Comparative morphology of the glossopodia of three North American Isoetes Ligules

One of the most distinctive features of the heterosporous lycopsids is the presence of a ligule. This structure, currently found only in Isoetes and Selaginella, is comprised of a basally embedded glossopodium and a free distal tongue. Previous studies on Indian species have demonstrated small variations in glossopodia suggesting the possibility that this structure could have taxanomic use. Serial cross, paradermal, and sagittal sections of glossopodia from three different North American species, representing three ploidy levels, were made. Three-dimensional digital rendering of the glossopodia provided comparative data for the three North America species and allowed comparisons with previously published descriptions. In general, the shape of the glossopodium is similar in all three North American species. There are several structural differences among them, such as the shape of the cornua, the length of the medimoles, and the angle of the glossopodium relative to the leaf axis.