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Callitriche
See list of 5 species in this genusReference: Fassett (1951).
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1a. Flowers and fruits not subtended by bracts; leaf blades ± uniform, never forming a floating rosette
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2a. Schizocarp 0.5–0.7 mm long, borne on a short pedicel 0.1–0.6 mm long, each mericarp with an inconspicuous wing-margin; leaf blades 2–5 mm long, spatulate to oblanceolate, rounded at the apex; plants annual, growing on damp soils
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2b. Schizocarp 1–2 mm long, subsessile, each mericarp with a conspicuous wing-margin; leaf blades 4–15 (–20) mm long, oblong-linear, rounded to retuse at the apex; plants perennial, typically wholly submersed
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1b. Flower and fruits subtended by 2 ± white or translucent bracts mostly 0.5–1.5 mm long; leaf blades often dimorphic—those along the stem linear and relatively distant, those near the stem apex spatulate, crowded, and forming a floating rosette
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3a. Schizocarp 1.5–2 mm long, each mericarp with a prominent, thin wing nearly 0.1 mm wide that is ± equally prominent from apex to base of fruit; floating leaf blades 3–8 mm wide, with 5–7 veins by branching of the lateral veins [Fig. 766]
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3b. Schizocarp 1–1.4 mm long, each mericarp unwinged or with a thin wing up to 0.05 mm wide that becomes less prominent toward fruit base and is absent in the basal ¼ or more of the fruit; floating leaf blades (when present) up to 5 mm wide, with 3 (–5) veins
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4a. Schizocarp 0.2 mm or more longer than wide [Fig. 765], the apical portion of each mericarp with a thin wing, also with an evident commissural groove between the mericarps; pit-like aereolae on surface of fruit tending to be aligned in vertical rows
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4b. Schizocarp ± as wide as long (i.e., not more than 0.1 mm longer than wide), lacking thin wings along the margin of the mericarp (or essentially so), without a commissural groove or with a shallow groove between the mericarps; pit-like aereolae on surface of fruit not at all aligned in vertical rows
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Each photo represents one species in this genus.