An order of the *Lycopsida
containing two genera, Isoetes (quillworts), with about 70 species, and the monotypic Stylites, both of which contain rushlike predominantly aquatic perennial plants. The stem is condensed into a fleshy rootstock from the top of which arises a bunch of long narrow ligulate leaves. Seasonal cambial activity occurs in a zone around the primary vascular tissue but is highly unusual in that it differentiates cortical rather than stelar tissue. Thus the oldest tissue occurs at the outer edge of the stem and is continually sloughed off as new material is generated from within. The megasporangia are borne in the axils of leaves formed early in the season while the later formed leaves bear microsporangia. The antherozoids differ from those of the Selaginellales and Lycopodiales in being multiflagellate rather than biflagellate.