

 nitrifying bacteria which oxidize ammonia into nitrous acid and 

 then into nitric acid; other bacteria oxidize hydrogen sulphide 

 into sulphuric acid; still others, hydrogen into water, etc. They 

 avail themselves of this energy of oxidation for the reduction of 

 carbon dioxide and the manufacture of substances building up 

 their body. Such a synthetic activity, based entirely on the trans- 



