The World Flora Online

SELECTED REFERENCES Jäger-Zürn, I. 2005. Morphology and morphogenesis of ensiform leaves, syndesmy of shoots and an understanding of the thalloid plant body in species of Apinagia, Mourera and Marathrum (Podostemaceae). Bot. J. Linn. Soc. 147: 47–71. Les, D. H. et al. 1997c. The phylogenetic placement of river-weeds (Podostemaceae): Insights from rbcL sequence data. Aquatic Bot. 57: 5–27. Philbrick, C. T. and A. Novelo R. 1995. New World Podostemaceae: Ecological and evolutionary enigmas. Brittonia 47: 210–222. Rutishauser, R. 1997. Structural and developmental diversity in Podostemaceae (river-weeds). Aquatic Bot. 57: 29–70. van Royen, P. 1951. The Podostemaceae of the New World. Part I. Meded. Bot. Mus. Herb. Rijks Univ. Utrecht, 107: 1–151. van Royen, P. 1953. The Podostemaceae of the New World. Part II. Acta Bot. Neerl. 2: 1–20. van Royen, P. 1954. The Podostemaceae of the New World. Part III. Acta Bot. Neerl. 3: 215–263.

Herbs, [annual] perennial, aquatic, attached to rocks and other solid substrata in rapids and waterfalls, submersed in vegetative stage, becoming reproductive as water level drops, exposing plants to air. Roots prostrate, elongate, root cap asymmetric. Stems often trailing in connate basally; stamens [1–]2[–many], [1–2-whorled or incompletely 1-whorled] restricted to 1 side of flower; filaments arising from andropodium [individually, not from andropodium], distinct or connate basally; ovary 2[or 3]-carpellate; stigmas 2[or 3], apical. Fruits capsular, 2-valved [3-valved in Tristicha], [valves persistent] 1 valve falling away after dehiscence. Seeds 0–40[–numerous] per capsule, orange-brown, minute, ovoid, outer integument expanded and sticky when wet.

Herbes aquatiques, rhéophytes, annuelles ou pérennes, dépourvues de racines, fixées directement sur la roche ou sur toute autre surface dure et stable par un thalle photosynthétique, foliacé ou rubané, qui joue le rôle de racine, sur lequel sont insérées des tiges, des feuilles ou même tout simplement des fleurs ; tiges souvent absentes, si présentes simples ou ramifiées, allant de quelques millimètres jusqu’à ± 1 m de longueur. Stipules très souvent absentes. Feuilles parfois réduites ou absentes, si présentes, caulinaires ou en rosette, alternes, distiques ou non, sessiles ou pétiolées, simples ou composées, engainantes ou non (parfois à 2 gaines opposées), parfois le sommet de la gaine allongé en appendices ressemblant à des stipules ; écailles foliacées (feuillettes) parfois présentes (et plantes anisophylles). Inflorescences axillaires, à fleurs solitaires, en groupes ou en épis de cymes, pédonculées ou sessiles, chaque fleur sous-tendue par 2 bractéoles ou contenue, avant l’anthèse, dans une spathelle membraneuse qui se déchire souvent plus ou moins irrégulièrement (hors du Gabon : fleurs parfois sans spathelle, ni bractéoles). Fleurs hermaphrodites, zygomorphes ; pédicelle exsert de la spathelle après l’anthèse ; tépales (1–)2 ou 3 – hors du Gabon, 0 à 20(–50) – , libres ou partiellement soudés, linéaires à filiformes ; étamines 1 à 3(–4) (hors du Gabon, jusqu’à 40), à filets libres, parfois sous-tendues par un androphore de longueur variable, anthères biloculaires, déhiscentes par une fente longitudinale ; ovaire supère, à 2 ou 3 carpelles (hors du Gabon, de 2 à 7) soudés, (1–)2- ou 3-loculaire, sessile ou porté par un gynophore plus ou moins développé, à placentation axillaire, avec de nombreux ovules ; stigmates 1 à 3, sessiles ou subsessiles. Fruits capsulaires, lisses ou ornés de côtes longitudinales, s’ouvrant en 2 valves égales ou inégales. Graines très petites, ellipsoïdes à ovoïdes, à testa réticulé et à cellules mucilagineuses, exalbuminées, à embryon droit.

Sans exception, ses membres sont des rhéophytes saxicoles immergées dans les eaux douces des chutes et rapides. Leur floraison n’a lieu qu’au moment de la baisse du niveau des eaux en saison sèche, quand la plante est soumise à une pression hydrique plus faible et à une luminosité plus importante. On a observé des cas où ce n’est qu’à cette période, généralement courte, de l’année (saison sèche) que la plante démarre sa croissance et la boucle rapidement depuis la phase végétative jusqu’à la floraison. On ignore combien de temps les graines peuvent rester viables, si pour une raison ou une autre cette fluctuation saisonnière n’avait pas lieu, tout comme on n’ignore combien de temps peut survivre la plante, saisonnière par définition, dans une phase purement végétative, si pour une raison ou pour une autre, le niveau de l’eau ne redescend plus.

Les Podostemaceae comprennent environ 49 genres et 280 espèces. Elles vivent immergées dans les chutes ou les rapides des cours d’eau continentaux des régions pantropicales, avec quelques représentants dans les zones tempérées d’Amérique du Nord et d’Asie. C’est la famille la plus importante des Angiospermes strictement aquatique. Beaucoup de ses membres sont endémiques d’un espace géographique restreint, parfois d’une rivière et même d’une seule chute. En Afrique, Madagascar inclus, on compte 16 genres et environ 86 espèces, dont quatre genres (peut être cinq, voir Note ci-dessous) et 19 espèces signalés jusqu’ici au Gabon.

Herbes'aquatiques submergées ayant l'aspect de mousses ou d'algues, fixées aux roches par des racines munies d'haptères, souvent rubanées ou thalloïdes, émettant des tiges souvent nombreuses et dichotomiques.'Feuilles'alternes, membraneuses, linéaires ou filiformes, souvent très divisées ou très réduites.'Fleurs'☿, très petites, le plus souvent renfermées avant l'anthèse dans une spathelle membraneuse qui se.déchire irrégulièrement par allongement du pédicelle; périgone de 5-3 ou le plus souvent de 2 segments très réduits, ou nul; étamines 3-1 (rarement nombreuses), à filets souvent soudés sur une certaine longueur (andropodium); pollen 1-2-cellulaire; ovaire souvent stipité, 3-2-carpellaire, 3-2-loculaire; styles 3-2, courts; ovules nombreux, anatropes, recouvrant un gros placenta.'Fruit'capsulaire s'ouvrant en 3-2 valves.'Graines très petites, exalbuminées.\n\t\t\tQuarante genres et 170 espèces des régions chaudes, surtout en Amérique du Sud; 4 genres et 6 espèces actuellement connues pour le Congo belge.\n\t\t\tLes Podostémacées sont les plus transformées des Dicotylédones; elles paraissent voisines des Saxifragacées par la structure de l'ovaire et des graines, mais en diffèrent par tant de caractères qu'on en a fait un ordre indépendant, monochlamydé. Elles vivent fixées aux rochers dans les cascades et les rapides, ou tout au moins dans les eaux très courantes, exigeant donc une eau très aérée, très éclairée et assez chaude. Les fleurs préparées dans le bouton attendent, pour allonger leurs pédicelles et s'ouvrir, d'être émergées par la baisse des eaux; la pollinisation est sans doute anémophile avec protandrie, mais la cléistogamie est évidente dans certaines espèces :'Leiothylax quangensis'var.'longifolia présente des fruits bien développés dans la spathelle. Des observations in situ à ce sujet sont désirables.\n\t\t\tLes Podostémacées congolaises sont mal connues, ayant sans doute échappé, en raison de leur habitat, à la plupart des collecteurs : on ne les connaît que pour 11 localités, disséminées dans cinq districts, bien qu'elles soient vraisemblablement très répandues dans tout le territoire (sauf dans les montagnes de l'Est), peut-être même, en dehors des vraies chutes et rapides, dans les ruisseaux à cours rapides, comme c'est le cas en Amérique du Sud. La récolte soigneuse de ces intéressants végétaux est chaudement recommandée1, spécialement dans les rapides de l'Uele où ils seraient très abondants.

Seeds minute, exalbuminous Aquatic herbs of rocks or stones in swift-flowing water or spray of waterfalls Plant-body often reduced to a thallus usually attached to the substratum by haptera Shoots (when present) arising from a thalloid base and bearing simple or deeply divided, alternate, exstipulate or minutely stipulate leaves Flowers solitary or cymose, bracteate, hypogynous, zygomorphic Perianth 3-phyllous or rudimentary and then usually of 2 minute more or less subulate tepals in which case the flower is enclosed (and often inverted) within a spathaceous bract (spathella) which ruptures irregularly on emergence of the flower Stamens 1–2 (rarely 3 or 4), filaments free or connate at the base; anthers 4-locular, opening lengthwise; staminode inserted between 2 stamens in Stonesia; pollen-grains solitary or geminate Ovary sessile or shortly stipitate, of 2–3 united carpels, 1–3-locular; stigmas 1–3; ovules numerous on axile placentas Fruit a septicidal capsule Aquatic herbs of rocks or stones in swift-flowing water or spray of waterfalls Plant-body often reduced to a thallus usually attached to the substratum by haptera Shoots (when present) arising from a thalloid base and bearing simple or deeply divided, alternate, exstipulate or minutely stipulate leaves Flowers solitary or cymose, bracteate, hypogynous, zygomorphic Perianth 3-phyllous or rudimentary and then usually of 2 minute more or less subulate tepals in which case the flower is enclosed (and often inverted) within a spathaceous bract (spathella) which ruptures irregularly on emergence of the flower Stamens 1–2 (rarely 3 or 4), filaments free or connate at the base; anthers 4-locular, opening lengthwise; staminode inserted between 2 stamens in Stonesia; pollen-grains solitary or geminate Ovary sessile or shortly stipitate, of 2–3 united carpels, 1–3-locular; stigmas 1–3; ovules numerous on axile placentas Fruit a septicidal capsule

Capsule brown, spherical to ellipsoid or fusiform, smooth or adorned with ± wide longitudinal ribs, dehiscing in dry air into 2–3 equal and sometimes caducous valves, or 2 unequal valves of which only the smaller is caducous Ovary sessile or on a gynophore, globose to ellipsoid, 1–2 or 3-locular with locules of equal or unequal size; locules ellipsoid to fusiform or subglobose, 2-lobed or not, placentation central or axile, bearing numerous anatropous ovules; styles 2 or 3, sessile or subsessile, usually free, variable in shape Plants of this family are moss-like freshwater herbs, almost always found in swift-flowing, permanent rivers, usually in turbulent waters of cataracts and waterfalls, or in continuous water-spray, often firmly fixed to rocks by a lichen-like thalloid basal part. Submerged and often annual the plants flower as they become exposed to air when water levels recede. Pollination is usually by insects, or by wind. Fruits develop rapidly after pollination. The seed coat is mucilaginous when wet and adheres to the substrate where it germinates Seeds reddish-brown to blackish, minute, slightly flattened-ellipsoid to ovoid, exalbuminous; testa reticulate; embryo straight Stamens hypogynous, 1 or 2 with filaments connate for at least one-third of their length; anthers 2-locular, dehiscing introrsely by a longitudinal slit Perianth of 3 segments (tepals) connate in their lower part, or perianth reduced to 2 minute free filiform structures, lateral to, and shorter than, the androecium, or perianth absent Leaves linear to filiform or reduced and scale-like (moss-like); linear leaves floating, entire or dichotomously, pinnately or laciniately divided, exstipulate or sometimes with 2 tooth-like stipules; scale-like leaves when present 3-ranked (tristichous) or 2-ranked (distichous), scattered or absent on the stems, and ± densely imbricate on branches and flowering shoots Stems simple or with abbreviated side shoots, leafless to ± densely leafy, and/or with reduced leaflets on flowering branches, or stems suppressed Plant base usually thalloid, variable in form and bearing endogenous buds on the margins and surface from which flowering shoots and stems arise Submerged freshwater herbs, firmly attached to rocks and stones in swift-flowing water or spray of waterfalls, often resembling mosses, liverworts or algae, habit also adapting to depth of water and receding water levels Flower hermaphrodite, small, actinomorphic or zygomorphic, without a spathella, or with a persistent membranous spathella (2 spathaceous bracts); spathella at first encloses the flower-bud and at anthesis tears irregularly at the apex allowing the pedicel to elongate bearing the flower erect beyond the spathella; flower-bud subtended by 2–3 free protecting bracts, or flower-bud at first inverted (reflexed) within a membranous spathella (also erect within the spathella outside the Flora Zambesiaca area) Ovary sessile or on a gynophore, globose to ellipsoid, 1–2 or 3-locular with locules of equal or unequal size; locules ellipsoid to fusiform or subglobose, 2-lobed or not, placentation central or axile, bearing numerous anatropous ovules; styles 2 or 3, sessile or subsessile, usually free, variable in shape Plants of this family are moss-like freshwater herbs, almost always found in swift-flowing, permanent rivers, usually in turbulent waters of cataracts and waterfalls, or in continuous water-spray, often firmly fixed to rocks by a lichen-like thalloid basal part. Submerged and often annual the plants flower as they become exposed to air when water levels recede. Pollination is usually by insects, or by wind. Fruits develop rapidly after pollination. The seed coat is mucilaginous when wet and adheres to the substrate where it germinates Seeds reddish-brown to blackish, minute, slightly flattened-ellipsoid to ovoid, exalbuminous; testa reticulate; embryo straight Stamens hypogynous, 1 or 2 with filaments connate for at least one-third of their length; anthers 2-locular, dehiscing introrsely by a longitudinal slit Perianth of 3 segments (tepals) connate in their lower part, or perianth reduced to 2 minute free filiform structures, lateral to, and shorter than, the androecium, or perianth absent Leaves linear to filiform or reduced and scale-like (moss-like); linear leaves floating, entire or dichotomously, pinnately or laciniately divided, exstipulate or sometimes with 2 tooth-like stipules; scale-like leaves when present 3-ranked (tristichous) or 2-ranked (distichous), scattered or absent on the stems, and ± densely imbricate on branches and flowering shoots Stems simple or with abbreviated side shoots, leafless to ± densely leafy, and/or with reduced leaflets on flowering branches, or stems suppressed Plant base usually thalloid, variable in form and bearing endogenous buds on the margins and surface from which flowering shoots and stems arise Submerged freshwater herbs, firmly attached to rocks and stones in swift-flowing water or spray of waterfalls, often resembling mosses, liverworts or algae, habit also adapting to depth of water and receding water levels Flower hermaphrodite, small, actinomorphic or zygomorphic, without a spathella, or with a persistent membranous spathella (2 spathaceous bracts); spathella at first encloses the flower-bud and at anthesis tears irregularly at the apex allowing the pedicel to elongate bearing the flower erect beyond the spathella; flower-bud subtended by 2–3 free protecting bracts, or flower-bud at first inverted (reflexed) within a membranous spathella (also erect within the spathella outside the Flora Zambesiaca area)

Fls perfect, much reduced, with superior or naked ovary, the sep vestigial or wanting; pet none; stamens in 1–several cycles; ovary mostly bicarpellate and bilocular, with axile placentation and distinct or basally connate styles; fr a septicidal capsule with numerous tiny seeds; embryo straight, with 2 cotyledons; endosperm none; submerged or partly floating aquatics of fast rivers with a stony bed, producing aerial fls and frs at times of low water; plants vegetatively highly modified, commonly ± thalloid. 40/200+.

Flowers and fruits produced aerially at low water Flowers bisexual, regular or irregular, small, solitary or in cymes, sometimes cleistogamous; bracteoles 2, enclosing the flower (‘spathella’) and then tearing irregularly at anthesis allowing the pedicel to elongate beyond the spathella, or spathella absent and bracts subtending the flower; tepals absent or 2–3(–5), ± connate or free, or consisting of a small annular scale; stamens hypogynous, 1–2(–4), with the filaments usually at least basally connate, dehiscing introrsely with a longitudinal slit Ovary sessile or stalked, (1–)2(–3)-locular with as many ± basally connate (sub-)sessile stigmas; ovules many, anatropous, on thickened axile placentas Fruit a stalked capsule, smooth or ribbed, dehiscing into 2–3 valves in dry air; seeds usually many, very small, often with mucilaginous testa Plant base usually thalloid, variable in form, without vessels, sometimes without xylem, bearing buds on the margins and surface from which shoots arise; thallus attached to the substrate by haptera, flattened disc-like organs excreting a cement-like substance, and sometimes with filiform rootlets; stem simple or with abbreviated side-shoots, sometimes suppressed Aquatic herbs, looking like mosses, lichens or algae and usually growing on rocks or stones submerged in fast-flowing water Leaves absent or, where present, alternate, entire to dissected, linear, filiform, or scale-like, 3-ranked or 2-ranked, sometimes densely imbricate on branches and flowering shoots; exstipulate or sometimes with 2 tooth-like stipules Flowers bisexual, regular or irregular, small, solitary or in cymes, sometimes cleistogamous; bracteoles 2, enclosing the flower (‘spathella’) and then tearing irregularly at anthesis allowing the pedicel to elongate beyond the spathella, or spathella absent and bracts subtending the flower; tepals absent or 2–3(–5), ± connate or free, or consisting of a small annular scale; stamens hypogynous, 1–2(–4), with the filaments usually at least basally connate, dehiscing introrsely with a longitudinal slit Ovary sessile or stalked, (1–)2(–3)-locular with as many ± basally connate (sub-)sessile stigmas; ovules many, anatropous, on thickened axile placentas Fruit a stalked capsule, smooth or ribbed, dehiscing into 2–3 valves in dry air; seeds usually many, very small, often with mucilaginous testa Plant base usually thalloid, variable in form, without vessels, sometimes without xylem, bearing buds on the margins and surface from which shoots arise; thallus attached to the substrate by haptera, flattened disc-like organs excreting a cement-like substance, and sometimes with filiform rootlets; stem simple or with abbreviated side-shoots, sometimes suppressed Aquatic herbs, looking like mosses, lichens or algae and usually growing on rocks or stones submerged in fast-flowing water Leaves absent or, where present, alternate, entire to dissected, linear, filiform, or scale-like, 3-ranked or 2-ranked, sometimes densely imbricate on branches and flowering shoots; exstipulate or sometimes with 2 tooth-like stipules

Herbs, annual or perennial, aquatic. Roots usually flattened, thalloid or filiform. Leaves distichous, scattered, or imbricate, base often sheathed, margin entire or dissected. Flowers bisexual, solitary, actinomorphic or zygomorphic, enclosed or not by a spathella or spathella lacking. Tepals 2-5, free or ± connate. Stamens 1-4, hypogynous; filaments free or partially connate; anthers 2-4-loculed, longitudinally or irregularly dehiscent. Ovary superior, 2- or 3-loculed; ovules numerous, placenta central. Styles 2 or 3. Fruit a septicidal capsule. Seeds numerous, minute, without endosperm.