CHAPTER XVII 

 ASSIMILATION OF CARBON 



In the previous chapter it has been shown that the green 

 parts absorb carbonic acid and return to the air the oxygen 

 contained in the carbonic acid. The green parts, therefore, act 

 as a reducing mechanism,* and it follows that in them some 

 organic compound is formed at the expense of the carbon 

 obtained. For let us remember that our definition of an 

 organic compound is a carbon-containing body which is capable 

 of combining with oxygen. The first step, in following the 

 career of the absorbed carbonic acid, is to ascertain what 

 organic bodies are found in the plants. 



First, there is no pure free carbon found in plants, so that 

 the absorbed carbonic acid does not simply lose its orygen, 

 and thus lead to the formation of pure carbon. There are 

 three universally present and important classes of organic 

 compounds found in all flowering plants. They are Proteids 

 (Albuminoids), Carbohydrates, and Fats or Fatty Oils. These 

 show their organic nature in being able to burn — that is, they 

 are able to combine with the oxygen of the air and give off 

 carbonic acid. 



I. Proteids are very complex compounds of carbon, con- 

 taining also hydrogen, oxygen, nitrogen, sulphur, and some- 

 times phosphorus. The white of an egg is a good example of 

 a proteid, and the " lean " of meat is mainly proteid. Proteids 

 stain deeply with many dyes : they assume a yellow or brown 

 colour with iodine. There is one substance which is mainly 

 composed of proteids, and which is the most important part of 

 a plant or animal — in fact, it is the only living part of living 

 beings, and is termed protoplasm. Continued growth of a 

 living being (plant or animal) implies an increase in the amount 

 of protoplasm. Hence, if a plant is to continue growing, it 



* By a "reducing mechanism" is meant a mechanism which wholly or 

 partially deprives certain oxygen-containing compounds of their oxygen. 

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