320 Biological Chemistry. 



the food supply of the higher animals. The materials 

 from which these foods (fats, carbohydrates, and proteins) 

 are built up are supplied by the atmosphere and the soil. 

 The carbohydrates are formed by the plant from carbon 

 dioxide and water vapour, whilst the nitrogen necessary 

 for the protein formation can be obtained from inorganic 

 substances such as the nitrates. It is probable that the 

 fats are formed by some kind of transformation of the 

 carbohydrates, from which the proteins are also formed 

 by the inter-reaction of a nitrogenous substance with 

 some derivative. The methods by means of which trans- 

 formations of this character are produced are at the 

 present moment simply matters of speculation, and prac- 

 tically nothing is known of the intermediary stages of 

 the synthesis of fats and proteins in the plants. The two 

 main facts which are known with certainty are (1) that 

 plants can synthesize carbohydrates from the carbon dioxide 

 of the air and water vapour, and (2) that they can utilize 

 inorganic nitrogen compounds and, indirectly under certain 

 conditions, the nitrogen of the air for building up complex 

 organic nitrogenous compounds. To a brief discussion of 

 these two facts the major portion of this chapter will be 

 devoted. 



SYNTHESIS OF CARBOHYDRATES IN PLANTS. 



The first important observation on the gaseous meta- 

 bolism of plants was made by Priestley in 1772. He 

 showed that if plants are grown under a bell-jar in air 

 which has been rendered unfit for respiration and for the 

 support of combustion by keeping an animal for a long 

 time within the space enclosed, this air, after a certain 

 period, could be altered by a plant in such a way as to 

 be fit again both for respiration and combustion. The 

 experiments of Priestley did not, however, yield constant 



