Phytochrome is a photoreceptor, a pigment that plants, and some bacteria and fungi, use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms. It also regulates other responses including the germination of seeds (photoblasty), elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings. It is found in the leaves of most plants. Biochemically, phytochrome is a protein with a bilin chromophore.Phytochrome has been found in most plants including all higher plants; very similar molecules have been found in several bacteria. A fragment of a bacterial phytochrome now has a solved three-dimensional protein structure.