380 VEGETABLE PHYSIOLOGY 
(fig. 158, 8). The leaves of many others take up still more 
curious positions, in some cases becoming twisted on their 
petioles, or folded together in various ways. In some, as 
in Nicotiana glauca (fig. 154), they rise instead of fallmg 
and become somewhat closely approximated to each other. 
These changes of position are generally spoken of as 
nyctitropic or sleep movements, though the latter term is 
misleading if it be interpreted to mean a sleep similar to 
Fie. 163.—Desmodium gyrans. (After Darwin.) 
4, stem with leaves as seen during the day; 3, a similar stem with leaves in 
the nocturnal position, pointing downwards. 
that of animals. The latter phenomenon is attended by a 
temporary suspension of sensitiveness and a diminution of 
rigidity, which is not necessarily the case with the move- 
ments which we are discussing. 
Ii is not difficult to prove that these curious changes of 
position are effected in response to the stimulation of the 
‘alternation of light and darkness, or to a rhythmic differ- 
ence in the amount of light which they receive. The 
accompanying rhythmic variation of temperature no doubt 
in some cases also plays a part in the stimulation. 
