Development of Knowledge of Auxins 9 



trations inhibit it. Knowledge of the inhibitory capacity soon led to 

 the discovery by Thimann and Skoog (1933) that auxins which are 

 formed at a plant apex inhibit the growth of lateral buds. This ex- 

 planation of the phenomenon of apical dominance clarified one of the 

 major correlative effects which had puzzled many earlier physiologists. 



Further dramatic demonstrations of the physiological roles of 

 auxins came from the experimental treatment of plants with chemical 

 compounds which had been found to possess auxin activity. Went 

 (1934) discovered that auxins can stimulate the formation of adventi- 

 tious roots (another correlative effect clarified). The full realization 

 of the developmental importance of auxin in morphological differen- 

 tiation was brought forth much later by Skoog and Tsui (1948), who 

 found that relative auxin levels in plant tissues play a crucial 

 role in determining what sort of growth will take place. Thus auxin 

 content in relation to other plant constituents determines whether 

 growth will be simple cell proliferation, or bud formation, or root 

 formation. 



Another role of auxin was discovered by LaRue in 1936. He 

 found that auxins applied to leaves could retard leaf abscission. Later 

 workers have found that abscission of all plant organs (leaves, flowers, 

 fruits, etc.) is correlated with low natural auxin content. 



The discovery that auxin plays a key role in parthenocarpy and in 

 fruit-set in general stems from the work of Gustafson (1936). 



In a very short time indeed, after the perfection of the Avena 

 test, we see that the essential framework of auxin functions in plants 

 had been brought to light. Auxin was found to be essential for 

 growth, and to be able either to stimulate or to inhibit growth, de- 

 pending upon the concentration. It was found to be the controlling 

 agent of tropic responses, the major control of apical dominance, and 

 a primary factor in organ formation, in the abscission or non-abscis- 

 sion of plant parts, and in the commencement of fruit development. 



Looking at the framework of auxin functions as an assembled 

 structure, the physiologist may see the higher plant as an organism 

 controlled and regulated in its growth and development by generally 

 continuous streams of growth hormones. These hormones, formed in 

 each of the plant extremities, course through the organism from the 

 extremities inward, where they not only control growth itself but 

 mediate the differentiation of new organs and tissues, the shedding of 

 old organs, and the stepwise progression of many of the stages in the 

 life cycle of the organism. In this view, then, the higher plant is not 

 a simple colony of cells but is an entity sychronized in its growth, 

 movement and development principally by streams of the growth 



