6 Fundamentals of Auxin Action 



a physiologist at Budapest, amply confirmed the earlier work and 

 added one simple but crucial point. Paal found that replacement of 

 the severed tip on one side of the coleoptile stump would produce 

 curvatures away from the treated side. This in fact was a replaceynent 

 of the effect of lateral light by the asymmetrical distribution of some 

 stimulus being produced by the tip. Paal then came to the conclusion 

 that "the tip is the seat of a growth regulating center. In it a substance 

 (or mixture) is formed and internally secreted, and this substance, 

 equally distributed on all sides moves downwards through the living 

 tissue. If the movement of this correlation carrier is disturbed on one 

 side, a growth decrease on that side results, giving rise to curvature 

 of the organ." 



By his careful research, Paal had come essentially to the explana- 

 tion of phototropism, but more important still, he had demonstrated 

 the existence of a "substance" or "correlation carrier," which could 

 control growth processes. 



The wider concept of Paal's "correlation carrier" as a growth 

 hormone intimately involved in all plant growth was left for two 

 other minds to grasp almost simultaneously. 



EMERGENCE OF THE HORMONE CONCEPT 



At the time that Darwin was studying the nature of the photo- 

 tropic stimulus, Sachs (1880) launched the first theory of substances 

 (which we would now call hormones) controlling plant growth. He 

 envisaged the existence of organ-forming substances moving in various 

 polar patterns, and controlling form and development. Fitting (1909) 

 actually extracted substances from orchid pollen which could cause 

 swelling of the ovary in a manner suggestive of fruit-set. He suggested 

 that these substances were hormones. After Paal's (1919) deduction 

 that specific substances produced in the coleoptile tip were responsible 

 for phototropism, Soding (1923) established that these same substances 

 were capable of stimulating straight growth as well. 



The demonstrations of a correlation carrier in oat tips attracted 

 many new workers to the field. Among these were Cholodny (1927) in 

 Russia and F. W. Went (1928) at Utrecht, who independently ex- 

 tended the correlation carrier theory to both phototropism and geo- 

 tropism. Each of them then came to the conclusion that all tropisms 

 were mediated by a growth hormone system which was essential to all 

 plant growth. "Ohne WuchsstofE, kein Wachstum"; without auxin 

 there is no growth. 



In carrying out his exploration of the role of auxin in growth. 

 Went did two things which opened the field of growth hormones to 



