ABSTRACT
The length of a day is known as a photoperiod and the responses of the plant development to a photoperiod is called photoperiodism. The first study on photoperiodism was published by Garner and Allard (1920). They noticed that a late-flowering variety of tobacco, Maryland Mammoth, which had been developed in the southern United States, did not flower during the growing season further north than Washington, D.C. The plants were always cut off by frost in the autumn before flowering occurred. Plants grown in the greenhouse during the winter, however, flowered and fruited freely. Garner and Allard also found that the plants in the greenhouse could be prevented from flowering in the winter by extending the day with artificial light.
