of Agricultural Research 18(11): 553-606 (March 1, 1920)
Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants.
W. W. Garner, Physiologist in Charge, and
H. A. Allard, Physiologist, Tobacco and Plant Nutrition Investigations,
Bureau of Plant Industry, United States Department of Agriculture1
The importance of the relationships existing between light and plant growth and development has been so long recognized and these relationships have been of so much interest to investigators that a very extensive literature on the subject has been developed. For present purposes it will not be necessary to attempt even a brief review of this literature, and only some of the leading features bearing upon the particular problems in hand need to be touched upon. For more extended discussions of the work in this field the monographs of MacDougal (18)2 and Wiesner (26) may be consulted. Three primary factors enter into the action of light upon plants-namely, (1) the intensity of the light, (2) the quality, that is, the wave length of the radiation, and (3) the duration of the exposure. Most phases of these three factors have been more or less extensively investigated. In the present investigation we are concerned chiefly with the general growth and development of plants and the reproductive processes as affected by the daily duration of the light exposure.
As regards intensity, it seems to be pretty well established that there is an optimum for growth in each species and that for many species this optimum is less than the intensity of the full sunlight on a clear day. Within limits, reduction in light intensity tends to lengthen the main  axis and branches and to increase the superficial area of the foliage of many species. Also, the thickness of the leaf lamina may be reduced, and there may be marked departures from the normal in internal structure, the tendency being toward a less compact structure. So far as is known, no important general relationships between differences in light intensity and reproductive processes have been experimentally demonstrated.
The comparative effects produced by different regions of the spectrum, including the ultra-violet, have been extensively investigated but with more or less conflicting results. The most extensive investigations on the subject, perhaps, have been made by Flammarion (8). It was found that there is abnormal elongation of the principal axis in several species under the influence of the red rays, while growth is markedly reduced under the green and especially under the blue rays. In some plants, however, such as corn, peas, and beans, growth is greatest in white light. Some plants blossomed considerably earlier in red light than in white. White light produced the greatest weight of dry matter. Leaves of Coleus developed decided differences in color patterns under differently colored lights. In subsequent work Flammarion has extended his studies to a large number of species.
The duration of the daily exposure to light needs to be considered in three separate phases—(1) continuous illumination throughout the 24-hour period, (2) continuous darkness throughout, leading to the phenomena of etiolation, and (3) illumination for any fractional portion of the 24-hour day. Under natural conditions continuous sunlight throughout the 24-hour period occurs, of course, only in very high latitudes. Schübeler (23) observed the behavior of several species transported from lower latitudes and grown in northern Scandinavia under continuous sunlight lasting for a period of two months. In the species under observation the vegetative period was shortened and the seeds produced were larger than the normal. It is stated, also, that there was an increased formation of aromatic and flavoring constituents. Another method of securing continuous illumination consists in the use of artificial light for illumination or in the supplementing of normal daylight with artificial light, though, of course, the quality and the intensity from the two sources will not ordinarily be the same. Using electric light alone, of an intensity one-third that of sunlight, Bonnier (6) observed a marked increase in chlorophyll formation which extended inwardly to unusual depths. He found also incomplete differentiation of the tissues, recalling, in this respect, the effects of continued darkness. In some instances the color of blossoms was deepened.
Etiolation, resulting from exposure to continuous darkness, has been the subject of much study. In this connection special mention should be made of the work of MacDougal (18) covering a very large number of species. This author also presents a comprehensive survey of previous  work on the subject. In most instances stems, and frequently leaves, exhibited negative geotropism in the absence of light. In all species investigated etiolated tissues show a lesser degree of differentiation than the normal. In this connection MacDougal points out that the differences exhibited between etiolated specimens and normal plants demonstrate the fact that growth, or increase in size, and development, or differentiation, are distinct processes capable of separation. For present purposes perhaps his most important observation is that in no plant investigated had the stamens and pistils attained functional maturity.
The effects of differences in the length of the daylight period, the subject of the present study, have not been so extensively investigated as most other phases of light action. Obviously the problem may be approached in any one of four ways: by comparing the behavior of plants when propagated in different latitudes, by growing plants at different seasons of the year in the same latitude, by supplementing the daylight period with artificial light, and by preventing light from reaching the plant for a portion of the normal daylight period. In the records of attempts to grow various plants in different parts of the world there are undoubtedly a great deal of available data bearing on the present problem; but apparently no systematic effort has been made to utilize this material, the reason probably being that the importance of the relative length of day in affecting plant processes, and, in particular, reproduction, has not been appreciated. Bailey (3, 4, 5,) carried out an extensive series of tests in which daylight illumination was supplemented by the electric are light applied for different portions of the night. The addition of the artificial light induced blossoming and seed formation in spinach. The additional light also favored the growth of lettuce. Rane (20), using the incandescent filament electric light, and Corbett (7), employing incandescent gas light, observed that certain flowering plants and some vegetables blossomed somewhat earlier when the normal daylight illumination was supplemented with artificial light In most of these tests the artificial light was applied for the entire night, but apparently the results so far as concerns reproduction were essentially the same as when the plants were darkened for a portion of the night. Tournois (24, 25) has reported the results of an interesting experiment with hemp (Cannabis sativa L.) and a species of hops (Humulus japonicus Sieb. and Zucc.) in which these plants were exposed to sunlight only from 8 a. m. to 2 p. m. daily. It had been shown by Girou de Buzareingues (10) as early as 1831 that when planted in the late winter or very early spring months the hemp plant first develops in the spring a number of abnormal sterile blossoms in the leaf axils and later produces normal flowers at the regular blossoming period. Following up this fact Tournois concludes from the above-mentioned experiment that the abnormal  blossoming period is induced by the short length of day prevailing in the early spring months.
In a few words, previous work on light action clearly indicates that permanent exclusion of light effectually prevents completion of the blossoming and seed-forming processes, while in certain cases lengthening the normal daily period of illumination by the use of artificial light or by propagation in far northern latitudes hastens the approach of the blossoming period, and, in the case of two species, shortening the daily exposure to light induces the formation of precocious blossoms. That the relative length of the day is really a dominating factor in plant reproduction processes, as is demonstrated in the present paper, seems not to have been suspected by previous workers in this field.
In 1906 there were observed in a strain of Maryland Narrowleaf tobacco (Nicotiana tabacum, L.), which is a very old variety, several plants which grew to an extraordinary height and produced an abnormally large number of leaves. As these plants showed no signs of blossoming with the advent of cold weather, some of them were transplanted from the field to the greenhouse and the stalks of others were cut off and the stumps replanted in the greenhouse. These roots soon developed new shoots which blossomed and produced seed, as did also the plants which had been transferred in their entirety. This very interesting giant tobacco, commonly known as Maryland Mammoth, which normally continues to grow till cold weather in the latitude of Washington, D. C., without blossoming, proved to be a very valuable new type for commercial purposes, but the above-mentioned procedure has been the only method by which seed could be obtained. The type bred true from the outset, and no matter how small the seed plant the progeny have always shown the giant type of growth when propagated under favorable summer conditions. It maybe remarked at this point that inheritance of gigantism1 in this tobacco has been studied by one of the present writers (2) and it has been shown that this character acts as a simple Mendelian recessive.
1 Throughout this paper, the term gigantism is used to signify a tendency toward more or less indefinite vegetative activity manifested by plants under certain favorable environmental conditions. Though an inherited characteristic, it may come into expression only under definite conditions of environment; and the present investigation seems to make it clear that the length of the daily light exposure is the controlling factor.
On one occasion it was observed that seedlings of the Mammoth transplanted to 8-inch pots in late winter blossomed in early spring after reaching a height of some 3 feet and developed an excellent crop of seed. From this it was at first concluded that growing the plant under conditions of partial starvation would induce blossoming, but this idea proved to be erroneous. Repeated attempts during the summer months to force blossoming by subjecting the plant to conditions which would permit only limited growth were futile. On the other hand, it was found that  seedlings grown in the greenhouse during the winter months invariably blossomed without regard to the size of the pot containing the seedling or the extent to which the plant was stunted by unfavorable nutrition conditions. The seedlings behaved, therefore, like the summer-grown giant plants which were transferred to the greenhouse late in the fall. Finally, it was observed that the shoots which were constantly developing from the transplanted roots of giant plants transferred to the greenhouse blossomed freely during the winter months, but as early spring advanced blossoming soon ceased and the younger shoots once more developed giant stalks. Obviously, then, the time of year in which the Mammoth tobacco develops determines whether the growth is of the giant character. During the summer months the plants may attain a height of 10 to 15 feet or more and produce many times the normal number of leaves without blossoming, while during the winter months blossoming invariably occurs before the plants attain a height of 5 feet. Naturally it became of interest from both a practical and a scientific standpoint to determine the factor of the environment responsible for the remarkable winter effect in forcing blossoming. It may be added just here that gigantism also has been observed in several distinct varieties of tobacco other than the Maryland-namely, in Sumatra, Cuban, and Connecticut Havana.
Again, in following out an investigation on the relation of the nutrition conditions to the quantity of oil formed in the seeds of such plants as cotton, peanuts, and soybeans, the present writers (9) had occasion to investigate the significance of the observation made by Mooers (19), that successive plantings of certain varieties of soybeans (Soja max (L.) Piper) made through the summer months, show a decided tendency to blossom at approximately the same date regardless of the date of planting. In other words, the later the planting the shorter is the period of growth up to the time of blossoming. In the course of the investigation on oil formation it became desirable to study the possible effects of temperature differences on the process. Since it is much simpler and cheaper to maintain temperature differences during the winter by the use of heat than during the summer by means of refrigeration, it was planned to make some tests with soybeans during the winter. It was soon found, however, that the plants began to develop blossoms before they had made anything like a normal growth, and the few blossoms produced were cleistogamous, so that it became necessary to abandon the plan of conducting the tests in question during the winter months. As is the case with the Mammoth tobacco, the time of year in which the plants are grown exerts a very profound influence on growth and reproduction in the soybean.
In seeking a solution of the problem as to why the behavior of these plants is radically different from the normal during the fall and winter months one naturally thinks of light and temperature as possible factors. It was observed, however, that both the Mammoth tobacco and the  soybeans still showed the abnormal behavior in the winter even when the temperature in the greenhouse was kept quite as high as prevails out of doors during the summer months. This observation seemed to dispose of temperature as a possible factor of importance in the "winter effect." It is dear that the quantity of solar radiation received by plants is less in winter than in summer, for both the number of hours of sunshine per day and the intensity of the light are reduced during the winter months. The quality of the light also is affected, since the angle of elevation of the sun's path during the winter is less than during the summer and the selective absorptive action of the atmosphere comes into play. It happened that in the investigation on oil formation in seeds a number of experiments had been made with soybeans to determine the effect of light intensity on this process and, incidentally, it was observed that in no case was the date of blossoming materially affected by the intensity of the light. It had been found, also, that partial shading was without decided effect on the blossoming of the Mammoth tobacco. In view of these experiences it hardly seemed likely that the other primary factor controlling the maximum amount of radiation received by the plant—namely, the length of the daily exposure—could be responsible for the effects in question. Nevertheless, the simple expedient of shortening artificially by a few hours the length of the daily exposure to the sun by use of a dark chamber was tried, and some very striking results were obtained, as detailed in the following paragraphs.
PLAN OF THE EXPERIMENTS
The first experiments with the use of the dark chamber were begun in July, 1918. A small, ventilated, dark chamber with a door which could be tightly closed was placed in the field. The soybeans used in the tests were grown in wooden boxes 10 inches wide, 10 inches deep, and 3 feet long. These containers have been extensively used in growing soybeans and other small plants under controlled conditions, and it has been found that normal plants are easily obtained in this way. The dark chamber arid the type of box used for growing soybeans and similar plants are shown in Plate 64, A. Larger plants like tobacco have been grown in large galvanized iron buckets or, in some cases, in ordinary flower pots. When the test plants have attained the desired stage of development the procedure has been to place them in the dark chamber at the selected hour in the afternoon each day. The plants were left in the dark chamber till the hour decided upon in the following morning, when they were again placed in the sunlight. This procedure was followed each day till the test was completed. Appropriate control plants were left in the open throughout the test in each case. By this method the number of hours of exposure to sunlight during the 24-hour period could be reduced as far as desired. 
In the preliminary tests of 1918 special means were provided for moving the boxes and pots containing the plants in and out of the dark chamber. In the spring of the present year a much larger dark house was constructed, and suitable facilities were installed for easily moving the test plants in or out of the house as often as desired. The dark house consisted of a rectangular frame structure 30 feet by 18 feet and 6 feet in height to the eaves and 9 feet to the ridgepole. All crevices by which light could enter were covered, tight-fitting doors were provided, and the interior was painted black. Means were provided at the bottom and top of the house for free circulation of air without the admission of light. A series of four steel tracks, each entering through a separate door, was provided; and on these tracks were mounted a number of trucks carrying the test plants in their containers. This equipment proved very satisfactory. A general view of the dark house, the trucks, and the test plants is shown in Plate 64, B.
It has been rather generally assumed that the pronounced changes in plant activities which come on with the approach of fall are due in some way to the lower mean daily temperatures or the wider daily range in temperature caused by cool nights. It seemed desirable, therefore, to compare the temperatures inside and outside the dark house, and for this purpose thermographs were installed. It was found that there were only slight differences in temperature. The temperature inside the dark house tended to run 2° or 3° higher than the temperature outside, particularly at night. Hence, any responses on the part of the plants resembling those appearing in the fall of the year could not be attributed to lower temperatures. To guard further against possible temperature effects, as soon as the above-mentioned temperature difference was discovered all doors of the dark house were opened as darkness came on each day.
In the various tests the length of the exposure to light was varied from a minimum of 5 hours per day to a maximum of 12 hours, 7 hours and 12 hours being the exposures chiefly used. For the shortest exposure the plants were placed in the dark house at 3 o'clock p. m. and returned to the light at 10 a. m.; for the 7-hour exposure the plants were darkened at 4p.m. and returned to the light at 9 a. m.; and for the 12-hour exposure they were in the dark house from 6 p. m. till 6 a. m. A further modification in exposure consisted in placing the plants in the dark house at 10 a. m. and returning them to the light at 2 P. M. In most instances the daily treatment began with the germination of the seed or in the earlier stages of growth and continued until maturity, but in some cases the plants were permanently restored to the open as soon as blossoming occurred, and in other cases the artificial shortening of the day was not begun until after blossoming had occurred. To facilitate discussion it will be convenient to use the expressions "long day" as meaning exposure to light for more than 12 hours and "short day" as referring to an exposure of 12 hours or less. The term "length of day"  as used in this paper refers to the duration of the illumination period for each 24-hour interval.
As a part of the present investigation a series of plantings of soybeans was made in the field at intervals of approximately three days throughout the season, in order that the effects produced by different dates of planting might be compared with those produced by artificially shortening the length of the daily exposure to light.
BEHAVIOR OF THE PLANTS TESTED
The initial experiment was made in the summer of 1918, and in this instance a box containing the Peking variety of soybeans in blossom and three pots containing Mammoth tobacco plants which had been growing for several weeks were first placed in the dark chamber at 4 p. In. on July 10 and removed therefrom at 9 a. m. the following morning. This treatment was continued each day till the seeds of the beans and tobacco were mature. All subsequent experiments were made during the year 1919. Details of the tests for both years follow.
SOYBEANS (SOJA MAX (L.) PIPER)
|1 Horticultural variety.|
(a) MANDARIN1 (F. S. P. I. No. 36,653), early maturing:
(1) Exposed to light from 10 a. m. to 3 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June 12 on test plants and June i5 on controls. Average height of test plants 6 to 7 inches and that of controls 18 to 20 inches. After blossoming, the growth and development of the seed pods was much more rapid in the test plants than in the controls.
(2) Exposed to light from 9 a. m. to 4 p. m. Planted May S, up May 17, and placed in dark house May 20. First blossoms appeared June 10 on test plants and June i on controls. Average height of test plants 9 to 10 inches and that of controls 19 to 20 inches.
(3) Exposed to light from 6 a. m. to 6 p. m. Planted and placed in dark house June II, up June 16. First blossoms appeared July 7 on test plants and July 14 on controls. Average height of test plants 14 to 15 inches and that of controls 32 to 33 inches. Six weeks after blossoming the seed pods and foliage were still green and the plants stocky, whereas, under the same conditions, the Peking variety, listed below, showed many brown, mature pods, foliage yellowing, and the plants slender.
(b) PEKING 1 (F. S. p. 1. No. 32,907), medium maturing:
(1) Exposed to light from 10 a. m. to 3 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June 12 on test plants and July 21 on controls. Seed pods on test plants  were turning brown by July 18, and all were mature before August 10. Average height of test plants to 6 inches and that of controls 42 to 43 inches. Test plants were restored to normal light exposure June 20.
(2) Exposed to light from 9 a. m. to 4 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June 10 on test plants and July 21 on controls. Average height of test plants 8 inches and that of controls 45 to 48 inches. See Plate 65.
(2a) Exposed to light from 9 a. m. to 4 p. m. Planted May 8, up May 17, placed in dark house June 7. First blossoms June 29. Average height of plants 16 to 17 inches.
(3) Exposed to light from 9 a. m. to 4 p. m. after blossoming. Planted May 7, blossomed July 9, and first placed in dark house July 10. By July 26 there were many full-grown pods on test plants while there were none on controls more than half-grown. By August 29 the leaves had yellowed and were falling, and some pods were fully ripe on test plants while control plants were still green. By September 7 all seeds were fully ripe on test plants, but those on controls did not fully mature till about October 1. See Plate 66.
(4) Exposed to light from 6 a. m. to 6 p. m. Planted and placed in dark house June 11, up June 16. First blossoms July 7 on test plants and August 6 on controls. Average height of test plants 14 to 15 inches and that of controls 39 to 40 inches.
(5) Exposed to light from daylight to 10 a. m. and from 2 p. m. till dark. Planted June 14, up June 19, and placed in dark house June 19. First blossoms July 29 on test plants and August 11 on controls. Average height of test plants 25 to 26 inches and that of controls 41 to 42 inches.
|1 Horticultural variety.|
(c) TOKYO,1 late maturing:
(1) Exposed to light from 10 a. m. to 3 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June 13 on test plants and July 29 on controls. Average height of test plants 7 to 8 inches and that of controls 49 to 50 inches. Test plants were restored to normal light exposure June 20.
(2) Exposed to light from 9 a. m. to 4 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June 13 on test plants and July 29 on controls. Average height of test plants 7 to 8 inches and that of controls 49 to 50 inches.
(2a) Exposed to light from 9 a. n. to 4 p. n. Planted May 8, up May 17, and placed in dark house June 7. First blossoms appeared July . Average height of plants 23 to 24 inches.
(3) Exposed to light from 6 a. m. to 6 p. m. Planted and placed in dark house June 11, up June 16. First blossoms appeared July 14 on test plants and August 21 on controls. Average height of test plants 17 to 18 inches and that of controls 42 to 43 inches.
(4) Exposed to light from daylight till 10 a. m. and from 2 p. m. to darkness. Planted June 14, placed in dark house June 16, up June 19. First blossoms appeared August 20 on test plants and August 23 on controls. Average height of test plants 24 to 25 inches and that of controls 42 to 43 inches.
|1 Horticultural variety.|
(d) BILOXI,1 very late maturing:
(1) Exposed to light from 10 a. m. to 3 p. m. Planted May 8, up May i', and placed in dark house May 20. First blossoms appeared June 16 on test plants and September 4 on controls. Average height of test plants 6 to 7 inches and that of controls 7 to 8 inches. See Plate 68, A. Test plants were restored to normal light exposure June 20.
(2) Exposed to light from 9 a. m. to 4 p. m. Planted May 8, up May 17, and placed in dark house May 20. First blossoms appeared June i on test plants and September 4 on controls. Average height of test plants ii inches and that of controls 57 to 8 inches. See Plate 67.
(2a) Exposed to light from 9 a. m. to 4 p. m. Planted June 10, up June i, and placed in dark house June 24. First blossoms July 22 on test plants and September i on controls. Average height of test plants 15 to 16 inches and that of controls 6 to 8 inches.
(3) Exposed to light from 6 a. m. to 6 p. m. Planted and placed in dark house June 11, up June 16. First blossoms appeared July 14 on test plants and September 8 on controls. Average height of test plants 23 to 24 inches and that of controls 54 to 55 inches. See Plate 68, B.
(4) Exposed to light from daylight to 10 a. m. and from 2 p. m. to darkness. Planted June 14, placed in dark house June 16, up June 19. First blossoms appeared September 6 on test plants and September 15 on controls. See Plate 69, A. Average height of test plants 39 to 4° inches and that of controls 47 to 48 inches. In all of the above-described tests with soybeans observations were made on from 20 to 23 individuals.
TOBACCO (NICOTIANA TABACUM AND N. RU5TICA L.)
|1 Horticultural variety.|
(1) NICOTIANA TABACUM;1 MARYLAND MAMMOTH, giant type:
(1) Exposed to light from to a. m. to p. m. Observations on 14 test plants and 10 controls. Planted March 6, transplanted to 6-inch pots May 10, and placed in dark house May 14. First blossoms appeared July 8 to August 14 on test plants and in last week of October on controls. Average height of test plants 14 to 16 inches and that of controls 3 to 5 inches.
(2) Exposed to light from 9 a. m. to 4. p. m. Observations on 7 test plants and 10 controls. Planted March 6, transplanted to 6-inch pots May 10, and placed in dark house May 14. First blossoms appeared July 18 to August 1 on test plants and in last week of October on controls.  Average height of test plants 12 to 14 inches and that of controls to 6 inches.
(2a) Exposed to light from 9 a. m. to 4 p. m. Observations on 8 test plants and 8 controls. Planted January 8, transplanted to 8-inch pots May 3, and placed in dark house May 14. First blossoms appeared July 5 to 25 on test plants and October 1 to 25 on controls. See Plate 70.
(2b) Exposed to light from 9 a. m. to 4 p. m. Observations on three test plants and four controls. Planted April 14, transplanted in steam-sterilized soil in 12-quart iron pails and placed in dark house June 10. First blossoms appeared August ii to 7 on test plants and August 30 to September 8 on controls. Average height of test plants 37 inches and that of controls inches.
(3) Exposed to light from 6 a. m. to 6 p. m. Observations on 6 test plants and controls. Planted April 14 and transplanted to 12-quart iron pails containing steam-sterilized soil and placed in dark house June 11. First blossoms appeared August 26 to September 4 On test plants and September 3 to 20 on controls. Average height of test plants 48 inches and that of controls 49 inches, See Plates 71 and 72, A.
|1 Horticultural variety.|
(b) N. TABACUM; STEWART 70-LEAF CUBAN,1 giant type:
(1) Exposed to light from 9 a. m. to 4 p. m. Observations on 6 test plants and controls. Planted April 14 and transplanted in steam-sterilized soil in 12-quart iron pails and placed in dark house June 10. First blossoms appeared August 16 to September 2 on test plants and September 24 to October 10 on controls. Average height of test plants 53 to 69 inches and that of controls 73 to 84 inches.
|1 Horticultural variety.|
(c) N. TABACUM; CONNECTICUT BROAFLEAF:1
(1) Exposed to light from 9 a. m. to 4 p. m. Observations on 11 test plants and 10 controls. Planted April 14 and transplanted to 14-quart iron pails and placed in dark house June . First blossoms appeared July 18 to 24 on test plants and July 17 to 22 on controls. Average height of test plants 38 inches and that of controls 34 inches. Average number of nodes on test plants 36 and same number on controls.
(1a) Exposed to light from 9 a. m. to 4 p. m. Observations on S test plants and 6 controls. Planted April and transplanted to 14-quart iron pails and placed in dark house May 28. First blossoms appeared July 13 to 20 on test plants and July 7 to 15, on controls. Average height of test plants 37 inches and that of controls 40 inches.
|1 Horticultural variety.|
(d) N. RUSTICA: CONNECTICUT BROADLEAF:1
(1) Exposed to light from 9 a. m. to 4 p. m. Observations on 5 test plants and 3 controls. Planted April 14, transplanted to 14-quart iron pails, and plants placed in dark house on June 2. Test plants blossomed July to 28 and controls July 1 to 12. 
ASTER LINARIIFOLIUS L.
A common wild aster found in dry, open situations from Maine to Wisconsin and southward. The normal blossoming period begins about September 1 and extends over a period of two or three months.
(1) Exposed to light from 9 a. m. to 4 p. m. Six individuals taken from the field May 13 and transplanted to boxes of the type used for soybeans, three plants to the box. One box of the plants placed at once in the dark house. The control plants soon resumed vegetative development, throwing out numerous axillary branches on the upper portion of the stems as the normal limit in height was approached, thus following the regular course of development in the field. The test plants, on the other hand, made little additional growth and by June 1 were showing tiny flower heads. First blossoms appeared June 18 on test plants and September 12 on controls. Average height of test plants on June 24, 8 to 10 inches and that of controls 14 to 15 inches. Test plants were permanently returned to normal light on June 20. See Plate 72, B.
(2) Exposed to light from 6 a. m. to 6 p. m. Three individuals transplanted from field to each of two 8-gallon iron cans June 10 and those in one can placed in dark house June 12. Tiny flower heads were showing on the test plants by July 2. First blossoms appeared July 19 On test plants and September 20 on controls. Average height of test plants 8 to 9 inches and that of controls 14 to 15 inches.
(3) Exposed to light from daylight to 10 a. m. and from 2 p. m. to darkness. Three individuals transplanted from the field to each of two 8-gallon iron cans on June 14 and those in one can placed in dark house June 16. Flower heads were showing on both test plants and controls by August 20. First blossoms appeared September 16 on test plants and September 18 on controls. Average height of test plants 11 to 12 inches and that of controls 14 to '5 inches.
CLIMBING HEMPWEED (MIKANIA 5CANDENS, L.)
A climbing composite, ranging from southern Maine to Florida and westward to Ontario, Mississippi, and Texas. The normal blooming period extends from late July to the latter part of September. The aerial summer growth perishes in the fall, and the plants are carried over the winter period by perennial underground shoots.
(1) Exposed to light from 9 a. m. to 4 p. m. A number of roots were transplanted from the field to 6-inch pots and placed in the greenhouse in November, 1918. These roots threw up shoots which made considerable growth during the winter months but did not blossom. On June 3 one plant was transferred to each of six 12-quart iron pails, three of which were placed in the dark house at once. The controls began blossoming in late July and continued to blossom profusely till the latter part of September. Some of the plants which had been left in the  greenhouse, where the temperature was much higher than out-of-doors, blossomed at the same time. The test plants behaved quite differently, for blossoming was completely inhibited throughout the summer. Moreover, the growth of the controls has been considerably greater than that of the test plants. See Plate 74.
BEANS (PHASEOLUS VULGARIS L)
Three lots of seed of a tropical bean—two of which came from Arequipa, Peru, and one from Oruro, Bolivia—were planted together in two boxes measuring 3 feet by 10 inches by 10 inches on June 16, and one box was placed in the dark house June 24. Exposed to light from 9 a. m. to 4. p. m. According to Dr. D. N. Shoemaker this bean when planted in the field at Washington has been found to make a very large growth without blossoming till late in the fall, but when propagated in the greenhouse in the winter months the plant promptly blossoms and sets seed. The test plants blossomed July 21 to 23, and some of the seed pods were mature by August 22, whereas the controls did not blossom till October 11. The average height of the test plants was 4 1/2 to feet and that of the controls 7 to 8 feet. See Plate 73.
RAGWEED (AMBROSIA ARTEMISIIFOLIA L)
Exposed to light from 9 a. m. to 4 p. m. Observations based on 6 test plants and 6 controls. Small plants taken from the roadside were transplanted to 6-inch pots on June 3, and a portion of these were immediately placed in the dark house. Staminate heads were showing on the test plants by June i', and the anthers were shedding pollen freely by July i. The controls did not begin blossoming till the last week in August, which is the normal period for the appearance of first blossoms on the plant. The average height of the test plants at the time of blossoming was 8 to 9 inches, while that of the controls on the same date was 11 inches and their final height 29 inches. The test plants were returned permanently to normal light exposure on July t. See Plate 75, A.
RADISH (RAPHANUS SATIVUS L)
|1 Horticultural variety.|
Exposed to light from 9 a. m. to 4 p. m. Planted May 15, up May 19, and placed in dark house on day of planting. The test plants grew more slowly than the controls for a time and then appeared to grow no further. All but two of the test plants, of which there were a large number, became diseased and finally died without forming seed stalks. The two survivors developed a crown of large leaves, and the roots also reached much larger proportions than those of the controls. Apparently enlargement of the roots had not ceased as late as October 15, when one  of them measured nearly 4 inches in diameter while its rosette of leaves measured 30 inches from tip to tip. Flower stems did not develop. The controls grew more rapidly from the outset, and all except three or four to be considered later formed flower stems in June, the first blossom appearing June 21. See Plate 75, B.
CARROT (DAUCUS CAROTA L.)
|1 Horticultural variety.|
Exposed to light from 9 a. m. to 4 p. m. Planted June 4 and at once placed in dark house. The test plants made a uniform but slow growth, and the roots, which were very small, appeared to be devoid of the yellow pigment, carotin, since they were almost snow-white in color. The controls grew and developed normally, the roots showing the normal yellow color. On August 19 the average height of the test plants was 8 to 9 inches and that of the controls 18 to 20 inches. See Plate 79, B.
LETTUCE (LACTUCA SATIVA L.)
|1 Horticultural variety.|
BLACK SEEDED SUMMER:1
Exposed to light from 9 a. m. to 4 p. m. Planted in dark house June 4. Germination was satisfactory, but the seedlings made very little growth, and after a time all died. The controls grew vigorously but under the stimulus of the long day the plants soon sent up flowering shoots and blossomed.
HIBISCUS MOSCHEUTOS L.
A wild perennial in marshes, ranging from Ontario to Florida and Texas. Normal blooming period July to September. Exposed to light from 9 a. m. to 4 p. m. Planted in November in greenhouse. Seed did not germinate till the following March. Seven plants transferred to 12-quart iron pails on June 6, three of which were placed in dark house June 7. The test plants did not blossom nor did they make any growth during the summer. The controls grew vigorously, and the first blossoms appeared August 22 to September 10. The average height of the test plants was 12 inches and that of the controls 29 inches.
CABBAGE (BRASSICA OLERACEA CAPITATA L.)
|1 Horticultural variety.|
EARLY JERSEY WAKEFIELD.1
Exposed to light from 9 a. m. to 4 p. m. Observations based on four test plants and four controls, Transplanted and placed in dark house on June 7. The test plants grew slowly but uninterruptedly throughout the season, although they showed little tendency to form heads. The control plants grew normally and formed large heads which eventually burst open, followed by the formation of new heads of small size. 
VIOLETS (VIOLA FIMBRIATULA SM.)
A common wild species ranging from Nova Scotia to Wisconsin and southward and growing in sandy fields and on dry hillsides. The normal blooming period comes in April. Exposed to light from 9 a. m. to 4 p. m. Two lots of six plants were transferred from the field to two boxes measuring 3 feet by 10 inches by 10 inches on June , and one of the boxes was placed at once in the dark house. The test plants showed flower buds as early as June 21 and were in blossom early in July, producing purple, petaliferous flowers and also cleistogamous flowers. The control plants produced numerous cleistogamous flowers but none of the purple, petaliferous type.
EARLY GOLDENROD (SOLIDAGO JUNCEA AIT.)
The earliest species of goldenrod, ranging from New Brunswick to Saskatchewan and south to North Carolina and Missouri. Blossoming normally extends from late June to September. Exposed to light from 9 a. m. to 4 p. m. Two lots of six plants were transplanted to two boxes measuring 3 feet by 10 inches by 10 inches on June 6, and one of the boxes was at once placed in the dark house. The test plants and the controls blossomed at the same time, late in August. The test plants however, were shorter and more compact than the controls. The heights of the test plants averaged 24 inches and those of the controls 38 inches. The test plants advanced toward maturation more rapidly than the controls after the flowering stage had been reached.
EFFECT OF RESTORING THE TEST PLANTS TO NORMAL LIGHT EXPOSURE AFTER BLOSSOMING HAD OCCURRED
In the experiments with soybeans, aster, and ragweed described above it has been made clear that after blossoming has occurred the effect of shortening the daily exposure to sunlight is to hasten greatly the ripening of the seed. In certain instances, however, as has been recorded under the several experiments, the test plants were restored to the normal light exposure as soon as blossoming had occurred.
Under these conditions seed pods of the soybeans ripened rapidly, the leaves turned yellow, and for a time it appeared that the plants would die as is normal for the soybean. Eventually, however, new branches developed under the influence of the long summer days. The renewed growth was especially well-developed in the Biloxi variety, and the final result was that these plants, still bearing the first crop of ripened seed pods, blossomed for the second time September 4 to 8. This date of blossoming, moreover, is also that for the first blossoming of the control plants which had been planted on the same date as the test plants and had been exposed to the normal daylight period throughout their development
Like the soybeans, the asters after a time responded to the long-day influence; and by July 20 the plants, though bearing ripened seed, were  developing new axillary branches. The new growth finally developed flower heads; and thus the plants blossomed for the second time during the first half of September, which is the time of blossoming of the original controls exposed to the normal daylight period throughout their development.
|FIG. 1.—Graph showing the shortening of the vegetative period preceding flowering In soybeans which results from progressively later planting during the growing season.|
The ragweed, likewise, resumed vegetative development after a time, and, in fact, under the influence of the full length of the daylight period the new growth exceeded in size that of the original plants. The plants blossomed the second time during the last week in August, which is also the time of blossoming of the original controls and of ragweed growing in the field. It may be noted, however, that while the original growth produced staminate spikes as well as pistillate flowers in the usual manner,  the second growth produced pistillate flowers almost exclusively and the leaves were mostly atypical.
RELATION OF DATE OF PLANTING TO DATE OF BLOSSOMING IN SOYBEANS
Through the spring and summer of 1919 a series of plantings of soybeans which included the four varieties used in the tests described above were made in the field at regular intervals of three days as nearly as conditions would permit. All plantings of each variety consisted of rows 10 feet in length. The date recorded as that when first blossoms appeared is in each case that when the majority of the individuals in the planting first showed one or two open blossoms. In most instances the greater number of the individuals in a planting showed their first open blossoms on practically the same date. The dates of planting, germination, and appearance of first blossoms, together with the number of days from germination till blossoming are shown in Table I.
|FIG. 2.—Graph showing changes In length of day during the growing season in the latitude of Washington. D.C. Ordinates indicate 2-hour intervals of the day and abscissae indicate 16-day periods of the growing season.|
The effect of the date of planting on the length of the period from germination to the blossoming stage for each variety is more easily seen in the curves of figure 1, in the construction of which the number of days from April 30 to dates of germination are used as ordinates and the number of days included in the periods of growth prior to blossoming are used as abscissae. The relative length of the day—that is, the time between sunrise and sunset, expressed in 2-hour periods—also is shown for the same period in figure 2. The relative heights of the plants in the consecutive plantings of the Biloxi variety are shown graphically in figure 3. 
|TABLE 1.—Effect of date of planting on date of blossoming of soybeans grown in field at Arlington, Va. 1919|
|FIG. 3.—Graph showing the progressive decrease in height attained by Biloxi soybeans as the dated planting is delayed beyond late spring|
DISCUSSION OF RESULTS
The results of the experiments which have been described show dearly that both the rate and extent of the growth attained by the plants under study and the time required for reaching and completing the flowering and fruiting stages are profoundly affected by the length of the daily exposure to sunlight. The behavior of some of the plants under the different exposures would seem to indicate that the action on the vegetative phase of development is more or less independent of that on reproduction, but only tentative conclusions can be drawn on these points at the present time. The effects of the different light exposures on these two phases of plant development can best be discussed separately.
LENGTH OF DAILY LIGHT EXPOSURE IN RELATION TO VEGETATIVE DEVELOPMENT
Under the conditions of the tests it was not possible to secure quantitative data on the various details of vegetative growth and development, but measurements of height and the photographic records will clearly indicate some of the differences resulting from the various light exposures. In general, the extent of growth was proportional to the length of the daily exposure to light; and this held true when the plants received two daily exposures to light, with an intervening period of darkening, as well as when there was only a single daily exposure to the light. Under the shorter exposures the plants were shorter and less stocky, and there were some indications of etiolation or chlorosis. Histological examination of the test plants was not undertaken, but in most species no very striking differences in gross anatomy resulted from the different exposures. Broadly speaking, the extent rather than the character of growth and vegetative development was chiefly affected. Table II is intended to bring out the relationship between size of plant and length of the exposure to the light for soybeans and the aster. This relationship is strikingly brought out for the Biloxi soybean in figure , which shows the decreasing heights of progressively later plantings. How length of exposure affects the Mandarin is shown in the foreground of Plate 78, B.
|TABLE II.—Effect of length of daily exposure to light on the height of soybeans and aster|
|a The relatively greater heights in proportion to the number of hours in the total daily exposures under this treatment are due to the fact that in this case the length of the growing period was not materially shortened by forced earliness in blossoming; they are not to be ascribed to an increased rate of growth.|
It may be worthy of note that in the tests under controlled conditions the height of the Biloxi plants under a 12-hour light exposure was practically the same as that of the latest field plantings shown in figure 3, while that of the controls was about the same as that of the early field plantings.
Since in many cases the length of the growing period was greatly curtailed by the forcing action of reduced light exposure on reproduction, the amount of growth was necessarily limited thereby in those plants having a determinate type of inflorescence; but, in addition, measurements made when the blossoming stage of the forced plants had been reached show that the rate of growth was greater as the length of the exposure to light increased. The measurements of height recorded under the several tests relate to the final heights attained by the plants. In the species tested no exceptions to the foregoing principle were encountered; but it is possible, of course, that other species will be found to act differently. It has been demonstrated by a number of investigators that when many green plants are transferred from light to darkness the immediate effect is an acceleration in the rate of growth; and, conversely, the first effect of exposure to light is a retarding of growth. These facts, however, bear on necessary relation to the total effect on rate of growth over a considerable period of time produced by differences in the relative length of night and day.
It remains to be pointed out that striking differences in sensitiveness to decreased length of the daily exposure to light were observed in the different species under investigation. Aside from considerable reductions in the rate of growth and slight chlorosis, soybeans, tobacco, aster, and some others showed no ill effects from the reduced length of illumination, while Hibiscus was not able to make any appreciable growth with the illumination period reduced to nine hours, and lettuce was much more seriously affected, all individuals having perished without making any material growth.
LENGTH OF DAILY LIGHT EXPOSURE IN RELATION TO SEXUAL REPRODUCTION
While the rate of growth of the species tested was markedly affected by change in the length of the daily illumination period, the effects on blossoming and fruiting are particularly interesting and important. The experiments with soybeans included four varieties which range from early to very late in maturing under normal conditions when grown in the latitude of Washington, D. C. Thus, for plantings in the field extending through the month of May the average number of days from germination to blossoming was approximately 27, 56, 70, and 105, respectively, for the Mandarin, Peking, Tokyo, and Bioxi, the last-named showing no open blossoms till early September. Table III brings out several important facts regarding the effects of reduced light exposure on these four varieties. 
|TABLE III.—Number of days required by soybeans to reach the flowering stage under daily light exposures of different lengths|
|a In those cases in which the plants were placed in the dark house after they had germinated, only the period elapsing alter they had bent transferred is taken into account, rather than that beginning with the date of germination.|
It is seen that when the daily illumination consists of a single exposure of 12 hours or less, the usual length of the growing period from germination to blossoming is only slightly shortened in the early variety, Mandarin; but the shortening effect is increasingly accentuated as the usual growing period increases, till, in the very lath variety, Biloxi, this period is reduced to less than one-fourth that of the control plants grown under full daylight exposure during the summer months. In reality, all varieties become early maturing ones under these conditions, and there is but little difference in the time required by the four varieties to reach the blossoming stage. These tests also show that reducing the length of the illumination period below 12 hours has no further effect in shortening the vegetative period, so that apparently there is a certain minimum period of light exposure, reduction of which is without action in hastening the appearance of the flowering stage. These results seem to indicate further that for each variety a certain minimum period of time (ordinarily one of vegetative activity) must elapse from the inception of the stimulating action resulting from the reduced light exposure before the flowering stage can be attained. The data in Table III suggest  that this minimum formative period is approximately 21 days for the Mandarin and Peking varieties, 24 days for the Tokyo, and 26 days for the Biloxi, although under suitable conditions these periods might possibly be somewhat further shortened.
Subjecting the plants to two periods of illumination daily, whereby the total daily exposure averaged 9 or 10 hours, was vastly less effective in inducing early blossoming than a single daily exposure of 12 hours; and, in fact, in the later varieties the effect was of little significance. This is true in spite of the fact that the plants were in darkness during the hours of most intense sunlight-namely, from 10 a. m. to 2 p. m. Obviously it is not merely the total number of hours of sunshine received daily by the plant that may induce such marked shortening of the vegetative period, but the continuity of the exposure also plays an important part. The two plantings of soybeans serving as controls, which first appeared above ground on May 17 and June 16, respectively, did not respond in the same manner to the prevailing seasonal conditions. The vegetative period of the Mandarin was lengthened by two days as a result of the later planting, while the later maturing varieties were affected in the reverse manner. These results are in accord with the fact that the average length of day during the vegetative period was longer for the later planting than for the earlier in the case of the Mandarin, while the reverse is true of the other varieties. The marked action of a decrease in the length of the day, within certain limits, in hastening the arrival of the blossoming stage is equally in evidence throughout the stages of seed formation and maturation. This fact is shown by numerous tests; but experiments (a) (1) and (b) (3) with the Mandarin and Peking varieties, respectively, may be cited specifically.
These tests under controlled conditions clearly show that so far as concerns sexual reproduction the Mandarin soybean is adapted to a relatively long day, since the time required by it to reach the blossoming stage during the long summer days can not be greatly reduced by shortening the length of the daily exposure to light. On the other hand, the Biloxi is distinctively a "short day" variety; and with a daily light exposure of 12 hours or less it blossoms almost as early as the Mandarin, whereas the control plantings show that it refuses to blossom during the long summer days when normally exposed to the light. It is interesting to note that, on the basis of these results, all of the four varieties tested should behave similarly when grown under a 12-hour day such as prevails at the equator. The action of the shortened period of daily light exposure in promoting sexual reproduction offers a satisfactory explanation of the fact that there is a marked progressive shortening of the vegetative period in successive plantings of medium and late maturing varieties of soybeans made during the summer months. In this connection an examination of figure 1, showing graphically this progressive shortening in the vegetative period, is of interest. It should be pointed  out here that the progressive decrease in the length of the vegetative period of all varieties apparent in the very early plantings which germinated during the early part of May is probably due to a gradual reduction in the retarding action of relatively low temperatures which prevailed at the time. Again, there is distinct evidence of the retarding influence of lower temperatures on the very latest plantings of the Peking and Biloxi varieties. Eliminating these portions of the curves from consideration, it is evident that the graph for the early variety, Mandarin, is practically horizontal, while there is a marked downward trend in the graphs for the remaining varieties which increases in pitch as we pass toward the later varieties, the drop being quite precipitate in the curve of the very lath variety, Biloxi. There is, in short, a marked tendency for the graphs to converge toward a common point as the summer season advances, a fact which is in full accord with the results of the tests under controlled conditions. Another interesting feature of these curves is that for the period around May 25 to June z there is a more or less well defined "hump" which is most strongly developed in the curve for the Peking, less prominent in that for the Tokyo, and hardly apparent in the curves for the Biloxi and the Mandarin. A possible explanation of this relative lengthening of the vegetative period of the Peking and Tokyo plantings which germinated during the dose of May and early June is to be found in the fact that these plants received the longest possible avenge light exposure. This would not affect the Mandarin or the Biloxi, since the length of the day is well above the "critical" for the Biloxi and below it for the Mandarin. Apparently field plantings can not be extended through the season in such a way as to bring the plants throughout the vegetative period under a light exposure below the critical in length and at the same time secure throughout the period a sufficiently high temperature (and possibly other favorable factors) to reduce the length of the vegetative period to that which experiments conducted under controlled conditions have established as apparently the physiological minimum requisite for sexual reproduction. There can be no doubt that decreasing temperature, within limits, will retard vital activities of the plant; and the fact should be emphasized that, as a rule, the action of decreasing temperatures as fall approaches must be retarding rather than accelerating in its influence on the attainment of the flowering stage by the plant. It should be pointed out here that the hastening effect of the shorter days on the final maturation of the seed of the soybeans is shown by the fact that in the late plantings there is an evident tendency for the early Mandarin and the later Peking varieties to progress toward maturity at the same rate.
As regards the critical length of day required for furnishing the stimulus which brings into expression the processes of sexual reproduction mentioned above, it should be stated that this has not been determined as yet for any of the plants under study, and it is not possible to state how  narrowly defined this maximum length of day capable of inducing sexual reproduction may be. The outstanding fact is that it is quite different for the four varieties of soybeans. In all cases, however, it is in excess of 12 hours.
Coming to tobacco, the contrast in behavior of the Connecticut Broadleaf and the Maryland Mammoth varieties is very striking. Sexual reproduction in the Connecticut Broadleaf is not materially affected by changes in length of day within the seasonal range for the latitude of Washington or southward. On the other hand, the Maryland Mammoth, which is presumably a mutation from a very old variety of Maryland tobacco and appears to be a typical example of gigantism, can not be forced into blossoming during the summer months by any method now known except artificial shortening of the duration of the daily exposure to light, while the character of gigantism is completely suppressed when the plant is grown during the short days of winter. A glance at Table IV shows that shortening the daily light exposure has not materially affected the Connecticut Broadleaf but has been effective in shortening the vegetative period of the Maryland Mammoth. The Cuban type of Mammoth was affected like the Maryland type, but it appears that the former has a somewhat longer vegetative period than the latter under similar conditions. The Maryland type blossoms readily under the influence of a 12-hour light exposure; but there is a suggestion that a time factor is operative here, for the plants seem not to blossom so promptly as when under the 7-hour exposure. It seems probable also that the Cuban Mammoth will blossom under a 12-hour exposure to light. The observation has been made by Lodewijks (17) that a giant type of Sumatra tobacco—grown under the influence of the 12-hour equatorial day—which may reach the extreme height of 24 feet, either does not blossom at all or forms only a few flowers and seeds. Gigantism in tobacco disappears when the plant is brought under the influence of short days such as prevail in the temperate zone during the winter months. Nicotiana rustica, so far as tested, behaves like the Connecticut Broadleaf.
Aster linariifolius, again, has given clean-cut results under the different light exposures, as is shown in the summarized data of Table IV. Its behavior is strictly comparable with that of the Biloxi soybean and the giant type of tobacco. It is a typical "short-day" flowering perennial. As with the Biloxi soybean, however, this maximum length of day capable of bringing into expression the flowering and seed-formation processes is in excess of 12 hours. Exposure to light twice daily was without effect, for the vegetative period of the test plants, counting from the beginning of the experiment, was 92 days and that of the controls (not shown in Table IV) was 4 days. Here, again, attention is called to the fact that the total daily exposure to light averaged only about 10 hours, and the plants were in darkness during the period of most intense illumination, 10 a. m. to 2 p. m. 
|TABLE IV.—Length of the Vegetative period of tobacco and aster as affected by the length of the daily exposure to light|
|a These controls and the test plants having a vegetative period of 52 to 72 days were in 8-Inch pots.|
The composite Mikania scandens L. is of interest as presenting a new type of plants so far as concerns behavior under long-day and short-day conditions. Under short-day conditions which were maintained for nearly 12 months this plant lost its power of blossoming. In other words, the plant became sterile. The early varieties of soybeans and the Connecticut Broadleaf tobacco blossom and fruit freely through the range of seasonal changes in the length of the day which obtains for the latitude of Washington, while the late varieties of soybeans, the giant types of tobacco, and the aster are essentially sterile when under the influence of the long summer days; and Mikania, on the other hand, is sterile during all seasons of the year except summer when long days prevail. It is worth noting that the Mikania was unable to develop flowers during the summer months when kept under the influence of a short daily exposure to light, notwithstanding that it had been growing in the greenhouse for several months previously.
The bean from the Tropics, Phaseolus vulgaris, included in the tests, brings us a step nearer to complete sterility in the latitude of Washington (approximately 39°), for whether it is able to blossom here will depend on the early or late occurrence of killing frost. Evidently it could not blossom very far northward of Washington. Under the influence of a 7-hour daily illumination this bean blossomed in 28 days, and one month later some of its seed pods were mature; yet under outdoor conditions  blossoming did not occur till October 11, 109 days after germination. The fact that this plant does not blossom here till the middle of October indicates that the critical length of day for flowering can not be much in excess of 12 hours; and the physiological minimum for the vegetative period appears to be approximately 28 days, about the same as for the Biloxi soybean. This bean would seem to be admirably adapted to tropical conditions.
The writers are informed by Dr. Shoemaker that in tests made by him at Washington this species in the greenhouse blossomed freely during the winter and developed seed. In the spring some of the plants, having been transferred to pots after the tops had been largely removed, were placed out of doors. New shoots developed, and these grew throughout the summer without blossoming. It is clear that this plant behaves like the Mammoth or giant type of tobacco toward differences in the length of day.
Ragweed is still another example of a short-day plant, for, under a 7-hour exposure, the anthers of the staminate heads were shedding pollen freely within 27 days after the beginning of the test, while under outdoor conditions blossoming did not occur till weeks later. Radish is a good example of the type requiring a long day for attainment of the flowering stage, for, like Mikania, it has not been able to blossom under a 7-hour exposure although the test was continued throughout the summer, while under outdoor conditions blossoms appeared one month after germination. Throughout the test the rosette type of leaf development was maintained under the shortened light exposure, and both leaf and root continued to grow; so here, once more, is apparently a manifestation of gigantism. Under the conditions of the tests, the two biennials, cabbage and carrot, showed no decided response to shortened light exposure so far as concerns flowering; but their behavior under normal conditions indicates that they are to be regarded as typically long-day plants. Hibiscus is a striking example of a long-day plant, for not only is it unable to blossom under a 7-hour light exposure but it is also unable to make any appreciable growth under these conditions. The behavior of Viola is of interest because of the habit of forming both cleistogamous and chasmogamous flowers, the two types appearing at different seasons. It appears that the later developing cleistogamous flowers are to be regarded as forming the more distinctively reproductive organs. Under a 7-hour light exposure, which was not begun till June 7, the plants showed open, purple, petalliferous flowers during the first week in July, although, of course, a previous crop of these blossoms had been produced earlier in the season. The cleistogamous blossoms appeared also on the plants at the usual time, in June. The early goldenrod used in the tests showed no shortening of the vegetative period under a 7-hour exposure.