Mar. 1. 1920 Effect of Length of Day on Plant Growth 6oi 



plants were grown, and 32 plants were grown in the illuminated house. 

 The dates on which the first blossoms appeared on the control 

 plants exposed to the short winter days extended over a range of 

 only about a week — from December 4 to December 10, inclusive. 

 On the other hand, the dates on which first blossoms appeared 

 on the plants exposed to the artificially lengthened day extended 

 over a period of about four weeks — from December 6 to January 

 2, inclusive. On February 12 the control plants averaged uniformly 

 only 24 inches in height and had practically ceased growing and 

 blooming. The plants in the artificially illuminated house, on the con- 

 trary, continued to grow vigorously and to flower freely, having attained 

 an average height of 58 inches, some of the taller being more than 9 feet 

 in height on February 12. These taller plants blossomed much later than 

 the others and produced very few blossoms, thus showing a tendency to 

 become giant forms in response to the artificially produced longer day. 

 The ever-blooming tendency of the pjants as a whole, however, was 

 much more marked under the influence of the lengthened illumination 

 period than in the control greenhouse. Again, although the control 

 plants showed very uniform behavior in the range of their earliest 

 blossoming, it is evident that the artificially lengthened period of illumina- 

 tion has in some manner led to a greatly extended range in the time of 

 blossoming. Whether this really represents an unequal response of 

 several more or less distinct, intermingled races to the artificially in- 

 creased length of day or may be due in part to a more profound physio- 

 logical variability which has been induced can not be determined until 

 systematic selection and breeding studies have been carried on. 



It will be evident that these data dealing with an artificially lengthened 

 illumination period obtained by means of the electric light greatly 

 strengthen the results of the experiments secured during the previous 

 summer by artificially shortening the natural period of illumination 

 through the use of dark houses. The results with the Maryland Mammoth 

 variety of tobacco, the several soybean varieties in question, and the 

 radish are of special significance since they were obtained by methods 

 the direct converse of those used during the summer. Although the 

 intensity of the electric light was undoubtedly far below that of normal 

 sunlight, it was sufficient to initiate or to suppress the reproductive 

 and vegetative activities of these three species as did the long days of the 

 summer time. With respect to the ever-blooming behavior of certain of 

 the plants under study, the results obtained indicate that this behavior 

 is likely to follow when an approximately constant daily illumination 

 period of a duration favorable to both growth and reproduction is main- 

 tained for a su.licient length of time. It thus seems possible that the 

 comparatively uniform length of day prevailing in the Tropics accounts 

 for the particular abundance of ever-bloomers in that region. 



160115°— 20 4 



