roots present (Azolla) or lacking (Salvinia); stems protostelic, dichotomously branched; leaves sessile, alternate,small (ca. 1–25 mm long), round to oblong, entire; veins free (Azolla) or anastomosing (Salvinia); spores of two kinds (plants heterosporous), large megaspores and small microspores, these globose, trilete; spore germination endosporic; x = 9 (Salvinia), the lowest base chromosome number known in ferns, 22 (Azolla).
Systematic remarks
Plants free-floating, subcosmopolitan; fossil record beginning in Cretaceous
