Polar Movement of Auxin in Shoots 



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CONCENTRATION OF lAA ADDED, P.RM. 

 Fig. 8. Evidence of saturation in auxin transport in young internode sections of 

 Coleus. The sections were prediffused for 2 hrs., ringed with vaseline (to obviate 

 surface leakage), then 5 mm. sections were cut from the slightly longer pre- 

 diffused sections. Transport time was 25 hrs. 



portance of the saturation capacity in the physiology ol the plant is 

 thus obvious: it could act as a controlling valve which prevents excess 

 amounts of auxin from being transported around the plant and thereby 

 disturbing the balanced coordination of normal development. 



That such a saturation capacity is not limited to Coleus is indi- 

 cated by the results in bean hypocotyl mentioned above and by trans- 

 port tests on Avena coleoptiles (Goldsmith, Ph. D. thesis, Harvard). 



From the earlier view of normal auxin transport as something 

 unique, strictly one-way, and unchangeable by various environmental 

 factors, a new view has thus developed since 1939. Auxin transport, in 

 these later researches, appears as a function which varies with the 

 physiological and histological state of the tissue, which is responsive 

 to changes in the environment and which is 7iot always strictly polar. 

 In addition, it has been shown not only that auxin transport controls 

 the relative polarity of various developmental phenomena, but also 

 that organs have a maximum capacity in the physiological range of 

 concentrations for transporting auxin, and that this set transport 

 capacity serves an important regulatory function by buffering the 

 plant from sudden increases in auxin level. 



What of the future? We need to know how general are the results 

 now known in detail for seedling organs and a few vegetative stems. 

 We need critical direct experiments on auxin transport in roots, 



