Bull. Sci. Soc. Philom. Paris sér. 3, 8: 186. 1821.
Plants usually rooted in soil. Sterile leaves lanceolate to lance-ovate to ovate to deltate or cordate. Petiole of sterile leaf 1–31 cm, not inflated. Blade of sterile leaf 1–3-pinnate, 2–41 × 2–20 cm; segments lobed or incised, elliptic to lanceolate to ovate or deltate, to 12.5 cm; proximal pinnae ± alternate. Fertile leaves lanceolate to ovate to deltate or cordate, 2–117 × 2–48 cm. Petiole of fertile leaf 1–46 cm. Blade of fertile leaf 3–4-pinnate proximally, 2-pinnate distally; terminal segments linear. Sporangia usually crowded between segment midvein and revolute margin, with 13–71 indurate annulus cells. Spores 32 per sporangium, 96–124 µm diam. 2n = 154, 156.
Habitat: Aquatic to semiaquatic in swamps, bogs, canals, ponds, lakes, ditches, marshes
Elevation: 0–200 m
Calif., Fla., La., Tex., worldwide in tropical areas except Africa.
Ceratopteris thalictroides is common in Florida but rare elsewhere. It is tetraploid (n = 77, 78), the two cytotypes reproductively isolated. It can be distinguished from the diploid C. richardii on the basis of spore number per sporangium. The single population in southern California may have been a recent introduction and apparently has not persisted. Several populations are of hybrid origin, with reduced spore viability and irregular meiotic pairing. These include some in southern Florida and Texas.