406 VV. F. Jacobs 



compare regeneration in plants in which only distal sources of auxin 

 are lett on (i.e., leaves 1, 2, and the apical bud) with plants with only 

 proximal sources of auxin (leaves 3 to 8), we see that the number of 

 xylem cells regenerated is nicely proportional to the observed auxin 

 transport (12). 



Our conclusion is that the acropetal auxin transport is of real 



physiological significance.^ 



Another idea is suggested by musing over the results with the 

 bean hypocotyl and Coleiis internode 2. In both cases lAA was added 

 at a concentration of 2 mg/1 of agar. And in both cases, very exact 

 relations could be observed between the amount of auxin transported 

 under these circumstances and the amount of growth or of xylem 

 regeneration. This suggests that, contrary to what one might expect 

 from the reports of van der Weij (20) or Went (23), there is 7iot a 

 steady increase in auxin transported as one increases the amount added, 

 but rather a plateau - and that 2 mg/1 is on the plateau. 



We are currently checking the hypothesis in two ways: by direct 

 assay with the Ave7ia curvature test of transport through isolated 

 sections ringed with vaseline (Figure 8), and by indirect test using 

 xylem regeneration in Coleiis again as a measure of how much auxin 

 is getting into the more intact plant. Xylem regenerated under the 

 influence of the shoot tip could be completely replaced by 1 per cent 

 lAA in lanolin. And as evidence of saturation was the fact that 10 

 per cent lAA in lanolin gave no further increase in the number of 

 xylem strands which regenerated [Figure 5 in Jacobs ct al. (15)]. 

 The Avena curvature results are not definitive, since they do not yet 

 include dilutions of the collected auxin. (This means that there is a 

 possibility that the Avena curvatures for the transport of 1.25+ 

 mg/1 are above the proportionality range of the Avena bioassay. We 

 are, of course, checking this possible artifact now.) But, so far as they 

 are valid, the two tests confirm the hypothesis and each other. Auxin 

 transport in Coleus seems to show a saturation effect at concentra- 

 tions of \-{- mg/1 of agar. 



Experiments already described, plus others ilescribed b) Jacobs 

 in 1954 (12), provided evidence that the transport capacity at satur- 

 ation normally limits how much auxin is available in the plant. Al- 

 though the leaves normally produce auxin at a level matching the 

 saturation capacity of the stem (12), their level of production is sub- 

 ject to fluctuation as the environment changes. The potential im- 



^The appearance, wiili flowering, of acropetal auxin transport in stems ot 

 Coleus has been reported (16), the report criticized on grounds of inadccjuate 

 technique (12), and found unconfinnable by Haupt (6). Professor Leopold has 

 told me that his attempts to repeat the experiments have given inconsistent 

 results. 



