50 



PHYTOHORMONES 



curves A and C, but since the angles are smaller on Curve C, 

 the percentage variation is greater. Hence the long period 

 between first decapitation and application of the block not 

 only increases the sensitivity, but also greatly reduces the 

 relative variabihty, so troublesome in Kogl's experiments. 

 It may also be noted that even the greatest variations ob- 

 tained at Pasadena are much less than those observed at 



9AM 12 15 18 21 24 3 6 9 12 15 



Fig. 23. Diurnal variation in sensitivity to auxin. Plants of uniform age 

 (77 hours from soaking) tested every 3 hours from 9 a.m., April 23, to 3 p.m., 

 April 24. Abscissa, time at beginning of test; ordinate, curvature. Curves A 

 and B, 3 decapitations at 2-hour intervals; curve C, 2 decapitations 40 min- 

 utes apart. Indole-3-acetic acid for curves A and C, 0.13 mg. per liter; for 

 curve B, 0.26 mg. per liter. 



Utrecht, so that the variability is evidently different in dif- 

 ferent places. Kogl (1933) concluded that the variability 

 is related to variations in the electrical field and ionization 

 of the air. However, there are reasons for believing that 

 other factors, such as light, are also involved (Kogl, Haagen 

 Smit, and van Hulssen, 1936). While the ultimate cause of 

 the variation is thus not known, it is probable that the 

 immediate cause is variation in the auxin content of the 



plants. 



Since the pure auxins have become available, the disad- 



