400 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
T shall not try my readers with either the detailed methods of study 
or the mathematics of the analysis, but shall simply summarize for 
you some of the principal facts. The most complete of our studies 
have been done on young potato plants, small pieces of potatoes with 
sprouting eyes prepared as illustrated in figure 4. Occasionally, while 
being studied in our recording apparatus, they even developed new 
potatoes. Potatoes were selected for this study for two reasons: (1) 
The potato tuber is essentially a generous reservoir of stored food, 
and hence there was no problem of feeding the organisms while they 
were sealed for long periods in constant, dark conditions; and (2) 
these seemed about as inauspicious a living thing as one could expect 
to find as far as rhythms were concerned. Briefer, parallel studies 
with other plants and animals have suggested that what we have found 
for the potato holds in a general manner for all other living things 
as well. 
It is well known that there are solar and lunar tides of the atmos- 
phere. These are reflected in rhythms of barometric pressure change. 
In the daily cycle (the upper curve in fig. 5A), the pressure, on the 
average, always rises during the early morning hours to a high about 
10 o’clock and then falls to the low point of the day, during the middle 
to late afternoon, the time of the low point depending on the time of 
year. This is a precise average 24-hour rhythm. The potato has 
similarly a precise average 24-hour cycle of metabolism (the solid line 
of the lower curves in fig. 5A.) with a minimum rate at midnight and a 
maximum rate at 6 in the afternoon. This average daily cycle can 
be shown to include the average of two kinds of daily oscillations: 
the dashed AA’A”’ and the dotted BB’B”’ ones in figure 5A. Both the 
barometric-pressure cycles and the potato cycles exhibit irregularities 
from day to day. The barometric-pressure cycles are distorted by 
Ficure 4.—Method of obtaining the potato plants used in the study of metabolic cycles. 
