THE POSITIVE RESPONSE 179 



a record of the positive response by means of a writing-lever. 

 The weight of the lever stands in the way of obtaining any 

 high magnification, especially in the case of leaflets, where 

 the force of responsive expansion or contraction is very 

 slight. By the employment of an extremely light recording- 

 lever made of aluminium, I was able to secure a magnifi- 

 cation of twenty times, which was the highest that could be 

 obtained in the circumstances. With very good specimens 

 this magnification is often sufficient to exhibit the positive 

 effect. In order to reduce friction and obtain time-records, 

 the recording-plate was made to oscillate to and fro once 

 in a second or once in 2 seconds. The successive dots 

 in the record thus indicate intervals of i or 2 seconds. 

 It has sometimes been possible to employ the Resonant 

 Recorder with long writing-index for recording the positive 

 response of Mimosa. The vibration-frequency of the writer 

 in such a case was five times in a second. 



Having thus secured two different methods of observa- 

 tion by means of optical and recording levers, I proceed to 

 demonstrate : — 



(i) That whatever may be the form of stimulation em- 

 ployed, two impulses are transmitted, of which the positive 

 travels with a higher velocity than the negative. 



(2) That such double impulses are exhibited not by any 

 particular plant but by various species of plants. The 

 phenomenon may therefore be regarded as universal. 



Dual Impulses under Different Stimuli 



The positive effect, as previously stated, may be separated 

 from the negative by taking advantage of the different rates 

 of transmission of the two impulses. The lag of one impulse 

 behind the other may be increased (i) by taking a specimen 

 in which the velocity of transmission of excitation is low, 

 and (2) by the appHcation of the stimulus at a distance 

 from the responding organ. It should be remembered 

 that the velocity of transmission is not only different in 



N 2 



