14 



Photoperiodism: An Outline 



initiation in a relatively few plants appears to be promoted by 

 successive exposures to the kinds of conditions promoting it in 

 classes 1 and 2, in an order depending upon the particular species. 

 Each requirement in a given species may have its own critical 

 daylength. Such plants have been little studied but may be valuable 



Fig. 2-1. Short-day response in morning glory (Ipomoea hederacea var. Scarlett 

 O'Hara). Plants are about 8 weeks old, all grown with 8 hours of sunlight per 

 day. In addition, the plant to the right received a further 8 hours per day of dim 

 (40 foot candles) incandescent light for a total photoperiod of 16 hours. (Photo- 

 graph from Hendricks [1956], American Scientist, 44: 229-247, by permission of 

 the board of editors of the American Scientist and courtesy of Drs. H. A. Borthwick 

 and S. B. Hendricks, U. S. Department of Agriculture.) 



in analyzing the photoperiodic mechanism. Some varieties of wheat, 

 Triticum vulgare, and rye, Secale cereale, may be short-long-day 

 plants; some Bryophyllum species and the night-blooming jasmine, 

 Cestrum nocturnurn, are long-short-day plants. 



5. Day-Neutral or Day length-lndifj event Plants: These simply 

 flower after reaching a certain age or size and apparently irre- 

 spective of daylength. Other processes, however, may be photo- 



