Morphogenetic determination of the axial cells of Antithamnion plumula (Ceramiaceae, Rhodophyta): early identification of the nature of the whorls

Abstract

Antithamnion plumula axial cells are either "cladomophoric" (bearing a lateral cladom) or "pleuridiophoric" (with pleuridia only). The time required for their formation (generation time) and their length were the two variables used for their early characterization. The data presented show that (i) the generation time (approximately 10 h) of the pleuridiophoric axial cell, which immediately succeeds a cladomophoric axial cell, is significantly longer than that observed for all the other axial cells (6–7 h); (ii) the initial cell length (approximately 9 μm) of the cladomophoric axial cells is significantly greater than that of the pleuridiophoric axial cells (approximately 6 μm); and (iii) during the first stages of cell elongation (when the distance to the apex does not exceed 12 cells), the difference in size between the axial cells remains significant; the lateral budding of the cladomophoric cell occurs earlier than that of the pleuridiophoric cell. Thus, the cladomophoric axial cells can be distinguished from the pleuridiophoric axial cells very early. Their state seems to be determined by that of the apical initial cell that generates them. The transition between two successive tagmas is always characterized by a significantly longer generation time. Key words: plant morphogenesis, Rhodophyta, Antithamnion plumula, ramification, apical growth, generation time.