632 



PLANT RESPONSE 



half of the pulvinus. We have just seen that in the case of 

 externally diffused stimulation both sides of the organ are 

 acted upon at the same moment, and the differential response 

 is downward from the beginning ; but when continuous 

 stimulus is applied on the dorsal surface, it is at first 

 unilateral, and only afterwards becomes internally diffused. 

 We therefore obtain in this case (fig. 256), as was theoretically 



inferred, all the phases of re- 

 sponse, positive, passing through 

 neutral, into negative. 



A very interesting feature of 

 this record is the after-effect, on 

 the cessation of stimulus, which 

 is represented by a dotted line. 

 In Chapter XXXIV., while deal- 

 ing with the detection of the 

 latent factor, it was explained 

 that there are two distinct after- 

 effects, positive and negative 

 (p. 457). In the former, the 

 movement is simply a continua- 

 tion of the effect seen when 

 stimulus was acting ; but the 

 latter, or negative, after-effect, 

 being due to the increase of 

 latent energy by absorption of 

 incident stimulus, finds expres- 

 sion in an opposite movement. In the case of the response 

 of growth we saw that the positive after-effect consisted of a 

 persistent retardation, and the negative of an acceleration, 

 of growth. In the case of pulvinated organs, however, the 

 negative after effect is generally indistinguishable from the 

 movement of recovery ; but in the present instance we find 

 it clearly exhibited in the fact that the after-effect not only 

 causes recovery to the original position, but carries the leaf 

 to a distance beyond. 



The positive hcliotropic response of the leaf, then, persists 



Fig. 256. Response of Pulvinus 

 of Mimosa to Action of Con- 

 tinuous Light from Above ap- 

 plied at Moment marked with j, 



Positive heliotropic movement 

 caused by excitation of upper 

 half neutralised by transmission 

 to distal side, and ultimately 

 reversed owing to greater ex- 

 citability of lower half. Dotted 

 line represents recovery on ces- 

 sation of light. Note final 

 erection of leaf above original 

 position as negative after-effect 

 by absorbed stimulus. 



