CHAP. VII. 



SLEEP OF LEAVES. 



385 



derable angle beneath the horizon at 10 P.M. In the case of four 

 youngish leaves, which were from 2 to 2 inches in length, 

 these angles were found to be 50, 56, 60, and 65. At the 

 end of August, when the plants had grown to a height of 10 to 11 

 inches, the younger leaves were so much curved downwards at 

 night that they might truly be said to be asleep. This is one 



Fig. 162. 



Nicotiana glauca : shoots with leaves expanded during the day, and asleep 

 at night. Figures copied from photographs, and reduced. 



of the species which must be well illuminated during the day 

 in order to sleep, for on two occasions when plants were kept 

 all day in a room with north-east windows, the leaves did not 

 sleep at night. The same cause probably accounts for the 

 leaves on our seedlings raised in the dead of the winter not 

 sleeping. Professor Pfeffer informs us that the leaves of 

 another species (S. Jorullensis ?) hang vertically down at night. 



