Phytochrome is a blue-green plant pigment which regulates plant development, including seed germination, stem growth, leaf expansion, pigment synthesis, and flowering. Phytochrome has been found in most of the organs of seed plants and free-sporing plants. It has also been found in green algae. Although phytochrome is an important plant pigment, it occurs in very low concentrations and is not visible unless chemically purified. In this respect, it is different from chlorophylls, carotenoids, and flavonoids. Phytochrome is a protein attached to an open chain tetrapyrrole (four pyrrole rings). The phytochrome gene has been cloned and sequenced and many plants appear to have five or more different phytochrome genes. The phytochrome tetrapyrrole absorbs the visible radiation and gives phytochrome its characteristic blue-green color. Phytochrome exists in two inter-convertible forms. The red absorbing form (Pr) absorbs most strongly at about 665 nm and is blue in color. The far-red absorbing form (Pfr) absorbs most strongly at about 730 nm and is green in color. When Pr absorbs red light, the structure of the tetrapyrrole changes and Pfr is formed; when Pfr absorbs far-red light, the structure of the tetrapyrrole changes and Pr is formed. Natural sunlight is a mixture of many different wavelengths of light, so plants in nature typically have a mixture of Pr and Pfr within their cells which is constantly being converted back and forth. There are three types of phytochrome reactions which control plant growth and development. The "very low fluence responses" require very little light, about one second of sunlight; the "low fluence responses" require an intermediate amount of light, about one sound of sunlight; and the "high irradiance responses" require prolonged irradiation, many minutes to many hours of sunlight. The low fluence responses exhibit red/far-red reversibility and are the best characterized type of response. For example, in the seeds of many species, a brief flash of red light (which forms Pfr) promotes germination and a subsequent flash of far-red light (which forms Pr) inhibits germination. When seeds are given a series of red and far-red light flashes, the color of the final flash determines the response. If it is red, they germinate; if it is far-red, they remain dormant.