[Chap. XIX SUBSTANCES MADE FROM FOOD 179 



in the system. Chemically, assimilation of foods consists of condensation 

 processes or of a combination of oxidation-reduction and condensation 

 processes. Assimilation is one phase of growth. About one-half of the 

 food made by a plant is converted into its own tissues. Perhaps the 

 amount is often greater than one-half. 



The known cases in which food, enzymes, vitamins, and hormones are 

 essential to certain basic processes exemplify an extremely intricate phase 

 of the chemistry of biological phenomena. Many of the substances that 

 affect the reactions in living cells are known, but relatively few of the 

 details of the many chains of reaction that occur are known. Enough 

 facts have been discovered, however, for one to learn ( 1 ) to relate cer- 

 tain symptoms of development to the presence or deficiency of particular 

 substances, and ( 2 ) to know many of the major links in numerous chains 

 of reaction. As this kind of knowledge increases, the bases of certain 

 types of superstitions and assumptions about living cells disappear. 



REFERENCES 



Booker, L. E., et al. Vitamin needs of man. Food and Life. Yearbook of Agri- 

 culture, U.S.D.A. 1939. Pp. 221-271. 



Clute, W. N. Useful plants of the world. W. N. Cliite Co. 1932. 



Daniel, E. P. Vitamin content of foods. Food and Life. Yearbook of Agricul- 

 ture,' U.S.D.A. 1939. Pp. 286-295. 



Good, R. Plants and human economics. Cambridge Univ. Press. 1933. 



Hitchcock, A. S. How plants are used by men. Pt. 1. Old and New Plant Lore. 

 Smithson. Inst. Series. 11:97-111. 1934. 



Saunders, C. F. Useful wild plants of the United States and Canada. Robert 

 M. McBride & Company. 1930. 



