CHAP. VI SLEEP MOVEMENTS. 283 



great advantage from such remarkable powers of 

 movement. 



The nyctitropic movements of leaves and cotyledons 

 are effected in two ways,* firstly, by means of pulvini 

 which become, as Pfeffer has shown, alternately more 

 turgescent on opposite sides ; and secondly, by in- 

 creased growth along one side of the petiole or 

 midrib, and then on the opposite side, as was first 

 proved by Batalin.j But as it has been shown by 

 De Yries J that in these latter cases increased growth 

 is preceded by the increased turgescence of the cells, 

 the difference between the above two means of move- 

 ment is much diminished, and consists chiefly in the 

 turgescence of the cells of a fully developed pulvinus, 

 not being followed by growth. When the move- 

 ments of leaves or cotyledons, furnished with a pul- 

 vinus and destitute of one, are compared, they are seen 

 to be closely similar, and are apparently effected for 

 the same purpose. Therefore, with our object in view, 

 it does not appear advisable to separate the above two 

 sets of cases into two distinct classes. There is, how- 

 ever, one important distinction between them, namely, 

 that movements effected by growth on the alternate 

 sides, are confined to young growing leaves, whilst those 

 effected by means of a pulvinus last for a long time. 

 We have already seen well-marked instances of this 

 latter fact with cotyledons, and so it is with leaves, as 

 has been observed by Pfeffer and by ourselves. The 

 long endurance of the nyctitropic movements when 

 effected by the aid of pulvini indicates, in addition to 

 the evidence already advanced, the functional iinr>ort- 



* This distinction was first Dassen in 1837. 

 pointed out (according to Pfefter, t * Flora/ 1873, p. 433. 



4 Die Periodischeu Bewegungen j ' Bot. Zeitung,' 1879, Doc. 



der Blattorgaue,' 1875, p. 1G1) by 19 ih, p. 830. 



