Polar Movement of Auxin in Shoots 



405 



and vaseline rings around all sections — did not affect the acropetal 

 movement of auxin (11, 12). With 2 mg/1 of lAA in agar added at 

 each end, the ratio of basipetal to acropetal auxin movement was 3:1. 

 What reason was there to think that this acropetal transport in 

 isolated stem sections was of significance in the more intact plant? 

 Evidence was obtained from parallel studies of xylem regeneration 

 in this same standardized internode. The leaves are the major source 

 of auxin in Coleus, and excising the leaves causes a decrease in the 

 number of xylem cells regenerated, the decrease being exactly pro- 

 portional to the amount of diffusible auxin produced by the leaf. 

 When synthetic lAA is substituted for the leaves so as to give exactly 

 the same amount of auxin as they produce, the lAA exactly replaces 

 the xylogenic effect of the leaves (11, 14). Thus we have detailed evi- 

 dence that auxin from the leaves is the factor normally limiting 

 xylem regeneration. Now, when regeneration occurs in plants with 

 all their leaves left on, their course of regeneration parallels the ob- 

 served 3:1 ratio of auxin transport — i.e., regeneration is mostly 

 basipetal, but with a significant amount of acropetal regeneration 

 occurring, too. And Figure 7 shows more quantitative data: if we 







AMOUNT OF AUXIN TRANSPORTED 

 20 65 



5.6 

 NUMBER 



OF XYLEM STRANDS 



I 



Fig. 7. Relation bet^veen amount of auxin transported through excised sections 

 of Coleus and the number of xylem strands regenerated in the plant when leaves 

 above and below are excised. From Jacobs (12). 



