21 



GROW^TH AND GROAA^TH HORMONES 



IN PLANTS 



Kenneth V. Thimann ^ 



In the fifty years since the Botanical Society of America was founded, plant 

 science has undergone some striking developments, but none of these has 

 been more revolutionary than the discovery of auxin and of the hormonal 

 control of the growth of higher plants. At every stage, from the fertilization 

 of the ovule through all the phenomena of vegetative and reproductive develop- 

 ment, the auxins play an important and often a controlling role. At the cellu- 

 lar level, the division of many types of cells, the enlargement of all t)^es 

 capable of enlargement, and the differentiation of xylem constitute the main 

 effects of the auxins. At the tissue level, the formation of roots on shoots, the 

 response of shoots to light and gravity, the growth of fruits, the interrelations 

 between buds and other organs which are included in the term "apical domi- 

 nance," all appear to be directly and quantitatively controlled by the same 

 hormone. Even the growth of algae (though insufficiently studied to allow 

 of certainty) appears to be subject to the same control. Other phenomena 

 such as the formation of leaves and the initiation of flowering are at least 

 strongly influenced by auxin, although other factors may be the prime causes. 

 Because this fundamental knowledge has led to many successful applications 

 in horticulture and in agricultural practices, the term "revolutionary" used 

 above can almost be transferred as well to these developments in the applied 

 plant sciences and arts. The following is a general summary of present knowl- 

 edge of the auxins and their physiology, omitting only those topics which are 

 to be treated elsewhere in this volume. 



As so often happens, the first developments took place in Europe, while 

 the later stages, though world-wide in distribution, have been pursued most 



^ Support of the author's researches, over a period of years, by the Committee on 

 Growth, acting for the American Cancer Society, and by the National Science 

 Foundation is gratefully acknowledged. 



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