372 LIFE MOVEMENTS IN PLANTS 



of the hypocotyl, (2) the positive curvature under direct 

 stimulation cannot proceed very far, since it is neutralised 

 by transverse conduction of excitation. 



It will be seen from the above that the illumination 

 of the tip practically inhibits the neutralisation and thus 

 restores the normal positive curvature. The question now 

 arises as to how this particular inhibition is brought 

 about. 



ALGEBRAICAL SUMMATION OF THE EFFECTS OF DIRECT 

 AND INDIRECT STIMULATIONS. 



An instance of inhibition, though of a different kind, 

 was noticed in the response of the tendril of Passijiora 

 (p. 296) ; the under side of the organ is highly sensitive, 

 while the upper side is almost insensitive. Stimulation 

 of the under side of the tendril induces a marked cur- 

 vature, but simultaneous stimulation of the diametrically 

 opposite side inhibits the response. This neutralisation 

 could not be due to the antagonistic contraction of the 

 upper side since the irritability of that side is very slight. 

 1 have shown that the inhibition results from the two 

 antagonistic reactions, contraction at the proximal side due 

 to direct stimulation and expansion caused by the positive 

 impulse from the indirectly stimulated distal side. 



"We have in the above an algebraical summation of the 

 effects of direct and indirect stimulations. The longitu- 

 dinally transmitted effect of indirect stimulus in Setaria 

 may, likewise, be summated with the effect of direct sti- 

 mulus. The phenomenon of algebraical summation is 

 demonstrated in a very striking and convincing manner 

 in the following experiment, which I have been successful 

 in devising. 



