W. W. Garner and H. A. Allards work on Maryland Mammoth, a variety of tobacco and Glycine max established the fact that it was the length of the day, which controlled flowering in these plants.
Garner and Allard suggested the term photoperiodism to designate the response of organisms to the relative length of the day and night and photo period to designate the favourable length of day for each plant.
According to their photoperiods, they classified plants into three groups:
a) Short day plants
Photoperiodism - Long-day, short-day and day-neutral plants
b) Long day plants
c) day neutral plants
The Day-length Requirements for Flowering in Three Categories of Plants
Examples: Nicotiana tabacum, soyabean, strawberry, oryza sativa, chenopodium, cocklebur and chrysanthemum.
Examples: Pisum sativum, Triticum aesativum, Beta vulgaris, Onion, carrot and spinach.
Examples: Tomatoes, cucumbers, sunflower, dandelions and cotton.The terms long day/short day plants are actually misnomers. Research has proved that when the long night period was interrupted by a brief exposure to light, the plants failed to flower. So the scientists concluded that what is essential for flowering is long and uninterrupted dark period rather than a short day length. A brief interruption of the dark period with light nullified the effect of long night.
So short day plants were called long night plants and long day plants were called short night plants.