Borthwick and Hendrick in 1952 reported the presence of a photoreceptive pigment called phytochrome, a bluish-green coloured compound isolated from the plasma membrane of alga.
There are two types of phytochromes in plants, named as phytochrome-red (Pr) and phytochrome-far-red (Pfr). The Pr is called as 660 nm absorbing form and the Pfr is 740 nm absorbing form. These two forms are inter-convertible, that means, Pr can be converted into Pfr. The Pr can be converted into Pfr if it is exposed to red light, which is faster. Similarly, the Pfr can be converted into Pr if it is exposed to far-red light, which is slow. Since, it has colour, it is called as a chromoprotein.