Abstract
The spectral distribution of radiant energy on the floor of three typical English woodlands (birch, beech and oak) has been monitored throughout the whole growing season. The results are presented in terms of the changes in the ratio of the photon fluxes at 660 nm and 730 nm (τ) and in the photosynthetically active radiation (PAR). The results are discussed in relation to the hypothesis that the role of phytochrome is to detect changed light quality due to mutual shading and to direct development accordingly.