414 



A. C. Leopold and S. L. Lam 



at the apical end for different periods of time. The donor blocks 

 and the stem sections were removed from the receptor blocks which 

 were then assayed for auxin. The concentration of lAA in the donor 

 block was IQ-^M, and the concentration of the NAA and IB A was 

 10-^M in each case. 



The results of velocity measurements are shown in Figure 3. It 

 can be seen that 40 minutes of transport time were necessary before 

 lAA could be detected in the receptor block. The stem section was 5 

 mm. long, and so the velocity can be estimated at 7.5 mm/hr. The 

 first NAA detectable in the receptor block appeared after about 45 

 min., indicating a velocity of 6.7 mm/hr. The first IB.\ detectable 

 appeared after about 95 min. indicating a transport velocity of 3.2 

 mm/hr. Each of these values was determined several times with 

 separate experiments and the variation was not in excess of 1 mm/hr. 



Another means of asking whether the three auxins are transported 

 by similar or dissimilar physiological processes is to measure the sen- 

 sitivity of each of them to inhibitors of transport. For these experi- 

 ments the following inhibitors were selected: 2,4-dichlorophenoxy- 

 acetic acid (2,4-D), a-methoxyphenylacetic acid (MOPA), 2,3,5-triiodo- 



160 



DONOR TIME, MINUTES 



Fig. 3. Determinations of the velocity of transport of three different auxins. Sun- 

 (loucr sections transporting the auxins are applied to receptor blocics for in- 

 ci easing periods of time and the time of initial response in subsequent Avena 

 test is indicated l)y tlie arrow. Symbols as in Figure 1. 



