Monthly Weather Review. July, 1920. p. 415
EFFECT OF THE RELATIVE LENGTH OF DAY AND NIGHT AND OTHER FACTORS
OF THE ENVIRONMENT ON GROWTH AND REPRODUCTION IN PLANTS.¹
W. W. GARNER and H. A. ALLARD.
(Abstract reprinted from Experiment Station Record. Dept. Agr., Washington, v. 42, no. 9. p. 518.)

The results are given of investigations carried on by the authors in the Bureau of Plant Industry, U. S. Department of Agriculture, in which a dark chamber was used for growing plants, by which the number of hours of exposure to sunlight could be controlled. As a part of the investigation, a series of plantings of soy beans was made in the field at intervals of three days throughout the season, in order that the effects produced by different dates of planting could he compared with those produced by artificial shortening of the daily exposure to light.

Tobacco, soy beans, and a large number of other plants were experimented with, and it was found that the relative length of the day was an important factor in the growth and development of the plants, particularly with respect to sexual reproduction. In some species it was found that the normal plant could attain flowering and fruiting stages only when the length of the day falls within certain limits. Consequently, these stages of development are ordinarily reached only in certain seasons of the year. In the absence of favorable length of day for bringing into expression the reproductive processes in certain species, vegetative development was said to continue more or less indefinitely, thus leading to the phenomenon of gigantism. On the other hand, under the influence of a suitable length of day, precocious flowering and fruiting may be induced. In this way certain varieties or species may act as early or late maturing, depending on the length of day to which they happen to be exposed. The species exposed to a length of day favorable to the growth and sexual reproduction have shown a tendency to assume an ever-blooming or ever-bearing type of development.

The relationship between annuals, biennials, and perennials was studied, and under artificial conditions it was found possible to change the nature of the plants materially. In all species studied the rate of growth was found directly proportional to the length of the daily exposure to light, but within the limits of the experiment light intensity was not found a factor of importance. With soy beans, limiting water, inducing temporarily wilting daily, was without effect on the date of flowering, although the drought hastened the final maturing of the seed. Interrelationships between length of day and prevailing temperatures of the winter season are said to control successful reproduction largely in many species and their ability to survive in certain regions. The authors point out that the relation between the length of the day and the time of flowering is of great importance in crop yields, and indicates the necessity for seeding at the proper time.

¹Journ. Agr. Research, U. S. Dept. Agr., Washington, 18 (1920), No. 11, pp. 553-606, pls. 16, figs. 3.