GEOTROPIC STIMULATION 403 



immediate and the rate was reduced from 14 to 3 in the 

 course of 8 minutes. On the cessation of exposure to Hght, 

 recovery was found to be complete after an interval of half 

 an hour. A second specimen gave very similar results. 



Experiment 223. Effect of indirect stimulation by light. — 

 In Thunbergia the normal rate of 20 was enhanced to 96 

 by indirect stimulation by light applied for I minute. 

 The recovery was complete in the course of half an hour. 

 In Porana, similarly, indirect stimulation by light enhanced 

 the rate from 8 to 38. The effect of indirect stimulation 

 by light is similar to that of indirect electric stimulation. 



Effect of Geotropic Stimulation 



In order to obtain a satisfactory explanation of the 

 effect of geotropic stimulation on autonomous torsion, it is 

 necessary to obtain a clear idea of the means by which that 

 stimulation is effected. The experiments that have been 

 described in previous chapters fully confirm the theory 

 that it is the statolithic particles in the statocysts which 

 cause geotropic stimulation {see p. 342). 



I have already shown that the apical ends of the stato- 

 cysts are more sensitive than the basal ends (p. 356). In 

 that case, a maximum variation in the geotropic action 

 should occur in two positions, erect and inverted — i.e. at 

 inclinations of 0° and 180°. The question now arises 

 whether this would be confirmed by observations made on 

 an organ performing torsional growth. 



Since in such organs the effect of more intense stimula- 

 tion is manifested by a greater retardation of the rate of 

 torsion (which may culminate in reversal), it is quite possible 

 to observe the effect of geotropic stimulation in the two 

 positions, erect and inverted. 



Experiment 224. — The mode of procedure is as follows : 

 The rate of torsion is observed by the optical method, first 

 when the stem is erect, and afterwards when it is upside 

 down. It is to be understood that the observer is looking 

 at the rotating apex of the stem from above in the erect, 



