74 THE FLOWERING PROCESS 



more efficient use of the available seeds. It is probably best to plant 

 cockleburs in sand, as the sand tends to pull the bur from the 

 seedling during germination. In the early days of work with cocklebur 

 it was thought that the seeds should be removed from the bur, or that 

 the seed should be scarified by trimming the horns off the end of the 

 bur. Such procedures may improve germination somewhat, but they 

 are much more trouble than they are worth. We plant seeds directly 

 with no previous treatment. Soaking the seeds for 24 hr before 

 planting makes them germinate exactly one day sooner after planting! 



3. Control of Day-length 



With short-day plants the day-length must be artificially extended 

 to maintain the plants in a vegetative condition prior to the experi- 

 ment. We do this with incandescent lights in reflectors (see Fig. 5-1). 

 Time switches turn the lights on about 4.00 p.m., in time for the 

 gardener to check them and replace burned out bulbs, and off about 

 1.00 a.m. Plants remain vegetative on dark periods of 8 hr or less, 

 but as a safety factor we seldom allow them to receive a dark period 

 exceeding 5 or 6 hr. 



With some long-day plants incandescent light is best for extending 

 the day-length to induce flowering. This is apparently due to the 

 mixture of red and far-red light produced by incandescent bulbs 

 (see Section III, 2, below). Some studies indicate that incandescent 

 light may also be best for maintaining short-day plants in a vegetative 

 condition, but fluorescent light seems to be quite adequate. Some 

 greenhouses use fluorescent tubes placed along the structural bars 

 supporting the glass, where they cause virtually no more shadow in 

 the daytime than would the structure itself. They are also cheaper 

 to operate, although the initial cost of installation is somewhat higher. 



In greenhouses where short-days must be provided regularly (e.g. 

 to maintain long-day plants in a vegetative condition) black curtains 

 are usually slid over a framework of pipe on the greenhouse bench. 

 The curtains are often pulled over the plants about 4.00 p.m. and 

 removed at 8.00 a.m. 



4. Temperature 



Temperature is relatively unimportant in photoperiodism studies 

 with cocklebur. No temperature treatment yet discovered will cause 

 flowering, but plants will flower on short days over a considerable 



