Chap. VII. SLEEP OF LEAVES 323 



on several occasions; whilst those on rather older plants sleep 

 in a conspiciTous manner. For instance, a leaf ('85 of an inch 

 in length) on a very young seedling 2 inches high, stood at noon 

 9° above the horizon, and at 10 p.m. at 28°, so it had risen on\y 

 19°; another leaf (I'd inch in length) on a seedling of the 

 same height, stood at the same two periods at 7° and 32°, and 

 therefore had risen 25°. These leaves, which moved so little, 

 had a fairly well- developed pulvinus. After an interval of some 

 weeks, when the same seedlings were 2J and 3 inches in height, 

 some of the young leaves stood up at night quite vertically, and 

 others were highly inclined ; and so it was with bushes which 

 were fully grown and were flowering. 



The movement of a leaf was traced from 9.15 a.m. on 

 May 28th to 8.80 a.m. on the 30th. The temperature was too 

 low (15° — 16° C), and the illumination hardly suflcient; con- 

 sequently the leaves did not become quite so highly inclined at 

 night, as they had done previously and as they did subse- 

 quently in the hot-house ; but the movements did not appear 

 otherwise disturbed. On the first day the leaf sank till 

 5.15 P.M. ; it then rose rapidly and greatly till 10.5 p.m., and 

 only a little higher during the rest of the night (Fig. 126). 

 Early on the next day (29th) it fell in a slightly zigzag line 

 rapidly until 9 a.m., by which time it had reached nearly the 

 same place as on the previous morning. During the remainder 

 of the day it fell slowly, and zigzagged laterally. The evening 

 rise began after 4 p.m. in the same manner as before, and on 

 the second morning it again fell rapidly. The ascending and 

 descending lines do not coincide, as may be seen in the diagram. 

 On the 30th a new tracing was made (not here given) on a 

 rather enlarged scale, as the apex of the leaf now stood 9 inches 

 from the vertical glass. In order to observe more carefully the 

 course pursued at the time when the diurnal fall changes into 

 the nocturnal rise, dots were made every half-hour between 

 4 P.M. and 10.30 p.m. This rendered the lateral zigzagging 

 movement during the evening more conspicuous than in the 

 diagram given, but it was of the same nature as there shown. 

 The impression forced on our minds was that the leaf was 

 expending superfluous movement, so that the great nocturnal 

 rise might not occur at too early an hour. 



Ahutilon Darwinii (Malvaceae). — The leaves on some very 

 young plants stood almost hoiizontally during the day, and 

 hung down vertically at night. Very fine plants kept in a 



