THE PRESIDENTIAL ADDRESS 13 
or on the surface of the living body ; the fact again that the recent 
isolation of vitamin C, and the accumulation of evidence for its 
nature started from the observation that the cortex of the adrenal 
gland displayed strongly reducing properties ; or yet again the proof 
that a yellow pigment widely distributed among plants, while not 
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 vitamin-like 
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 mole- 
cular 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 
generalisations have been demonstrated. It should be remembered, 
however, that until quite lately only the structure of the drug could 
be considered. With increasing knowledge 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 labelled ‘ 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 structure. 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 pigments, one of which a carotene, so called, is 
the substance which I have mentioned as being convertible into the 
