560 



Cams, Addicott, Baker, and Wilson 



rosis. Explains were cut when the seedlings were 2 to 3 w-eeks old. 

 In most experiments the cotyledons were debladed 24 hrs. before the 

 explants were cut. 



The explant technique was a modification of that described by 

 Addicott et al. (3). The explants consisted of 5 mm. slumps of coty- 

 ledonary petioles and stem, with 10 mm. of the hypocotyl. Explants 

 were placed in plastic or stainless steel holders in petri dishes contain- 

 ing a sheet of moistened filter paper and kept in the dark at 25° C. 

 Gibberellic acid was applied in agar blocks of the size used in the 

 standard Avena coleoptile assay (2.7 X 2.7 X 1-0 mm.). Application 

 was either distal (to cotyledon petiole stumps) or proximal (to stem 

 stump). The blocks w^ere removed after 24 hrs. to facilitate testing for 

 abscission. Abscission was determined by means of an abscissor (a 

 spring instrument) calibrated to deliver a force of 5 g. 



The gibberellic acid used was provided by Merck and Company 

 as samples No. 57-RTS-931 and No. 57-RTS-lOOO. 



EXPERIMENTS AND RESULTS 



in the first experiments, gibberellic acid was applied distally to 

 explants from debladed seedlings and in concentrations of 0.01, 0.1, 

 1.0, and 10 /^g. per block; blank agar blocks were applied proximally. 

 In the controls, blank agar blocks were applied both distally and 



100 



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DAYS 



Fg. 1. Acceleration of abscission by distal application of gibberellic acid. Each line 

 is the average of three expcrinienis totaling 100 abscission zones. 



