SLEEP OF PLANTS. 255 



which had its leaves violently agitated by the wind dur- 

 ing the day was thereby prevented sleeping by night. 



In some instances, as has been detailed, the leaves, 

 or leaflets, are elevated at night, and in others they 

 are depressed. Of the genera examined experi- 

 mentally there are thirty-seven in which the leaves or 

 leaflets rise and thirty-two in which they sink at 

 night. In a species of Bauhinia from Brazil the 

 nocturnal movement was different from any others to 

 which we have alluded. In this plant the leaves are 

 large and broad, with a deep notch at the ends. At 

 night the two halves rise, and close together, with the 

 upper faces closely applied to each other, like closing 

 a book. In young plants the petioles rise also at the 

 same time. Owing to the closing up of the leaves in 

 this manner, the plants have a much more compact 

 appearance at night than during the day. 



Without attempting to go over all the argument, 

 either as to the causes which operate in producing 

 these nocturnal movements, or their utility in the 

 economy of the plant, to the latter of which we have 

 already alluded, we may refer to one point which has 

 not been evident from our narrative, but which is 

 insisted upon by Mr. Darwin in his summary. This 

 is in reference to the continuous movement, to a 

 greater or a less extent, during night as well as day, 

 except where the close pressure or imbrication of the 

 leaves prevents motion. 



