Most of us know a friend or relative who isn’t content to follow the standard path in life. Why do things the simple easy way when there’s a more complicated option available? Maybe you’re even that person yourself. If so, you’ll appreciate the pollination strategy of milkweed plants.
Most flowering plants that attract insects as pollinators have adopted a fairly simple strategy for getting their embryos fertilized. They use color, patterns, and/or scent to draw bees or other insects to themselves and then reward them with nutritious pollen, nectar, or both. As a tradeoff, when those hungry insects come in to feed, they rub against anthers and pick up tiny pollen grains that lodge in their hair. Upon visiting another flower, some of those pollen grains rub off the insect onto strategically placed stigma and there you have it – fertilization.
That basic process is tried and true, and responsible for seed production in thousands upon thousands of species.
“Yawn,” say milkweed plants.
Or maybe they say, “Hold my beer.”
Milkweed flowers make lots of nectar, and most also emit an attractive scent. They produce pollen too, but instead of loose pollen grains, milkweed pollen grains are glommed together in a kind of waxy sack. Botanists call that sack a pollinium, and milkweed flowers create them in pairs (pollinia), connected by two arms stretching out from a small dark structure called a corpusculum. The entire combination looks like a pair of yellow saddlebags.
As a quick aside, the creation of these waxy pollinia isn’t unique to milkweed plants. Orchids create them too. If they could, I bet orchids and milkweeds would very much enjoy swapping tales about how they trick insects into hauling those saddlebags around for them. You can find lots of stories about orchids and their tricks on the internet – they have no shortage of fans who like to talk about how beautiful and special orchids are. No argument here, I just think those orchid fans might consider throwing a little love toward milkweed too.
If you’ve ever really inspected a milkweed flower, you’ll have noticed that it looks pretty different from how a kindergartner would draw a flower. It has its own unique structures, perfectly suited to carry out its crazy pollination strategy. As part of that strategy, the milkweed flower doesn’t put those pollinia (saddlebags) on display where any old insect could pick them up and carry them off. Instead, they hide them behind a vertical slit on the side of the stigma – the central ‘hub’ of the flower.
Sorry, another quick aside here. The botanical term ‘pollinarium’ refers to the entire package of two pollinia, arms, and corpusculum. I mean, that’s fine. But wouldn’t ‘pollinarium’ be the perfect name for the compartment on the side of the stigma in which the milkweed plant stores the pollinia? “The pollinia can be found in the pollinarium.” That just sounds right. But no. That compartment is called a ‘stigmatic chamber’. Such a waste…
The milkweed’s nectar – the whole reason insect visitors come in the first place – is located in each of five sections of the colorful star-shaped ‘corona’ that surrounds the central stigma where the pollinia are hidden. Visiting insects walk around the flower, sticking their tongues down into the openings where the nectar is stored. For most flowers, this is the part of the story when insects get pollen all over themselves as they search for nectar. In the case of milkweed flowers, those pollinia are hidden away within the ‘stigmatic chamber’ (sigh) far away from the fuzzy faces and bodies of those insects. Seems like a dumb design, right?
However, as insects crawl around on milkweed flowers, moving from one to another nectar bucket, there are only so many places to step. The stigma is one of those few places, and now and then, a leg inadvertently slips into the slit where the pollinia are stored. It’s a tight fit, and insects have to struggle a little to yank their leg back out. As they do, they often catch their foot on the corpusculum between the saddlebags and when that happens, the whole package pops out and dangles from the leg.
“Aha!” says the milkweed, “told you it would work.” (I like to imagine that milkweed flowers say this every single time it happens.)
I want to be absolutely clear that an insect’s leg sliding into that slit is a complete accident. Insects are not going this on purpose. In fact, it’s not unusual for an insect’s leg to get so securely stuck in the slit that it has to pull its leg off to escape – or die while trying.
Ok, the milkweed has scored its first points, but the game is only at halftime.
In order to complete the pollination process, pollen from one flower has to enter the stigma of another flower and fertilize the embryo. In the case of milkweed, that means the same insect that accidentally stepped into a stigmatic chamber (ugh) on one flower has to make the same mistake again on another flower. With the same leg.
More specifically, for all this to work, a milkweed flower needs an insect to:
- accidentally slide one of its legs into a slit on the side of a milkweed flower’s stigma
- successfully extract the leg again (don’t die!) and snag the pollinia on the way
- leave the pollinia on its leg (insects often attempt to pull them off but they seem to stick pretty well)
- visit a different flower
- accidentally slide THE SAME LEG into a slit on that flower
- successfully extract the leg again, this time dislodging the pollinia and leaving it inside
There are, of course, structural features of the flower that ‘encourage’ insect legs to slide into the correct slits. Similarly, there are structures that help ensure the leg will hook the pollinia as it is pulled back out (or leave a pollinia behind the second time). In other words, pollination is not all due to random chance. But still. It’s a pretty crazy process.
Depending upon your perspective, the alternate strategy employed by milkweed flowers is either risky and overly complicated or quirky and fun. Milkweed plants probably don’t care which terms any of us choose to describe their approach. The countless milkweed seeds floating around each autumn is testament to their success.
Well done milkweed; here’s your beer back.