CONTROL OF LEAF GROWTH 



175 



auxin may be involved in leaf expansion. These experiments also 

 indicate that there is a basic difference in the reactions affected by 

 adenine and thiopurinesuccinic acid, since a red-light effect is evident 

 in the presence of adenine following x-irradiation, but not in the 

 presence of thiopurinesuccinic acid. 



DISCUSSION 



What do these results contribute to the understanding of the many 

 faceted phenomenon of growth as controlled by light? First of all, as 

 suggested earlier in this presentation, because of the varied nature of 

 the final response, it appears highly unlikely that a single series of 

 reactions controls all the growth processes shown in Fig. 2. It seems 

 probable that the initial phases of all these processes are quite similar 

 because of their control by red and far-red light. We can logically ask, 

 then, at what point between the initial light reaction and the final 

 response — flowering, leaf expansion, etc. — do the chemicals discussed 

 above have their action? In the endeavor to answer this question, we 

 may begin to get an insight into the biochemical nature of each of 

 these processes and to determine how many reactions they may have in 

 common; or to put it another way, we may learn at what point these 

 reactions begin to diverge. 



It may be profitable to take each of the chemicals discussed above 

 and relate its action to the hypothetical sequence shown in Fig. 2. 



A — B^D — F — 



LEAF EXPANSION 

 HYPOCOTYL HOOK UNBENDING 

 COLEOPTILE ELONGATION 

 SEED GERMINATION 



FLOWERING 



MESOCOTYL ELONGATION 



HYPOCOTYL ELONGATION 

 INTERNODE ELONGATION 

 SEED GERMINATION 



Fig. 2. Proposal concerning possible reaction sequences involved in 

 photoperiodically controlled responses. The cycle represents the reactions 

 closely allied with the primary light reaction. G" represents from one to 

 hundreds of reactions, possibly not on the same pathway, which lead to 

 the final response — flowering, etc. 



