Cypripedium acaule Aiton

Pink lady's slipper, pink moccasin-flower

The specific epithet acaule is the Latin meaning "stemless," in reference to the leafless stem of this species.

DESCRIPTION: Plant arising from a rhizome with a fascicle of fibrous roots. Leaves two, basal, plicate and pubescent; elliptic, 10-25 cm long and (5)8-15 cm wide. Flower solitary, terminating a stalk 25 to 40 cm long that arises from between the two leaves; flower subtended by a green bract 3-5 cm long by 1-1.5 cm wide. Sepals apparently two (the result of the fusion of the two lateral sepals behind the labellum), lanceolate, 3-5 cm long and 0.5-2 cm wide, purplish-brown to brown, often striped with green. Petals similar in size and appearance to the sepals, 4-6 cm long and 1-1.5 cm wide. Labellum pouch-shaped, inflated, 4-6 cm long by 2.5-4 cm wide, opening by a slit (with inrolled edges) at the front; magenta to whitish-pink, often whitish pink with darker pink venation, rarely all white.

Cypripedium acaule could probably be confused only with other pinkishCypripedium species such as C. arietinum and C. reginae. However, it is easily separated from these two as they have cauline leaves, whereas the leaves of C. acaule are always basal.

Cypripedium acaule can be found in two distinct habitat types in Wisconsin: dry, acidic woods, typically on slopes or steep hillsides, and in sphagnum bogs.

May 20 to July 20, blooming earlier in the southern part of the state and later in the north.

Cypripedium acaule, like the other Cypripediums in Wisconsin, has a deceptive pollination system. Bumblebees are lured into the pouch of the labellum through the slit in the front, attracted by the bright color and sweet scent of the flower. Once inside, they find no reward, and discover that they are trapped--with only one point of escape. Inside the pouch, there are hairs which lead to a pair of openings, one beneath each pollen mass. First, however, the bee must pass under the stigma, so if it bears any pollen from a visit to another flower, it will be deposited before picking up a fresh load, thus preventing self-pollination. Unfortunately, the bees quickly learn from their experiences and soon avoid C. acaule flowers. Thus, like several other orchids in our flora, they are dependent on naive bees, and generally experience very low pollination rates (Davis 1986).

DISCUSSION: Fuller (1933) noted that C. acaule always grows with its roots in acid soil; root horizon soil pH tested at a number sites in Wisconsin ranged from pH 4-5. He also records that the white-flowered form has been collected in Adams and Rock counties.

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