NYCTITROPISM 511 



itself up, as Schwendener (1897) has already shown, but in the periodic move- 

 ments it droops in the evening. This contradiction Pfeffer naturally did not 

 fail to notice, and he has also attempted to explain it. He assumed that the 

 drooping arose from a marked increase in the statical moment of the leaf which 

 was necessarily associated with the forward movement of the secondary petioles 

 described above, and he found that his supposition was confirmed by experiment. 

 When movement in the secondary petioles was rendered mechanically impossible 

 the evening drooping gradually ceased, but came back again when the secondary 

 petioles were allowed freedom of movement. In spite of this ingenious experi- 

 ment we have certain criticisms to advance against the correctness of the explana- 

 tion. In nature the erection of the petiole takes place during the night and it 

 may amount to from 40° to 80° in 6 to 8 hours. Schilling (1895) has, however, 

 noticed that the compression of the articulation and sinking of the petiole (the 

 statical moment of which had been greatly increased by hanging a weight upon 

 it) was compensated in 10 to 15 minutes ; and it is not obvious why the plant 

 should require several hours to compensate evening drooping, if this drooping 

 be due to purely mechanical causes. Schwendener (1897) has advanced 

 additional arguments against Pfeffer's explanation, so that we may regard 

 the sinking and rising of the petiole of Mimosa as a phenomenon of periodic 

 movement not yet explained, and possibly as a phenomenon which has some 

 special biological function to fulfil. 



We cannot enter into a discussion of the mechanics of periodic movement ; 

 we may only notice that, according to Pfeffer, in nutation movements as in 

 variation movements the opposite sides behave in contrary ways during the 

 after-effect ; this is especially well seen in the articulation because the resistance 

 to flexion of the entire articulation remains unaltered owing to the expansion of 

 the convex side and the simultaneous contraction of the concave side. 



In conclusion we may ask ourselves what is the biological significance of 

 nyctitropic movements ; for that they have nothing to do with the ' sleep ' 

 of animals goes without saying. We must keep distinct in our minds the move- 

 ments of foliage-leaves and those of floral- leaves. The position which foliage- 

 leaves take up in the evening is essentially vertical ; it is a matter of less moment 

 manifestly, whether they bend upwards or bend downwards. The question is 

 what is the advantage of this nocturnal vertical position to them ? Darwin (1881) 

 has drawn attention to the fact that the leaf in the vertical position is less subject 

 to heat radiation during the night than if it were in the horizontal position. But 

 this radiation as such could injure the leaf only during cold nights, and yet at 

 low temperatures nyctitropic movements cease ; on the other hand, they occur 

 during warm summer nights and are especially evident in tropical plants where 

 injury due to cooling is out of the question. Hence Stahl (1897) has sought to 

 explain the night position by considering it as a means of preventing the deposit 

 of dew, and he has supported this idea, as far as it was possible to do so, by means 

 of experiments. He looks upon the deposition of dew as injurious to the plant 

 inasmuch as it prevents transpiration so long as the leaves are wet. 



As to the biological significance of nyctitropic movement in flowers little 

 trustworthy is as yet known. It is scarcely possible that we have to do here 

 with any transpiration phenomena, as in the case of foliage-leaves ; in all pro- 

 bability quite different explanations must be given of the movement of floral- 

 leaves in different groups of plants. We may imagine that in the case of spring 

 flowers which are especially sensitive to heat stimulus, the closing of the flowers 

 is a protection against cold. Flowers which open in the evening are obviously 

 adapted to the visits of those insects which effect pollination and which are 

 nocturnal in habit ; insects which are active during the day would thus be 

 entirely excluded from such flowers. Other types again which show opening 

 and closing movements may thereby attain benefits with which we are as yet 

 entirely unacquainted. 



