RHYTHMIC NATURE OF ANIMALS AND PLANTS—BROWN 397 
Such a resemblance to a recording system was lucidly shown by 
Professor Biinning for the bean seedling. If we have a bean seedling 
displaying in constant low light a daily sleep rhythm, drooping its 
leaves at night, and now give it a brief brighter light stimulus during 
the nighttime phase, we see that not only does the light cause a momen- 
tary brief elevation of the leaf but now the plant continues to do it 
day after day at the same time even in constant darkness. From the 
standpoint of rhythmicity, the basic question is, what keeps the re- 
cording system going at an essentially uniform speed, sometimes quite 
precisely one circuit per day? One widely held view, as has been 
mentioned earlier, is that the living thing possesses within itself some 
machinery for measuring off quite independently from the environ- 
ment periods of time, but since all known living machinery is pre- 
dominantly chemical in nature, this would clearly have to depend 
upon the rates of chemical reactions. 
There is a very general rule, the van’t Hoff rule, applying equally 
to chemical reactions and to living processes, which states that as the 
temperature rises, the rate of chemical processes speeds up, and as we 
lower temperature, the rate slows down. The rate of biological 
processes usually more than doubles for every 18° F. rise in tempera- 
ture. This law is the basis of the universal use of refrigerators to 
reduce the rate of food spoilage or bacterial decay. Now if the 
rhythmic timing mechanism were biochemical and wholly inside the 
organism, one would expect a speeding up of the rhythm with in- 
creasing temperature, and a slowing down with decreasing tempera- 
ture. For example, if at 70° F. the period of the rhythm was 24 hours, 
at 90° F. we would expect it to be about 8 or 10 hours and at 50° F. 
about 50 hours. When we studied critically this problem with fiddler 
crabs about 10 years ago, we found to our amazement that through a 
wide range of temperature the period of the rhythm remained the 
same, precisely 24 hours. The animals had available some method of 
time measuring that was independent of temperature, a phenomenon 
quite inexplicable in any currently known mechanisms of physiology, 
or, in view of the long period lengths, even of chemical reaction 
kinetics. Following this discovery it was rapidly shown in our lab- 
oratory and in those of others that the daily cycles of other animals 
and plants were similarly temperature independent. The most 
spectacular recent demonstration of temperature independence in- 
volved a study of dried plant seeds. These have been shown to have 
an annual rhythm in their capacity to germinate even when stored in 
constant conditions. Biinning and his associates in Germany recently 
found that dried seeds showed an accurate annual rhythm whether 
they were stored at 3° below zero F. or at 120° F. At the former 
temperature, 35° below the freezing point of water, all vital processes 
would normally have been expected to be at least temporarily halted. 
