How Stanhopea Flowers Work

Now that you've gotten a handle on how a Stanhopea flower is constructed, you're probably wondering,  How does it work?

Stanhopea is one of a group of orchids (including about two dozen genera in the subtribe Stanhopeinae) native to the tropical Americas which are pollinated by male euglossine bees. Their large waxy flowers offer no food or reward for bees other than powerful fragrances containing compounds like 1, 8-cineole, methyl salicylate, benzylaldehyde and methyl benzoate. Each Stanhopea species has a fairly unique fragrance mixture. Individual plants of a particular species are often widely scattered in the forest.

Euglossine bees are large tropical bumblebee relatives, often iridescent blue, green or bronze. They are fast fliers, capable of covering long distances in search of a particular fragrance. They feed on nectar from a variety of tubular flowers. The males also gather scent by scratching flowers and mopping up the liquid fragrance with the feathery brushes on their front tarsi. They transfer the liquid to their hind legs and carry it in their inflated hind tibiae. The fragrance is believed to play some role in mating.

Stanhopeas like S. jenischinana have ‘fall-through flowers.’ Their flowers point downward. The bee flies directly to the source of the scent at the center of the flower, entering through the side and alighting on the base of the lip (the hypochile). The bee falls either when he loses his grip on the slippery surface of the lip, or when he attempts to launch into the air. He then falls backwards with his back tracing the length of the column. The horns on either side of the flower’s lip channel him past the tip of the column, where the pollinarium with its adhesive disk is deposited on his back. At a subsequent flower the pollina are pressed against the sticky stigmatic surface as the bee falls through the flower.

Making Sense of Stanhopea Flowers

Seeing a Stanhopea in flower for the first time was an unforgettable experience for me. The plant was growing on a bench in our greenhouse and had sent its long spike down through the potting mix and the bench mesh so that the flowers opened below the bench. The flowers were enormous and birdlike. They had glossy black eyespots. And the fragrance was vanilla creme icing. It was sublime. One by one, our Conservatory staff crawled on hands and knees under the bench for a closer look.

But I felt a growing sense of unease looking at those flowers. After everyone else had left the room I turned to my boss, Ron Determann, and pointed to the confusing configuration of flower parts. Which are the petals?

Et voilà. If you were to hold a Stanhopea bud and peel back the successive layers like an onion, the uppermost layer would be the sepals. Orchids have three sepals. In primitive flowering plants the sepals are drab and leafy and their purpose is to protect the inner layers from hungry beetles. In Stanhopea, a relatively advanced flower, the sepals are showy and attractive to pollinators.

The next layer consists of three petals. In Stanhopea two petals are narrow strips reflexed backwards over the top of the sepals. The third petal is the lip. The trumpet lip of a Cattleya is iconic --it's what makes Cattleya instantly recognizable. But a Stanhopea lip with its horns and eyespots is even more interesting.

Take a closer look at the lip. If you draw an imaginary line horizontally across its midsection, the basal part is the hypochile (hypo = below), the center of scent production, and the terminal part is the epichile (epi = above). Two horns project from the midsection, the mesochile. The horns make sure that the bee is correctly positioned to receive the pollen.

At the very center of the flower is the column. The column is one of the defining characteristics of the orchid family (and our blog's namesake!). It combines male and female parts into a single structure. The stamens and pistils that you expect to find at the center of many flowering plants are absent in Stanhopea. Likewise the pollen grains.

In Stanhopea the pollen is clustered into masses. Two pollen masses, or pollinia, are united at the end of a slender filament, or stipe, that has a large adhesive disk at its base. Perfect for attaching to a bee. The whole package, called the pollinarium, is tucked away under a cap at the end of the arching column.

Just above the anther cap is the sticky stigmatic surface where incoming bees leave pollinaria from other Stanhopea plants.The pollen germinates on that surface, releasing sperm cells that travel the length of the column to the ovary (barely visible in the fourth photo) located above the sepals. After fertilization of the ovules the ovary swells and becomes the seed capsule.