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W. P. Jacobs 



During the last 15 years, 1 have been investigating the movement 

 of auxin in plants and organs other than the Avena coleoptile — 

 with the aim of checking just how generally applicable were the 

 results obtained from this histologically simple, highly specialized 

 structure of determinate growth. 



A sharp gradient in the amount of added auxin transported basi- 

 petally was found along the axis of a nearly mature 8-day bean 

 hypocotyl (Figure 2). At the level of the transition from stem to root 

 structure, none of the added auxin was transported. No acropetal 

 transport of added auxin was found at any level of the hypocotyl (8). 

 Went suggested that less auxin might be getting through the more 

 basal sections because of a mechanical blocking by more end walls. 

 However, quite the reverse was foiuid (Figure 3): There was a clear 

 positive relation between the number of cells in a 5 mm. section 

 and the amount of auxin that the section transported. A similar 

 steep basipetal giadient of auxin transport can be seen in the data 

 for the Avena coleoptile (20, 25), but the authors minimize the dif- 

 ference. Graphing the data of Went and White with cell-length 



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Fig. 2. Gradient in basipetal auxin transport along the 8-day bean hypocotyl 

 [figure is from Jacobs (8)]. Data give the amount of auxin, calculated as lAA, 

 collected in 3 hrs. at one end of 5 nun. sections when lAA at 2 mg/1 of agar was 

 applied at the opposite end. Parallel collections of the native auxin gave amounts 

 at each end which were equivalent to that shown in this figure for base toward 

 apex transport plus diffusion. Hence, there was no detectable acropetal transport. 



