DAILY RHYTHMS IN CERTAIN ROOTS 
397 
activity. After the plants had been in the refrigerator for nine hours, and 
from that time until the end of the period of refrigeration, the amount of 
elongation of the individual plants ranged from 0.018 to 0.079 i^im. per hour. 
In six hours after being taken from the refrigerator and incubated at a tem- 
perature of 24°-25° C. these same control plants had regained their normal 
rate of elongation for that temperature. At the end of the refrigeration 
period the seedlings from which root tips were to be cut were also incubated 
at a temperature of 24°-25° C. Series 33 (curve 33) was removed from the 
refrigerator at 9 A.M., and series 35 (curve 35) was removed at i P.M. 
A comparison of the two curves (table 9) shows that there are present, again, 
Table 9. Pisum sativum. Correlation of Maxima and Minima of Curves jj, 35, and 27 
33 
35 
27 
Removed from Re- 
frigerator at 9 A. M. 
Removed from Re- 
frigerator at I P. M. 
DifF. from 33 
Germination Began 
at 9 A. M. 
Diff. from 33 
Difif. from 35 
Maxima 
5 P.M. 
II P.M. 
5 A.M. 
9 P.M. 
I A.M. 
9 A.M. 
4 
2 
4 
3 P.M. 
9 P.M. 
I A.M. 
2 
2 
4 
6 
4 - 
8 
Minima 
I P.M. 
9 P.M. 
I A.M. 
7 P.M. 
II P.M. 
5 A.M. 
6 
2 
4 
II A.M. 
7 P.M. 
II P.M. 
2 
2 
2 
8 
4 
6 
three waves, and that the times of two of the maxima and one of the minima 
are just four hours later in curve 35 than in curve 33, while the 7 P.M. 
minimum of curve 35 is six, instead of four, hours later than the i P.M. 
minimum of curve 33; and that the 11 P.M. minimum and i A.M. maximum 
of curve 35 are each but two hours later than the corresponding minimum 
and maximum of curve 33. Hence, in general, these curves also differ 
from each other by a time interval equal to the difference in time between 
their initiation of activity after refrigeration. 
A comparison of curves 33 and 27 (table 9) shows that with but one 
exception the maxima and minima of curve 33 occur just two hours later 
than the corresponding waves of curve 27. This exception is found where 
the 5 A.M. maximum of curve 33 comes four, instead of two, hours later 
than the i A.M. maximum of curve 27. While the particular amount of 
difference in time between waves in curves 33 and 27 has no special signifi- 
cance, the fact that the time interval between waves of one curve is the 
same as that between waves of the other curve, taken together with the 
relation we have just seen existing between all these other curves of Pisum, 
proves that these rhythms are regular and definite and not mere chance 
variations. It further indicates the truth of the contention that the time 
of occurrence of maxima and minima is related to the time of initiation of 
activity, and not to actual time of day. 
We note from this study of cell division in Pisum that (i) once activity is 
