A family of Pteridophyta, mostly aquatic vascular cryptogams with 2 types of spore (see heterospory). They have a short, stout, basal stock from which arise the roots below, and a dense rosette of usually tubular, quill-like leaves, which bear the sporangia sunken in their expanded bases. The first leaves of the season produce megasporangia in their bases which contain large megaspores that develop into female gametophytes. Microsporangia are produced in later leaves and contain tiny microspores which develop into minute male gametophytes. There are 2 genera, Isoetes (quillworts), with some 70 species of cosmopolitan distribution, and Stylites, with 1 species in the Peruvian Andes.
Subjects: Plant Sciences and Forestry.