DAILY RHYTHMS OF ELONGATION AND CELL DIVISION 
IN CERTAIN ROOTS^ 
Ray C. Friesner 
Introduction 
The subject of periodicity of growth activities in plants is by no means a 
new one ; in fact, it is one of the oldest. But a careful review of the available 
literature shows that there are still certain phases of the work which have 
not yet been thoroughly investigated. Two of these are embodied in the 
present paper, viz., rhythms of elongation and rhythms of cell division in 
roots under constant environmental conditions. 
Historical 
Elongation 
Aerial parts. Sachs (29,30) gives an historical account of the older 
literature up to his time. No attempt will be made to reproduce it here 
except to point out that the work before his time was all done on large 
plants which grew rapidly and which in most cases had to be observed in 
the open where external factors could not be controlled. Hence, the work 
was only of the grossest nature and led to no definite general conclusions. 
In 1872 Sachs (29) published the results of his study of the elongation of the 
stem in various plants, including Dahlia variabilis, Fritillaria, Polemonium, 
etc. In general he found that plants exposed to the alternation of darkness 
and light exhibited a single daily wave of elongation in which the maximum 
occurred shortly after sunrise, and the minimum shortly after sunset. 
This he formulated into his so-called "universal law." He further found 
that this daily periodicity is entirely absent from plants grown continually 
in the dark. 
In 1873 Prantl (27) found, in studying the rate of growth in leaves, that 
curves for increase in width are very similar to those for increase in length, 
and that under normal conditions the maximum is reached in the morning 
from 6 to 9 and the minimum in the evening from 6 to 9. He found, further, 
that by changing the hours of illumination and darkness he could shift the 
times of maxima and minima at will, since for each change in the time of 
illumination and darkness there was a corresponding change in the times of 
maxima and minima. These results show clearly that the daily periodicity 
here is an induced one. In continuous darkness this periodicity was absent. 
In 1878 Stebler (33) published the results of similar observations on the 
^ Papers from the Department of Botany of the University of Michigan, no. 180. 
380 
