320 RESEARCHES ON IRRITABILITY OF PLANTS 



a diminished amplitude of pulsation : in other cases an 

 augmentation. It has been supposed that this nerve 

 contains two different kinds of fibres, the accelerators and 

 the inhibitors. In the case of lower animals, such as the 

 tortoise, these have been isolated by Gaskell. But the 

 accelerator fibres have not been distinguished in the mam- 

 malian vagus. It has been found that, generally speaking, 

 in a vigorously beating heart the effect of vagus-stimulation 

 is to induce depression, whereas in a sluggish heart the same 

 stimulation is apt to induce an augmentation. 



To return to the case of the pulsating pulvinule of 



Fig, 164. — Inhibitory effect of transmitted excitation on the pulsa- 

 tion of vigorous leaflet of Desmodium. The line below indicates 

 duration of transmitted excitation ; note the gradual removal 

 of inhibitory effect on cessation of stimulation. 



Desmodium, the question arises whether the organ is in 

 communication with any conducting channel by which 

 distant excitation might be transmitted to it. The fact 

 of such transmission, if it occurred, would be tested by its 

 modifying influence upon the normal pulsation. In order, 

 then, to subject this question to the test of experiment, I made 

 suitable electrical connections — one contact being on the 

 petiolule, 5 mm. below the contractile pulvinule, and the 

 other still lower down on the petiole. Normal responses 

 were first taken, after which indirect stimulation was applied 

 by means of tetanising electrical shocks of moderate intensity. 

 The first specimen employed was very vigorous, as will be 

 observed from the amplitude of its pulsations (fig. 164). It 



