372 



VEGETABLE PHYSIOLOGY 



(fig. 153, 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. 154), 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 

 misleading if it be interpreted to mean a sleep similar to 

 that of animals. The latter phenomenon is attended by a 



FIG. 153. 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. 



temporary suspension of sensitiveness and a diminution of 

 rigidity which is not necessarily the case with the move- 

 ments 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 

 accompanying rhythmic variation of temperature no doubt 

 in some cases also plays a part in the stimulation. 



