582 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1920. 



greenhouses, and by the latter part of December the plants in the 

 control house were in full bloom, having attained a height of about 30 

 inches. In the lighted house the plants continued to grow throughout 

 the winter without flowering and late in spring after long clays had 

 arrived they were placed out of doors. In the following fall when 

 the days had shortened these plants finally flowered after reaching a 

 height of 15 feet. Plate 15 illustrates the behavior of similar lots of 

 plants in the two greenhouses. Varieties of lima bean (Phaseolus 

 hinaims L.) imported from the Tropics, which had failed to flower 

 in the field during the summer, promptly flowered and developed 

 fruits abundantly when transferred to the unlighted greenhouse in 

 October. In the house lighted with electricity the plants grew vigor- 

 ously, but were sterile throughout the winter, a few blossoms finally 

 appearing in April. Plates 16 and 17 bring out the contrast in be- 

 havior under the two sets of conditions. The Maryland Mammoth 

 tobacco and the several varieties of soy bean previously discussed be- 

 haved just as would have been expected from the results obtained by 

 shortening the day during the summer season — that is, the use of elec- 

 tric light to prolong the day inhibited or delayed the appearance of 

 blossoms just as do the long days of summer. These examples will 

 suffice to show that artificial light may be successfully used either in 

 inducing or in inhibiting flowering and fruiting of many plants 

 when it is so applied as to prolong the daily illumination period. 



It is interesting to compare the intensity of the electric light used 

 in these tests with that of sunlight. In summer the intensity of the 

 sunlight at midday may reach as high as 10,000 foot-candles, while 

 in winter the intensity will be something like half this value. The 

 ordinary 40- watt electric light bulb is rated at about 39 foot-candles, 

 which signifies that the indicated intensity applies at a distance of 

 1 foot from the filament. Since the intensity varies inversely as the 

 square of the distance, the intensity of the light of one of these bulbs 

 at a distance of 2 feet would be about 10 foot-candles and at a distance 

 of 10 feet the intensity drops to less than one-half foot-candle. The 

 plants used in the tests under discussion stood at varying distances 

 from the bulbs, but in most cases the distance to the nearest lights 

 probably averaged 4 to 6 feet. Few of the plants, therefore, received 

 a higher intensity of electric light than 5 to 10 foot-candles and many 

 of them considerably less. It is obvious that after the plants above 

 mentioned have been exposed to natural sunlight during the day, 

 artificial light of an intensity as low as one-thousandth that of the 

 sunlight will suffice to bring about the same results with regard to 

 flowering and fruiting as does continuous exposure to the sunlight 

 for the same length of time. In other words, artificial light of very 

 low intensity may be successfully used in lengthening the short 



