Acceleration and Retardation of Abscission 



561 



proximally. Results are summarized in Figure 1. With the lowest 

 concentrations ot gibberellic acid, 0.01 /xg. per block, abscission was 

 appreciably faster than in the controls, and the rate increased with 

 increasing gibberellic acid. In similar experiments higher concentra- 

 tions of gibberellic acid were used; 100 /xg. per block (the highest 

 concentration which would remain in solution) produced somewhat 

 more rapid abscission than 10 /xg. 



The second group of experiments was similar to the first except 

 that lower concentrations were applied: 0.0001, 0.0005, 0.001, and 

 0.005 /xg. per block. Results are summarized in Figure 2. Slightly 

 increased abscission followed the 0.005 /xg. application. Results from 

 the other applications were very close to the controls. 



In the third group of experiments gibberellic acid was applied 

 proximally and blank agar blocks distally; other conditions were 

 identical with those of the first two groups of experiments. The re- 

 sults (Figure 3) were closely similar to those of the first group of ex- 

 periments. The rate of abscission increased with increasing concen- 

 trations of gibberellic acid. 



In a few experiments the proximal applications included 100 /xg. 

 of gibberellic acid. As with distal applications these produced some- 

 what more rapid abscission than 10 /xg. However, another quite un- 

 expected result also occurred: The stem stump abscised in some 



iOO 



80 





- 60 



O 



cn 



m 

 < 



o 



UJ 



a. 



DAYS 



Fig. 2. Abscission following distal application of small amounts of gibberellic acid. 

 Each line is the average of five experiments totaling 200 abscission zones. 



