THE COMPONENTS OF PLANTS AND ANIMALS 41 



tained in the material originally or, especially in the case of 

 human foods, artificially added or developed during cooking. 

 Besides ethereal oils, stock -feeds contain a great variety of 

 bitter or astringent substances, gums, waxes, resins, etc., etc., 

 of whose properties and physiological effects little or nothing is 

 known. The flavor and palatability of feed, as already indi- 

 cated, are usually dependent upon these accessory ingredients, 

 while the fact that palatability is an important factor in nu- 

 trition aside from any direct nutritive effect will appear in later 

 discussions. 



72. Vitamins: Growth substances. Much attention has 

 been devoted during the past few years to an important but as 

 yet rather ill-defined group of food constituents called by some 

 investigators vitamins and by others growth substances. These 

 substances, however, are known by their effects rather than by 

 their chemical properties and may therefore be more appropri- 

 ately considered in their relations to the requirements for 

 maintenance and growth (438, 498, 738). 



