Kinetics of pleuridial growth in Antithamnion plumula (Rhodophyceae)

The filamentous and branched thallus of Antithamnion plumula is constitued of two different kinds of branches with apical growth: the cladomial axes with a continuous or indefinite growth, and the pleuridia with a limited growth. The size of the pleuridia depends on their position with respect to the lateral cladomial axes.The growth kinetics of 35 pleuridia were analysed using Nelder's generalized logistics. Each sigmoidal curve, which was divided into four growth stages from the instantaneous acceleration variations, was thus characterized by ten kinetic parameters: lengths at the time of the changes in growth stage, durations of the growth stages and maximum growth rate.The data showed that all curves displayed a marked left dissymmetry which reflected the brevity of the accelerated growth stage (1/5 of total growth duration). The maximum acceleration stage occurs between the individualization of the pleuridial initial cell and the first mitosis. Previously known experimental data were related to this quantitative result so as to put forward a biological meaning. New research pathways were suggested.The kinetic parameters investigated were used for a classification of pleuridia. The smallest ones displayed the slowest growth: they needed twice more time to reach their maximum size than the largest pleuridia whose growth was the fastest. This association of traits was related to the occurrence of acropetal fluxes along the main axis of the thallus.