240 PLANT RESPONSE 



marks on the revolving drum — one when the discharge-key 

 is pressed, at the moment of application of stimulus, and 

 another when the spot begins to move, that is to say, at the 

 commencement of response. It is then easy, knowing the 

 rate of movement of the drum and the distance between the 

 two marks, to determine the exact time-interval between 

 the two. 



There then remains only the question of allowing for the 

 loss of time due to the latent period of the responding organ. 

 This is accomplished by means of a separate experiment, in 

 which the stimulus is directly applied at the base of the 

 motile organ. The latent period thus ascertained is sub- 

 tracted from the time-interval already determined, and we 

 have thus the true time of transmission of excitation through 

 the given distance ; from this the velocity, or rate of trans- 

 mission per second, may be deduced. 



Exact determination of velocity.-— I now give an 

 account of an actual experiment for the determination of 

 the velocity of transmission of excitation in the petiole of 

 Biophytum. The two points A and B are connected in the 

 circuit of a condenser through the usual non-polarisable 

 electrodes (fig. 14). An indicating leaflet, L, is attached to 

 the Optic Lever, by which the exact moment of its response 

 may be recorded on the revolving drum. If now we make B 

 kathode, during the charge of condenser, an excitatory wave 

 will start from B and travel inwards towards L, in this par- 

 ticular case in a centripetal direction, i.e. towards the main 

 stem. A mark is made, as already explained, on the re- 

 volving drum, at the exact moment when the tapping-key 

 excites the plant. In this case, the capacity of the condenser 

 was *oi microfarad, and E.M.F. twelve volts. The time of 

 the stimulus reaching L, as indicated by the movement of the 

 spot of light, is also marked, as explained, on the revolving 

 drum. As has been said before, the time-interval is accu- 

 rately determined from the speed of the revolving drum. As 

 an additional precaution, the same time-interval is taken by 

 means of a stop-watch. 



