504 THE POPULAR SCIENCE MONTHLY. 



sleep does not so much concern the inquiry. The night-movements of 

 leaves result from circumnutation modified by changes of light and 

 darkness, and also by heredity ; and, as Darwin has proved, they are 

 chiefly of use to the plant in diminishing the loss of heat by radiation. 

 Therefore, he says, no movement deserves to be called nyctitropic 

 unless it has been acquired for this purpose. As some leaves and coty- 

 ledons bend upward only a little at night, the question arises. At what 

 angle does the diminished radiation warrant the use of this term ? He 

 takes an arbitrary limit of 60, above or below the horizon, as any less 

 angular rise and fall would be of slight significance. Nyctitropic 

 movements are easily affected by surrounding conditions of moisture 

 and temperature, and in many genera it is indispensable that the 

 leaves should be well illuminated during the day. From the very 

 wide list of genera experimented upon, it follows that the habit of 

 sleeping " is common to some few plants throughout the whole vas- 

 cular series." 



Darwin first considers the sleep of cotyledons, having observed the 

 positions during the day and night of the representatives of one hun- 

 dred and fifty-three genera. Some ten pages are given up to remarks 

 upon this subject. We give his account of the behavior of the coty- 

 ledons of Tinfoliwrn strictum as it is illustrated by a picture of the 

 diurnal and nocturnal positions they assume. 



" On the first day after germination, the cotyledons stood at noon 

 horizontally, and at night rose to only about 45 above the horizon. 

 Four days afterward the seedlings were again observed at night, and 

 now the blades stood vertically and were in contact, excepting the 

 tips, which were much deflexed, so that they faced the zenith. At 

 this age the petioles are curved upward, and at night, when the 

 bases of the blades are in contact, the two petioles form a vertical 

 ring around the plumule. The cotyledons continued to act in nearly 



A. B. 



Fig. 6. Oxalis acetosella : A, leaf seen from vertically above ; B, diagram of leaf asleep, also 



seen from vertically above. 



the same manner for eight or ten days from the time of germination, 

 but the petioles had become straight and were much lengthened. 

 After tweWe or fourteen days the first true leaf was formed, and 

 during the next fortnight a remarkable movement was repeatedly 

 observed. At I, Fig. 5, we have a sketch made in the middle of the 

 day, of a seedling a fortnight old. The two cotyledons, of which 



