Most Isoetes spp. develop fleshy lobed corms with, very rarely, vegetative propagation by adventitious plantlets in a sporangial or subsporangial position in sterile specimens, or by branching from the upper or basal portions of the vertical axis. The discovery of Stylites expands the morphological concept of the Isoetaceae as a polymorphic, clinal, monogeneric family. Recognition of Isoetes tegetiformans sp. nov., Heggies Rock, Columbia County, Georgia, USA, in a 20 m2, 30 cm deep pool with pH 7 supports this concept. Unique characteristics include a sexual maturity in permanently distichous plants, a shoot apex which produces leaves and sporophylls only, a tristichous rhizotaxis of demonstrably adventitious roots, unbranched, frequently dimorphic roots, sympodial stem vasculature, a protrate intercalary growth habit and the formation of cauline, adventitious buds. The new taxon is recognized as a member of the sect. Tuberculatae on the basis of its megaspore sculpture and is close to the I. melanospora Engelm. complex of the southeastern USA. Stylites spp. and Isoetes triquetra (A.Br.) Kubitzki et Borchert are assignable to this section and show some morphological variability in vegetative habit. I. melanospora may be a neotenic stage in the development of plants assignable to I. piedmontana (Pfeiffer) Reed, but this determination requires further study.