178 RESEARCHES ON IRRITABILITY OF PLANTS 



which the velocity of transmission of excitation is relatively 

 slow. 



(3) As the velocity of transmission of true excitation is 

 greater under stronger stimulus, a feeble stimulation should 

 be employed for the exhibition of positive effect. 



The above are the results obtained from the electric 

 mode of investigation. We have seen that the electric 

 variation of galvanometric negativity corresponds to the 

 excitatory reaction of negative turgidity variation, con- 

 traction, and concomitant motile-response by the fall of the 

 leaf. The positive electric response should connote effects 

 the very reverse of these — ^positive turgidity variation, 

 expansion, and concomitant motile-response of erection of 

 the leaf. Hence if in consequence of stimulation two 

 distinct impulses are sent out with different velocities, we 

 should be able to demonstrate their existence in an alto- 

 gether different manner — namely, by means of two distinct 

 mechanical responses of erection and of fall of the leaf. 



The difficulty that confronts us in this demonstration 

 lies in the relatively small amplitude of the positive response, 

 which is liable to pass unnoticed unless a high magnification 

 be employed. It is easier to demonstrate the existence of 

 the positive impulse by the employment of a magnifying 

 optical lever. A very light mirror is fixed to the fulcrum 

 rod of a lever, one arm of which is attached to the motile 

 leaf or leaflet. Under excitatory fall there is produced 

 a rotation of the fulcrum rod with its attached mirror ; 

 a spot of Hght reflected from the mirror thus exhibits a 

 responsive down-movement, indicative of normal negative 

 response by contraction. An erectile positive response, on 

 the other hand, is recognised by the movement of the spot 

 of Hght upwards. Responsive movement may in this 

 manner be magnified from a hundred to a thousand times. 

 The optical method is simple and efficient. It is also 

 well suited for purposes of demonstration before a large 

 audience. 



Great difficulties are, however, encountered in obtaining 



