50 
Proceedings of the Royal Society of Edinburgh. [Sess. 
increased, but for exactly opposite reasons. In the former the growth 
of the shoot is restricted by prolonged illumination, in the latter the 
growth of the root is stimulated by illumination, and the after-effect of 
the preceding illumination is seen in the increased ratio during the night. 
In (c), during the winter, the ratio is reduced because the shoot growth 
is not so much retarded by the short period of daylight, and the longer 
night period of weaker electric light is not sufficient to retard its swing, 
while it is not strong enough to stimulate the root appreciably. It there- 
fore compares with the normal conditions in the summer. 
Table III. 
Winter. 
Summer. 
Root. 
Shoot. 
Root. 
Shoot. 
Day : Night. 
Day : Night. 
Day : Night. 
Day : Night. 
Normal 
DO ; DOS 
1-0 : D3 
1-0 : D29 
DO : 1T5 
{a) Intermittent 
1-0 : D92 
DO : D5 
1-0 : 1T4 
DO : 1T2 
(6) Dark . 
1-0 : D71 
10 : D24 
DO : D21 
DO : -93 
(c) Light . 
DO : D5 
DO : 1-43 
DO : -82 
1*0 : D26 
General Results from Table III. 
In root and shoot respectively, the night rate is greater than the day 
rate under ordinary conditions, both in winter, when there is a long night, 
and in summer, when the night is short. 
This holds even when the conditions are changed as already formulated. 
Two exceptions only are noted when the ratios tend to equality : (1) 
in the etiolated stem during the summer, where the shoot is under uniform 
external conditions of darkness ; and (2) in the root under the converse 
conditions — constant illumination in the summer, when the electric light 
during the night gives a weaker growth for the root than the stronger light 
in the daytime. 
Correlation in Growth Rhythm. 
Two main views can be held with regard to growth periodicity in 
plants — either that it is autonomic, that is, due to internal changes inherent 
in the plant (but moulded by external conditions), or that it is an induced 
phenomenon. 
The former was upheld by Sachs, who regarded the appearance of 
periodicity in shoots of Brassica rapa produced from the tuber in darkness 
