Chap. VII. SLEEP OF LEAVES. . 377 



in one instance to the extent of 16° This pinna, therefore, 

 circumnutated. Later in the evening the four pinnae approach 

 each other, and the one ■which was observed moved inwards 

 59° between noon and 6.45 p.m. Ten observations were madf 

 in the course of 2 h. 20 m. (at average intervals of 14 m.) 

 between 4.25 and 6.45 p.m. ; and there was now, when the leal 

 was going to sleep, no swaying from side to side, but a steady 

 inward movement. Here therefore there is in the evening the 

 same conversion of a ciroumnutating into a steady movement 

 in one direction, as in the case of the main petiole. 



It has also been stated that each separate leaiJet circnm- 

 nutates. A pinna was cemented with shellac on the summit ol 

 a little stick driven firmly into the ground, immediately beneath 

 a pair of leaflets, to the midribs of both of which excessively 

 fine glass filaments were attached. This treatment did not 

 injure the leaflets, for they went to sleep in the usual manner, 

 and long retained their sensitiveness. The movements of one 

 of them were traced during 49 h., as shown in Fig. 158. On the 

 first day the leaflet sank down till 11.30 a.m., and then rose 

 till late in the evening in a zigzag Hue, indicating circum- 

 nutation. On the second day, when more accustomed to its 

 new state, it oscillated twice up tind twice down during the 

 24 h. This plant was subjected to a rather low temperature, 

 viz., 62° — 64° F. ;' had it been kept warmer, no doubt the move- 

 ments of the leaflet would have been much more rapid and 

 complicated. It may be seen in the diagram that the ascending 

 and descending lines do not coincide ; but the large amount of 

 lateral movement in the evening is the result of the leaflets 

 bending towards the apex of the leaf when going to sleep. 

 Another leaflet was casually observed, and found to be con- 

 tinually circumuutftting during the same length of time. 



The circumnutation of the leaves is not destroyed by their 

 being subjected to moderately long continued darkness ; but the 

 proper periodicity of their movements is lost. Some very young 

 seedlings were kept during two days in the dark (temp. 57°^59° 

 F.), except when the circumnutation of their stems was occa- 

 sionally observed ; and on the evening of the second day the 

 leaflets did not fully and properly go to sleep. The pot was 

 then placed for three days in a dark cupboard, under nearly the 

 same temperature, and at the close of this period the leaflets 

 showed no signs of sleeping, and were only slightly sensitive to 

 ■ a touch. On the following day the stem was cemented to a 



