The Effect on Certain Plants of altering the 
Daily Period of Light. 
BY 
J. ADAMS, 
Central Experimental Farm, Ottawa, Canada. 
P LANTS growing within the Arctic Circle complete their development 
during the summer months at a time when there is continuous illu¬ 
mination during the whole twenty-four hours. From this it may be inferred 
that a daily alternation of light and darkness is not essential for the growth 
of certain plants. 
On the other hand, plants growing at places between the Equator and 
the Arctic Circle are subject to an alternation of light and darkness during 
each day, the relative length of which varies according to the latitude and 
the time of year. At a spot half-way between the Equator and the pole— 
that is, in latitude 45 0 —during the summer months the period of daylight is 
much in excess of the daily period of darkness. In such localities, assuming 
that the rainfall and soil are suitable, there is an abundant development of 
natural vegetation. The question then arises—Is this alternation of light 
and darkness necessary for the growth of such vegetation, and what would 
be the result if the relative length of day and night were artificially altered, 
other factors being left as far as possible the same? Supposing the periods 
of light and darkness were made approximately equal during each 24 hours, 
what would be the effect on plants as regards (1) the height of the stem, 
(2) the total weight of the plant, (3) the time of coming into flower ? It was 
to obtain some further information on these subjects that the experiments 
detailed below were undertaken. 
Before describing these experiments in some detail it will be advisable 
to review briefly some of the conclusions arrived at by previous investigators. 
Kjellman ( 3 ) experimented with Lepidium sativum in the Arctic 
regions, allowing some plants to remain exposed to continuous illumination 
for 24 hours, while other plants were kept dark from 8 p.m. to 8 a.m. After 
two months he found that the illuminated plants were both taller and 
heavier than those which had been kept in the dark for 12 hours daily. 
Similar differences, but even more pronounced, were found by him in his 
experiments with Arctic plants such as Cochlearia fenestrata and Catabrosa 
algida. 
Curtel’s experiments ( 5 ) in Norway during a period of continuous 
[Annals of Botany, Vol. XXXVII. No. CXLV. January, 1923.] 
