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ELEMENTS OF BIOLOGY 



parous forms, the food available for growth is dependent upon the 

 eflForts of the animal and the supply in its surroundings. Rapidity 

 and extent of growth may therefore depend to a considerable 

 degree upon environmental conditions. 



The Vitamins. It has been found in recent years that not only 

 quantity but also certain qualities of food are necessary for growth. 

 Growth in the young and the maintenance of normal metabolism 

 in the adult are dependent, in part at least, upon the presence in 

 the food of small quantities of substances termed vitamins. Five 

 of these have been detected, all apparently necessary to normal life. 

 Their exact chemical structures are not yet known. They have been 

 named in the order of their discovery. 



Vitamin A is found in fats, particularly cod liver oil, butter, and 

 in the leaves of plants. When it is absent from the diet, young ani- 

 mals fail to grow. Its absence from the diet of adults in some way 

 causes the degeneration of epithelial tissues, resulting in ulcera- 

 tions of various types. 



Vitamin B is water soluble and occurs in green plants, germinat- 

 ing seeds, fruits, eggs, milk, and other foods. In its absence beriberi, 

 a serious degenerative disease, develops. 



Vitamin C is found in fruits, particularly the citrous fruits, lemons 

 and oranges, and in most vegetables. When it is absent from the 

 diet, scurvy, a degenerative disease, formerly common among sailors 

 on long voyages and among explorers in polar regions, will develop. 



Vitamin D, like Vitamin A, is found in cod liver oil and else- 

 where with Vitamin A. It is concerned with the metabolism of bone 

 and in its absence bone growth and maintenance are interfered 

 with, a condition known as rickets. 



Vitamin E is found in green leaves and germinating seeds. In its 

 absence growth and general health appear to be normal but de- 

 generation of the gonads occurs, particularly those portions which 

 have to do with the maturing of the germ cells. Its deficiency is 

 one of the causes of sterility. 



