680 BIOLOGICAL EFFECTS OF RADIATION 



the growth and development of the individual species or variety, often- 

 times to a remarkable degree, but the type and the grade of response to 

 relatively long or short days may be very different in different species 

 and varieties. In this paper particular attention is given to length of 

 day in its relation to duration of the vegetative stage, that is, initiation 

 of reproductive processes, including both the quantitative and quahta- 

 tive phases of the subject. 



As a background for their work these investigators previously had 

 been concerned for some time with the peculiar seasonal behavior of 

 certain varieties of tobacco and soy beans. A newly developed form of 

 Maryland tobacco was found to maintain exceptional vegetative vigor 

 through the open growing season without attaining the reproductive 

 stage when propagated in the latitude of Washington, D. C. When 

 grown in the greenhouse during the winter months this form of tobacco 

 made only moderate growth and flowered and fruited abundantly. With 

 certain varieties of soy beans, plantings at wide intervals through spring 

 and early Summer tended to flower at approximately the same date in 

 late summer, the vegetative period being progressively shortened with 

 advance in date of planting. Experiments in varying the temperature 

 and light intensity having failed to materially affect the length of the 

 vegetative stage in the tobacco and soy beans, the simple expedient of 

 shortening by a few hours the midsummer daily exposure to sunlight by 

 use of a dark chamber was tried and very striking results were obtained. 

 The shortened daylight period quickly initiated flowering in the tobacco 

 plants and greatly hastened formation and ripening of seeds in cultures 

 of the Peking variety of soy beans which had already flowered when the 

 test was begun. 



The tests were extended to a wide range of species and varieties and 

 various day lengths were used. The technique employed was simple. 

 During the open growing season large pot or box cultures of the test 

 plants were excluded from daylight for portions of each day according to 

 a fixed schedule and control cultures received the full daylight period. 

 In regulating the daily light exposure a ventilated light-proof structure 

 was employed and the cultures were transferred into and out of the 

 structure by means of trucks mounted on steel tracks. Field plantings 

 of early-, medium-, and late-maturing horticultural varieties of soy beans 

 also were made at intervals through the late spring and summer period to 

 observe the effects of changing day length. During the winter months 

 bench or pot cultures in the greenhouse received each day supplementary 

 artificial illumination of low intensity from late afternoon till midnight, 

 supplied by tungsten-filament incandescent lamps. Control cultures 

 were exposed to the natural daylight only. No effort was made to 

 maintain fixed conditions, but as far as could be ascertained variation 

 in the daily illumination constituted the only important difference in 



