368 



K. V. Thimann and N. Takahashi 



The inhibiting action of 3 X 10-^M Co*^ jj indeed detectable at 9 

 hrs. (Figure 2B), and the promotion by lO-^M and 3 X 10-^M Co^^ 

 at almost the same time. However, it is true that the increments 

 only become really large after 12 hours' growth. 



Another difficulty in regard to Busse's interpretation is that if 

 the promotion of giowth by low concentrations of Co"- is due to 

 inhibition of the deposit of cell-wall, no explanation is offered for 

 the inhibition of growth by higher concentrations. That 3 X 10"^Af 

 Co+2 should strongly promote a process which lO'^M Co^^^ equally 

 strongly inhibits seems most unlikely. For these reasons the conclu- 

 sion of Busse can only be accepted with reserve at present. 



The Action of Chelating Agents 



The essential results of Heath and Clark (10) could readily be 

 confirmed. For Avena coleoptile sections the optimum concentration 

 of EDTA was found to fall somewhat below 10-^M, and lO'^M was 

 generally inhibitory. The increment caused by EDTA occurred in all 

 growth media tested; in CaCU its value was not significantly different 

 from that in KCl, buffer, or water. In the absence of sucrose it aver- 

 aged about 10 per cent, in 2 per cent sucrose about 5 per cent. Since 

 these increments are small differences between two fairly large num- 

 bers, they are subject to fairly wide variation; nevertheless the data 

 agree rather well. 



Other chelating agents have been studied less thoroughly. AVith 

 8-hydroxyquinoline (8-HOQ) the optimum concentration lay near 

 3 X 10-^i\i^ i.e., about one-third of that with EDTA. A representa- 

 tive group of three experiments is shown in Table 1; the increments 

 (in 2 per cent sucrose) are comparable to those obtained with EDTA. 

 A few trials with 9,10-phenanthroline, on the other hand, did not 

 show significant growth promotion. 



In the hope of improving the magnitude of the effect, sections 



Table 1. Effect of 8-hyclroxyquinoline on elongation oi Avena coleoptile sections. 



Medium consisted of 2 per cent sucrose plus O.OLU phosphate buffer at pH 5.0. 



