RHYTHMIC NATURE OF ANIMALS AND PLANTS—BROWN 389 
A final kind of illustration to familiarize one with roles of rhythms 
in organisms comes from studies of the time sense of organisms. 
Beling found that if honey bees are fed sugar water at a particular 
place and time of day for a few days, they will continue to come back 
to the same place at the same time of day. They even continue to come 
back for several days when food is no longer provided. This kind of 
time training can be accomplished only on a 24-hour periodic basis. 
One cannot teach them, say, to feed at regular 19-hour intervals. This 
learning process, therefore, in some manner involves their 24-hour 
rhythm. 
The foregoing selected examples of biological rhythms suffice to 
illustrate their widespread distribution and a few of the innumerable 
ways they may be put to use by living things. Now let us pass on to 
some studies of the mechanisms of the rhythmicities. 
The most obvious explanation of the daily rhythms that occurs to 
one is that the organisms are simply responding to the daily cycles in 
light, temperature, or humidity, and that the lunar-tidal rhythms of 
shore dwellers are simply direct responses to the rhythmic ebbing and 
flooding of the tides. This hypothesis was easily testable, and was, in 
fact, first tested about 200 years ago. Many plants were known to show 
a sleep rhythm involving the drooping of the leaves by night and their 
elevation by day. Zinn, in 1759, found that plants would still show the 
sleep rhythm when no light or temperature changes occurred. This 
observation was confirmed and studied extensively early in this cen- 
tury using the bean seedling, and such work is now continuing actively 
in the laboratory of Professor Biinning and his associates at the 
University of Tiibingen, in Germany. A general method of recording 
automatically the leaf movements on a slowly rotating drum is 
illustrated in figure 1. 
The persistence of the daily rhythmic processes in living things in 
conditions constant with respect to all factors generally conceded to 
influence the organisms has now been demonstrated by numerous 
investigators for living things representing the gamut of the animal 
and plant kingdoms, and ranging from single-celled microorganisms 
to the most complex of multicellular larger ones. 
Fiddler crabs in nature start to blacken their skin about sunrise in 
the morning to hide themselves better from their predators, and to 
screen their delicate internal organs from the intense sunlight of the 
unshaded beaches, and about sunset they rapidly blanch until their 
bodies are pale silvery gray. This is more evident for their legs 
than for their bodies, since the shell covering the legs is quite trans- 
parent. Even crabs captured and maintained in constant conditions 
in a photographic darkroom continue for weeks or months to darken 
