Hydrocharitaceae

Abstract

Monoecious, dioecious or trioecious, perennial or annual, aquatic herbs. Roots simple or, in two genera, branched. Stems monomorphic, dimorphic or polymorphic, elongate or contracted, simple or branched, when branched, then often complex. Leaves heteroblastic or ontogenetically constant, spiral, spirodistichous or spirotristichous, distichous or whorled, scalelike or leaflike, simple; stipules, when present, membranous, translucent, either single and median or paired and lateral, united below to the petiole; petioles, when present, short or elongate, rigid or flexuose, sometimes sheathing below, winged or unwinged, armed with spines or unarmed, sometimes not clearly delimited from the leaf blade; leaf blades, when present, membranous to coriaceous, submerged, floating or rarely emergent, linear to orbicular or cordate; venation acrodromous or reduced to 1 or 3 simple longitudinal veins, cross-veins often present; margin entire with or without spines, flat to undulate or crisped; apex with or without spines. Squamulae intravaginales usually 2–10 or more, or rarely 1 in each leaf axil. Inflorescence scapose or sessile, developing within 1 or 2, free or united, prophylls (bracts) collectively called a spathe; spathes submerged or emergent, membranous to coriaceous, armed or unarmed, winged or unwinged, persistent or evanescent, containing 1–100 or more flowers, arranged in 1–3 monochasia or with more complex systems below the terminal flower; scape when present often curving or spiralising and thus pulling the developing fruits under water. Flowers bisexual or unisexual; actinomorphic or with a tendency to zygomorphy in ontogeny (clear in Nechamandra, Maidenia and Vallisneria); male flowers borne on evanescent pedicels or abscising from parent before anthesis; bisexual flowers often cleistogamous. Sepals 3 or very rarely absent, free, caducous or persisting in fruit. Petals 3 or less or occasionally absent, minute to large and showy, not persisting. Stamens fertile or staminodial, in up to ± 6 whorls of 3 or sometimes more or less, in some plants reduced to 1; staminodia very variable in number, form and function depending on pollination mechanism; anthers basifixed or dorsifixed, with 1–4 microsporangia or by fusion apparently 6-locular. Nectaries, when present, usually 3, rarely more, borne at the bases of the styles or stylodia (probably androecial in origin). Ovary inferior, sometimes apocarpous, 3–20-(or more)-carpellate; placentas parietal with or without dissepiments; styluli or style branches 3–20 or more, simple or forked; stigmas Dry, papillose; ovules few to numerous; amphitropous, anatropous or atropous, placentation laminar, the ovule position often varies from ovule to ovule within a carpel; pistillodium of male flowers when present usually 3-lobed, the lobes often divided, in entomophilous species often nectarial. Fruit a berry or a somewhat fleshy or membranous to coriaceous capsule dehiscing regularly or breaking up at maturity. Seeds ellipsoidal to cylindrical or fusiform, smooth or with highly complex outer testa, exalbuminous, or (Ottelia) with scanty endosperm; at time of germination the embryo often with several leaves; at germination the seedling usually with a well-developed hypocotyl and radicle; hypocotyl often with unicellular hairs.