472 EFFECT OF COMPLEX AGENTS ON GROWTH [Ch. XIX 



The phototropic limiting angle is constant for all cases in 

 which gravity and light act at a right angle, whether the plant 

 axis be originally placed vertically upright, inverted, or hori- 

 zontal. When the direction of the light ray makes some other 

 angle with that of gravity, the resulting position of the plant 

 may be quite different, as Czapek ('95) has shown. 



If the light falls upon the plant vertically from heloiv, in such 

 a way therefore as directly to oppose gravity, the resulting 

 position of the plant varies according to its original position, in 

 a way generally explicable on the ground that one of the two 

 tropic influences has come to prevail, but ^^'ith diminished 

 power. Thus, when the axis of the seedling is horizontal it 

 will, if like Avena it is very sensitive to light, turn its tip 

 slightly upwards for a time, and then definitely down. A 

 slightly phototropic plant, like Helianthus, assumes its limiting 

 angle of 45° from the zenith. When the seedling is exactly 

 inverted, plants highly sensitive to light grow down, but 

 Helianthus places itself at 135° from the zenith, hence no longer 

 at the limiting angle. When the axis of Helianthus is placed 

 obliquely downwards its tip becomes inclined at 45° from the 

 zenith. 



If the ray of light falls upon the plant obliquely from below, 

 the result on the erect, horizontal, or inverted seedling is a 

 response to the active component of the light ray. The erect 

 seedling is affected in the same way as by the horizontal ray; 

 the horizontal seedling responds as though the light came verti- 

 cally from below, and all inverted plants place themselves in 

 the axis of the infalling ray. 



If the ray of light falls obliquely from above, all plants, even 

 Helianthus, place themselves in the axis of the ray. This is 

 what might have been anticipated, since gravity affects the 

 inclined plant much less than the horizontal one,, and so the 

 active component of the light ray is relatively more effective 

 in this oblique position than in the horizontal one. 



The foregoing experiments show that, in general, the geo- 

 tropic reaction is modified by the simultaneous action of light. 

 I have hitherto assumed that the final position of the plant is 

 the resultant of the action of two tropic agents ; that the pho- 

 totropic and geotropio reactions interlock. Another cause of 



