134 GROWTH 



is a question of the conduction of stimuli rather than the 

 movement of tangible substances. Indeed, the evidence re- 

 garding the occurrence of hormones in plants is nothing like 

 so conclusive as the proof of their existence in the animal ; ac- 

 tually, hormones have never been demonstrated in the plant ; 

 their presence is inferred in order to explain certain very com- 

 plicated physiological processes otherwise inexplicable. 



VITAMINS. During recent years increasing attention has 

 been given to a class of substances known as vitamins, or 

 accessory food factors, which play a highly important part in 

 the dietary of man and animals since their deficiency or ab- 

 sence may give rise to a variety of so-called deficiency diseases 

 such as rickets, beri-beri, and scurvy. 



Three vitamins are at present recognized and are known 

 as fat soluble or vitamin A, water soluble or vitamin B, and a 

 third which may or may not be identical with one of the other 

 two. As none of these substances have as yet been isolated 

 in a state of purity, no definite knowledge exists regarding 

 their chemical composition, and for this reason they are fre- 

 quently designated by the function which they appear to per- 

 form in the economy of life. Thus vitamin A is described as 

 the growth-promoting or anti-rachitic vitamin, whilst vitamin 

 B is known as the anti-neuritic or anti-beri-beri vitamin, and 

 the third, for convenience, is known as the anti-scorbutic 

 vitamin. 



The present occasion is not appropriate for a consideration 

 of the significance and properties such as thermostability re- 

 sistance to oxidation,* and the chief reason for drawing atten- 

 tion to vitamins here is the fact that they appear to be 

 produced only in the vegetable world ; in every case the vita- 

 min content of animal tissue is dependent on the animal's 

 nutrition with plant material containing the requisite substance ; 

 no animal is able independently to produce vitamins for its own 

 use. 



Fat soluble A or the growth-promoting vitamin is, as its 

 name implies, closely associated with fats or oils and is found 

 in the animal kingdom in the largest quantities in the oils of 

 the livers of fish which, in all probability, derive them from 



*See " Medical Research Committee" No. 38, H.M. Stationery Office, 1919. 



