Metallic Ions, Auxin Action, and Chelating Agents 377 



21. Thimann, K. V. Sludles on the growth and inhibition of isolated plant parts. 

 V. The effect of cobalt and other metals. Amer. Jour. Bot. 43: 241-250. 1956. 



22. , Craigie, J., Krotkov, G., and Cowie, L. Utilization of uniformly 



labeled C"-galactose by etiolated Avena coleoptiles. Amer. Jour. Bot. 45: 



295-297. 1958. 

 23. , and Schneider, C. L. The role of salts, hydrogen-ion concentration, 



and agar in the response of the Avena coleoptile to auxins. Amer. Jour. Bot. 



25: 270-280. 1938. 

 2!. , Slater, R. R., and Christiansen, G. S. The metabolism of stem tissue 



during growth and its inhibition. IV. Growth inhibition without enzyme 



poisoning. Arch. Biochem. 28: 130-137. 1950. 



25. Weinstein, L. H., Meiss, A. N., Uhler, R. L., and Purvis, E. R. Growth-pro- 

 moting effects of ethylenediamine tetra-acetic acid. Nature. 178: 1188. 1956. 



DISCUSSION 



Dr. Nitsch: At the time Dr. Thimann reported the very large 

 effects he obtained with cobalt as an lAA synergist on Avena coleop- 

 tiles (Amer. Jour. Bot. 43: 241. 1956), we tried the effect of cobalt 

 on oat first internodes. To our great surprise, we got little, if any, 

 stimulation (J. P. Nitsch and C. Nitsch. Plant Physiol. 31: 94. 1956). 

 We generally got a small cobalt effect with coleoptiles. We repeated 

 these experiments several times but never obtained the large cobalt 

 promotion which has been reported, although manganese gave results 

 which agreed with those of other workers. The explanation for the 

 discrepancy between Dr. Thimann's and our results came only this 

 year when Busse's article appeared (Planta. 53: 25. 1959). Busse has 

 shown that the cobalt effect is large under two conditions: (1) that the 

 sections be long, and (2) that the test last for more than 24 hrs. We 

 used 4 mm. sections, taken 3 mm. below the tip of the coleoptile, or 

 2 mm. below the node (in the case of first internodes), whereas Dr. 

 Thimann used 10 mm. sections. In addition, we always measured 

 elongation 20 to 24 hrs. after the start of the treatment whereas Dr. 

 Thimann measured elongation after 48 hrs. These differences in 

 technique, differences in the oat variety (we used 'Brighton'), and the 

 fact that we added a citrate-phosphate buffer to the sucrose solution, 

 explain the discrepancies observed in the results. They indicate that 

 cobalt may retard a sort of aging process which allows lAA to act 

 longer on older parts of the coleoptile. 



Dr. Thimann: I have never done any large number of experi- 

 ments with cobalt using 4 mm. sections. We have found perfectly 

 good cobalt effects in 24 hrs. with both Avena coleoptiles and either 

 'Alaska' or 'Laxton's Progress' pea stem sections, although it is true 

 that we have observed greater responses in 48 hrs. Busse's data on 

 coleoptile sections show cobalt promotions from about 6 hrs. on. His 

 conclusions are essentially that the effect of cobalt appears when the 



