Aquatic herbs, perennial or rarely annual, glabrous, rooted in the substrate. Vegetative shoot system comprising horizontal shoots, mostly stoloniferous, non chlorophyllous (here called rhizomes) and vertical shoots, erect or sometimes ± floating, often densely branched, chlorophyllous (here called stems); tubers or turions sometimes present on the horizontal and/or the erect shoots. Leaves alternate or subopposite, rarely ternate, sessile or petiolate, simple, served by 3 traces, parallel-veined or with a single midvein; stipules present throughout or only on young shoots ( Groenlandia ), often forming a tubular sheath around the stem and young inflorescence, either free or adnate to the base of the blade; intravaginal scales 2– several, situated in leaf axils, mostly linear to subulate, rarely ovate, entire and acute; bladeentire, denticulate or serrate. Inflorescences pedunculate, with 2 opposite flowers ( Groenlandia ) or more than 2 flowers in a dense or interrupted spike ( Potamogeton ); peduncle rigid and mostly slightly elevated above the water level, but sometimes flexuous, the flowers resting then on the water surface, rarely submerged, but often withdrawing the fruits below the surface; bracts abortive or absent. Flowers& #2695; (hermaphrodite), small, regular, hypogynous, tetramerous. Perianth: a singlewhorl of 4 valvate shortly clawed tepals. Stamens 4, opposite the tepals and basally adnate to the claw; anthers sessile, bilocular, extrorse, opening by longitudinal slits. Gynoecium superior, of (1– 3) 4 (5– 8) carpels, sessile, free or shortly connate at the base, alternating with the stamens; styles usually short; stigma unicellular-papillate and strongly secretory or smooth and dry or weakly secretory; ovule solitary in the carpel and attached to its ventral margin, orthotropous at first, but becoming campylotropous at maturity, pendant and filling the locule, bitegmic, crassinucellate. Fruit apocarpous consisting of distinct sessile fruitlets, ± drupaceous, but opening with a dorsal lid, with a rather fleshy parenchymatous mesocarp and a sclerified endocarp, the latter multi-layered ( Potamogeton )or 1-layered ( Groenlandia ). Seed without endosperm; embryo unciform or spiral (i.e. coiled more than 1 complete turn), with a large hypocotyl and a single obliquely terminal cotyledon that encloses the plumule.
Aquatic herbs of fresh, water. Leaves alternate or opposite, those immersed thin, those above water often leathery, sheathing at the base, sheath free or partially adnate to the petiole. Flowers hermaphrodite, small, arranged in pedunculate axillary spikes; peduncle surrounded by a sheath at the base; bracts absent. Perianth of 4 free rounded shortly clawed valvate segments. Stamens 4, inserted in the claws of the segments; anthers extrorse, 2-locular, sessile. Carpels 4, free, 1-locular; stigmas sessile or on short styles; ovule solitary, on the adaxial angle. Fruits free, 1-seeded, indehiscent. Seeds without endosperm.
Notes: One genus, Potamogeton, widely distributed.
Perennial or annual, monoecious or rarely dioecious herbs, growing in fresh or brackish waters, sometimes stranded by receding water levels. Roots fibrous, few, nonseptate, at lower nodes of stems. Stems slender, branched or unbranched, often dimorphic, the lower stems rhizomatous or stoloniferous, occasionally with tubers, the upper erect, leafy, the tips often modified into turions. Leaves floating or submersed, alternate, (the upper 2-12 subopposite, Potamogeton), opposite or pseudo-whorled, entire to serrulate, scale-like or foliaceous. Scale-like leaves without vascular tissue. Foliaceous leaves sessile or petiolate; base somewhat clasping the stem to forming a tubular sheath; 1-many-veined, veins parallel or arching, often connected by cross-veins; ligule present or absent; stipular sheath present or absent. Submersed leaves thin, linear to orbicular; floating blades often leathery, lanceolate to elliptic or ovate. Axillary intravaginal scales present, membranous. Inflorescences pedunculate, axillary, or terminal, with 2 opposite flowers or more than 2 flowers in a capitate to interrupted spike, with a subtending spathe; or an axillary cluster of 2-several flowers, subtended by reduced bract-like leaves. Flowers bi- or unisexual. Bisexual flowers sessile, actinomorphic; tepals (2-) 4, in one series, distinct, rounded, flat to convex, short-clawed, greenish, reddish, or brownish; stamens 4, rarely 2-3; anthers extrorse, 2-locular, dehiscing by longitudinal slits; filaments adnate to the perianth claw; pollen grains shed singly, spherical; ovary superior, the carpels unilocular, sometimes with an indistinct stipe; ovules solitary, attached to the ventral margin of the carpel; styles short; stigmas capitate. Unisexual flowers short-pedicellate. Male flowers reduced to 1 stamen; perianth mostly absent or rarely with 3 minute scales; anther 2-12-loculed, dehiscing by longitudinal slits, pollen often contained in gelatinous matrix. Female flowers often enclosed by a membranous spathe-like envelope, perianth a small cup-like sheath or of 3 separate segments; carpels 1-8, separate, short-stipitate, 1-locular; ovule solitary, bitegmic, ventral-pendulous, anatropous, placentation apical; styluli short to long; stigma enlarged, funnel-shaped, feathery, or peltate. Fruits usually drupelets with a membranous exocarp, a massive, hard fleshy mesocarp, and a very thin (1 or 2 cell layers) thick stony endocarp, rarely berries. Opening by decay of the pericarp or by tearing, by the enlarging embryo, of a c. 1 cell layer thick basal “seam” with very thin cell walls that connects a lower “hinged” flap in the pericarp to the main fruit. Alternatively, fruit a very thin, hard egg-shell-like pericarp (Zannichellia, Althenia) enclosing an elongated embryo; the “shell” being ground down by the surrounding sand and water. Seeds solitary, exalbuminous, with coiled or slightly curved embryo; seed coat in both groups bitegmic and ephemeral; this is to be expected in the presence of a tough pericarp. Endosperm: Helobial in development, absent in mature seed.
With kind permission from Springer Science+Business Media: This work is subject to copyright. All rights reserved, whether whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, broadcasting reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under German Copyright Law.All Rights Reserved