122 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



are not due to any particular mode of stimulation but to 

 stimulation in general, I carried out the following additional 

 experiments. Two fine pin-electrodes were inserted 5 mm. 

 apart into the stem of Mimosa exactly opposite to the 

 particular responding leaf. After a suitable period allow- 

 ing for recovery from mechanical irritation, a tetanising 



Fig. 65. Effect of transverse transmission of impulse under 

 photic stimulation. Preliminary erectile response due to 

 positive impulse (down-curve), followed by fall of leaf (up- 

 curve) due to transverse conduction'of excitation. (Mimosa.) 



electric current of moderate intensity was passed through 

 the electrodes. The responsive effects on the distal side 

 of the stem were precisely similar to those induced under 

 unilateral photic stimulation — that is to say, the first effect 

 was an erectile movement of the leaf brought about by 

 the positive hydraulic impulse ; the excitatory negative 

 impulse then reached the distal side after an interval of 

 31 seconds and caused a fall of the leaf (fig. 66). The 

 velocity of conduction of excitation was o-o6 mm. per 

 second. 



