592 



W. S. Hillman and W. K. Purves 



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GA 



S5 



10"^ M GA 





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GA 



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10 



-7 



10' 



10 



-5 



NO lAA 



lAA. M 



Fig. 1. Effect of distance from apex and of lAA on response of etiolated pea 

 epicotyl sections to GA. Lower histogram: response of SI sections (apical) to 10-" M 

 GA in the absence of lAA. Curves: response of S5 sections (subapical) to GA in the 

 presence of various lAA concentrations. Upper histograms: magnitude of GA re- 

 sponses (GA-treated value minus corresponding control). Divisions of bars and pairs 

 of points represent replicate lots of ten sections. Control medium: phosphate buffer, 

 pH 6.1 plus 1 per cent sucrose. Incubation: 20 hrs. in darkness, about 30° C. 



lAA responses, and thus inferentially the auxin relations of the tissues. 

 Two such experiments are shown as Figures 3 and 4. Figure 3 shows 

 the effects of adding a-(p-chlorophenoxy)isobutyric acid (PCIB) to the 

 test medium on the control elongation and on the GA and lAA re- 

 sponses of SI sections. PCIB has been described as an anti-auxin (3), 

 but the interpretation of this experiment does not depend on the 

 correctness of this view. It is evident from Figure 3 that a level of 

 lAA which has essentially no effect in the absence of PCIB greatly 

 promotes elongation in its presence, returning the PCIB-inhibited 

 sections to (or occasionally above) the control. PCIB thus inhibits con- 

 trol elongation and brings about a greatly increased auxin response. 

 The additional elongation conferred by GA, however, is essentially 

 the same in the presence or absence of PCIB, whether or not lAA is 

 present as well. 



Very similar results are obtained by an entirely different treat- 

 ment which also alters the auxin response of SI sections. This treat- 

 ment consists of decapitating the seedlings several hours before the 



