1. Isoetes L. (quillwort)

Plants perennial, heterosporous. Stems fleshy corms, 2- or 3-lobed. Leaves quill-like, linear, hollow (with 4 chambers), the bases broadened and convex, bearing the sporangium in a shallow pit on the inner side and a thin scale, the ligule, above the sporangium. Sporangia thin-walled, partially covered along the margin by a flap of tissue, the velum. Megasporangia with 50–300 trilete megaspores, each 0.25–0.80 mm in diameter, trilete. Microsporangia with 150,000–1,000,000 monolete microspores, each 20–55 mm long, monolete. Gametophytes endosporic, retained within the megaspore walls. One hundred to 150 species.

Species of Isoetes are best distinguished by characters of the megaspores, leaf bases, and habitat. A hand lens is necessary to determine the texture of the megaspores, and microspores are visible only under higher magnification Thus the species can be difficult to identify. Most species undergo an annual cycle of leaf production in which the first leaves of the season are sterile, followed by leaves with megasporangia, then leaves with microsporangia, followed by late season leaves with abortive sporangia.