The Garden Magazine 32(2): 98-99 (Oct 1920)
Do Plants Need the Dark?
Strangely Dependent Upon It They Assuredly Are, According To the Latest Discoveries
That Follow Exhaustive Investigation and Experiment on the Part of Government Experts

NOTHING in the world is more dramatic than the denouement 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 normal 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 proportion to the number of hours of darkness which they enjoy. Too long a day as well as too short a day, will prevent many kinds of plants from ever arriving at the stage of flowering and fruiting. It has also been found that, although daylight in too great proportion for flowering and fruiting may stimulate profuse vegetative growth, plants will not reproduce except when exposed 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 alternate 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 capable 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 teachings than theories which assume that birds "go south" as a matter of instinct.


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 Shorter days meant a longer—
and more decorative—seedstalk!

Furthermore, it has been shown conclusively 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 by artificial light if the day is too 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 daylight. 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 probably applicable to animal life as well.

Long series of tests have been made with a large variety of vegetables and flowers. By employing dark chambers 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 delayed 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 several hours. That is, they were exposed to the light only from 10 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 on the dark-house plants on June 16. On the plants that were left in the light all day no blossoms appeared until September 4, or 80 days later. But, while the dark-house plants averaged only 6 or 7 inches in height, those that were left in the light all day grew to an average height of 57 or 58 inches. This test proved conclusively that the Soy Bean requires a short day and a long night for flowering and seed bearing.

In tests with other plants, just the opposite was found to be true. The plants that were left in the light all day did not grow luxuriantly, but produced flowers and seeds, while those that. were kept in the dark a part of the day made abundant growth. The latter, however, were either greatly retarded in producing seed or produced none at all. Temperature moreover, appeared to exert no influence in the tests. A striking illustration of the relative unimportance of temperature lies in the fact that plants kept in the dark for a part of the day underwent, in midsummer, the changes that naturally come in the fall and that, heretofore, have been attributed to lower temperatures. This was true even when the dark houses registered a higher temperature than the outside temperature.

Equally interesting were the results obtained by artificially extending the period of light instead of shortening it. In a test with Ins, for example, the artificial illumination was so arranged as to give 18 hours of continuous light in a greenhouse during the winter, a large number of tungsten filament incandescent lights being used for the purpose. Control plants were kept in a similar greenhouse with no artificial light. This test was begun on October 20, 1919. In the greenhouse where daylight was supplemented with electric light the plants made rapid growth, soon attained normal size and produced blossoms on December 24. On the other hand, the plants in the greenhouse where no artificial light was used, remained practically dormant and showed no tendency to blossom as late as February 12, 1920, notwithstanding that this greenhouse was kept at the same temperature as the other.

Another test was conducted with the seed of Spinach, which was sowed November 1, 1919, and came up in both greenhouses on November 6. The plants in the control house, 20 to 25 in number, grew very slowly, producing low, compact, leafy growths or rosettes, and gave no evidence of blossoming as late as February 12, 1920. The plants in the lighted house, however, elongated very rapidly, soon developed flower stalks, and all blossomed in the period between the dates of December 8 and 23, 1919. These continued to elongate more or less throughout January and February, 1920, blossoming and shedding pollen continuously, thus becoming in effect "ever-blooming" plants.

The advance in agricultural practice which may come through this new discovery will have to be brought about largely by plant breeders and other crop specialists. For instance, it will prove of material significance in the future planning of cropping systems for different regions, especially where consideration of new crops from different latitudes is necessary. The new principle undoubtedly explains the erratic behavior which has been observed with many crops when they are shifted to different latitudes, and may also clear up the conflicting results of variety tests and field tests conducted with the same crops in different regions.

The experiments have shown, for instance, that Ragweed requires for flowering a stimulus that is afforded by the shortening of the days and lengthening of the nights. It does not come into flower until the period of daylight falls below 15 hours. In the latitude of Washington D. C., that comes about July 1. But if Ragweed should be taken to northern Maine and planted, the plants would not experience a proper length of day until after August 1 and, though the vegetative growth might be very rank, they could not mature seed before killing frosts intervened. The long days, therefore, might make it impossible for Ragweed to perpetuate itself in that latitude. On the other hand, plants that get their flowering stimulus from a long day could not perpetuate themselves through seed formation at the equator, where the day never exceeds 12 hours.

This principle seems to clear up the puzzling fact that many plants grow most luxuriantly near the northern limit of their range. The long northern day allows them to attain their maximum growth before the shorter day intervenes to check vegetative growth and start the reproductive process.


This is conclusively proved by the response of plants (on left) raised under usual conditions, and the marked dejection of those (on right) whose light supply was restricted to seven hours daily The almost tropical abundance of the plants (on right) matured in comparative night (their daily light-exposure was only from ten to three) convincingly indicates the magical properties of darkness