A. W. GALSTON AND RAVINDAR KAUR 705 



(iii) Etiolated sections respond optimally to very low concentra- 

 tions (ca. 10-^' M) of gibberellic acid and indole acetic acid, while 

 green sections require ca. 100 to 1000-fold higher concentrations of 

 these substances for optimal growth. 



(iv) The autotrophic growth of green sections is inhibited by the 

 substituted urea herbicides, their heterotrophic growth being essen- 

 tially unaffected by these compounds. In shari^ contrast, the (hetero- 

 trophic) growth of etiolated sections is markedly promoted by the 

 urea herbicides. 



(v) Green pea plants grown under continuous light produce sec- 

 tions which grow better in the light and worse in the dark than those 

 from daily illumination regimes, including some darkness. They are 

 thus to be considered as "light-adapted." Conversely, plants grown 

 under short periods yield sections which grow better in the dark and 

 worse in the light than sections from longer photoperiod plants. This 

 implies that such sections are still partially dark-adapted, and that 

 light induces a progressive shift from the etiolated to the fully green 

 condition. 



An explanation of the causes of any of these phenomena would 

 bring us considerably closer to an understanding of the mechanism 

 of the action of light on plant growth. 



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