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Keyed in multiple places:

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Key to Myriophyllum

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1 Leaves reduced to small scales or absent; stems short, erect from substrate; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
1 Leaves well-developed, pinnately divided with filiform segments; stems elongate, suspended in the water column and/or floating.
..2 Flowers/fruits absent and emersed shoots with leaves closely similar in size and shape to submersed ones; widespread alien; [subgenus Myriophyllum; section Pectinatum]
..2 Flowers/fruits present; emersed shoots present or not.
....3 Flowers/fruits in axils of leaves.
......4 Leaves whorled; emersed stems present and with feathery leaves; [subgenus Myriophyllum; section Pectinatum]
......4 Leaves strictly alternate or subopposite; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum].
........5 Flowers/fruits on submersed stems; fruits tuberculate
........5 Flowers/fruits on submersed stems (forma capillaceum) or on emersed stems with pinnatifid or pectinate leaves (forma natans); fruits smooth
....3 Flowers/fruits in erect spikes emersed from water, flowers/fruits subtended by bracts much smaller than the normally submersed leaves.
..........6 Uppermost flowers/fruits alternate; leaves alternate or whorled or both; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum].
............ 7 Bracts much shorter than floral internodes, varying from pectinate to entire; fruit surface smooth or papillose
............ 7 Bracts usually longer than floral internode, pinnatifid to pectinate; fruit surface strongly tuberculate
..........6 Uppermost flowers/fruits opposite; leaves whorled (technically pseudo-whorled in many M. heterophyllum) (note that early season plants of M. pinnatum may have flowers opposite, but at least some leaves will be alternate).
............ ..8 Bracts usually > 2× as long as pistillate flowers; stems drying brown, pale brown, or reddish.
............ ....9 Bracts throughout inflorescence pectinate to pinnatifid; winter buds scattered along stem, clavate, falling by early winter; [of DE and northward]; [subgenus Myriophyllum; section Myriophyllum; subsection Isophylleae]
............ ....9 Distal bracts subentire to serrate, proximal bracts pectinate to serrate; winter buds developed only at base of stem or on rhizomes, usually persisting; [widespread]; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
............ ..8 Bracts usually < 2× as long as pistillate flowers; stems drying pale tan or whitish]; [subgenus Myriophyllum; section Myriophyllum; subsection Myriophyllum].
............ ......10 Midstem leaves with 11 or fewer segments on each side of rachis; leaves rounded at apex; stem diameter more-or-less uniform; stem tips usually green; winter buds produced; [native, of DE and northward]
............ ......10 Midstem leaves with 12 or more segments on each side of rachis; many leaves appear truncate or clipped at apex; stem diameter below inflorescence is up to 2× diameter of lower stem; stem tips usually reddish; no winter buds; [widespread alien]

Alternate key: Subkey in Myriophyllum

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1 Leaves reduced to small scales or absent; stems short, erect from substrate; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
1 Leaves well-developed, pinnately divided with filiform segments; stems elongate, suspended in the water column and/or floating.
..2 Flowers/fruits produced in axils of submersed leaves; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
....3 Fruits tuberculate
....3 Fruits smooth
..2 Flowers/fruits produced in axils of emersed leaves or on emersed shoots with bracts (reduced bracteal leaves).
......4 Emersed shoots with feathery leaves about same size and shape as submersed leaves; flowers/fruits rarely produced; [widespread alien] [subgenus Myriophyllum; section Pectinatum]
......4 Emersed shoots with bracts subtending flowers/fruits; these bracts much different in shape than submersed leaves. [stranded plants may produce bracts and leaves of similar size and shape, but these not feathery].
........5 All flowers/fruits alternate; fruits smooth; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
........5 All flowers/fruits opposite or whorled (or the lower opposite and the upper alternate in M. pinnatum).
..........6 Bracts usually longer than the internodes; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum].
............ 7 Leaves whorled or pseudo-whorled; fruits with low bumps
............ 7 Leaves strictly alternate; fruits strongly tuberculate
..........6 Bracts usually shorter than the internodes.
............ ..8 All bracts pectinate to pinnatifid; [subgenus Myriophyllum; section Myriophyllum; subsection Isophylleae]
............ ..8 Bracts vary from entire to pectinate.
............ ....9 Leaves alternate, pseudo-whorled, or both; plain green; [of se. VA and southward] ; [subgenus Brachytheca; section Tessaronia; subsection Spondylastrum]
............ ....9 All leaves whorled, grayish green; [collectively widespread]; [subgenus Myriophyllum; section Myriophyllum; subsection Myriophyllum].
............ ......10 Midstem submerged leaves with (3-) 5-11 segments on each side of rachis; leaves rounded at apex; stem diameter more-or-less uniform; stem tips usually green; turions (specialized winter buds) produced late in the season; [native, of DE and northward]
............ ......10 Midstem leaves with 12-25 segments on each side of rachis; many leaves appear truncate or clipped at apex; stem diameter below inflorescence up to 2× the diameter of the lower stem; stem tips usually reddish; turions (specialized winter buds) not produced; [widespread alien]
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