ORGANIC SUBSTANCES AS PLANT FOOD 



201 



In starchy seeds, on the contrary, there is usually very little 

 fat, as may be seen from these figures: 



In his agricultural activity, man cultivates primarily plants 

 with starchy seeds. These give him cereals, which form the major 

 part of his diet. In general, however, plants with oily seeds pre- 

 dominate in nature, being produced by 90 per cent of plants on 

 earth. As they contain less oxygen and have a high energy value, 

 of which details will be given later on, oils form more compact 

 reserves of nutritive substances than carbohydrates. 



Protein compounds always form a smaller part of a seed's 

 reserves, rarely reaching 25 per cent of its total weight. Besides, 

 oily seeds usually contain a larger percentage of proteins than 

 starchy seeds. Hemp seeds contain 18 per cent of proteins; flax, 

 23 per cent; poppy, 19 per cent; almond, 22 per cent; nuts, 17 

 per cent; and sunflower seeds, up to 30 per cent, whereas in wheat 

 there is but 12 per cent; in buckwheat, 10 per cent; corn, 10 per 

 cent; and rice, 7 per cent. The starchy seeds of leguminous plants, 

 however, are very rich in proteins, very much like the oil-contain- 

 ing seeds. Thus, peas contain 29 per cent protein; kidney beans, 

 23 per cent; soybeans, 34 per cent; and lupines as much as 40 

 per cent. This high protein content of leguminous seeds is un- 

 doubtedly connected with their ability to fix atmospheric nitrogen 

 owing to the activity of the tubercle bacteria. Having at their 

 disposal during the course of their lives an unlimited supply of 

 nitrogen, leguminous plants supply their progeny with a liberal 

 amount of nitrogenous substances. 



The composition of reserve substances in the seeds of different 

 types of plants is not constant. It is subject to considerable fluc- 

 tuations, depending upon external conditions. Especially strik- 



