Abstract
Petalonia fascia (O. F. Müll.) Kuntze, P. zosterifolia (Reinke) Kuntze and Scytosiphon lomentaria (Lyngb.) J. Ag. have been cultured in white, blue and red light of equal quantum irradiance at 15°C. Hairs, knot-filaments and Ralfsia-like crusts were formed only in blue light, whereas the prostrate system of red-grown plants consisted entirely of sparingly-branched and mostly uniseriate filaments. The production of erect thalli from the prostrate system was controlled or stimulated by red light, but these erect thalli became fertile only in the presence of blue light. All three species exhibited all of these types of response, although specific differences in the degree of certain types of response were observed.