EFFECT OF LIGHT ON PLANTS — GARNER AND ALLARD. 579 



RESPONSE OP OTHER PLANTS TO REGULATED LENGTH OF DAY. 



Artificial control of the length of day has now been applied to 

 a large number of flowering plants, and it has been found to be 

 an important factor of very wide application in flowering and 

 fruiting. It is not to be expected that all plants would respond in 

 the same way or to any particular length of day. On the contrary, 

 there is the greatest diversity of response in different species and 

 varieties to seasonal change in day length. Additional examples of 

 plants behaving like soy bean and the Maryland Mammoth tobacco, 

 which flower as a result of decrease in length of day, may be first 

 considered. The common wild aster, Aster linariifolius L., which 

 normally begins to flower about September 1, may easily be made 

 to blossom in June by artificially shortening the day to 12 hours 

 or less, as shown in plate 6. Like the soy bean and tobacco, the 

 aster is not forced into early flowering by interposing a period 

 of darkness in the middle of the day, even though the total number 

 of hours of sunlight daily is thereby reduced to as low as 10. A 

 variety of bean {Phaseolus vulgaris L.) imported from the Tropics 

 began to blossom October 11, 109 days after germination, when 

 grown under the full length of day. When given only 7 hours of 

 sunlight daily it began to flower in 28 days, and a month later 

 some of its seed pods were mature. These differences in be- 

 havior are shown in plate 11. Coming from the Tropics, this plant, 

 which is accustomed to a comparatively uniform length of day, 

 with a maximum of about 124 hours, flowers with equal readiness 

 in our hottest July weather or in the cool weather of October, pro- 

 vided only that the day-length is favorable. The common ragweed 

 (Ambrosia artemisiifolia, L.), the date of flowering of which is so 

 well known to sufferers from autumnal hay fever, was shedding 

 pollen within 27 days when exposed to 7 hours of sunlight daily, 

 beginning with June, but required 7 weeks to reach the same 

 stage when exposed to the full day-length. (See pi. 12.) Speci- 

 mens of one of the familiar wild violets of spring (Viola fimbria- 

 tula Sm.) which had already flowered in the field at the usual time 

 in April were transplanted to boxes on June 9 and thereafter ex- 

 posed to sunlight daily from 9 a. m. to 4 p. m. By July these plants 

 were again displaying purple blossoms similar to the typical petalif- 

 erous blossoms of early spring, while control plants exposed to the 

 sunlight throughout the day produced only the cleistogamous flowers 

 which are characteristic of this species during the long days of 

 summer. A very late variety of dahlia known as John Ehlich, 

 which normally flowers about October 1st at Washington, was 

 exposed to 10 hours of light daily from May 12, and under these 

 conditions was in blossom as early as July 8, as shown in plate 13. 

 A striking example of this response to decrease in length of day 



