51 
1914-15.] Studies on Periodicity in Plant Growth. 
as an evidence of its autonomic nature ; while Barenetzsky took the latter 
view because he found the periodicity disappeared in two shoots, after 
three and fourteen days respectively, in continuous darkness. He ex- 
plained the appearance of the periodicity in Brassica shoots as a pheno- 
menon of correlation, the tendency to periodicity being transferred from 
the tuber to the stem as an after-effect. 
The results of this research support Sachs’ view that there is an 
autonomic growth rhythm in all higher plants. This may occur in some 
lower plant forms as a primitive rhythm. 
Roots, being under more uniform conditions than stems, might be 
expected to show a rhythm most nearly approximated to this primitive 
type, due to internal causes, but modified by correlation with the shoot. 
If both root and shoot of a plant grown from seed in continuous 
uniform conditions show periodicity after a considerable time, we are 
justified in considering this periodicity autonomic, even though the first 
few days’ rhythm may be an after-effect transferred from the seed to the 
new plant. Also the shoot rhythm of a plant grown from the seed in the 
dark would tend to revert to the rhythm of the root after the after-effect 
of the light and darkness period has passed off. 
It is therefore possible that when grown in continuous darkness some 
kind of adjustment will take place between the root and shoot rhythms — 
we should expect the shoot rhythm first to alter considerably, being under 
abnormal conditions, while the root periodicity will also alter in correlation 
with that of the stem, and both will tend to produce a fundamental rhythm 
which will be present after a considerable time in the dark, being due to 
internal causes. 
In the same way, if a plant is grown in continuous light we should 
expect a change to take place in the rhythm. The root is now under 
abnormal conditions — in continuous light — and may be expected to respond 
first to the altered conditions, while the shoot will also alter to adjust its 
rhythm to the new external light conditions and the correlation with the 
altered root periodicity. 
That these changes take place is clearly seen from a study of the 
graphs of the growth measurements taken two-hourly under the various 
conditions of light and darkness already mentioned (Plates I and II, 
figs. I, 2, 3, and 4). 
The time of the maximum changes under the various conditions of 
light and darkness, and its variation, provide an easy method of noting the 
change from the normal in the periodicity of both roots and shoots when 
grown in continuous light or continuous darkness or other conditions. 
