The pigment that detects light for flowering and germination is phytochrome. Its discovery came from experiments by Borthwick and Hendricks at the US Department of Agriculture in the 1950s.They used lettuce seeds, which need light to germinate. Soaked seeds were exposed to flashes of light of different wavelengths. Red light (about 660 nm) stimulated germination. Far-red light (about 730 nm) inhibited it.When seeds were given alternating flashes, the result depended only on the last flash:R → germinate
R, FR → do not germinate
R, FR, R → germinate
R, FR, R, FR → do not germinate 
The same red and far-red reversal was seen for flowering. A night break of red light stopped a short day plant from flowering, but a flash of far-red light straight after the red cancelled its effect.This photoreversibility suggested a single pigment existing in two interconvertible forms, one absorbing red light and one absorbing far-red light. Absorbing light of one wavelength converts the pigment into the other form. The two forms were named Pr (or P660) and Pfr (or P730). In 1959 the pigment was extracted from plants and shown to be a blue-green protein, which was named phytochrome.The experiments also showed that it is the form of phytochrome present, not simply whether light has been received, that controls the plant's response.
R, FR → do not germinate
R, FR, R → germinate
R, FR, R, FR → do not germinate
The same red and far-red reversal was seen for flowering. A night break of red light stopped a short day plant from flowering, but a flash of far-red light straight after the red cancelled its effect.This photoreversibility suggested a single pigment existing in two interconvertible forms, one absorbing red light and one absorbing far-red light. Absorbing light of one wavelength converts the pigment into the other form. The two forms were named Pr (or P660) and Pfr (or P730). In 1959 the pigment was extracted from plants and shown to be a blue-green protein, which was named phytochrome.The experiments also showed that it is the form of phytochrome present, not simply whether light has been received, that controls the plant's response.