208 



CHEMICAL AGENTS AND GROWTH 



Dork 



(inner) 



Red 



Fig. 1. Camera lucida tracings of photomicrographs of sections through 

 the center of excised bean hook (x700). The hooks were marked and 

 then exposed to 20 hr of development in the dark and to 1 /j,w/cm- of 

 red energy (625-700 mix). The two upper sections are from the outer 

 part of the hook; lower sections are from the inner portion. Note the 

 marked elongation of the cells on the inner portion when the hook was 

 exposed to red radiant energy. 



lAA and Red Irradiance 



The relationship between 3-indoleacetic acid and red energy is pre- 

 sented graphically in Fig. 2 for concentrations of lAA from 10~^ to 

 10~^M and red irradiance (625-700 miJ.) from 10~^ to 10 /xw/cm-. 

 In complete darkness, lAA caused closure of the hook, which in- 

 creased in magnitude with concentration. All concentrations of lAA 

 decreased the effect of the photoreaction. Low concentrations of lAA 

 between 10~^ and 10 *^M did not alter either the Hnearity of the open- 

 ing response to the log of the irradiance or the slopes of the curves 

 above a 10° opening value. 



Effect of Gibberellin on Hook Response 



Hooks developed in the dark with gibberellin responded negatively 

 at all concentrations greater than 10~^M. Hooks exposed to both red 

 radiant energy and gibberellin opened to a lesser degree than the 

 water controls, if the concentration of gibberellin was greater than 

 lO^^'^M. These results indicate that gibberellin opposes the opening of 



