374 LIFE MOVEMENTS IN PLANTS 



reached the responding pulvinus after 30 seconds and 

 induced an erectile movement of the leaf ; the excitatory 

 negative impulse reached the organ 4 minutes later and 

 caused a rapid fall of the leaf. The record (Fig. 138) 

 shows further that the previous action of direct stimulus 

 which brought about neutralisation, does not interfere with 

 the effects of indirect stimulus. The individual effects of 

 direct and indirect stimulus are practically independent 

 of each other; hence their joint effects exhibit algebraical 

 summation. 



We are now in a position to have a complete under- 

 standing of the characteristic response of Paniceae to 

 transmitted phototropic excitation. 



(1) Local stimulation of the tip gives rise to two im- 

 pulses, positive and negative. The former induces a transient 

 negative movement (away from light) ; the latter causes 

 a permanent and increasing positive curvature towards 

 light. 



(2) Local stimulation of the growing hypocotyl gives 

 rise to positive curvature, subsequently neutralised by the 

 transverse conduction of excitation to the distal side. The 

 absence of tropic effect in the growing region is thus due 

 not to lack of power of perception, but to balanced anta- 

 gonistic reactions of two opposite sides of the organ. 



(3) The effects of direct and indirect stimulations are 

 independent of each other ; hence, on simultaneous stimula- 

 tions of the tip and the growing hypocotyl, the effects of 

 indirect stimulus are algebraically summated with the 

 effect of direct stimulus (neutralisation). The indirect 

 stimulation of the tip on the right side gives rise to 

 two impulses, of which the expansive positive reached 

 the right side of the responding region earlier, inducing 



