80 PENHALLOW ON MECHANISM 
Our observations have been almost wholly confined to the true nyctitropic movement, 
the principal data of which we have collected in connection with M. Chapman and G. E. 
Cooley. 
Sleep-movement usually begins within half an hour after sunset, though this period 
appears to become longer as the season advances. In all cases, it has been most 
conspicuous that leaves at an elevation, e.g. those on the tops of trees, assume the 
sleep position much sooner than those at a lower level. No general rate of change, 
however, can be stated for all the leaves, since it is found that the leaflets fall into the 
nocturnal position at very irregular intervals, some assuming this position very early in 
the evening, while others remain in their diurnal position until quite late. The sleep- 
movement is generally completed by 10.30 p.m. From that time on there is no change 
in position, until the actual awakening occurs, which begins just before dawn. With 
reference to this, the following extract from our notes will give the general features of 
the change :— 
“ At 2 o’clock am. the maximum darkness and minimum temperature have just 
passed, and the older leaves show the first signs of awakening, many of the leaflets being 
expanded 5° or 10° from their former sleep position. At 3.30 the leaflets have opened to 
to an angle of 45°, and at 4 o’clock they are well expanded, the most marked change 
occurring within the latter hour. From an examination of the accompanying table the 
general conditions throughout the night may be obtained. The temperature was taken 
from an exposed thermometer, hung at the height of, and among, the plants observed. 
This will account for the variations of temperature noted in one or two instances :— 



Hour. Tee. REMARKS. 
7.30 p.m. 16.00° C. |Leaves closing. 
8.00 “ 16.50 Sky clear all night. 
SoD 16.25 
9.00 “ 15.25 
9:30 15.50 
10.00 “ 16.00 
10:30 16.25 Leaves all closed. 
0059 16.75 
IUT 15.00 
12:00) 14.30 | 
12.30 a.m. 12.75 | 
100 11.25 | 
HES (ame 11.20 Max. darkness. 
200m os 12.75 Leaves begin to open. 
2.30 “ 13.00 
GHOD), | 13.50 | 
20 13.50 | 
ASI) « & 13.25 Leaves all open. 




The leading features of the awakening are, that the process begins at or immediately 
following the periods of maximum darkness and minimum temperature, and that it is 
completed before the sun rises above the horizon. As in going to sleep, the first change is 
