Abstract
Three species of the phaeophycean genus Padina were investigated with the electron microscope. Calcification is initiated in P. japonica and P. pavonica at the inrolled thallus margin which contains chloroplasts and acts as a semi-enclosed space. In these species, the apical cells are covered with a pilose layer with which small aragonite crystals are associated. The non-calcareous P. arborescens has a similar anatomy but here the apical and young cells possess a smooth cuticular layer instead of a pilose layer and it is suggested that nucleation of aragonite is inhibited by this layer. In P. pavonica, carbonate deposition continues at the thallus surface and reaches a maximum, on a percentage dry weight basis, approximately mid-way along the lamina (mean CaCO3 content 9·3% dry wt). The deposits show concentric banding, but this could not be correlated with differences in photosynthetic activity associated with tetrasporangium production.
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