52 
Proceedings of the Eoyal Society of Edinburgh. [Sess. 
In the winter, in February and March, when the light and darkness are 
approximately equal periods, under the normal conditions, the maximum of 
the shoot occurs in the early morning, 5 a.m.-7 a.m., while that of the 
root is much the same — the range being greater, extending back to mid- 
night and on towards mid-day. 
Under the intermittent light conditions the shoot is very much the 
same as under normal conditions, though there is an occasional earlier 
maximum in the evening, and in the root a change to an earlier 
maximum between 8 p.m. and midnight. It may be this change in the 
root — due to being in the light during the daytime — which affects the 
shoot rhythm. 
When grown in continuous darkness, the early morning maximum 
gradually gives place to a much earlier one, about fourteen hours or more 
earlier, occurring during the early afternoon, and often followed by a 
secondary maximum during the early morning, 5 a.m.-7 a.m., as though 
an attempt were being made to revert to former maximum. 
This earlier maximum in the darkness is evidently due to the previous 
long period of exposure to the dark stimulating the shoot. 
The same thing is seen in the root, though there is evidence to show 
that the change takes place first of all in the shoot, and is followed by a 
change in the root to an earlier evening maximum followed by a secondary 
maximum as in the shoot. 
A daily periodicity is still evident in the Bean root after seventeen 
days’ and in the shoot after nineteen days’ continuous darkness. 
Under the continuous ligiit conditions, a change also takes place, the 
usual normal early morning maximum giving place to an earlier one, 
though not so early as in the dark — in this case about eight hours 
earlier. It occurs first in the root, being the organ most affected by the 
strange conditions, and is followed by the shoot. 
Periodicity was still evident in the Bean root after eleven days’ and in 
the shoot after nineteen days’ continuous illumination. In this case the 
light at night was diffuse and probably favourable to growth. 
Seasonal changes affect the maximum growth. In the summer : — 
(a) The normal maximum is earlier than in winter — because at 
this time the amount of light to darkness is 18 : 6 hours, only about one- 
third the amount of darkness present in February and March, so that 
there is a quicker rise to a maximum after the very short period of 
stimulation. 
(b) The maxima are more variable than in the winter. The root 
maximum also varies more, but is usually from midnight to 5 a.m. 
