452 LIFE MOVEMENTS IN PLANTS 



for perceptible displacement of the statoliths should be very 

 short, about a second or so, and the latent period of 

 perception of stimulus should be of this order. 



The mechanical indication of response to stimulus 

 is delayed by a period which is somewhat indefinite ; for 

 the initiation of responsive growth variation will necessarily 

 lag behind the perception of stimulus. 



Experiment 173. — The mechanical response with its draw- 

 backs is thus incapable of giving an accurate value of the 

 latent period. The electrical method of investigation labours 

 under no such disadvantage, since the excitation is here 

 detected even in the absence of movement. The perception 

 of stimulus will thus be followed by response without 

 undue delay. I shall in this connection give a record of 

 electric response of the quickly reacting petiole of Tropceo- 

 lum, wlien the angle of inclination is increased from zero 



Fig. ir>8.— Geo-electiic response of the petiole of Tropceohim. 



to 90^. The responsive movement of the galvanometer 

 spot of light was initiated in less than 5 seconds and 

 the maximum deflection was reached in the course of 90 

 seconds. The angle was next reduced to zero, and the 



