Cypripedium arietinum
 
Ram's-Head Lady's-Slipper

Flower Side Annotated

Eastern Saskatchewan
18-June-2009

The term dorsal usually refers to the lower surface of a structure.  This is also the surface that is facing away from the axis, assuming the structure is at least slightly pointing upward.  Similarly, ventral usually refers to the upper surface of a structure.  However, in many species of orchids (including this one) the ovary twists 180 degrees as it matures, resulting in the dorsal structures being on top and the ventral structures being on the bottom.  The annotations in this image are the ones generally encountered in the literature.  To remove ambiguity, however, one should refer to developmentally dorsal and developmentally ventral if they are used as above.  An orchid whose ovary undergoes this 180 degree twist is referred to as resupinate.

Note that the two ventral sepals are not united (they commonly are united in this genus).

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Arietinum: Answers to key questions in Budd's Flora leading to this species. 
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lower sepals separate and spreading; NOT [lower sepals united, usually 2-veined or 2-toothed]

 
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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]