43^ COMPARATIVE ELECTRO-PHYSIOLOGY 



it is not till after the lapse of a period, which is sometimes 

 as much as an hour in duration, that there is a reversal of this 

 downward movement into the normal apo-geotropic move- 

 ment. Fig. 268 gives a curve which exhibits this prelimi- 

 nary movement persisting for nearly forty minutes before its 

 ultimate reversal into the usual normal apogeotropic curva- 

 ture upwards. 



This delay in the appearance of the characteristic response 

 may, however, be explained from the known fact that steady 

 tension increases, whereas compression retards, the rate of 

 growth. Thus, in a horizontally-laid shoot, there is a ten- 

 dency to curve down by its own weight, the upper side being 



FIG. 268. Record of Apogeotropic Response in Scape of Uriclis Lily' 



The up curvature due to apogeotropic action proper commenced forty 

 minutes after the specimen was laid horizontally. 



in this way subjected to tension, and the lower to compression. 

 The effect of these is an increased rate of growth on the 

 upper, associated with a decreased rate on the lower, sides of 

 the specimen, giving rise to a downward curvature. The fact 

 that geotropic stimulus has to overcome this action before its 

 own characteristic effect can be exhibited may account for 

 the observed delay in its appearance. A crucial experiment 

 in support of this explanation will be given presently. 



We have seen in the last chapter that the presence of 

 internal excitation is capable of detection by electrical indi- 

 cations, and by taking advantage of this fact we have an 

 independent means of coping with the obscurities of the 

 present problem. I have demonstrated, by means of experi- 

 ments already referred to, that it is the upper side which, 



