CONDUCTIVITY AND EXCITABILITY 221 



applied on the petiole, it will be found that this particular 

 leaflet will not respond. This loss of excitability will, how- 

 ever, be temporary, disappearing as the leaflet returns to its 

 ordinary temperature, when it will be found to respond as 

 usual. 



A moderate application of cold does not altogether 

 abolish the response, but the molecular sluggishness induced 

 is shown in the prolongation of the latent period of response. 

 It was found, for example, in an experiment on Biophytum 

 that the latent period was sometimes prolonged by several 

 seconds (p. 268). 



(c) By effect of fatigue. — We have already seen (p. 113) 

 how the motile excitability of the plant-tissue is diminished 

 by fatigue, as shown in the diminution of successive responses, 

 when the intervening periods of rest are not sufficient for 

 complete recovery. We have seen, too, that under strong 

 and long-continued excitation the motile excitability is 

 abolished ; and that it can be restored after the lapse of a 

 sufficiently long resting period. 



Variation of conductivity. — We shall next examine how 

 the transmission of stimulus from point to point is affected 

 by various external agencies. And, first, we shall refer back 

 to the mechanical model (fig. 95). We there* saw how the 

 sluggishness, induced in the intermediate molecules by 

 plunging the dampers to a greater or less depth in a viscous 

 liquid, retarded the transmission of disturbance through them. 

 When this induced sluggishness is slight, the propagation will 

 merely be slowed below the normal ; but when the sluggish- 

 ness induced is great, the disturbance will not reach the 

 responder R. 



(a) By effect of cold. — We shall now proceed to investi- 

 gate the effect of induced molecular sluggishness on the 

 conductivity of a plant-tissue ; and for this purpose we 

 shall first observe the influence of cold. In an experiment 

 on Biophytum, I found that the normal velocity of trans- 

 mission, depending on the conductivity, was 37 mm. per 

 second ; but on subjecting the tissue to moderate cold, the 



