The genus Ceramium : taxonomic, morphological and cultural investigations

Developmental studies on Ceramium rubrum showed a highly plastic phenotype that is easily modified by environmental conditions. The morphology of cortical bands varied from ''rubrum' type cortication to that of C. rubriforme Kylin and C. areschougii Kylin. These differences are related to the photoperiod under which the plants are grown. Plants cultured under short daylengths (8-16, 10-14 and 12-12 hr) developed 'rubrum' type cortication whereas plants in long photoperiods (16-8 and 24-0 hr) developed 'rubriforme' and 'areschougii' types of cortication. Longer daylengths induced the elongation of axial cells, and during early stages, inhibited the growth of cortical cells restricting them to well defined cortical bands. As plants became older, cortical cells began to divide spreading either 1) acropetally from the node or 2) basipetally and acropetally from the node. Plants eventually became completely corticated and resembled species of Ceramium from Europe. These results suggest that the subgenus Mesoceramium should be merged with Euceramium, and that C. areschougii and C. rubriforme as well as a host of European Ceramium taxa be considered as synonomous with C. rubrum. Heteromorphism between gametophytes and tetrasporophytes in C. rubrum and C. diaphanum occurs in both nature and culture. Male plants of both species in culture were smaller than tetrasporophytes and this may explain their apparent absence in the field. Gametophytic plants of C. rubrum tend to have more tightly curved apices, a higher degree of branching, and larger interior angles at points of dichotomies than tetrasporophytes. Statistical analysis of branching patterns between female gametophytes and tetrasporophytes showed many differences that were significant at the 0.001 probability level. The morpholgy of C. diaphanum was influenced by low light intensities and light quality. Under low light intensities of white light and in blue light cortical bands became highly reduced, resembling C. strictum. Under red, blue and green light this species produced abundant rhizoids thoughout the plant and assumed a prostrate growth form, resembling Ceramothamnion Richards and C. codii (Richards) Mazoyer.