1912-13.] Studies on Periodicity in Plant Growth. 
95 
Growth Periodicity in Roots and its Correlation with that 
of the Stem. 
When a shoot is exposed to the stimulus of variation in light intensity, 
consequent on the alternation of light and darkness and the various other 
accompanying conditions, it exhibits its daily periodicity of growth in those 
cases where the shoot is phototonic, i.e. where light is a tonic condition of 
growth. When such a shoot is placed in continuous darkness, according to 
Sachs and Baranetzsky, the daily rhythm disappears. This may he taken 
as due to two factors : — 
(a) Withdrawal of the stimuli, or rather the interference with the 
sum of the stimulatory conditions consequent on the withdrawal of 
light; and 
( b ) Because of the inability of the mechanism to act on account of its 
being in a non-phototonic condition due to the withdrawal of the tonic 
light factor. 
Experiments on such stems, when etiolated, are of questionable value 
from this point of view, as the material must be regarded as being in a 
pathological condition. It is possible, however, that such stems by adapta- 
tion might develop a periodicity in response to the sum of the external 
daily factors minus light, i.e. a special darkness-periodicity might develop. 
Such a periodicity, modified by correlation with the stem, is what might 
be expected in the case of roots, and also in those “ permanently etiolated 
stems ” of certain climbers recorded by Sachs. On the latter we have not 
yet made any observations in this connection. 
In roots light is not a necessary condition of growth, and any periodi- 
city they exhibit, on the analogy of the shoot, may depend on — 
(1) The sum of the external daily stimuli exclusive of light. 
(2) A correlative effect with the periodicity of the shoot. 
The periodicity of the root ought to have a reactive influence on the 
shoot when the plant is placed in continuous darkness, after the preliminary 
disturbance, due to the change of the environment of the stem, has passed 
off. Baranetzsky ’s experiments on shoots show that as an after-effect 
periodicity continues to be manifested in the shoot for some time after it 
has been subjected to continuous darkness, giving way ultimately to small 
spontaneous variations. His observations broke off at this point. The 
suggestion is now made of the possibility of the appearance of a darkness- 
periodicity after a period of adaptation has elapsed, induced by the 
new set of conditions, exclusive of light, and in correlation with root 
periodicity. 
