Procedure: 
1. When the plants are large enough to use, divide them into lots A and B. 
2. Both lots of plants should receive 8 to 10 hours of light in the light 
chamber each day. 
3. Place lot A in darkness at the close of each daily light period, Turn off 
fluorescent lamps. Place lot B 3 to 4 feet from a 40-watt incandescent- 
filament lamp, which is now turned on, 
4, If an electric time switch is available, give lot B a total light period of 
18 to 20 hours (8 to 10 hours fluorescent light and 8 to 10 additional 
hours incandescent light). 
5. If an electric time switch is not available, leave the incandescent lamp 
on throughout the night. 
6. Return both lots A and B to the light chamber each morning. 
7. Continue these daily treatments until flower buds are obvious. During 
this period of development both lots of plants should receive photoperiods 
of 18 hours or more. If the treatments are discontinued now, the flowers 
will usually continue to develop. 
Observations: 
Record date of planting, date treatments began, length of the light and dark 
periods, and include the number of short days required to induce flower 
formation, 
Supplementary Reading: 
Doorenbos, J., and Wellensiek, S. J., Photoperiodic control of floral in- 
duction. Ann, Rev. Plant Physiol. 10: 147-184. 1959. 
Lang, A. Physiology of flowering. Ann, Rev. Plant Physiol. 3: 265-306. 
1952: 
Liverman, J, L. The physiology of flowering. Ann. Rev. Plant Physiol. 
62.97 7=210. 1955). 
Naylor, A. W. The control of flowering. Sci. Amer. 186: 49-56. 1952. 
Parker, M. W., and Borthwick, H. A., Influence of light on plant growth. 
Ann, Rev. Plant Physiol. 1: 43-58. 1950. 
Salisbury, F. B. The flowering process. Sci. Amer, 198: 109-117. 1958. 
DEMONSTRATION D-2: Photoperiodic control of flowering of long-day plants. 
Materials: 
1, A light-equipped chamber (a minimum oftwo 40-watt fluorescent lamps). 
2. A dark chamber. 
3. Plants of tuberous-rooted begonia, petunia, or barley. 
Procedure: 
1, Divide the plants into lots AandBas soon as they emerge from the soil. 
2. Both lots should receive 8 to 10 hours of light daily in the light chamber. 
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