The Lying Lady's Slipper
We’ve all met people who lie, but have you met a flower that lies? And how would you know you’ve met a deceitful plant? Such plants are quite crafty in their deception, just like lying people. And they certainly have their reasons, also like people.
Plants are necessary for the very existence of much of life on earth. They are primary producers(1), organisms capable of synthesizing their own food using light or chemical energy and molecules like carbon dioxide. Without plants, the rest of us wouldn’t have food to eat.
Over the past 470 million years since they appeared on earth, plants have evolved ways to survive despite the onslaught of plant consumers – insects, other grazing animals, humans, etc. – who are higher up the food chain.
Plants are slow movers at best and need a way to transfer pollen between male and female flowers for reproduction(2). Consequently, a prominent plant survival ploy is the “You scratch my back and I’ll scratch yours” strategy(3) that many flowering plants and their pollinators(4) have coevolved. This reciprocal relationship is typified by honeybees(5), who fly from plant to plant gathering pollen and nectar to feed themselves and their growing larvae. In essence, nectar is a prime attractant as a high-energy food source containing primarily sugars and pollen is a high protein food. Additionally, brightly colored flowers, including ultraviolet hues which humans cannot see, and floral scents advertise the plants’ nutritional offerings.
Fundamental to the plants’ and pollinators’ relationship is the belief(6) that the arrangement is mutualistic, that both parties will benefit. The pollinators assume they are getting nutritious food – the sweet nectar or pollen. The plant hopes that whoever visits the flower will also take some pollen and transfer it to other flowers during the pollinator’s rounds(7). But what if one or both parties cheat?(8)
The false is nothing but an imitation of the true. - Cicero
Pink Lady’s Slippers (Cypripedium acaule) are in the orchid family with beautiful flowers blooming in New England woods in late spring or early summer. The flowers produce a subtle aroma attracting bumblebees who push their way into the pouch-like “slipper” through an opening in the front of the flower. The door closes behind the bumblebees and its recurved lips make the entrance effectively one-way. The bumblebee must exit the flower via openings deeper within the pouch aided by plant hairs that lead the pollinator out. As the bumblebee leaves it deposits any pollen it is carrying, thus pollinating the flower, and picks up pollen to carry to other flowers as well. Once out, the bumblebee says, “Phew, that was a lot of work!” and flies off.
But, wait! What about the reward that the pollinator counted on – the sugary nectar. This is the big lie – there is no reward. The Pink Lady’s Slipper has saved the energy it would have expended in manufacturing the nectar, yet it still managed pollination by misleading the bumblebee. Alas, poor bumblebee!
Bait and Switch
Then there are the smelly plant tricksters. Carrion (decaying animals), dung, or other substances give off strong(9), characteristic odors that attract a variety of insects. Flies can be drawn to a cadaver within minutes of death, following a gradient of airborne chemicals that promises a land of plenty. So carrion plants have evolved putrid perfumes(10) that smell just as bad as the real thing to us but, more importantly, are potent attractants to certain insects and other arthropods.
Unlike the bee that sips sweet nectar for sugar, a fly drawn to a cadaver expects that it will find a substrate to lay eggs on and for its larvae (maggots) to consume. But the carrion plant lies. Instead of the nutritious carrion, the plants don’t provide an appropriate meal of protein, carbohydrate, or fat. Rather, the maggots of flies that are duped into laying eggs on the plant, generally die.
The Skunk Cabbage (Symplocarpus foetidus) relies upon insect pollination and may hedge its bets by combining deception and mutualism:
- The flesh-colored flowers appear early and actually melt(11) their way through the snow that still covers the ground. Its odor chemicals include the appropriately-named cadaverine and appeal to beetles and flies seeking rotting flesh. The flower deceives most of these visitors and offers no reward yet still coats the deceived fly with pollen to carry to the next flower.
- There are a few carrion-seeking pollinators whose larvae consume the flower and thrive. Details of this mutualistic relationship between the plant and these select pollinators are not yet understood.
So, why don’t all flowering plants conserve energy by deceiving pollinators with these tactics? Because there must be enough “honest” plants to maintain pollinator population health so they can’t all be lying liars.
Don't cry for me
Insects have evolved their own treacherous way out of the contract. Some bees will chew holes in the flower’s base to obtain nectar and bypass the flower’s natural opening and its pollen-producing structures. In these cases, it may be simpler than crawling through the plant’s reproductive structures to access the nectar. Similarly, there are pollen-robbers who eat pollen and may even destroy the plant’s reproductive organs to obtain their food. In each scenario, the flower’s expectations of quid pro quo are left unfulfilled. Scientists who study this pollinator betrayal fittingly call it “floral larceny”(12).
To my knowledge, plants have not lied to me. Were they to do so, I would forgive them and continue to appreciate their elegance, aromas, and complexity. The relationships they have evolved and continue to evolve with their pollinators and nectar- and pollen-robbers are intriguing facets of plant life that are never-ending sources of fascination.
1 Insects, birds, dogs, you, and I are consumers who acquire nutrition from mainly plant or animal matter.
2 Fair warning: this is literally a “birds and the bees discussion.”
3 They have also evolved chemical defenses, like bitter or poisonous substances, and physical defenses, like thorns, to thwart consumers, which are topics for another essay.
4 The pollen of many nonflowering plants like pine trees, and some flowering plants like ragweed is spread by the wind rather than by pollinators.
5 Honeybees are not native to North America. There are many native bee pollinators that do not enjoy the fame of honeybees. Other pollinators include beetles, flies, birds, and bats.
6 The words “believe” or “expect” don’t necessarily mean that the plant or insect has an expectation the same way I or my dog might.
7 The pollen often just hitches a ride on the pollinator. The pollinator is unaware of its role as floral matchmaker.
8 There are also plants that lie by looking like the mate of a pollinator. They won’t be discussed here.
9 We often find these odors offensive but anyone who has a canine companion knows how attractive they can be to other animals.
10 The odors produced by the plants are actually chemically similar to those produced by rotting meat.
11 The temperature within the flower can be more than 15 degrees centigrade warmer than the surrounding air.
12 Birds and other pollinators can engage in floral larceny as well.