664 PLANT RESPONSE 



in the two halves, by the reduction of excitability of the 

 upper, is virtually a plagiotropic or dorsi-ventral organ, and 

 we can thus see why a true plagiotropic organ, laid hori- 

 zontally, has a tendency to remain in that position. It 

 would remain in that position absolutely if the excitability 

 of the outer tangential wall of the lower side were actually 

 zero, and the general excitability of the upper half of the 

 dorsi-ventral organ had also completely vanished ; but if 

 these two values were not both zero, various effects would 

 occur, according to the relative differential excitabilities of 

 the two halves. 



I shall now describe the line of investigation by which 

 it is possible to demonstrate the fact that the two halves of 

 an anisotropic organ possess different excitabilities with 

 regard to geotropic stimulus ; but before entering upon this, 

 it is necessary to obtain a clear idea of the direction of 

 response, in relation to direction of the force which causes 

 stimulation of gravity. In the case of an apogeotropic stem, 

 laid horizontally, we have vertical lines of force striking the 

 organ from above, and the curvature induced makes it turn 

 upwards to meet, as it were, these lines of force. We have 

 here an example which is analogous to the response of the 

 organ to the rays of light, in which the responsive move- 

 ment consists in a similar curvature upwards to meet them. 



The fact that the upper and lower halves of an aniso- 

 tropic organ are unequally excited by light has just been 

 demonstrated by means of torsional response, where it was 

 shown that lateral excitation induced a twisting movement 

 by which the less excitable was made to face the incident rays. 

 By a similar torsional response of the anisotropic organ we 

 are able not only to demonstrate the unequal geotropic 

 excitability of the upper and lower halves, but also, by 

 noting which of the two is made to face the lines of gravita- 

 tional force, to determine which it is that is the less excitable 

 to this particular form of stimulus. 



Torsional response to lateral geotropic stimulation. — 

 For this experiment I took a leaf of Erythrina i/tdica, whose 



