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Genus Myriophyllum Family Haloragaceae

Description: Annual or perennial herbs, aquatic to fully emergent; usually glabrous, stems weak, often glabrous.

Leaves whorled, opposite, alternate or irregularly arranged, submerged leaves often deeply pinnatifid forming regular narrow segments, emergent leaves entire or toothed.

Flowers mostly unisexual, rarely bisexual, usually solitary in the axils of the emergent leaves but sometimes in axillary clusters of 2 or 3; in monoecious plants the male flowers in the upper part of the flowering shoot. Male flowers mostly 4-merous; sepals usually present; petals hooded; stamens usually 8, sometimes 4. Ovary and styles vestigial or absent. Female flowers often lacking both petals and sepals; ovary usually of 4 carpels; styles free, one per carpel; stigmas usually fimbriate.

Fruit splitting into 1-seeded mericarps.


Distribution and occurrence: World: c. 60 species, cosmopolitan. Australia: 36 spp. (31 spp. endemic, 5 spp. naturalized), all States.

Text by Peter G. Wilson
Taxon concept:

 Key to the species 
1Lower or submerged leaves always whorled2
All leaves opposite, alternate or irregularly arranged12
2Emergent leaves pinnately divided3
Emergent leaves entire or toothed
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5
3Emergent leaves glaucous, in whorls of 4–6, mostly with 10–18 segments on each side of the leafMyriophyllum aquaticum
Emergent leaves green to reddish, usually in whorls of 2–4, with up to 6 segments on each side of the leaf
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4
4Mericarps ovoid, smooth or sparsely papillose; leaves often crowded at the tips of the stemsMyriophyllum alpinum
Mericarps oblong, angular, distinctly warty; leaves not crowded
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Myriophyllum verrucosum
5Emergent leaves ovate or oblong, entire or with a few short teeth towards the apex6
Emergent leaves terete or lanceolate, entire or toothed
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8
6Mericarps oblong to ovoid, smooth to warty7
Mericarps cylindrical, spinose
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Myriophyllum caput-medusae
7Mericarps ovoid, smooth or minutely papilloseMyriophyllum salsugineum
Mericarps oblong, distinctly warty
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Myriophyllum verrucosum
8Emergent leaves terete, linear or lanceolate, entire or toothed9
Emergent leaves lanceolate, shortly toothed in the upper half
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Myriophyllum latifolium
9Flowers and fruits solitary; styles white to pink; leaves in whorls of 3–8, emergent leaves usually more or less entire10
Flowers and fruits in axillary clusters; styles mauve to purple; leaves in whorls of 4–6, all emergent leaves toothed
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Myriophyllum papillosum
10Emergent leaves in whorls of 3–6; flowering shoot more or less cylindrical with a truncate tip; stem glabrous11
Emergent leaves in whorls of mostly 5–8; flowering shoot tapering to the tip; stem usually bearing scattered, small, crisped hairs
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Myriophyllum crispatum
11Fruit yellow-brown, c. 1.5 mm long, mericarps cylindrical; emergent leaves in strict whorls of usually 5; stigmas white; aquatic plantMyriophyllum variifolium
Fruit reddish purple, c. 1 mm long, mericarps ovoid; emergent leaves in whorls of mostly 3 or 4, sometimes irregularly so; stigmas pink (fading to white); aquatic or semiterrestrial plant
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Myriophyllum simulans
12Leaves all, or predominantly, alternate or irregular13
Leaves all, or predominantly, opposite
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17
13Plants perennial, adventitious roots produced along the stems14
Plants annual, more or less erect
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Myriophyllum glomeratum
14Male flowers with sepals; aquatic or terrestrial plants15
Male flowers lacking sepals; mat plants on wet ground
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16
15Lowermost leaves pinnately divided with 8 or more segments on each side of the leaf; mericarps ovoidMyriophyllum simulans
Lowermost leaves entire, trifid or pinnately divided with 2 or 3 segments on each side of the leaf; mericarps shortly cylindrical
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Myriophyllum gracile
16Mericarps papillose; leaves always entire; plants strictly dioeciousMyriophyllum implicatum
Mericarps weakly ribbed by linking of papillae; leaves entire or pinnately lobed; plants monoecious or dioecious
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Myriophyllum striatum
17Mericarps ovoid with an attenuate apex, smooth or warty; leaves linear to trifid, mostly 4–20 mm longMyriophyllum pedunculatum
Mericarps cylindrical with a pale dorsal crest, smooth or papillate; leaves linear, rarely over 3 mm long
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Myriophyllum lophatum

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