The World Flora Online

Herbes'dioïques, submergées acaules, à souche tubéreuse émettant de grandes feuilles pennées ou bipennées, rarement simples, couvertes d'émergences verruqueuses, subulées ou ramifiées.'Inflorescences'en épis denses.'Fleurs'♂ ♀, très petites, nues, abritées par une bractée persistante, les ♂ à 1 étamine à filet très court; les ♀ à ovaire 2-carpellaire, à placentas pariétaux; styles 2, longs, filiformes soudés à la base.'Capsule enveloppée par la bractée concave, s'ouvrant en 2 valves; graines nombreuses, très petites.\n\t\t\tUn genre avec environ 15 espèces, la plupart à Madagascar, les autres en Afrique tropicale, au Sud de l'Équateur, 1 seule dépassant celui-ci.\n\t\t\tHabitant exclusivement les chutes d'eau et les rapides, fixées aux roches dans les courants les plus violents, les Hydrostachyacées ont été longtemps réunies aux Podostémacées, avec lesquelles elles n'ont de commun que l'habitat; ENGLER en fit, en 1930, un ordre distinct. Elles ne fleurissent qu'émergées, quand les eaux baissent. Leur pollinisation est sans doute anémophile.\n\t\t\tLes Hydrostachyacées congolaises ne sont connues que par de rares exemplaires, ne provenant que de six localités, dont une seule est située au Nord de l'Équateur (rapides de l'Aruwimi). D'abondantes récoltes de ces curieux végétaux (en y comprenant du matériel en liquide conservateur) et des observations sur le vif, sont hautement désirables.

Seeds small, numerous Fruit a small capsule enclosed by the persistent bract, 2-valved Female flower reduced to a 2-carpellate unilocular ovary; ovules numerous, anatropous; styles 2, filiform, exserted from bracts, persistent on the fruit Male flower reduced to a single subsessile stamen; anther thecae 2, dehiscing longitudinally Aquatic herbs attached to rocks in waterfalls and fast-flowing fresh water, usually dioecious, rhizomatous or sometimes stoloniferous Rhizomes discoid or tuberous; roots numerous, radiating Flowers unisexual, solitary in the axil of an enclosing bract, sessile; bracts spirally arranged, imbricate, differing in form between the sexes, distal part of bract variously margined or flanged, proximal part or spur [the éperon of Cusset] concave; sepals and petals absent Inflorescences emergent, scapiform, densely spicate (resembling fruiting spikes of Plantago), produced as water level recedes; peduncle verrucose, rarely smooth or becoming so (on dried specimens at least) as emergences are shed Leaves submerged or emergent, basal, usually rosulate, often fern-like in appearance with a dorsal (abaxial) and ventral (adaxial) surface, 1–2-pinnate, 2–3-pinnatisect (entire or lobed), dilated and ligulate at the base; petiole and rhachis ± dorso-ventrally flattened and ± densely covered with wart-like or tongue-shaped outgrowths (emergences); pinnae (primary divisions of the leaf) numerous, subopposite, ± cylindrical in outline, bearing simple to variously divided emergences all around the pinna axis, sometimes also with secondary branches (pinnules) Fruit a small capsule enclosed by the persistent bract, 2-valved Female flower reduced to a 2-carpellate unilocular ovary; ovules numerous, anatropous; styles 2, filiform, exserted from bracts, persistent on the fruit Male flower reduced to a single subsessile stamen; anther thecae 2, dehiscing longitudinally Aquatic herbs attached to rocks in waterfalls and fast-flowing fresh water, usually dioecious, rhizomatous or sometimes stoloniferous Rhizomes discoid or tuberous; roots numerous, radiating Flowers unisexual, solitary in the axil of an enclosing bract, sessile; bracts spirally arranged, imbricate, differing in form between the sexes, distal part of bract variously margined or flanged, proximal part or spur [the éperon of Cusset] concave; sepals and petals absent Inflorescences emergent, scapiform, densely spicate (resembling fruiting spikes of Plantago), produced as water level recedes; peduncle verrucose, rarely smooth or becoming so (on dried specimens at least) as emergences are shed Leaves submerged or emergent, basal, usually rosulate, often fern-like in appearance with a dorsal (abaxial) and ventral (adaxial) surface, 1–2-pinnate, 2–3-pinnatisect (entire or lobed), dilated and ligulate at the base; petiole and rhachis ± dorso-ventrally flattened and ± densely covered with wart-like or tongue-shaped outgrowths (emergences); pinnae (primary divisions of the leaf) numerous, subopposite, ± cylindrical in outline, bearing simple to variously divided emergences all around the pinna axis, sometimes also with secondary branches (pinnules)

The ? flower is reduced to a single stamen; anther with 2 divergent thecae dehiscing longitudinally; pollen in tetrads Leaves emergent or sometimes floating, often fern-like, borne in a rosette on the rhizome, developing when the water is low, dilated and ligulate at the base, at first small and entire, becoming large, entire, lobed or 2–3-pinnate, the petiole, rhachis and subdivisions covered with emergences of very diverse shape Partly submerged aquatic dioecious (but in one species monoecious, and another monoecious or dioecious) herbs with tuber-like rhizomes, sometimes stoloniferous, the roots attached to rocks in waterfalls and rapidly running water Fruit a small capsule enclosed by the bract, opening by 2 equal valves along the median suture, many-seeded, bearing the persistent styles The ? flower reduced to a unilocular but 2-carpellate ovary; placentas 2, parietal with numerous anatropous tenuinucleate unitegmic ovules; styles 2, glabrous, erect or divergent Seeds ovoid, very light; endosperm cellular, divided into 3 parts but transitory, soon used by the developing embryo and ± nil in ripe seeds Sepals and petals absent Flowers unisexual in dense spikes (synflorescence polytele), sometimes appearing precociously, usually when water low, each flower borne in the axil of a bract consisting of a distal and proximal part, the latter usually termed the spur (éperon) Leaves emergent or sometimes floating, often fern-like, borne in a rosette on the rhizome, developing when the water is low, dilated and ligulate at the base, at first small and entire, becoming large, entire, lobed or 2–3-pinnate, the petiole, rhachis and subdivisions covered with emergences of very diverse shape Partly submerged aquatic dioecious (but in one species monoecious, and another monoecious or dioecious) herbs with tuber-like rhizomes, sometimes stoloniferous, the roots attached to rocks in waterfalls and rapidly running water Fruit a small capsule enclosed by the bract, opening by 2 equal valves along the median suture, many-seeded, bearing the persistent styles The ? flower reduced to a unilocular but 2-carpellate ovary; placentas 2, parietal with numerous anatropous tenuinucleate unitegmic ovules; styles 2, glabrous, erect or divergent Seeds ovoid, very light; endosperm cellular, divided into 3 parts but transitory, soon used by the developing embryo and ± nil in ripe seeds Sepals and petals absent Flowers unisexual in dense spikes (synflorescence polytele), sometimes appearing precociously, usually when water low, each flower borne in the axil of a bract consisting of a distal and proximal part, the latter usually termed the spur (éperon)