140 LIGHT, VEGETATION AND CHLOROPHYLL 



plants and removed at seven o'clock in the morning. The 

 "black-out" must be done with great care; the screens should 

 be well arranged and opaque enough to reduce the illumi- 

 nation on all parts of the plant to -^^ of the normal 

 illumination. 



Other plants which require long, uninterrupted nights to 

 make them flower are begonia, cosmos and Poinsettia 

 pulcherrima (the night temperature must be maintained 

 between 14° and 10° C). 



There is also the economically important Mammoth 

 tobacco of Maryland, which does not flower or produce seeds 

 in the summer in the State of Maryland (latitude 39° N.) 

 because the days are too long, but does so when it is cultivated 

 in a glasshouse in the winter with long nights. In the south 

 of Florida (latitude 26° N.) it also flowers in the winter. There, 

 again, low supplementary illumination at night is sufficient 

 to prevent the flowering entirely. 



In our latitudes, plants which form buds and flowers 

 when they are subjected to long nights find these conditions 

 naturally at the end of autumn, in the winter and at the 

 beginning of spring. In the summer, it is necessary to lengthen 

 the night artificially to make them flower. 



Long-day Plants 



Some plants, on the contrary, require long days and short 

 nights and begin to flower naturally only in summer. 



Nasturtium {Tropaeolum majus), cultivated at a temperature 

 of 17° to 18° C, does not flower if the days are short. 



China-aster {Callistephus chinensis) flowers at a temperature 

 lower than 24° C. only with short nights. When the tem- 

 perature is above 24° C, the length of the day makes no 

 diff'erence. 



Centaurea cyanus, imperialis and suaveoleus flower only 

 during the long days. Supplementary lighting from October 

 to March hastens the flowering and makes it more abundant. 



Other plants like lupin, Buddleia and Browallia are 

 indiff'erent to the length of the day and night. 



