Pink Lady’s Slipper – So lovely, so deceptive!

Pink Lady’s Slipper (Cypripedium acaule) emerging from its winter blanket of leaves
Pink Lady’s Slipper (Cypripedium acaule) emerging from its winter blanket of leaves

On a recent trip to Vermont, Pink Lady’s Slipper orchids (Cypripedium acaule), also called Moccasin Flower, were just emerging from their winter blanket of leaves, shyly raising their showy pink blossoms, some still partially obscured by the protective sepal and bract draped over the flowers from above.

Pink Lady’s Slipper orchids (Cypripedium acaule)
Pink Lady’s Slipper orchids (Cypripedium acaule)

Each of these lovely orchid plants has two deeply-veined leaves at its base, from which a single flower stem emerges, topped by a spectacular pink slipper (or moccasin) shaped flower.

Pink Lady’s Slipper orchids (Cypripedium acaule)
Pink Lady’s Slipper orchids (Cypripedium acaule)

Lady’s Slipper orchids have three petals, one that forms the ‘slipper’, while the other two are shaped like slightly curly ribbons or ties, positioned slightly above the slipper in the perfect location to secure it around a slender ankle. One sepal projects directly above the slipper, adding to the floral display, while two more sepals are fused and extend down the back of the flower. The sepals acted as bud scales protecting the flower before it opened.

Pink Lady’s Slipper orchid (Cypripedium acaule)
Pink Lady’s Slipper orchid (Cypripedium acaule)

Pink Lady’s Slippers invest a lot of energy to produce these lovely flowers to entice pollinators to assist with the cross pollination that the plants are unable to achieve on their own. In addition to their alluring appearance, the flowers produce a mild scent to add to the attraction for insect pollinators. Insects are not altruistic, however. They expect a reward in return for their efforts, in the form of nectar and pollen.

Veining on the flower adds to the attraction, and helps steer potential pollinators to the flower’s entrance at its front. The entrance is also typically highlighted with striping that acts as a directional signal (or nectar guide) for floral visitors.

Pink Lady’s Slipper orchid (Cypripedium acaule). The striped veins direct pollinators to the the flower's entrance at its front.
Pink Lady’s Slipper orchid (Cypripedium acaule). The striped veins direct pollinators to the the flower’s entrance at its front.

The most likely pollinators for Pink Lady’s Slipper are queen Bumble Bees of several species. These bees have the strength required to muscle their way through the narrow slit that offers access to what they anticipate will be a floral reward.

Once inside, however, bees may begin to have second thoughts about the enterprise. The entrance to the flower is one way.  They can’t exit the same way they entered, because the edges of the entryway are curved inward, making an exit impossible. They are trapped inside until they find a different way out, one engineered by the plant to require traversal of a snug passage past the flower’s reproductive parts. The flower’s pistil (female reproductive part) and two fertile stamens (male reproductive parts) are tucked behind the shield-shaped flower part pointing downward at the back of the slipper. This flower part is a modified stamen, called a staminode.

Pink Lady’s Slipper orchid (Cypripedium acaule). Queen Bumble Bees enter the flower through the slit bordered by striped veins in the center of the slipper.
Pink Lady’s Slipper orchid (Cypripedium acaule). Queen Bumble Bees enter the flower through the slit bordered by striped veins in the center of the slipper.

Hairs inside the slipper direct the hapless bee toward the back of the slipper, its reproductive parts, and finally the flower’s exits. There are two possible exits, one on each side of the staminode, and each partially obstructed by an anther from which pollen is dispensed. To get to an exit, the bee first has to brush against the flower’s stigma at the tip of the pistil. This is the spot where pollen must be placed in order for pollination to occur.  If the bee is bringing in pollen, it will be deposited on the stigma as the bee squeezes past it. The exits are within sight now, but before reaching one, the bee will brush against an anther, from which a pollinium (a package with thousands of tiny grains of pollen) will be attached to its back. Then, freedom!

One of two exits available for a bee to escape from a Pink Lady’s Slipper (Cypripedium acaule). The round, cream-colored, seed-like object obstructing the exit is the anther that deposits the pollinia on the bee as it escapes the flower.
One of two exits available for a bee to escape from a Pink Lady’s Slipper (Cypripedium acaule). The round, cream-colored, seed-like object obstructing the exit is the anther that deposits the pollinia on the bee as it escapes the flower.

Throughout this adventure, no nectar was provided to the flower’s visitor. Pollen is an important food for bees, but when it is packaged in pollinia as it is in the Lady’s Slippers, it isn’t accessible for bees to eat. The bees visiting these flowers are seduced by false advertising into assisting the Pink Lady’s Slipper with cross-pollination, but they receive no reward for their efforts. Hopefully, they’ll attempt the quest for food again with another Pink Lady’s Slipper, but it may not take many visits before a bee gets wise to the deception, and stops visiting these flowers.

If Pink Lady’s Slipper’s duplicitous plot succeeds and pollination takes place, a fruit capsule will develop, like those in the photo below. Making the most of this success, each capsule contains thousands of dust-like seeds. In order to obtain the soil nutrients it needs to germinate and grow, each seed needs to find a fungus of the genus Rhizoctonia with which to partner in the location where it lands. Without this partnership, the seeds won’t be viable, and the plant won’t develop. The fungus must be present throughout the Pink Lady’s Slipper’s life to enable the plant’s survival. In return, when the plant is mature enough, it will provide payment to the fungus in the form of carbohydrates.

Pink Lady’s Slipper (Cypripedium acaule) in bloom. At left, mature fruit capsules from successful pollination of a flower from the previous season.
Pink Lady’s Slipper (Cypripedium acaule) in bloom. At left, mature fruit capsules from successful pollination of a flower from the previous season.

Pink Lady’s Slipper is also capable of sending up additional shoots from its rhizome (underground stem), so you may sometimes see it growing in large groups, or colonies.

Colony of Pink Lady’s Slippers
Colony of Pink Lady’s Slippers

The color of the slipper can vary from a deep to pale pink, sometimes even white.

Pink Lady’s Slipper's color can vary from deep to pale pink, sometimes even white.
Pink Lady’s Slipper’s color can vary from deep to pale pink, sometimes even white.

Pink Lady’s Slipper is found in acidic soil in various habitats including deciduous woods or mixed forests of hardwood and coniferous trees, often with pine or hemlock, and in bogs. It is native in Canada in the Northwest Territories and from Alberta to Newfoundland, in the United States from Minnesota to Maine, then south as far as Alabama and South Carolina.  

Pink Lady’s Slipper growing with ferns and Canada Mayflower (Maianthemum canadense)
Pink Lady’s Slipper growing with ferns and Canada Mayflower (Maianthemum canadense)

Look for it blooming near you!      

Pink Lady’s Slipper orchid (Cypripedium acaule) in bloom
Pink Lady’s Slipper orchid (Cypripedium acaule) in bloom

Related Posts

Yellow Lady’s Slipper – Like Winning the Lottery

Resources

USDA Forest Service – Plant of the Week

USDA NRCS Plant Database

North American Orchard Conservation Center

Orchids of the North: the life of the Pink Lady’s Slipper

Davis, Richard W. The Pollination Biology of Cypripedium Acaule (Orchidaceae)

Flora of Newfoundland and Labrador

Minnesota Seasons