. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Investigation of natural auxins and growth inhibitors were obtained. They show that coleoptiles in 50 y/l. of lAA (phis buffer and sucrose) continue to grow for a long time after they have been put in the solution, whereas the first internodes in the auxin solution stop growing after 18 hours. A short incubation time has the great advantage of minimizing the danger of bacterial contamination in the solutions. T
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Investigation of natural auxins and growth inhibitors were obtained. They show that coleoptiles in 50 y/l. of lAA (phis buffer and sucrose) continue to grow for a long time after they have been put in the solution, whereas the first internodes in the auxin solution stop growing after 18 hours. A short incubation time has the great advantage of minimizing the danger of bacterial contamination in the solutions. Therefore, an incubation period of about 20 hours was adopted and was found adecjuate in subsequent work. We can now attempt to obtain a comparison between the response of coleoptiles and that of internodes to increasing concentrations of lAA. As. 0 1 s 10 so too 300 -JOOO sooo Concenfraf/on of IA4(x5x70' 9 Y\) Figure 18. Growthcurvesofoatcoleoptile (C) and first internode {M) sections in buffer^ sucrose+ Tiveen 80 (0-1 per cent) with various concentrations of I A A. The scale on the left ordinate gives the growth in mm over the initial 4 mm; the scale on the right ordinate gives it in per cent of the origiiial length. The scale giving the LAA concentration is logarithmic. The coleoptile sections had been floated 5 hours on MnS04 • HjO (1 mgjl.) and the internode sections 1 hour on HjO prior to being put in the auxin solutions. Figure 18 shows, the amplitude of the response is larger in internodes tlian in coleoptiles. A clearly detectable increase over the growth of the controls can be obtained with the first internodes at a concentration of 10~^ M lAA, or 1-75 }'/l. The lowest amount of lAA detectable is approximately 2 y/1. in 0-5 , or one thousandth of a gamma. The lowest amount of lAA detect- able in the Avena curvature test, for which about 0-3 of agar has to be used in order to make 12 small blocks, is 10 y/1. in 0-3 , or 3 X 10~^ y. The new internode test ca
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