Temporal morphology and cap formation in Acetabularia--I. Light-dark cycle perturbations.

Cap formation, a major developmental process in the alga Acetabularia, is influenced by a single perturbation of the entraining light-dark schedule and thus, presumably, of the circadian rhythms. This perturbation is brought about several weeks before cap formation, the most conspicuous expression of morphogenesis in Acetabularia. The effect is more pronounced on cap formation than on growth. It varies in importance with the circadian time at which the perturbation was brought about. The effect is dependent on the developmental state of the alga: transfer carried out during the logarithmic phase of growth produces a delay whose importance decreases with time. When carried out during the phase of slow terminal growth, the transfer induces a transitory acceleration of cap formation. When the algae approach their final length, no effect is elicited. Photoperiodism seems to be involved.