Abstract Research on the regeneration in Pterocladia capillacea (Gmel.) Born, et Thur. cultured in vitro. II. Light intensity and development of the erect frond. — The thallus of Pterocladia capillacea consists of two different portions: the erect frond and the creeping axis. The former essentially assimilative and reproductive function while the latter exerts the function of perennation, attachement and vegetative propagation of the alga. As previously shown, both apical and intercalary segments, in culture, grow apically and generate adventitious buds from the cut-surfaces. If the culture are exposed to low light intensities, both the adventitious buds and the preexisting apical meristems give rise to creeping filaments. However, if the light intensity is increased over a threshold level (750–1000 lux) and the light treatment is continued for 12 hours a day, the « distal » tips, both the newly formed adventitious tips and preexistig ones, develop into regular erect fronds. On the contrary, the adventiti...