Polar Movement of Auxin in Slioots 403 



supply, tlifferentiates a system for transporting auxin. Confirming 

 this interpretation is the high and statistically significant correlation 

 between the growth of the hypocotyl and the amount of added auxin 

 transported basipetally through the tip. 



In other words, the ability of the tip to transport auxin apparently 

 controls the growth pattern of the 3- to 8-day hypocotyl (9). 



The rate of auxin transport in the sections from Avena coleoptile 

 was shown to be 9 to 12 mm/hr in the critical paper by Went and 

 White (25). The old and unresolved question as to whether auxin 

 moved faster through the vascular strands of the coleoptile cylinders 

 than through the other cells was answered by them in the negative — 

 although their published figures seem to show a slight increase in 

 transport ^vhen the added auxin was over the vascular strand. 



Since the coleoptile is famous for being nonmeristematic, I 

 thought it would be interesting to study the transport properties of 

 a meristem. Accordingly, the intercalary meristem of the gynophore 

 of Arachis was investigated (7, 10). Auxin transport was strictly and 

 basipetally polar, and the rate of transport was 10 mm/hr (Figure 1). 

 In view of the results mentioned below, it should be noted that vascu- 

 lar tissues are maintained intact across the elongating meristem (7). 



Other determinations of rate are few. Guttenberg and Zetsche 

 (5), using a somewhat different method, provided evidence that the 

 rate of auxin transport in sections cut from the hypocotyls of Helian- 

 thus annuus was also 10 to 12 mm/hr. Using a third method, Gregory 

 and Hancock (4) estimated the rate at 5 mm/hr in sections from 

 apple stems; however, a critical examination of their paper makes 

 clear that this is a minimum rate — the maximum is 10 mm/hr, the 

 same as for the other two organs studied. This uniformity is quite 

 remarkable, considering the differences in the material used. 



Gregory and Hancock had also been impressed by the flimsy 

 basis of van der Weij's conclusions about temperature effects on auxin 

 transport, so they examined the point with apple stems. Contrary 

 to van der AVeij's conclusions, though not his data, they found that 

 increasing temperature resulted in increasing rate of transport. 



Although the earlier work on coleoptiles left the impression that 

 strictly polar transport was a characteristic of all cells, we had reason 

 to think otherwise. During the period that auxin transport was 

 gradually appearing in the tip of the bean hypocotyl, the only obvious 

 histological change was the differentiation of vascular tissue. Similarly, 

 the course of xylem regeneration in Coleiis would be easier to under- 

 stand if auxin were normally moving in the vascular tissue rather 

 than through all the stem tissues. Accordingly, transport tests were 

 run in which various areas of the stem were excised. No transport 



