Morphogenesis and evolution in theAmphiroideae(Rhodophyta, corallinaceae)

Using light- and scanning electron microscopy of Lithothrix aspergillum and Amphiroa spp., homologies in vegetative anatomy and morphogenesis in the two genera are resolved. The genicula of these genera are functionally comparable non-homologous structures. The long-celled medullary tiers are developmentally equivalent in Lithothrix and Amphiroa as are the short-celled tiers, the latter remaining meristematic to produce cortical sheaths. In Amphiroa the boundaries of the cortical sheaths are manifested at the macroscopic level by transverse markings across thalli in terminal intergenicula, and at the microscopic level by overlapping or otherwise disrupted cortical cells. Thus an intergeniculum of Amphiroa is equivalent to many fused genicula and intergenicula of Lithothrix. The evolutionary implications of this homology are discussed.