Herbaceous, grass-like aquatics or semi-aquatics. Corm branched or unbranched, 2–4-lobed. Leaves crowning corm, ligulate; base dilated and enclosing sporangia. Sporangia large, covered by thin membranous veil (velum) or naked, containing either megaspores or microspores. Megasporangia on outer leaves; megaspores large, usually white when dry, surface ornamented by tubercles, spines or ridges, rarely almost smooth. Microsporangia, if present, on inner leaves; microspores minute, surface smooth, tuberculate or spiny. Gametophyte unisexual, formed within spore wall; apogamous growth of sporophytes (without fertilization) common.