RHYTHMIC NATURE OF ANIMALS AND PLANTS—BROWN 395 
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Ficure 3.—Shaded area indicates time of day of running activity of one male white rat 
during 70 days in dim, constant illumination followed by a 25-day period of continuous 
darkness, 
When, at the end of this time, the rat was placed in darkness (the 
last 25 days in fig. 3), the rhythm became an accurate 24-hour one, the 
rat running at the same time each day. In other words, the rat ap- 
peared able to indicate accurately the time of day by its running only 
when it was in continuous darkness. 
In the bean seedling not only may the sleep rhythm often have 
natural periods a little different from 24 hours in constant darkness at 
ordinary room temperature, but the length of the periods seems to be 
inherited ; some genetic strains have longer-period natural rhythms in 
these constant conditions than others. One strain whose rhythmicity 
has been studied very extensively possesses a 28-hour cycle. 
The clocks of man are simply instruments with built-in cyclic 
changes corresponding to the length of the natural daily cycle, and 
by their appropriate divisions into hours, minutes, and seconds, enable 
us to know at any given instant at just what point in the daily cycle 
we have arrived. The daily cycles of living things are often referred 
to as depending upon “biological clocks,” since the daily rhythms of 
all living things imply the possession by the organism of a means of 
measuring accurately the period of the day. 
