28o RESEARCHES ON IRRITABILITY OF PLANTS 



to devise some arrangement by which the recording-plate 

 could be maintained in a state of oscillation. The recording- 

 plate was consequently made as light as possible, using the 

 glass plate employed for covering magic -lantern slides ; the 

 carrying-frame was made of aluminium. By means of an 

 electric motor and an eccentric device, the plate-carrier 

 was made to oscillate to and fro, the frequency of oscillation 

 being regulated by an adjustment of the electrical current 



(fig. 131). 



The plate-carrier has small wheels which run between 

 horizontal rails, above and below. The recording-plate 

 thus travels in a horizontal instead of vertical direction. 

 There is also a knock-over key or trigger arrangement, 

 not shown in the figure. During a particular part of the 

 travel of the plate the trigger is released, causing a single 

 break-induction-shock to pass through the plant. The 

 moment of stimulation is marked in the usual manner on the 

 travelling-plate, and the number of dots intervening between 

 this mark and the beginning of response enables us to deter- 

 mine the latent period when the stimulation is direct, or 

 the velocity of transmission when the stimulation is indirect. 

 In the latter case it is of course necessary to know the 

 distance between the point of application of stimulus on 

 the midrib and the responding leaflet. 



For the determination of the latent period, and of the 

 velocity of transmission, the oscillation-frequency of the 

 plate should be about 10 times in a second. But for the 

 mere obtaining of response-records the frequency of oscilla- 

 tion need not be high. 



Determination of the Latent Period 



Taking a record of Biophytum on a fast-moving plate, 

 with a recorder having a frequency of oscillation of 10 times 

 a second, it is seen (fig. 132) that there are four spaces between 

 the incidence of stimulus and the initiation of response ; it 

 would thus appear that the latent period of the leaflet is -4 of 



