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Sparganium

Angelo and Boufford (2000) reported Sparganium americanum ×S. fluctuans for MA. Though this collection may be accurately determined, it is not included here until further study is taken given that these two species below to different subgenera and Cook and Nichols (1986, 1987) did not mention the occurrence of this hybrid. References: Cook and Nicholls (1986, 1987).

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S. eurycarpum
1a. All or most flowers and fruits with 2 stigmas; all or most ovaries with 2 locules; body  of achenes truncate or abruptly tapering to the beak, with 3–7 angles (i.e., not terete in  cross-section), not constricted near center, 4–8 mm wide
See list of 6 species in 1b
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1b. Flowers and fruits with 1 stigma; ovary with 1 locule; body of achenes tapering to beak, terete or weakly angled in cross-section, often slightly constricted near center, up to 4 mm wide
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  S. natans
  2a. Flowering stems with 1 staminate spike; fruiting spikes 5–12 mm in diameter; beak  of achene 0.5–1.5 mm long
- See list of 5 species in 2b
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  2b. Flowering stems with 2–20 staminate spikes; fruiting spikes 10–35 mm in diameter; beak of achene 1.5–7 mm long
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    S. fluctuans
    3a. Beak of mature fruit flattened and strongly curved; tepals adnate to fruit  stipe for ca. ½ their length; anthers (0.5–) 0.6–0.7 (–0.9) mm long; stigmas  (0.4–) 0.7–0.8 (–0.9) mm long
  - See list of 4 species in 3b
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    3b. Beak of mature fruit ± terete, straight or only slightly curved (sometimes strongly curved in S. androcladum); tepals free from fruit stipe; anthers 0.8–1.5 mm long; stigmas (0.6–) 0.8–4 mm long
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      4a. Both the sessile carpellate spikes of the main axis and the peduncle bases of  the lateral branches borne directly in the axil of leaves or bracts [Fig. 306]; tepals  ± opaque, with a dark brown pad of tissue near apex, the apical margins emarginate to entire
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        S. androcladum
        5a. Fruiting spikes 25–35 mm in diameter; achenes usually dull in basal half and lustrous in apical half, with a beak (4–) 4.5–7 mm long; anthers 1–1.6 mm long; lateral branches of inflorescence, when present, bearing only staminate spikes
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        S. americanum
        5b. Fruiting spikes 15–25 mm in diameter; achenes usually dull throughout, with a beak (1.5–) 3–4 (–5) mm long; anthers 0.8–1.2 mm long; lateral branches of inflorescence, when present, bearing 1–3 carpellate spikes in addition to staminate spikes [Fig. 306]
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      4b. Some of the sessile carpellate spikes of the main axis and/or the peduncle  bases of the lateral branches borne above the axil of leaves or bracts [Fig. 307]; tepals ± translucent, without a pad of dark tissue at the apex, the apical margins distinctly erose
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        S. emersum
        6a. Leaves usually erect and emergent, though sometimes flaccid and floating,  the distal portion with a prominent abaxial keel or triangular in cross-section;  beak of fruit 2–4.5 (–6) mm long; at least some of the staminate spikes separated by short internodes [Fig. 307]
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        S. angustifolium
        6b. Leaves flaccid and floating (or submerged), the distal portion flat, without an abaxial keel; beak of fruit 1.5–2 (–2.2) mm long; staminate spikes contiguous

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Show photos of: Each photo represents one species in this genus.