116 



L. J. Audiis and J. K. Bakhsh 



250 



-CO 



LOG lAA 

 G./ML, 



UJ 



o 



UJ 



a 



150 



100- 



40' 



to 



o 

 cc 

 o 



z 

 llJ 



Fig. 2. Growth response of segments from roots grown in 2,4-dichlorophenoxyacetic 

 acid solution (3 x 10-° g/ml). A and B. Total extension in 48 hrs. of normal 

 (open circles) and 2,4-D-grown (solid circles) segments. (A) Responses to \\\. (B) 

 Responses to 2,4-D. Vertical lines indicate the least significant difference (5 per 

 cent) between means. C and D. Epidermal cell length distribution before and 

 alter 48 hrs. of extension. (C) Normal segment responses to IA.\. (D) 2,4-D-grown 

 segment responses to lAA. Open parallel lines— segments immediately after exci- 

 sion. Blacked-in parallel lines — segments after 48 hrs. in 0.5 per cent sucrose solu- 

 tion. These lines are plotted at distance of cr/VN above and below tlie respective 

 mean of successive groups of cells. 



g/ml) on normal segments, which is of the order of 60 per cent. This 

 suggests that either 2,4-D is rapidly lost from the segments inuler 

 growth conditions in sucrose, about 80 per cent loss being necessary 

 to explain the much lower inhibition, or that there might have 

 occurred a direct loss of sensitivity to the persistent 2,4-D, of a nature 

 reminiscent of bacterial adaptation to drug action. The latter altern- 

 ative is somewhat favored by observations on the growth of such 



