CHEMICAL ASPECTS OF LIFE HOPKINS 141 



the vitamin itself, can be converted within the body into vitamin A ; 

 these and other aspects of vitamin studies will stand out as interesting 

 chapters in the story of scientific investigation. 



In this very brief discussion of hormones and vitamins I have so 

 far referred only to their functions as manifested in the animal body. 

 Kindred substances, exerting analogous functions, are, however, of 

 wide and perhaps of quite general biological importance. It is 

 certain that many micro-organisms require a supply of vitaminlike 

 substances for the promotion of growth, and recent research of a very 

 interesting kind has demonstrated in the higher plants the existence 

 of specific substances produced in special cells which stimulate 

 growth in other cells, and so in the plant as a whole. These so- 

 called auxines are essentially hormones. Section B will soon be 

 listening to an account of their chemical nature. 



It is of particular importance to my present theme and a source 

 of much satisfaction to know that our knowledge of the actual molec- 

 ular structure of hormones and vitamins is growing fast. We have 

 already exact knowledge of the kind in respect to not a few. We are 

 indeed justified in believing that within a few years such knowledge 

 will be extensive enough to allow a wide view of the correlation 

 between molecular structure and physiological activity. Such 

 correlation has long been sought in the case of drugs, and some 

 generalizations have been demonstrated. It should be remembered, 

 however, that until quite lately only the structure of the drug could 

 be considered. With increasing laiowledge of the tissue structures 

 pharmacological actions will become much clearer. 



I cannot refrain from mentioning here a set of relations connected 

 especially with the phenomena of tissue growth which are of par- 

 ticular interest. It will be convenient to introduce some technical 

 chemical considerations in describing them, though I think the 

 relations may be clear without emphasis being placed on such details. 

 The vitamin, which in current usage is labeled "A", is essential for 

 the general growth of an animal. Recent research has provided much 

 information as to its chemical nature. Its molecule is built up of 

 units which possess what is known to chemists as the " isoprene struc- 

 ture." These are condensed in a long carbon chain which is attached 

 to a ring structure of a specific kind. Such a constitution relates it to 

 other biological compounds, in particular to certain vegetable pig- 

 ments, one of which a carotene, so called, is the substance which I have 

 mentioned as being convertible into the vitamin. For the display of 

 an influence upon growth, however, the exact details of the vitamin's 

 proper structure must be established. Now turning to vitamin D, 

 of which the activity is more specialized, controlling as it does the 

 growth of bone in particular, we have learned that the unit elements 

 in its structure are again isojirene radicals; but instead of forming 



