STIMULATION AND ITS EESULTS 375 



night which they appreciate ; it is rather the difference 

 between the illumination they receive during the two 

 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 paren- 

 chyma on its upper and lower sides (fig. 157), 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 accom- 



