404 
RAY C. FRIESNER 
must ensue until the food supply is again adequate, after which the same 
processes may be repeated. In other words, may there not be a certain 
inertia inherent in these vital processes, so that once they are in operation a 
certain force is required to check them, and, once slowed down, a certain 
force is again required to accelerate them? This might explain oscillations 
in either process independently of the other, or in the sum of the two pro- 
cesses, but it would not explain the reciprocal relation between the two 
processes. The possibility of growth rate exceeding that of enzymatic 
activity is apparent in the exhaustion effects found at higher temperatures 
in seedlings of Zea Mays and Pisum sativum by Lehenbauer (19) and Leitsch 
(20). _ 
It is necessary, also, to distinguish between the terms "periodicity" and 
"rhythm." By "periodicity" the earlier workers meant a regular oscil- 
lation which was caused by the alternation of day and night or by other 
external changes, and which was lost when the environmental conditions 
were rendered constant; while the term, "rhythm" in the present paper is 
restricted to mean any oscillation in activity which is definite and regular 
and not related to any external influences. Thus these roots in their 
development exhibit "rhythms" in the absence of changes in environment, 
but not a "periodicity" in the sense in which .the older writers used the 
term. 
Summary 
1. Under constant uniform conditions elongation in all plants studied 
proceeds in a rhythmic manner, two or more waves occurring during the 
24-hour period. 
2. Nuclear and cell division proceed in a similar rhythmic fashion. 
3. The times of occurrence of maxima and minima are dependent upon 
the time of initiation of metabolic activity and not upon the time of day 
by the clock. 
4. Elongation and cell division, as regards time of maxima and minima, 
are, in general, reciprocals of each other. 
5. This reciprocal relation existing between elongation and cell division 
accounts for a large share, at least, of the rhythms found in these plants. 
The writer desires to take this opportunity of expressing his appreciation 
to Professor F. C. Newcombe, under whose direction this work was done, 
for his constant encouragement and helpful criticism: also to Professor J. B. 
Pollock and Professor R. M. Holman for helpful criticism and suggestions. 
LITERATURE CITED 
1. Amelung, E. tlber mittlere Zellengrossen. Flora 77: 176-207. 1893. 
2. Baranetzky, J. Die Periodicitat des Blutens. Halle. 1873. 
3. •, Die tagliche Periodicitat in Langenwachstum der Stengel. Mem. Acad. St. 
Petersburg. VII. 27: 1-9 1. 1879. 
4. Braun, A. tlber die Erscheinung der Verjungung in der Natur, pp. 240,^^241 . Leipzig, 
1851. 
