Photoperiodic Behavior of Sunflower

1. The sunflower, Helianthus annuus L., var. Mammoth Russian, has been shown to be a short-day type plant. It will, however, flower under a wide range of photoperiodic conditions, including excessively long days (as long as 20 hours) and a regime in which the daily dark periods are interrupted by 1-hour light periods if it be permitted to grow for a sufficiently long period. 2. Short photoperiods promote flowering both by hastening the initiation of flower primordia and by hastening the development of primordia into macroscropic floral structures. 3. Interruption of the dark periods by light is effective in inhibiting both the initiation of flower primordia as well as the development of primordia into macroscopic floral structures. 4. Sunflower thus exhibits a wide range of photoperiodic conditions under which flowering may take place plus an unusual variability in the time of flowering among individual plants in a given population. This may be related to the phenomenon pointed out by Habermann and Wallac...