Polar Movement of Auxin iji Shoots 



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Fig. 3. Relation between auxin transport and mean cell length in 8-day-old bean 

 hypocotyl. Data from Jacobs (8). 



data derived from Avery and Burkholder (1) indicates that in the 

 Avena coleoptile a similar relation exists to that in the bean hypo- 

 cotyl - more cells correlate with more transport (Figure 4). 



The development of auxin transport through the tip of young 

 hypocotyls 2 to 8 days old was then studied. No auxin was transported 

 acropetally in any stage. Basipetal transport first appeared in the 

 5-day tip and steadily increased in amount between the 5- and 8-day 

 stages (Figure 5). 



But does this gradual development of auxin transport, found 

 in excised sections, have any significance in the normal development 

 of the plant? We found evidence that it does (9). There are two 

 regions of maximum elongation in the hypocotyl: the first occurs 

 in the tip in the very early stages (2 to 3 days after germination); 

 the second develops just below, and becomes the major zone of giowth 

 by 6 to 7 days. Studies on excised sections show that growth in this 

 second zone is limited primarily by auxin, and yet this zone does not 

 produce any significant amount of auxin itself — it gets auxin from 

 the upper parts of the plant. But it cannot get the auxin it needs 

 for growth until the tip region, standing between it and the auxin 



