Plants monitor changes in day length with a bluish, light-sensitive protein pigment called phytochrome. Phytochrome exists in two forms, based on the wavelength of the light that it absorbs. It is generally produced in meristematic tissues in very minute amounts. The two stable forms can be converted to each other by absorbing light. Pred (Pr) that absorbs red light and Pfar-red (Pfr) that absorbs far-red light. In the daylight, more Pr is converted to Pfr (the active form) than vice versa. Pfr will convert back to Pr over several hours in the dark where it would be stable indefinitely. The conversion in light is almost instantaneous. The phytochrome mechanism is what transforms the crook in the hypocotyls of the emerging seedling into a straight stalk. Stem elongation appears to be inhibited by Pfr. However, if light levels are low, the shaded stems of a tree, for example, more far-red light will reach them and cause the conversion to Pr which lowers inhibition and allows the stems to grow longer and out from under the shade.
The interconversion abilities of phytochrome: