390 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
SLOWLY ROTATING DRUM 
COUNTERWEIGHT 
WRITING ARM 
PLANT SLEEP MOVEMENT 
Ficure 1.—Apparatus for automatically recording sleep movements in the bean seedling. 
and lighten each day in synchrony with their relatives still free in 
their natural day-night environment. 
Another, and very interesting, property of these rhythms persisting 
in darkness is that they may be easily reset so that events in the 
rhythm occur at times of day other than the normal ones. This may 
be illustrated by the crab color-change rhythm. If this is proceeding 
normally in constant darkness and temperature (the top line of fig. 2), 
showing maximum darkening at noon and blanching at midnight, and 
we simply, for two or three days, leave lights on in the darkroom 
from midnight to noon, we find at the end of this treatment that the 
crabs still possess an accurate daily rhythm of color change which 
persists in darkness indefinitely, but now (as in the second line in 
fig. 2) it has been shifted so that the color changes occur at an earlier 
time of day, by 6 hours. They now darken most at 6 a.m. and blanch 
most about 6 p.m. In a comparable manner we may set the actual 
time of the color changes in the 24-hour rhythm to any time of day, 
and there it will remain until reset. The third line of figure 2 shows 
crabs which were subjected to three lighted nights and darkened 
days; the cycles are inverted. An alternative way to reset the cycles 
is through giving the crabs, while in constant darkness, experimental 
cycles of abrupt temperature changes. High temperatures act like 
light, lower temperatures like darkness. A third way to reset them is 
simply to place crabs with normal cycles in constant darkness on ice 
cubes for the number of hours one wishes to set back the cycles. When 
taken off the ice the crabs’ color-change rhythm picks up right where 
it left off when they were first iced (fourth line of fig.2) and hence the 
phases are slow by a time correlated with the period of chilling. It 
is as if self-starting 24-hour clocks were stopped near freezing 
temperatures, 
