562 



Cams, Addicott, Baker, and Wilson 



100 



80 



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^ 60 



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CD 



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Z 40 

 UJ 



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 tr 



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DAYS 



Fig. 3. Acceleration of abscission by proximal application of gibberellic acid. Each 

 line is the average of four experiments totaling 160 abscission zones. 



(but not all) of the explants. The anatomical changes in this stem 

 abscission were found to be identical with the changes which nor- 

 mally occur in the abscission zones of cotyledonary petioles. 



In the fourth group of experiments the seedlings were not de- 

 bladed prior to cutting the explants; gibberellic acid was applied 

 proximally in low concentrations. In these experiments (Figiue 4) 

 gibberellic acid retarded abscission. The highest concentrations of 

 gibberellic acid, 0.005 /^g. per block, retarded the rate only slightly, 

 but from 0.001 down, retardation was appreciable. The rate of ab- 

 scission decreased with decreasing concentrations of gibberellic acid. 



DISCUSSION 



These experiments show that gibberellic acid can affect abscission 

 when applied close to the abscission zone, and can either accelerate 

 or retard abscission depending on concentration and site of applica- 

 tion. 



Under the conditions of these experiments, gibberellic acid prob- 

 ably reaches the abscission zone and directly influences the process 

 of abscission. Under other conditions, the effects of gibberellic acid 

 on abscission appear to be indirect. For example, the retarded leaf 

 abscission of sprayed branches (6) and increased fruit-set (9, 12) were 

 probably indirect effects resulting from increased vigor or growth 



