The Function of Phytochrome in the Natural Environment—i. Characterization of Daylight for Studies in Photomorphogenesis and Photoperiodism

The daily changes in spectral energy distribution of natural daylight, with special reference to the red and far‐red wayelength bands, are outlined. The effects of solar elevation and sky condition are described. The changes in the proportions of red and far‐red radiation in daylight which occur at sunrise and sunset are discussed in relation to the possible function of phytochrome in photoperiodic timing.