TECHNIQUE OF AUXIN DETERMINATIONS 45 



compounds, which retard the growth of such organs as the 

 coleoptile, is not clear. Tryptamine (see VII E) may give 

 slight positive curvatures within 2 hours after application 

 (Skoog, 1937), but within 6 hours these become strongly 

 negative, doubtless due to transformation of the amine 

 into indole-acetic acid. Czaja (1934) found that tannic 

 acid, or plant tissues rich in tannins, cause positive curva- 

 tures, which seem to be due to shrinkage of the cells. Meyer 

 (1936) obtained positive curvature with lanoline mixed 

 with certain plant extracts. In lower concentrations the 

 paste caused normal negative curvatures, so that apparently 

 increasing the concentration brought the toxic or osmot- 

 ically active substances up to a concentration at which 

 their effect more than offset the growth-promotion effect. 

 If agar blocks with low auxin concentration are applied to 

 Avena coleoptile stumps immediately after the (first) decap- 

 itation, small positive curvatures may appear, but these 

 change to negative curvatures after about 1 hour (u). 

 Very high auxin concentrations will retard coleoptile growth 

 (Bonner, 1933) but not under the conditions of the Avena 

 test. Under ordinary test conditions the above cases are ex- 

 ceptions, and we may conclude that all substances active in 

 the Avena test have growth-promoting effect. The growth- 

 inhibiting action of auxins on roots and buds is a phenomenon 

 of quite another type. 



C. 7. The Maximum Angle 



It will be seen that above a certain concentration the 

 curvatures obtained in 90-110 minutes do not increase at 

 all. This Hmiting curvature or ''maximum angle" varies 

 between 15° and 35°, according to experimental conditions. 

 The zone of transition between the two arms of the curve of 

 Figure 19 is exceedingly short. This type of curve indicates 

 that two independent factors are here limiting the growth, 

 providing an excellent example of Blackman's ''limiting 

 factors" (1905). On the left-hand arm the limiting factor 



