Abstract
The elongation of etiolated Avena mesocotyls is inhibited by red light (660 mμ). Immediately after exposing mesocotyl sections to varying doses of red light the ensuing concentrations of phytochrome in the far-red absorbing form (P730) were measured. The extent of mesocotyl inhibition observed 5 days later is proportional to the logarithm of P730 concentration in mesocotyl tissue at the time of red light exposure.