284 PLANT RESPONSE 



in the leaf of Mimosa. In this case the second response 

 occurred four minutes after the first. 



We have seen that multiple electrical response is obtained 

 when any form of strong stimulation is employed. I now 

 tried to find out whether multiple mechanical response could 

 be produced by chemical stimulation. One of the middle 

 leaflets of a Biophytum leaf was attached to the recording 

 Optic Lever, and I applied a drop of sulphuric acid 1 cm. 

 away from the recording leaflet in the direction of the tip 

 of the leaf. This was found to give rise to five vigorous 

 recurrent pulsations. 



Thus, though a single moderate stimulus evokes but 

 a single response, yet under strong stimulation we obtain 

 not only the immediate consequent response, but also a 

 surplus of energy which remains over and is held latent in 

 the tissue, to be given out later, after a shorter or longer 

 interval, in the form of recurrent responses. From these 

 experiments it is clear that a rhythmic series of effects 

 need not have a periodic antecedent cause. As regards 

 these pulsatory movements, it was shown on page 47 that 

 the fall of the motile leaflet was due to a pulse of diminution, 

 and its erection to a pulse of restoration, or increase, of 

 turgidity. In these multiple responses, then, we have the 

 expression of rhythmic variations of turgidity initiated by 

 stimulus. 



Among ordinary responses — that is to say, single response 

 to single stimulus — we have observed three types, depending 

 on the excitability of the tissue. When the excitability 

 remains uniform, the responses are uniform. When the 

 excitability undergoes a gradual diminution, there is a cor- 

 responding depression of response — that is to say, fatigue 

 supervenes. And when the excitability increases by degrees, 

 there is a correspondent enhancement of response known 

 as the 'staircase' effect. In addition to these, I have, as 

 explained before, also noticed some curious instances in which 

 the excitability of the tissue appeared to undergo periodic 

 fluctuations, in consequence of which successive responses to 



