This paper shows that the hypothetical yellow-light-absorbing pigment P580 is an unnecessary postulate for describing the photobiology of fern filaments. The existence of P580 was originally predicted on the basis of action and response spectra that assumed that filament elongation is the growth parameter subject to direct photocontrol. The present work supports an alternative concept, that the cross-sectional area at the base of the apical dome is the photocontrolled parameter. Far-red irradiation reverses the effects of both red and yellow light, and dose-response curves for yellow light parallel but lag behind the curves for red light. These observations indicate that the responses of fern filaments to the entire long wavelength spectral region (yellow to far-red) can be attributed to absorption of light by phytochrome alone.