382 VEGETABLE PHYSIOLOGY 



periods which constitutes the stimulus, for some of them 

 will not assume the nocturnal position unless they have 

 been brilliantly illuminated during the day. The degree 

 of sensitiveness in this case is not so great as in those where 

 the diurnal and nocturnal positions are always regularly 

 assumed. 



The peculiar movements which the leaves perform in 

 response to this stimulus are brought about by different 

 -mechanisms in different cases. In young leaves they are 

 attendant upon growth, and are brought about by varia- 

 tions of turgescence upon the two sides of the leaf or its 

 petiole, which are frequently followed by growth. We 

 have seen that during growth the internal turgescence 

 varies rhythmically, and leads to the curious movements 

 of nutation or circumnutation. The actual nyctitropic 

 movement is in these cases a modification of the extent of 

 the circumnutation, the original rhythm being affected by 

 the stimulus. The leaves which exhibit it can be seen 

 by careful observation to be circumnutating during the 

 day. When they assume their nocturnal position it is 

 generally effected by their describing a much longer ellipse 

 than that of their ordinary movement. In some cases 

 only a single ellipse is described during the twenty-four 

 hours ; in others two ellipses, the nyctitropic one being much 

 the greater in amplitude. In yet other cases, several 

 ellipses may be described in the same time. 



Adult leaves which show this movement do so by virtue of 

 a special pulvinus, a kind of motile organ which is developed 

 at that part of the leaf -stalk which joins the stem. This 

 structure has special developments of parenchyma on its 

 upper and lower sides (fig. 155), which become alternately 

 turgid, and cause the leaf to droop and to rise accordingly. 

 These leaves generally exhibit the movement for a much 

 longer period than those in which it is brought about by 

 variations of turgescence accompanying or preceding 

 growth. This naturally follows from the fact that the 

 growth of leaves is not as a rule very prolonged. 



