666 



PLANT RESPONSE 



vinus — artificially rendered the less excitable of the two — 

 that was found turning upwards, to face the rays of incident 

 force of gravity. 



The converse of this experiment would consist in still 

 further reducing the excitability of the upper half of the 

 pulvinus, in which case the normal differential excitability 

 of the two halves would be increased, and the geotropic 

 response enhanced. The right-hand record in fig. 271 shows 



that this is what actually 

 occurs. The first part of 

 the record displays the 

 normal torsional response, 

 and that which follows, 

 after the local application 

 of chloroform on the 

 upper surface — indicated 

 by the arrow from above 

 — exhibits, by its in- 

 creased steepness, the en- 

 hanced character of this 

 response. 



We have thus seen, 

 by means of torsional re- 

 sponse, that the differ- 

 ential character of the 

 excitability of the two 

 halves of an anisotropic 

 organ under the stimulus 

 of light is in every way paralleled by their differential ex- 

 citability to the stimulus of gravity. The geotropic response 

 of the organ, as a whole, is neither simple positive nor 

 negative, but may be more fittingly described as differential. 

 And as we saw with regard to photic stimulus that there was 

 no need for the assumption of any specific dia-heliotropic 

 sensibility in plagiotropic stems or in leaves — their move- 

 ments being fully accounted for by the mechanical con- 

 siderations arising out of their differential excitability to 



Fig. 271. Records showing Torsional 

 Response to Geotropic Stimulus and 

 Induced Modifications in Terminal Leaf- 

 let of Erythrina indica 



In the figure to the left is seen the normal 

 torsion under lateral geotropic action, 

 and its reversal when the differential 

 excitability of the organ is reversed by 

 the application of chloroform to the 

 lower half (f ). In the figure to the 

 right is seen the enhancement of the 

 torsional response when the natural 

 differential excitability is increased by 

 the application of chloroform to the 

 upper half of the organ (|). 



