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 thig case ig 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 bemg 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. 
