EFFECT OF LIGHT ON PLANTS — GARNER AND ALLARD. 58l 



cussed which require relatively short days in order to attain the 

 flowering and fruit stages. The difference between the two groups, 

 however, is primarily one of degree and not of kind. In fact, for 

 some plants it is possible to have a day too long as well as one too 

 short to permit normal flowering and fruiting. 



USE OF ELECTRIC LIGHT TO INCREASE THE LENGTH OF DAY. 



The experiments in forcing the flowering and fruiting of plants 

 thus far considered have to do with shortening the daily illumination 

 period during the comparatively long days of summer through the 

 use of dark chambers into which the plants may be placed for a por- 

 tion of the day. During the winter months the days, of course, are 

 much shorter than in summer, and as a consequence many plants 

 which normally flower in summer are unable to do so in winter, even 

 though all other conditions be favorable. The question naturally 

 arises whether these plants can be forced to flower out of season 

 by using artificial light to extend the length of day so as to furnish an 

 illumination period equal in length to that naturally prevailing in 

 summer. If artificial light thus applied is effective, it should be 

 possible in the winter to reverse the results obtained by use of dark 

 chambers during the summer — that is, it should be possible both to 

 prevent the flowering of those plants normally responding to a short 

 day-length and to force flowering of those plants which normally re- 

 spond to a long day. Tests were made along these lines in the usual 

 type of greenhouse fitted with a series of ordinary 40-watt electric 

 bulbs evenly distributed below the glass roof, a total of 34 such lights 

 being contained in a greenhouse 50 feet long and 20 feet wide. The 

 lights were turned on about sunset each day and turned off at mid- 

 night. A specimen of Iris florent'ma L., which normally flowers in 

 May, was placed in the illuminated greenhouse late in October and 

 soon began growing vigorously. By Christmas the plant was in 

 bloom, while a similar specimen in a greenhouse without electric light 

 remained dormant all winter. Spinach planted in the artificial^ 

 lighted house on November 1 was in bloom by the middle of Decem- 

 ber, while a similar planting in the control house did not flower till 

 late in the spring when the days had lengthened. Seed of Spanish 

 Needles {Bidens frondosa L.) planted in the unlighted greenhouse 

 began to flower when less than 2 inches in height, while under the 

 influence of the electric light the plants continued to grow through- 

 out the winter till the experiment was discontinued, having attained 

 a height of some 5 feet without flowering. Plants transferred from 

 the artificially lighted house to the control house when they had 

 attained a height of about 9 inches promptly flowered. Seed of cos- 

 mos {Cosmos bipinnata Cav.) was sowed November 1 in the two 



