Myriophyllum L.

Aquatic or terrestrial, glabrous herbs, perennial or (not in Mal.) sometimes annual. Stems mostly branched, erect or decumbent-ascending, often freely floating in aquatic forms, sometimes creeping in terrestrial forms, often with minute outgrowths (‘myriophyllin-glands’) leaving brown circular scars in the older parts. Leaves often very constantly flanked by 1 (-3) ‘stipular’ outgrowths, which are filiform to subulate, mostly short, becoming dark, caducous; nearly always dimorphous in aquatic forms; immersed ones nearly always whorled, nearly always pinnately divided into long, filiform or ligulate lobes, very rarely scale-like reduced or absent; aerial leaves mostly whorled in the lower part of the stem, upwards often alternate, sometimes opposite, mostly wider than the immersed leaves and more or less entire, not rarely bract-like. Spikes aerial, mostly terminal, sometimes forming an apical tuft of sterile leaves after fruit setting which may or may not perpetuate growth in the next season, or pushed aside by an axillary shoot by which a sympodial structure is emerging, almost always solitary, rarely 1-4 additional axillary in the upper leaf axils. Flowers mostly sessile, 1 or more in the axil of a bract or leaf, each with 2 sometimes very inconspicuous bracteoles; ♀, often polygamous with the upper ♂ and the lower ♀, or plant monoecious, rarely dioecious. ☿ Flowers: 4(-2)-merous. Sepals 4 or 0, mostly very small, erect. Petals 4 or 2, whether or not caducous, in the ♀ flowers strongly reduced and covered by the styles or 0. Stamens 8 or 4 epipetalous, or 1, in the ♀ flowers 0; anthers often mucronate, rather large. Ovary more or less urceolate, nearly always (mostly alternisepalous) 4-sulcate, 4-2-celled, in ♂ flowers reduced or 0; styles 4 or 2. Fruit mostly ± urceolate, breaking up into 4 or 2 one-seeded mericarps; pericarp often ± tuberculate, rather thin, the 4 mericarps very hard by the thick endocarp which has a soft tissue in an upper axial spot.

Nearly all species are growing in aquatic or at least in swamp or damp habitats, locally gregarious, but dry periods are mostly tolerated. Terrestrial forms occur in most species; these are often more abundantly flowering and bear more ripe fruits than the aquatic forms.Dispersal for most aquatic species may be vegetative, either by detached parts of stems or in temperate species by winterbuds. Endozoic dispersal of fruits by ducks and other birds is recorded from M. verticillatum L. and M. spicatum L. of which the fruits float for a short time. The hard and rather thick endocarp may stand passage of the guts.