166 PHYTOHORMONES 



moved inside the plant by electric potentials, no direct 

 evidence to prove this has ever been brought forward. 

 In the experiments of Koch (1934) it appears that auxin 

 is moved within Helianthus hypocotyls by the application 

 of 10-100 mv. externally, but this deduction is founded 

 only upon curvatures and not on direct auxin determina- 

 tions. It is possible that these curvatures away from the 

 + pole are due to the production of acid around this pole 

 which would set free auxin (see VIII i^). As has been indi- 

 cated on p. 103 the evidence that electric potentials play a 

 part in auxin transport is as yet inconclusive. 



III. PHOTOTROPISM 

 F. Auxin REDisTmsuTiON 



We have seen that in geotropism the Cholodny-Went 

 theory quantitatively explains the observed facts. In photo- 

 tropism, however, the situation is far more complicated, 

 and it will therefore be desirable to differentiate clearly 

 between the auxin distribution effect and the other phe- 

 nomena. We shall therefore give up the historical order of 

 treatment and consider first the simple auxin redistribution 

 effect and afterwards the many complicating factors. 



As a beginning let us consider the Avena coleoptile. In 

 the first place, the sensitivity of the extreme tip (the upper- 

 most 0.25 mm.) to hght is about 1000 times as great as 

 that of the lov/er zones (Sierp and Seybold, 1926; Lange, 

 1927). In the second place the curvature varies periodically 

 with the amount of light (see curve. Figure 49). Small 

 illuminations up to 4000 MCS ^ produce the so-called "first 

 positive curvature"; the maximum curvature is reached at 

 about 200 MCS. Above this and up to about 40,000 MCS 

 the plant curves away from the light — ''first negative curva- 

 ture." With still more Hght there occurs a second and then 

 a third positive curvature separated by a zone of indiffer- 

 ence or of slight negative curvature. The third positive 



1 MCS = meter-candle seconds, 25 meter-candles at 4360 A.U. being equal 

 to 1 erg/cm^. 



