Isoetes (ī́-sō′ ḗ-tēz), a small genus of aquatic plants known as quillworts, which belong to the pteridophytes. By some the genus is put with ferns and by others with club-mosses. It resembles a bunch of fine grass growing in shoal water or mud, but the leaves enlarge at the base and overlap one another on the very short tuberous stem. In each enlarged leaf base there is a single sporangium of interesting structure. Isoetes has attracted a great deal of attention because it is heterosporous, and has the general appearance of monocotyledons. There are some who believe that the monocotyledons have been derived from isoetes or from forms closely allied to it.
ISOETES A quillwort (A) showing microspore (D), sperm (E), megaspore (B), and a female gametophyte (C) with an archegonium (a).
