Isoetes riparia and Its Variants

Abstract

The genus Isoetes has long presented systematists with some difficult problems. Because of the morphological simplicity of the plants (apparently a derived rather than a primitive condition2) and the lack of constant and striking differences in outward form from species to species, taxonomic differentiation has, perforce, been based primarily on the configuration of the gynospore walls. Correlations of other characters have been mostly unsuccessful. An additional difficulty is that collections from many regions have been scanty, likely due to the inconspicuousness of the plants rather than to their rarity. In few parts of the world have the quillworts been more intensively collected and studied than in the northeastern United States; yet even here many entities remain poorly understood. With this in mind, the present attempt is made to reorganize the taxonomy of the Isoetes riparia complex, so as to bring out more clearly the natural relationships involved. Isoetes riparia and its apparent relatives are usually included in the somewhat arbitrary and artificial section Cristatae, characterized by crested rather than tuberculate, spiny, or reticulate gynospore-prominences. Within the section, sub-groups cannot be clearly differentiated, as there are exceptions to most separating criteria. Generally speaking, the I. riparia complex can be distinguished by the plants each having 2 corm-lobes (exception: I. riparia var. amesii); by an amphibious habitat with accompanying constant presence of stomata, which