Cypripedium parviflorum
 
Yellow Lady's-Slipper

Flower Front

Riding Mountain National Park
21-June-2010

Note the two lower sepals are united almost to the tip, forming a synsepal.

Note that Budd's Flora and Flora of Alberta refer to this species as C. calceolus.  Three varieties have been recognized in North American.  However Flora of the Great Plains states that there are intermediates between all of them. 

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Calceolus: Answers to key questions in Budd's Flora and Flora of Alberta leading to this species. 
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flowering stem leafy, with 2-6 alternate leaves nearly to the top; NOT [flowering stem a naked scape, with 2 subopposite leaves at the base]

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flowers generally 1; NOT [flowers generally 2]

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flowers yellow; NOT [flowers brownish] (FOA)

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lower sepals united, usually 2-veined or 2-toothed; NOT [lower sepals separate and spreading] (BF)

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sepals often longer than the lip; NOT [sepals not longer than the lip] (FOA)

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sepals lanceolate, attenuate; NOT [sepals obovate or oval] (FOA)

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sepals yellowish; NOT [sepals greenish, brownish, or white]

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petals greenish yellow to brownish (BF)

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lip 2-4 cm long; NOT [lip 1-2 cm long]

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lip with a rounded opening; NOT [lip cleft down the front, drooping]

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lip yellow; NOT [lip pink with red veins], NOT [lip white]

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staminode triangular; NOT [staminode ovate or obovate]

 
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Cypripedium: Answers to key questions in Budd's Flora  and Flora of Alberta leading to this genus. 
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plants with 2 or more leaves; NOT [plants with a single (basal) leaf]

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flowers large, to 8 cm long; NOT [flowers mostly smaller]

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lip a large inflated sac or pouch, moccasin-shaped); NOT [lip concave or flat, not moccasin-shaped]

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fertile anthers 2; NOT [fertile anther 1]