TECHNIQUE OF AUXIN DETERMINATIONS 47 



as already mentioned, consists of the new production of 

 auxin from the uppermost cells of the stump. No observ- 

 able morphological change is associated with it. At 25° 

 the time between decapitation and the beginning of re- 

 generation is close to 150 minutes, as has been shown di- 

 rectly by straight growth measurements (see Figure 7). As 

 stated above, the presence of a block of agar delays the 

 onset of regeneration considerably, so that plants bearing 

 a block of agar on one side will, 150 minutes after decap- 

 itation, regenerate more auxin upon the opposite side and 

 hence will tend to curve towards the block. In Avena curva- 

 ture tests, therefore, when auxin is in the block, this one- 

 sided regeneration will have the effect of tending to reduce 

 the auxin curvature. 



To avoid the effects of the factors described under 2 and 

 3, which operate to reduce the auxin curvatures obtained, 

 it is an advantage to decapitate the plants shortly before 

 application of the block. This is the reason for the second 

 decapitation 3 hours after the first, recommended above. 



When very high concentrations of auxin, such as 0.01 mg. 

 indole-acetic acid per cc. are used, the curvature obtained 

 is less than the maximum angle (?^). Figure 19 (p. 41) shows 

 the broad range of concentrations over which the maximum 

 angle is approximately constant, and its decrease at very 

 high concentrations. To explain this we have only to con- 

 sider that if auxin sufficient to cause maximal growth on 

 the far side reaches that side by leakage, especially across 

 the cut surface (Laibach and Kornmann, 1933a), no curva- 

 ture can result. This can only occur if the concentration 

 of auxin applied is very high. Evidently, also, all inter- 

 mediate stages between the normal maximum angle and 

 no curvature at all can result from suitable auxin concen- 

 trations. It is a characteristic of such "supramaximal" 

 angles that the individual curvatures are extremely irregular, 

 as can be seen from the photograph (Figure 22). This 

 irregularity is doubtless due to variations in the moisture 

 film at the cut surface. 



