286 CHAP. XXVI. GEOTROPISM 



Experiment 156. Geotropic response of petiole of 

 Tropaeoliim. — I expected to obtain a shorter latent period 

 by choosing thinner specimens with less weight. I therefore 

 took a cut specimen of the petiole of Tropaeolum and held 

 it at one end. The lamina was also cut off in order to 

 reduce the considerable leverage exerted by it. The 

 response did not now exhibit any preliminary down-move- 

 ment, the geotropic up-movement being initiated within 

 a few seconds after placing the petiole in a horizontal 

 position (fig. 167). The successive dots in the record are 

 at intervals of 20 seconds and the second dot already ex- 

 hibited an up-movement ; the latent period is therefore 

 shorter than 20 seconds. It has been thought hitherto 

 that the latent period for geotropic reaction is a matter of 

 many minutes ; this very high value must have been due 

 to the crude methods employed for the determination. 



The Complete Geotropic Curve 



The characteristics of the geotropic curve are similar 

 to those of other tropic curves. That is to say, the sus- 

 ceptibility for excitation is at first feeble ; it then increases 

 at a rapid rate ; in the third stage the rate becomes uniform ; 

 and finally the curvature attains a maximum value and the 

 organ reaches the state of geotropic equilibrium. The period 

 of completion of the curve varies in different specimens 

 from one to several hours. 



Experiment 157. — The following record was obtained 

 with a bud of Crinum, the successive dots being at intervals 

 of 10 minutes. After overcoming the effect of weight 

 (which took an hour), the curve rose at first slowly, then 

 rapidly. The period of uniformity of movement is seen to 

 have been attained in this case after 3 hours and continued 

 for nearly 90 minutes. The final equilibrium was reached 

 after a period of 8 hours (fig. 168 V 



For studying the effect of an external agent on geo- 

 tropic reaction, the period of uniform movement is the most 



