OBSERVATIONS: 



Record date treatments began and length of the light and dark neriods; also record how many short days 

 were required to induce dormancy or to stop growth of the main axis. Measure at daily intervals and plot 

 the length of the main axis of plants from both lots against time in days. 



SUPPLEMENTARY READING: 



Borthwick, H. A. Light effects on tree growth and seed germination. Ohio Jour. Sci. 57: 357-364. 1957. 

 Downs, R, J., and Borthwick, H. A. Effects of photoperiod on growth of trees. Bot. Gaz. 117: 310-326. 



1956. 

 Thimann, K. V. The physiology of forest trees. Pp. 529-583. Ronald Press, New York. 1958. 

 Wareing, P. F. Photoperiodism in woody plants. Ann. Rev. Plant Physiol. 7: 191-214. 1956. 



DEMONSTRATION D-4: Photoperiodic control of bulb formation of onions. 



MATERIALS: 



1. Onion seeds. Plants of southern varieties White Bermuda, Crystal Wax, Eclipse, Excel, and Granex 

 hybrid will bulb on 12-hour days. Plants of northern varieties Australian Brown, Sweet Spanish, Yellow 

 Globe (Early Yellow Globe, Yellow Globe Dan vers, Downing Yellow, and Globe), and Elite hybrid 

 will bulb on 15-hour days. 



2. Sterilized soil. 



3. For each variety, four wooden boxes 8 to 10 inches wide, 10 inches deep, and 12 inches long, with drain- 

 age holes. 



4. A light-equipped chamber. Illuminate the chamber with at least two 40-watt fluorescent lamps. 



5. A dark chamber. 



PROCEDURE: 



1. Fill boxes with sterilized soil and level the soil surface. 



2. Make two shallow furrows one-quarter inch deep and 4 inches apart, lengthwise of the box. 



3. Plant the seeds thinly in the furrows, cover the seeds with soil, and water carefully. Label each box with 

 the name of the variety, the date of planting, and the daylength treatment. 



4. When seedlings are well established, thin the plants to 2 inches apart in the row. 



5. Germinate the seeds and grow the plants at room temperature (70° to 80° F.). 



6. Place two boxes on short days and the remaining two boxes on long days immediately after planting 

 the seeds. 



7. Additional varieties and intermediate daylengths can be used to broaden the experiment. 



OBSERVATIONS: 



Note any differences in the top growth or plant habit at regular intervals during the course of the demon- 

 stration. Differences should become apparent in about 60 days. Carefully remove a few plants at random 

 from a box on each daylength and note any differences in bulbing. Do this at regular intervals to determine 

 the time of bulbing and the treatment on which it occurred. When bulbing is definitely apparent, the ex- 

 periment may be terminated. The plants in the remaining box on each daylength may be harvested and 

 the extent of bulbing on each daylength noted and recorded. 



SUPPLEMENTARY READING: 



Boswell, V. R., and Jones, H. A. Climate and vegetable crops: Onions. In Climate and man. U.S. 



Dept. Agr. Yearbook of Agriculture 1941: 388-389. Government Printing Office, Washington, D. C. 

 Jones, H. A. Onion improvement: Varietal adaptation. In Better plants and animals, II. U.S. Dept. 



Agr. Yearbook of Agriculture 1937: 235-236. Government Printing Office, Washington, D. C. 

 Magruder, R., and Allard, H. A. Bulb formation in some American and European varieties of onions as 



affected by length of day. Jour. Agr. Res. 54: 719-752. 1937. 

 Piringer, A. A. Photoperiodic responses of vegetable plants. Campbell Soup Co. Plant Sci. Symposium 



Proc. 1962: 173-184. 1962. 



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