THE COMPONENTS OF PLANTS AND ANIMALS 39 



abundant, therefore, where growth is going on most rapidly. 

 Young and succulent feeding stuffs, such as pasture grass, green 

 soiling crops and the like, accordingly contain a considerable 

 proportion of their nitrogen in the non-protein form. As plants 

 approach ripeness, the proportion of non-protein to protein 

 nitrogen becomes less, so that mature hay, straw and the like 

 are relatively poor in non-proteins. This is especially true of 

 seeds, whose nitrogen is contained chiefly in the reserve pro- 

 teins, although small amounts of various non-proteins are also 

 found. One class of feeding stuffs relatively rich in non-protein 

 is the roots and tubers, in which the conversion of inorganic 

 nitrogen into protein seems to be incomplete and in which the 

 non-protein serves as a nitrogenous reserve for the growth of 

 the succeeding year. Finally, feeding stuffs which have under- 

 gone fermentation, such as silage, show a relative increase of the 

 non-protein nitrogen over that of the original material. 



67. Nitrates and ammonium salts. Occasionally somewhat 

 considerable amounts of nitrates or of ammonium compounds 

 are found in vegetable material, especially when the supply of 

 soluble nitrogen compounds in the soil is abundant. In such 

 cases they are to be regarded as materials taken up in excess 

 of the immediate needs of the plant. 



6. SUNDRY INGREDIENTS 



68. Number and significance. In the foregoing sections 

 the groups of substances which constitute the animal body or, 

 in the form of feed, supply the matter and energy for its growth 

 and maintenance have been considered. It is hardly necessary 

 to say, however, that these four groups, the carbohydrates, 

 fats, proteins and non-proteins, are very far from comprising all 

 the constituents of animals or plants. 



In the animal body the physiological chemist has recognized 

 relatively small amounts of a vast number of substances of 

 the most varied nature. Some of these are derived quite di- 

 rectly from the proteins, fats or carbohydrates and these will be 

 considered to a greater or less extent in studying the changes 

 which these substances undergo in the body. Others, while of 

 great physiological importance, have little direct relation to the 

 processes of nutrition. 



