DO PLANTS NEED THE DARK? 
ROBERT H. MOULTON 
Strangely Dependent Upon It They Assuredly Are, According To the Latest Discoveries 
That Follow Exhaustive Investigation and Experiment on the Part of Government Experts 
S OTHING in the world is more dramatic than the de- 
nouement which often comes at the close of a long 
series of patiently conducted experiments, that have 
been carried on sometimes so gropingly that the 
layman finds himself wondering if the experimenter has any 
idea at all of what he is trying to do! And dramatic is the result 
at which careful workers have arrived in recently completed 
experiments, covering a period of two years, concerned with the 
effect on plants of light. Everyone — scientist and layman — has 
believed for generations that sunlight was necessary for the nor- 
mal growth of most kinds of plants ; but no one has ever supposed 
that the darkness played any especial part in the process. 
Yet it is now demonstrated that, quite apart from possible 
injury from burning by a too hot sun, plants may be adversely 
affected by too much 
daylight; or, in other 
words, too many hours 
of daylight in propor- 
tion to the number of 
hours of darkness which 
they enjoy. Too long 
a day as well as too 
short a day will pre- 
vent many kinds of 
plants from ever arriv- 
ing at the stage of 
flowering and fruiting. 
It has also been found 
that, although daylight 
in too great proportion 
for flowering and fruit- 
ing may stimulate pro- 
fuse vegetative growth, 
plants will not repro- 
duce except when ex- 
posed to a favorable 
length of day. Length 
of day favorable to 
both reproduction and 
growth seems to be the 
factor that results in the “ever-bearing" type 
of fruits. 
Plant life depends, in short, on light rather 
than temperature for growth, and may be 
controlled by regulating the hours of light and 
darkness. The principle is revolutionary; but 
it rests on actual experiments in which it was 
demonstrated that plants subjected to alter- 
nate periods of light and darkness in carefully 
determined proportions, could be brought to 
maturity at any time of the year. Going even 
further, the experts say that eventually it may 
be found that the animal organism also is capa- 
ble of responding to the stimulus of certain 
day lengths. They believe that the migration 
of birds may be an illustration, on the ground 
that direct response to such a stimulus would 
be more in line with modern biological teach- 
ings than theories which assume that birds 
“go south” as a matter of instinct. 
Furthermore, it has been shown conclu- 
sively that the intensity of the light has 
very much less influence upon the growth of the plant than 
has usually been supposed. Rather it is the number of 
hours; for the experiments show that the flowering and 
fruiting period of practically any plant can be made to take 
place at any time of the year by darkening the greenhouse 
in the morning and evening if the day is too long, or by 
lengthening the day bv artificial light if the day is top short. 
This application of the newly discovered principle will, of course, 
be used by florists and other greenhouse operators. For 
example, Violets bloom naturally only during the comparatively 
short days of spring; but if Violet plants are covered with 
light-proof boxes at night and not uncovered until the sun is 
about “half a house” high each morning during the summer 
time, they can be forced to bloom again in the summer. 
Spring flowers and 
spring crops happen to 
be spring flowers and 
spring crops because the 
days at the season of 
their flowering have the 
necessary proportion of 
hours of daylight to 
bring these particular 
things to flower or to 
maturity at this time. 
Correspondingly, the 
early summer flowers 
and crops must have a 
longer period of day- 
light. This has been 
proven as to a large 
number of plants; and 
scientists believe that 
the principle will hold 
not only throughout the 
higher forms of plant 
life, but that it is prob- 
ably applicable to ani- 
mal life as well. 
Long series of tests 
have been made with a large variety of vege- 
tables and flowers. By employing dark cham- 
bers to shorten the period of light and artificial 
lights to extend it, they have shortened or 
lengthened the life cycle of plants and forced 
some of them to complete two cycles in one 
season. They have brought others into flower 
and fruit months in advance of their regular 
time and, with still others, have greatly de- 
layed and even completely prevented fruiting. 
A test made with Soy Beans is typical of 
the experiments carried on and shows how the 
principle works. Both test and control plants 
were used in order to check up results. For 
the test plants the day was shortened by sev- 
eral hours. That is, they were exposed to the 
light only from io o’clock in the morning until 
3 o’clock in the afternoon. They were placed 
in the dark house May 20, 1919. The control 
plants, otherwise treated exactly like the test 
plants, were left exposed to the light from 
dawn until dark. The first blossoms appeared 
MAKING ASTERS BLOOM TO ORDER 
Note the superior vegetation of plants which lived a normal out-of- 
doors existence, but show no desire to bloom (on right), whereas those 
under artificial light treatment (on left) are already in full blossom 
A DECORATIVE RADISH 
Shorter days meant a longer — 
and more decorative — seedstalk! 
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