172 CHEMICAL AGENTS AND GROWTH 



there is a strong suggestion of a similar phenomenon. It now appears 

 that this chemical may be used for control of the tomato dormancy 

 problem on a field scale, particularly if combined with other chemicals. 



Let us now turn to another interesting area of the chemical control 

 of the light-induced response in leaves. On the basis of experiments 

 with the Avena coleoptile, James Bonner and I postulated a number 

 of years ago that a cyclic mechanism was involved in the red-far red 

 reaction concerned with growth of the coleoptile as affected by auxin 

 (Liverman and Bonner, 1953). There has been some question of the 

 validity of the conclusions that we drew from a few data, and I do not 

 wish here to discuss the correctness of our conclusions and hypotheses 

 — all I want to do now is to present some additional information 

 which bears upon this topic and which may lead us eventually to the 

 right answer concerning the nature of light actions. Let me summarize 

 very briefly the experiments on Avena. 



It is known that auxin is required for the growth of an oat coleoptile 

 and that red light promotes its growth. Our experiments showed that 

 far-red light had its effect only if both red light and auxin were 

 supplied before the far-red exposure. It was concluded that red light 

 generated something which led to additional growth and that, whatever 

 the nature of this substance, its effect was erased by exposure to far red. 

 These results were interpreted in terms of a cyclic mechanism as shown 

 in Fig. 1. Attempts to demonstrate the existence of such a system in 

 leaf discs by using auxin were never successful at that time. This 

 result is not too surprising, however, since it is known that young 



AUXIN 

 RESPONSES 



y 



TEMPERATURE 

 DEPENDENT 



Fig. ]. The morphogenetic photocycle proposed by Liverman and Bon- 

 ner (1953) as a working hypothesis for studying the Hght-controlled reac- 

 tions in photoperiodism. 



