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 



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 c.c, or one thousandth of a gamma. The lowest amount of lAA detect- 

 able in the Avena curvature test, for which about 0-3 c.c. of agar has to be 

 used in order to make 12 small blocks, is 10 y/1. in 0-3 c.c, or 3 X 10~^ y. The 

 new internode test can thus be 3 times more sensitive than the Avetia curvature 

 test. Moreover, as shown on Figure 18, its proportionality range is wider than 

 that of the curvature test (about a hundred fold, from 2 to 200 y/1.), higher 

 concentrations giving irregular results. The accuracy, however, is less than 

 that obtained with the Avena test in which the curvature is directly propor- 

 tional to the auxin concentration, whereas here, the elongation is pro- 

 portional to the logarithm of this. 



23 



