Sparrow's egg lady slipper is a tall orchid with white, conspicuously inflated flowers that grows in mossy forests and river terraces in Denali. The inside of the flower's lip is purple-speckled, like a bird egg. Stems grow 12-40 cm tall from fibrous roots, with several alternate stem leaves. The leaves are clasping at the base, elliptic to ovate-lanceolate in shape. The stems and leaves are sparsely long-haired. At the top of the stem, plants produce one flower (rarely two). The flower is bilaterally symmetrical with specialized morphology, as in all orchids. The lower lip is inflated, slipper-like. The upper sepal is green and leaf-like, the lateral sepals are white. The fruit is an ellipsoid capsule containing hundreds of minute seeds. In fruit, sparrow's egg lady slipper can be confused with spotted lady's slipper (Cypripedium guttatum), but that species has only two sub-opposite leaves, not several alternate leaves up the stem (neither of these species are common in the park so count yourself lucky if you found either one!).
Sparrow's egg lady's slipper is early flowering.
The lady slipper orchids are deceptive pollinators, smelling sweet but offering no reward of nectar or pollen. Flowers have extremely high seed set (99% of undamaged flowers set seed in the Lake Superior area) and are self-compatible, indicating the species is possibly automatically self-fertilizing (Keddy et al. 1983). Thousands of minute seeds are released from a capsule, which are wind and gravity dispersed. Seeds contain almost no food reserves for the embryo, and are dependent on mycorrhizae to germinate. Plants may grow for over 15 years before they begin flowering, based on a count of stalk scars (Keddy et al. 1983).
Cypripedium passerinum is endemic to boreal North America, occurring from Alaska across Canada, and southwards into Montana. In Alaska, this species occurs from the Seward Peninsula and the Brooks Range to Prince William Sound and northern Southeast Alaska. In Denali, Cypripedium passerinum occurs scattered localities in the northern lowlands, usually in river terrace environments.
Sparrow's egg lady-slipper is an uncommon species in Denali, found in the subalpine (average altitude 694 m). This species overwhelmingly occurs on flat terrain, only two specimens found on slopes above 5 degrees.