BIOLOGY OF THE
PINK LADY'S SLIPPER

Some Orchid Basics

On a worldwide basis, the orchids (Family: Orchidaceae) are a huge group of plants with over 18,000 species, containing plants that boast some of the most spectacular flowers on earth. As inventories or tropical rainforests continue, there are sure to be many more species discovered. Orchids are monocotyledonous plants. Uh, oh...there was a big word in that sentence! Here's what it means. "Mono", you should know, means "one" or "singular". A "cotyledon" is a "seed leaf", an organ in plants associated with the seed. The two halves of a pea, bean or peanut are the cotyledons. And the two main groups of flowering plants (the Angiosperms -- see Pussy Willows, in this issue, for an explanation of that term) are distinguished as having one cotyledon (monocotyledonous) or two cotyledons (dicotyledonous) on their seeds. The two groups are usually referred to as "monocots" and "dicots". In monocots, the cotyledon usually serves to absorb food (endosperm) from the seed as the new plant germinates, while in the dicots, it is the site of food storage before germination.

Some of the other characteristics of the monocots, which includes the grasses, orchids and lilies, are:

1) their flower parts are usually in 3's
2) the leaf veins are parallel, and
3) their stem vascular bundles (the main "trunks" of the vein system that moves water and nutrients around the plant) are scattered.

As opposed to the dicots, groups like asters and peas (most of the more familiar plants), which have:

1) flower parts in 4's or 5's,
2) branching or net-like leaf veins, and
3) stem vascular bundles in a ring near the outside of the stem.

Orchids are all herbaceous (non-woody) perennial plants. Most individual orchid plants may live for many years. They grow in just about every imaginable habitat on the planet. Their often fantastic flowers have lead them to become prized specimens in botanical gardens and within the horticultural trade. Orchid fanciers and "Orchid Societies" are as abundant as the orchids themselves are varied.

Manitoba has 37 species of wild orchids, plus a number of distinct varieties within certain of the species. There are orchids in just about every kind of habitat in this province, from wetlands to boreal forest, and from tall grass prairie to the tree line.

The lady's slipper orchids, of which their are 7 species in Manitoba, are easily identified by their inflated slipper-like lip. They are our most recognizable wild orchids. The group contains some of our most common and our rarest orchids. The small white lady's slipper (C. candidum) is one of the few plants in Manitoba to be declared an endangered species. The yellow lady's slipper (C. calceolus) is very common, often popping up in roadside ditches throughout southern Manitoba.

Description

The pink lady's slipper is a herbaceous perennial with thick fibrous roots. It has two large, basal leaves; that is, the leaves seem to grow from the base of the plant, next to the ground. The leaves, up to 20 cm long, are strongly veined, elliptic in shape, and are thinly pubescent (have a thin covering of short hairs). They are a medium green in colour and may have lighter coloured spots. A single flower stalk arises from between the two leaves growing to as much as 40 cm above the ground. Each stalk produces a solitary flower. The large (3 - 6 cm long), inflated lip of the flower (the slipper part) is usually dark pink in colour, but the shade will vary, and white varieties are known. The sepals and petals surrounding the lip are usually reddish brown, but may be tinged with green. The sideways pointing petals are twisted into a spiral.

Close-up of top of lip.

Classification

The taxonomic classification of the pink lady's slipper goes like this:

Name Derivation

Scientific: The Generic name "Cypripedium" is from the Greek, "Kypris", a name for Venus or Aphrodite; and "pedion", also Greek, an anklet, instep, or having to do with the foot. Presumably then, "Cypripedium" translates as Venus' Slipper, or Aphrodite's shoe. The specific name, "acaule" derives from the Latin prefix "a", meaning without or lacking, and "caul", from the Latin "caulis" for the stem of a plant. So, "acaule" means "stemless", a reference to the lack of a stem in this species. (The flower stalk is not considered to be the plant's stem.)

Common: The pink lady's slipper is so named as it is the only lady's slipper with a lip (slipper) that is completely pink. Others may have pink on the lip, but their will be other colours as well. This species has a variety of other common names including stemless lady's slipper (for obvious reasons) and moccasin flower, owing to it's resemblance to certain aboriginal foot wear.

The Showy Lady's Slipper (Cypripedium reginae),
is often confused with the Pink.

Distribution

The pink lady's slipper occurs in eastern North America, from Newfoundland to Georgia along the Atlantic coast, then in a broad band angling north-westward across southern Canada and the northern United States, reaching to northern Alberta and the southwestern part of the Northwest Territory. In Manitoba, it is found throughout the central forested part of the province, perhaps as far north as near the tree-line.

Range of the Pink Lady's Slipper
(after: H.W. Art, 1986)

Habitat

This species grows in a number of coniferous forest habitats in Manitoba. It can be found in dry, sandy soil in jack pine stands, in moist glens in white spruce / balsam fir stands or in black spruce bogs. It seems to prefer poor, acid soils (around pH 4-5) whether these are wet or dry. It seems to do best in light to moderate shade. Elsewhere in North America, it grows in a wide range of habitat types, but still seems to be found most commonly in association with conifers.

Populations

In good years, in the right habitat, at the right time of year (late June,) pink lady's slippers can seem to be everywhere. It wouldn't surprise me to find several hundred in a hectare in such a situation. But, of course, in other years, you may not find any. There have been no serious inventories, that I know of, for this species in Manitoba. I think it's still common enough not to warrant any concern.

Life Cycle

The pink lady's slipper, like all other orchids, starts life as a very tiny seed. Orchid seeds are little more than a few cells constituting an undifferentiated embryo. The seeds must germinate in the right habitat and in the presence of a symbiotic fungus. The fungus grows within the tissues of the orchid and helps it to take up nutrients from its environment.

Growth in orchids is slow, and it may take several years for a plant to grow large enough to flower. In Manitoba, the pink lady's slipper sends up flower stalks in June and the flowers open in mid-late June in the south, and probably into July in the north.

Pollination in lady's slippers involves deception and entrapment! It's pretty much the same process in all species of lady's slippers. The flowers have little or no nectar to reward insects, but their bright colours are attractive. The shape of the slipper part of the flower (the lip) encourages insects, usually bees, flies or beetles, to crawl inside. They enter through the fissure in the front of the lip, then find that they cannot exit the way they entered, owing to the infolded margins. The inside of the lip is designed to guide the insect to the only exit, out the base of the lip, where it must pass by the flower's stigma, depositing any pollen the insect may have been carrying. Then as it leaves the opening in the lip it brushes past the anthers, collecting more pollen, which hopefully (from the plant's perspective), will be carried to another lady's slipper of the same species.

If the lady's slipper is successfully pollinated, a capsule containing many thousands of tiny seeds will develop and ripen by late summer. The sides of the capsule then split open and the seeds are shed to the breeze. Successful pollination is not that common in most lady's slippers, however, and when it occurs, seed production can be quite a drain on the plant. Studies of artificially pollinated pink lady's slippers show that plants often fail to bloom in the year following the production of a seed pod, and can even lie dormant below ground, not even putting up any leaves in the following year. (Dormancy in orchids and other plants is surprisingly common. Much to the consternation of people studying these plants, large portions of a population may lie quiescent beneath the ground in any given year. What evolutionary advantage this has isn't clear.)

Grow Your Own?

I know you wouldn't dare go out and dig up a pink lady's slipper, or any other kind or orchid, to bring home to put in your garden, but you want one, don't you? Well, I've no experience growing lady's slippers, but there are people around who have. There's a web site listed in the references below that deals with this topic. The only thing I'll add is that growing orchids is not a project for the weekend gardener; only serious green-thumbers should even think about it.

"Native Orchid Conservation Inc."

Manitoba has its own native orchid conservation group! Native Orchid Conservation Inc. is dedicated to promoting awareness and aiding in the conservation of Manitoba's native orchids.

Visit their web site: Native Orchid Conservation Inc.

Final Thoughts

As the summer solstice approaches get out to your favorite coniferous woodland and scout out some pink-lady's slippers. Go on a warm, sunny day when the bumblebees are going to be out. Take a lunch, sit down next to a patch of lady's slippers, and wait. When a bee or some other pollinator shows up, watch carefully. That big, pink lip is not there for our enjoyment; it is there for the species' survival. Enjoy the flowers and be amazed at nature's grand schemes.

Acknowledgments

Some information for this article was obtained from the following sources:

Native Orchids of North America (North of Mexico). By D. S. Correll. 1950. Published by Chronica Botanica Co. Waltham Mass., USA.

A Garden of Wildflowers: 101 Native Species and How to Grow Them. BY. Henry W. Art. 1986. Storey Communications Inc. Pownal Vermont, USA.

Budd's Flora of the Canadian Prairie Provinces, by J. Looman and K. Best. 1987. Research Branch, Agriculture Canada. Publication 1662.

Biology of Plants, 2nd Edition. 1971. By P. Raven, R. Evert and H.Curtis. Worth Publishers, Inc.

Cost of reproduction in the pink lady's slipper orchid (Cypripedium acaule, Orchidaceae): an eleven-year experimental study of three populations. Richard Primack and Elizabeth Stacy. 1998. American Journal of Botany Issue No. ? (Sorry, couldn't re-find the reference)