SUPPLEMENTARY LIGHTING OF YEAR-ROUND CHRYSANTHEMUMS
In the first experiment we examined the effects on flowering of various constant light levels given throughout the period of growth. The four levels we chose (table 1) covered the range of daily totals of visible radiation received in glasshouses in the South of England during the winter period from September to April.
Flower development was assessed against the scale of developmental stages outlined in table 2 (Cockshull and Hughes, 1967), and the results from this experiment (figure 1) showed that flower initiation and development proceeded normally at the two higher light levels of 60 and 30 cal/cm2/day and the plants were in flower (stage 8) after 10 weeks of shortday treatment, whereas flower development at the other two light levels was delayed. The plants grown at 15 cal/cm2/day, for example, reached our flower stage 8 after 13 weeks of short-day treatment.
In the next experiment, plants were transferred to the 'very low light level' for short periods at different stages in their development. We adopted a light régime of 30 ca1/cm2/day with a carbon dioxide level of 900 ppm CO2 as our 'standard' and sample plants were transferred from this to a 'very low light' régime of 7.5 ca1/cm2/day also at 900 ppm CO2 at weekly intervals. The plants remained in the transfer conditions