436 LIFE MOVEMENTS IN PLANTS 



gravity impinge) that undergoes excitation. The vertical 

 lines of gravity must therefore be the direction of incident 

 stimulus. This conclusion is supported by results of three 

 independent lines of inquiry : (1) the algebraical summa- 

 tion of effect with that of a different, stimulus whose 

 direction is known, (2) the relation between the directive 

 angle and geotropic reaction, and (H) the torsiotuil response 

 under geotropic stimulus. 



EFFECT OF ALGEBRAICAL SUMMATION. 



Experiment 167. — A flower bud of Grinmn is laid 

 horizontally, and record taken of its geotropic move- 

 ment. On application of light on the upper side at L, 



c:^ 



Fig. 16-J. Fig. 163. 



Fig. 162. — Stimulu? of light or gravity, represented bj' arrow, induces up cur- 

 vature aa .seen in dotted figure. 



Fig. 163. — The effect of super-imposition of photic stimulus. The first, third, and 

 fifth parts of the curve, give normal record under geotropic stimulus. Rate, of 

 up-movement enhanced under light L. 



the responsive movement is enhanced, proving that 

 gravity and light are inducing similar etfects. On the 



