3o8 CHAP, xxvni. geotropic torsion 



on restoration of the pulvinus to the normal position there 

 is a recovery shown by the down-curve. 



Experiment i6g. Anti-clockwise torsion in b position. — 

 The experiment was repeated with this difference, that the 

 opposite flank was now exposed to the vertical lines of force 

 of gravity. The record (fig. 184) shows by the down-curve 

 the anti-clockwise movement, and subsequent recovery on 

 removal of stimulation. 



It has already been explained that the direction of 

 incident stimulus can be found from the responsive torsion 

 by which the less excitable side of the organ is made to face 

 the stimulus. The results of experiments just described 

 prove conclusively that the direction of geotropic stimulus 

 must be the vertical lines of force of gravity — a conclusion 

 which is of great theoretical importance. 



Algebraical Summation of Geotropic and 

 Phototropic Effects 



Experiment 170.— If the direction of the incident geo- 

 tropic stimulus is vertical, and should the leaf be in the 

 a position, then the stimulus of Hght acting from above 

 should enhance the previous torsional response due to 

 geotropism. In the above case the directions of the lines of 

 gravity and of the rays of light coincide. The effect of rays 

 of light acting from below should, on the other hand, oppose 

 that of gravity. The additive effect of stimulation by both 

 light and gravity is seen in the record (fig. 185). The first 

 part of the curve is the record of pure geotropic torsional 

 movement. Light from above is applied at L ; the rate of 

 movement is seen to have become greatly accelerated. On 

 cutting off the light the enhanced rate induced by it is found 

 to disappear, the response curve being now solely that of 

 geotropic reaction. The effect of phototropism in opposi- 

 tion to geotropism is demonstrated by the following experi- 

 ments, where the opposing action of light of different 



