Abstract
P. urceolata, P. ferulacea and P. denudata, representing species of cool temperate, tropical and warm temperate affinities, respectively, were investigated in cross-gradient light-temperature culture to determine the effects of light intensity and temperature on growth and reproduction. The patterns of seasonal periodicity of growth and reproduction observed in nature were discussed in light of the results of culture studies. The life histories of all 3 spp. in culture were of the Polysiphonia-type, and explanations were given for the observed preponderance in nature of tetrasporophytic plants for P. denudata and P. urceolata.