628 PLANT RESPONSE 



able side which, under strong unilateral stimulation of either, 

 will become concave. Supposing the lower side to be the 

 more excitable, then, strong stimulation, whether of dorsal 

 or of ventral, will bring about concavity of the lower ; but 

 here we must bear in mind the possibility of an effect which 

 will be the result of moderate stimulus. The differential 

 effect, which brings about the concavity of the lower, even 

 when it is the dorsal surface that is excited, depends on 

 the internal diffusion of stimulation, and this in turn is 

 dependent not only on the conductivity of the tissue, but 

 also on the intensity of the stimulus. If, then, a feeble or 

 moderate stimulus be applied on the dorsal surface, even of 

 a highly conducting organ, the result will be a concavity of 

 that surface. The long-continued action of moderate stimu- 

 lus will, however, bring about a gradual percolation, and 

 the first response due to localised, will give place to the 

 differential effect of diffuse, stimulus. If the stimulus again 

 be very much stronger, this reversal will take place much 

 more quickly. We thus see that responsive movements may 

 be positive, neutral, or negative, according to the strength 

 and duration of the stimulus. 



Pulvinated organ showing positive heliotropic move- 

 ment : (a) Terminal leaflet of Desmodium. — We shall now 

 take the case of a pulvinus in which the conductivity is 

 feeble, and in which we should therefore expect to obtain 

 positive response. I have already given response-curves in 

 illustration of this positive response, whether it be the upper 

 or lower surface of the pulvinus which is subjected to light 

 (p. 586). It was also shown that the leaflet moved towards 

 the light, and that in such cases it was the pulvinus, and not 

 the lamina, which was both the perceptive and responding 

 region. • 



(b) Response of leaflet of Robinia. — The leaflets of Robinia, 

 under the vertical light of the noonday sun, exhibit what is 

 known as diurnal sleep — that is to say, they fold themselves 

 upwards. This is simply an instance of the positive helio- 

 tropic effect common to all those pulvinated organs in which 



