TROPISMS 169 



effect, phototropic curvatures continue to increase for 6 hours 

 or so after illumination (see X H) . 



The above considerations hold only for auxin redistribu- 

 tion in the tip. Since the extreme tip is sohd {cf. Figure 6, 

 p. 23), it is clear that auxin redistribution can take place 

 much more readily in it than in the lower, hollow zones. 

 Thus, it was shown by Sierp and Seybold (1926) and by 

 Lange (1927) that the minimum amount of light necessary 

 to produce curvature is, if the upper 0.2 mm. are illumi- 

 nated, 20 MCS; in the 0.2 mm. zone 0.8-1.0 mm. from the 

 tip, 500 MCS; while if a 0.2 mm. zone 2 mm. below the tip 

 is illuminated 20,000 MCS are required. We can now explain 

 the experiment of Brauner (1922), mentioned above, in 

 which unilluminated tips, replaced on pre\dously illumi- 

 nated stumps, caused a curvature. This result is due to 

 the persistence of the auxin-redistributing system after uni- 

 lateral illumination has ceased. The fact that the illumi- 

 nated stumps not provided with tips curved scarcely at all 

 is due to the necessity of auxin for growth (and curvature). 

 Instead of the tip a block of agar containing auxin is equally 

 effective in allowing curvatures to take place in the stump 

 (Seubert, 1925; Boysen Jensen, 1933; Reinders, 1934). The 

 effect of the decapitation itself in temporarily inhibiting 

 lateral transport (Nuernbergk, 1933) results, however, in a 

 delay in the onset of such curvatures. 



The fact that phototropic curvature can only occur if 

 sufficient free-mo\4ng auxin is present {i.e. curvature is 

 dependent upon growth) is brought out very effectively by 

 the work of Tsi Tsung Li (1934). A large number of coleop- 

 tiles were decapitated and then illuminated unilaterally at 

 different time intervals after decapitation. With very short 

 time intervals between decapitation and illumination they 

 still gave a slight curvature, due doubtless to redistribution 

 of the free-mo\'ing auxin still present, but afterwards the 

 ability to curve became extremely small, i.e. the ''free" or 

 redistributable auxin (see VIII F) almost disappears. Fi- 

 nally, as soon as auxin was regenerated the curvatures 



