| 1. Leaves subtending flowers pinnately dissected, like cauline leaves; spike 0; dioecious |
| M. aquaticum |
| 1' Leaves subtending flowers generally simple, entire to lobed, < cauline leaves, leaf- or bract-like; spike emergent; monoecious |
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| 2. Emergent inflorescence-leaves generally 415 mm, > 2 × flower, entire to lobed |
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| 3. Emergent leaves generally ± entire to serrate |
| M. hippuroides |
| 3' Emergent leaves generally pinnately lobed to divided |
| M. verticillatum |
| 2' Emergent inflorescence-leaves < 3 mm, < flower, bract-like, margin entire to finely toothed |
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| 4. Submersed leaves with < 26 divisions; divisions thread-like, generally not paired |
| M. sibiricum |
| 4' Submersed leaves with > 28 divisions; divisions linear, paired |
| M. spicatum
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