1898] NUTATION OF HELIANTHUS ANNUUS 399 
again brought with their upper surfaces at right angles to the 
light. 
Of course there is considerable difference to be observed in 
different individuals. Some regain the vertical position after 
sunset much sooner than others, and some begin to turn east- 
ward long before I o’clock A.M. The nutations described here 
are not exceptional, but under ordinary conditions they occur in 
nearly all individuals, some nutating more prominently, how- 
ever, than others. As soon as side branches are developed 
they partake of the movements to nearly as great an extent as 
the main axis. 
If the bending of the stem and the diaheliotropism of the 
leaves is due to the direct stimulus produced by sunlight, then 
itis easy to see why the nutation should be considerably less in 
the morning than in the evening, since the light has much less 
time to produce the stimulus. But it may be that other causes 
have an influence on nutation. Thus at 11 o’clock P.m., while 
the leaves are all drooping, the crowns can frequently be seen 
turning toward the east which just two hours before were nutat- 
ing 90° west. At 1 o’clock a.M., when it is evident no direct 
light can come from the sun, the tops of the stems are fre- 
quently seen nutating toward the east 30° or more, and the 
: leaves are beginning to rise and stand up rigidly. Nor does it 
- Seem that the period of repose is brought on entirely by the 
falling of the temperature. From sunset until midnight there is 
_ *tapid fall in temperature, and during this period the “sleep” 
‘Position is assumed. But there is also a rapid fall from 1 o'clock 
_ 4M. until sunrise, when the plant responds exactly in the ss et 
Site way. The drooping of the leaves then is not to be expisites 
; ‘ntirely as a means of protection against excessive radiation, for 
When the temperature is the lowest the leaves are already stand- 
Mg up tigidly and turning toward the east. Whether fluctua- 
tions in the humidity of the atmosphere had anything to do 
ith this action, I did not attempt to investigate. I believe that 
this action is largely due to the fatigue of the protop 
- Must have expended an enormous amount of energy to keep up 
lasm which 
