374 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 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 
being much the greater in amplitude. In yet othtr 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. 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 accompany- 
ing or preceding growth. This naturally follows from the fact 
that the growth of leaves is not as a rule very prolonged. 
