Photoperiodic Responses of Hemp

1. Flowering of hemp, Cannabis sativa L., variety Kentucky, occurred in all plants at photoperiods of 8-14 hours, inclusive. At photoperiods of 16-20 hours flowering was incomplete and greatly delayed. 2. Seedling plants 3-5 weeks old flowered within about 2 weeks when shifted from 16-hour to 8-hour photoperiods. Five-week-old plants required fewer 8-hour photoperiods than did 3-week-old plants for equal promotion of flowering. 3. When one branch of a two-branched plant was subjected to short photoperiods, the production of flowers was restricted to that branch. Temperatures as low as 55⚬ F. applied during the dark periods did not inhibit flowering. 4. Intensities of light from an incandescent-filament lamp greater than 0.03 ft-c. inhibited flower-bud initiation of hemp when applied daily during the 16-hour periods between successive 8-hour periods of natural light outdoors. 5. Pollination of female plants with pollen from male flowers produced on female plants resulted in seed that produced only female plants. This "female" seed was produced in such abundance that subsequent experiments were done with plants from this source. 6. The relative abundance of nitrogen in the nutrient solution did not greatly modify the extent to which male flowers were formed on female plants, but it influenced their rate of development. 7. Full intensity of natural light resulted in production of male flowers on a greater percentage of female plants than did lower intensities of natural light. 8. Transfer of plants from long photoperiods to photoperiods appreciably shorter than that critical for flowering resulted in production of male flowers on a greater percentage of female plants than occurred when they were transferred to photoperiods only slightly shorter than the critical one. 9. A low temperature during or immediately prior to photoperiodic induction resulted in more extensive production of male flowers on female plants than did a higher temperature. 10. The tendency of plants of certain female lines to form male flowers was significantly greater than that of plants of other lines, indicating that selection for abundant production of male flowers on female plants might be successful. 11. Detailed dissection of all the buds on plants that had failed to flower when grown on long photoperiods revealed the presence of a few floral primordia in the axils of certain fully expanded leaves. Both male and female flower buds were present in these positions, but they failed to continue development except when subjected to short photoperiods.