DAYLENGTH AND DEVELOPMENT IN 4 SPECIES OF CERAMIACEA (RHODOPHYTA)

Developmental patterns in 4 spp. of Ceramiaceae [Ceramium echionotum, C. rumbrum, C. shuttleworthianum and Antithamnion spirographidis] were determined using excised thallus apices grown under a range of light periods. Models of thallus development and organization based on these patterns are presented. Increased rates of apical cell division, greater growth of apical fragments and increased average cell size were found with increasing number of hours light per day between 8-16 and 16-8 h. No aspect of growth investigated was associated with photoperiodic phenomena and growth occurring during the light break (8-7.5-1-7.5 h) was intermediate between that in 8-16 and 12-12 h. Patterns (3) of cell elongation were found in 4 spp. in which cell age; cell age and position; and cell age, cell position and light period determined cell length at different axial cell positions. Elongation was followed within cells, along axes of A. spirographidis for plants grown under different day lengths. Regions (3) of development were found along main axes: an apical region in which basipetal expansion was greater than acropetal expansion; a zone of stability with equal elongation in apical and basal growth region of cells and a basal region with greater acropetal expansion. With increasing daylength, the zone of stability was extended to greater ranges of cell length.