278 



M. K. Bach and J. Fellig 



mm.-long bean stems were used for the first six lines of this table, and 

 the results are the averages of four replicate determinations. The 

 stems were exposed to lO^Af radioactive 2,4-D and varying amounts 

 of unlabeled 2,4-D to the final concentrations given for a period of 

 6 hrs. As can be seen, the incorporated radioactivity remains con- 

 stant above the lowest concentration of 2,4-D supplied, indicating 

 that the uptake of 2,4-D was directly dependent on the concentration 

 applied. The last two lines of this table depict the effect of the cross- 

 sectional area of exposed surface on the uptake of 2,4-D. The same 

 weights of tissue were used in both these conditions, but in the upper 

 line the stems were cut into 10 mm. lengths and on the lower line 

 into twice as many 5 mm. lengths. It can be seen clearly that the 

 uptake is directly dependent on the exposed surface. 



One would expect to find exposure to 2,4-D in liquid a much 

 more effective method of inducing proliferation, since the area ex- 



r-^ 



4.0 



a 

 o 



-I — I — I I I I 



-| 1 — I I I I I I 



-1 1 — I I I I I 



\ \ 



2.0 



{ X. 



^ 



I. I J 



I I I I 



I 10 '00 



HOURS 



Fig. 1. Tlie overall changes in radioactivity of bean stem sections witli lime after 

 application of lC"-2,4-D. Closed circles, stems in contact with 2, ID for the en- 

 tire experiment. Open circles, 2,4-D removed at the end of 3 hrs. 



