282 PLANT RESPONSE 



rhythmic responses in a plant. Thus on applying a thermal 

 stimulus to the petiole of Biopliytum near its insertion on the 

 stem, an excitatory wave was found, as usual, to produce 

 successive depressions of leaflets in a centrifugal order. But 

 after a while the existence of a second excitatory wave 

 became evident, by a second series of closures of leaflets. 

 That the wave was due — not to excitation reflected in some 

 way from the tip of the leaf, but — to a second wave starting 

 from the original point of stimulation, was made evident by 

 the fact that the successive fall of leaflets was again centrifugal, 

 from near the stem to the tip of the leaf. From the records 

 now obtained by means of the Optic Lever, attached to one 

 of the leaflets, it was interesting to observe the very numerous 

 rhythmic pulsations, often as many as twenty, now made 

 visible. 



As an instance of the simplest form of such multiple 

 response, I give the following record, which was obtained 

 under the thermal stimulation of the electrothermic stimu- 

 lator. The point of application was close to the responding 

 leaf. The average period of these pulsations was about 

 thirty seconds (fig. 1 16) It may be stated here that the 

 period of multiple responses of Biopliytum is found to vary 

 from about fifteen seconds to three minutes or so, depending 

 on the condition of the plant and the intensity of the 

 stimulus. 



The question suggests itself, since multiple electromotive 

 and electrotactile excitations are observed in Mimosa, why 

 should not this plant also exhibit them mechanically? The 

 answer to this is probably found in the fact that the Biophy- 

 tum leaflet is light, and easily exhibits fluctuating impulses, 

 whereas the impulsive fall of the heavier Mimosa leaf persists, 

 owing to greater momentum, till it is more or less complete. 

 Again, unless an organ has at least partially recovered from 

 fatigue, it is not susceptible of fresh excitation. The period 

 of full recovery in the pulvinus of Mimosa is very long, 

 being about seven minutes. We saw, in studying fatigue in 



