1. Phytochromes are proteinaceous pigments present in cell membrane of green cells.
2. Phytochromes receive photoperiodic stimulus and induce flowering in plants in response to light duration.
3. Hendrick and Borthwick observed that in SD plants flowering is inhibited if dark period is interrupted by flash of red light (660 nm).
4. If flash of far red light (730 nm) is given then again flowering is observed in SD plants.
5. Pigment system of plant receives photoperiodic stimulus and these pigments exist in two interconvertible forms Pfr and Pr
