characteristics 32 1 



Characteristics of Geo-Electric Response 



There are certain phenomena connected with the electric 

 response under geotropic stimulation which appear to be 

 highly significant. According to the statolithic theory : 



' Geotropic response begins as soon as an organ is 

 deflected from its stable position, so that a few starch-grains 

 press upon the ectoplasts occupying the walls which are 

 underneath in the new position ; an actual rearrangement 

 of the starch-grains is therefore not an essential condition 

 of stimulation. As a matter of fact, the starch-grains 

 do very soon migrate on to the physically lower walls, when 

 a positively or negatively geotropic organ is placed hori- 

 zontally, with the result that the intensity of stimulation 

 gradually increases, attaining its maximum value when all 

 the falling starch-grains have moved on to the lower region 

 of the ectoplast. The time required for the complete 

 rearrangement of the statoliths may be termed the period 

 of migration ; its average length varies from five to twenty 

 minutes in different organs.' ^ 



Stimulation, according to the statolithic theory, is 

 induced by the displacement of solid particles. The dia- 

 meter of the geotropically sensitive cells is considerably 

 less than o-i mm. ; and the stimulus will be perceived 

 after the very short interval taken by the statohths to fall 

 through a space shorter than -i mm. This may be some- 

 what delayed by the viscous nature of the plasma, but in 

 any case the period for perceptible displacement of the 

 statoliths should be very short, about a few^ seconds, and 

 the latent period of perception of stimulation 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 incidence of stimulus. 



Experiment 177. — The mechanical response is thus 



1 Haberlandt, Physiological Plant Anatomy (1914), p. 598. 



Y 



