Il8 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



travels considerably faster than the nervous, and is not 

 dependent upon the conducting power of the tissues in 

 which it travels. 



There are thus two distinct impulses initiated by stimu- 

 lation, the universaHty of which is demonstrated : 



1. By the identical effects induced by modes of stimu- 



lation so diverse as thermal, electric, and photic ; 



2. By the longitudinal and the transverse transmission 



of two impulses generated by stimulation ; 



3. By the opposite reactions induced in growing organs 



by the two impulses ; and 



4. By the electric detection of the two impulses. 



It is necessary to explain here that it is not the stimulus 

 but its effect that is transmitted to a distance. The 

 phrase transmission of stimulus has, however, come into 

 general use, and will be occasionally employed in the place 

 of the more correct phrase, transmission of impulse. 



These points are well illustrated in the following experi- 

 ments. Compared with that of the petiole, the conducting 

 power of the stem of Mimosa is relatively feeble, as evidenced 

 by the fact that the velocity of transmission of excitation 

 is only 2 mm. as against 30 mm. per second in the petiole. 

 The semi-conductivity of the stem makes it easier to obtain 

 records of the effects of the two impulses. 



Experiment 68. Longitudinal transmission of impulse 

 under radio-thermal stimulation. — The method of experi- 

 mentation is illustrated in fig. 63. The distance of the 

 point of application of stimulus on the stem from the 

 responding leaf was, in successive experiments with an 

 identical specimen, reduced from S to Si and finally to Sg. 

 The application of stimulus at a point opposite to the 

 indicating leaf is represented by S3. The radio- thermal 

 stimulus of moderate intensity was kept constant, the 

 intervening distance in successive experiments being re- 

 duced from 80 mm. to 30 mm., and finally to 10 mm. 

 The record was taken on a fast-moving plate, the successive 



