Photoperiodism in Plants.

Flowering of plants depends upon the length of the night. This control of flowering is one of the methods of adaptation of species by which an unfavorable season is anticipated. It implies a time-measuring system that distinguishes between light and darkness through mediation of a pigment. Ways of finding the pigment and explanations of some of the features of seasonal response are described. The pigment, now called phytochrome, is a blue or a bluish-green protein that exists in two forms interconvertible by light. Phytochrome is present to the extent of about 1 part in 10 million in many plant tissues. 18 references, 4 figures.

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