CLIMATIC FACTORS : RADIANT ENERGY 141 



in the spring, but asters, goldenrods, and chrysanthemums are ex- 

 pected to flower in late summer or fall. When a fruit tree occa- 

 sionally blossoms in the fall, the occurrence is considered unusual. 

 The controlling factor in such periodic phenomena was not recog- 

 nized until Garner and Allard 104 published results of their studies 

 of photoperiodism, or responses of organisms to the relative length 

 of day and night. Their investigations developed from difficulties 



FlG. 72. The effect of long day (15 hours), left, and short day (9 hours), 

 right, on flowering of henbane {Hyoscyamus niger), a long-day plant. All 

 plants received 9 hours of natural radiation. The supplemental light of the 

 15-hour lot was obtained from 100-watt incandescent lamps, which gave an 

 intensity of only about 30 foot candles.— Photo by courtesy of H. A. Borth- 

 ivick, Bureau of Plant Industry, U. S. Dept. Agr. 



experienced in growing new varieties of tobacco and soy beans in 

 the vicinity of Washington, D. C. The tobacco grew vigorously 

 and did not flower under field conditions, but in the greenhouse, 

 during the winter months, it flowered and fruited abundantly. 

 The soy beans flowered and set fruit at about the same date in late 

 summer regardless of how long they had been in the vegetative 

 condition, as determined by plantings spaced at wide intervals 

 during the spring and early summer. When the length of daylight 

 period was shortened for these plants by enclosing them in a dark 

 chamber for a few hours each day, the tobacco flowered very 

 soon and the formation of seeds in the soy beans was hastened 

 materially. 



Some Applications '.—It can readily be seen why garden plants 

 grown for vegetative parts, if they are long-day species, develop 

 best in spring and late fall and, if grown in summer, bolt to form 



