The Dia-Heliotropic Attitude of Leaves. 171 



now diphasic, since excitation reached the two points in succession (fig. 9). 

 Discontinuity of the nerve stops the transmitted impulse, as will be seen 

 below. 



Fig. 9. — Galvanometric record of transmitted excitation in the nerve of Helianthus. 

 The first is in response to electric stimulus, the second and the third to thermal and 

 chemical stimulus. Note in these multiple responses due to strong stimulation. 

 The fourth exhibits diphasic response (see text). 



9. The Directive Action of Propagated Impulse in Heliotropic Leaf -adjustment. 



In Mimosa and in Helianthus I have traced the nervous channels from 

 the receptor to the effector, and showed how the nervous impulse is 

 propagated along definite channels. The most difficult problem that 

 confronts us now is to explain the responsive movement and torsion of the 

 motor organ, by which the expanded leaf surface faces the light. I shall now 

 describe the motor reaction when different parts of the leaf surface are locally 

 stimulated, not only by light, but by diverse modes of stimulation, 

 (a) Mimosa pudica. 



Experiment 6. — For this experiment I first took specimens of Mimosa leaf 

 having two sub-petioles. The right sub-petiole was stimulated by feeble 

 tetanising electric shock. The response was by right-handed torsion. The 

 latent period was 2 seconds, and the torsional movement continued for 

 20 seconds, even on cessation of the stimulus, after which there was a 

 slow recovery, not shown in the record (fig. 10, a). The propagated impulse 

 has thus followed its definite path, and reached the right flank of the 

 pulvinus or the right-effector. We saw that the characteristic response 

 of this particular effector is by a right-handed torsion. Thus the same 

 response takes place, whether the effector is directly stimulated or by 

 transmitted excitation. This finds strongest confirmation from the following 

 experiment, where the responsive movement is made to undergo reversal. 



