678 PLANT RESPONSE 



further (p. 635) we saw that it constituted a true instance of 

 phototropic reaction, and that this particular movement of 

 fall in a definite direction was due to the stimulus being 

 diffused, whether internally or externally, and by differential 

 action inducing concavity of the lower and more excitable 

 half of the pulvinus. 



In the assumption by the leaf at evening, then, of its 

 lowest position, we have nothing which is specifically different 

 from this responsive movement, as caused by the action of 

 light on the anisotropic organ ; but this same fall, when it 

 attains its maximum in the evening, as a phase of the 

 nyctitropic movement, is usually said to be due, not to the 

 stimulus of light, but to the variation in that stimulus, or to 

 the effect of on-coming darkness. Now, if it had been true 

 that the diminution of light, or on-coming of darkness in the 

 evening, had acted in some unknown manner as a stimulus 

 to bring about the fall, then we should have found that such 

 a fall was at that time extremely rapid, and that it persisted 

 on the withdrawal of light during the night. As a matter of 

 fact, however, it is found that this movement of the petiole 

 downwards has been taking place progressively throughout 

 the day, and that the fall of the leaf at evening is merely a 

 continuation of this previous movement, and not markedly 

 more rapid at that time than before, if the increased 

 mechanical moment due to the closing of the secondary 

 petioles and their leaflets be eliminated by their removal. 

 Nor does the petiole remain in this depressed position, but 

 begins after an interval to erect itself, till in the morning, or 

 earlier, it has attained its highest degree of erection. So far, 

 again, from darkness being efficient to cause the depression 

 of the leaf, it is well known that on artificially darkening the 

 Mimosa plant the leaves respond by erection. 



It is thus clear that we must seek some other explanation 

 of the nyctitropic movement, and as it is known that such 

 periodic movement persists for a time, even when the plant 

 is kept for days in continuous darkness, the inquiry resolves 

 itself into the two questions: (i) how are these diurnal 



