THE NATURE OF SEEDS 345 



use fertilizers. Manure, for example,' is rich in nitrates 

 and ammonia, both of which contain nitrogen. 



In that part of the book which deals with the lower forms 

 (Chapter X) you will learn of certain microscopic plants 

 that live in the soil and are very important in keeping up 

 the supply of nitrates. This is because they are able to 

 use the nitrogen of the air which penetrates the surface of 

 the soil. From it and other common substances they 

 manufacture nitrates which are used by the higher plants. 



Proteins, then, are nitrogenous foods, while fats and car- 

 bohydrates are non-nitrogenous foods. Fats differ from 

 carbohydrates in that they contain a much lower percentage 

 of oxygen than the latter do, and a much higher percentage 

 of carbon. They are like carbohydrates, however, in that 

 carbon, hydrogen, and oxygen are the three elements of 

 which they are composed. Just as carbohydrates usually 

 exist in plants in the form of sugar and starches, so fats 

 usually exist in plants in the form of oils. Very small 

 drops of oil are frequently found in plant cells, and in cer- 

 tain tissues they are of regular occurrence. Proteins exist 

 in various forms. Like starch, they are insoluble, a property 

 which is evidently desirable for a storage form of food. 



The gluten of wheat and the aleurone grains of corn and 

 rice are almost pure protein. It is common for the outer- 

 most layer of the endosperm of grains to be particularly 

 rich in proteins. (See Figures 149 and 154) It is the 

 gluten in them which makes the grains of wheat form a 

 gummy mass when chewed. It is the gluten in wheat 

 flour which prevents the gas which arises from the yeast 

 from escaping, and so gives lightness to wheat bread. 

 Since corn does not contain gluten, light bread cannot be 

 made from corn flour. 



