32 Science of Plant Life 



liquid fats are commonly called oils. They are probably 

 made directly from the carbohydrates. As there appears 

 to be present in the cell no special fat-producing apparatus 

 for bringing about this chemical change, it is probably effected 

 by the protoplasm and fat can therefore be formed in any 

 living part of the plant. Although fats are widely distributed 

 in the plant body, they are especially abundant in seeds and 

 fruits. Some of the commonest fats and oils of commerce 

 derived from plants are corn oil, coconut oil, cottonseed oil, 

 linseed oil, castor oil, olive oil, peanut oil, and cocoa butter. 



The making and use of proteins. The proteins are the third 

 class of foods. They too are constructed in large part from 

 the carbohydrates ; but their molecules are vastly more com- 

 plex than are the molecules of carbohydrates and fats, and, 

 in addition to carbon, hydrogen, and oxygen, they contain 

 the elements nitrogen and sulfur, and occasionally phos- 

 phorus. In protein synthesis the amount of sulfur and 

 phosphorus consumed is small, but a very large amount of 

 nitrogen is required. Furthermore, nitrogen in the gaseous 

 condition in which it occurs in the air does not readily unite 

 with other substances; so, although it makes up four fifths 

 of the atmosphere, green plants cannot take it directly from 

 the air. For the nitrogen needed for protein-making they 

 must depend, therefore, on the supply which comes from the 

 soil in the form of nitrates. This is carried to the cells 

 with the water that is absorbed by the roots. Protein syn- 

 thesis, like the synthesis of fats, is probably effected by the 

 protoplasm. It may occur in nearly all parts of a plant, but 

 it takes place for the most part in the leaves. Proteins, be- 

 cause of their complex composition, are especially used in 

 building up and repairing the protoplasm. They are trans- 



