STIMULATION AND ITS RESULTS 



373 



as night draws on, the terminal leaflets droop till they 

 assume a position almost or quite parallel to the stem 

 (fig. 155, B). 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. 156), they rise instead of falling 

 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 



FIG. 155. Desmodium gyrans. (After Darwin.) 



A, stem with leaves as seen during the day ; B, a similar stem with leaves in the 

 nocturnal position, pointing downwards. 



misleading if it be interpreted to mean a sleep similar to 

 that of animals. The latter phenomenon is attended by a 

 temporary suspension of sensitiveness, which is not neces- 

 sarily the case with the movements which we are discussing. 

 It 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 



