Every other Friday on Morning Edition, Outside/In Host Sam Evans-Brown answers listeners' questions about the mysteries and quirks of the outside world.
Donna from Center Ossipee asks: “I’m calling in regard to the acorn tassels, which I never see on the trees but they’re always on the ground this time of year. And there’s thousands of them! They cluster together, move like tumbleweeds or like a snake bundle. Does each tassel represent an acorn? How does that pollination thing work?”
For starters, those tassels, as Donna calls them, have a scientific name: “catkins.” The catkin is the male bit.
“One very interesting thing about oaks is that they have separate male and female flowers,” says Mario Vallejo-Marin, who studies plant evolution at the University of Stirling in Scotland. “Most flowering plants have both sexes in the same flower, but a fraction of flowering plants including oaks have separate males and females.”
There are some plants that have separate male and female plants, but oaks — and approximately 6 percent of all flowering plants — are hermaphroditic. As an evolutionary strategy, the upside is it’s easier for them to fertilize their flowers if they are the only tree of their species in an area, but the downside is they sacrifice some genetic diversity and are more at risk of inbreeding.
(There’s a whole coda here on the astonishingly diverse sex-lives of plants — which are much more varied than that of boring old mammals [have you heard of buzz pollination? I mean, come on!] — but alas, that is too much of a digression.)
Perhaps the reason our listener has never noticed oak flowers on the tree is because they put out their flowers quite early in the year, before the leaves are out. They flower early because they don’t want the leaves to get in the way of wind. The purpose of the lazy dangly shape of the catkin is to maximize the ability of the wind to deliver pollen from the male flowers to the female ones.
Interestingly, while wind pollination was the earliest method of plant reproduction, plants as a whole abandoned the wind about 100 million years ago, because relying on insects is so much more efficient. However, wind pollination re-emerged in times of environmental change, when there were fewer insects around to do the job.
What does that mean for these 10 percent of plants that now rely on the wind? Unlike bees or hummingbirds, the wind blows pollen indiscriminately… inaccurately.
“And the evolutionary solution to this problem of making sure the pollen grains reach the female flowers is to produce a very vast amount of pollen grains,” says Vallejo-Marin, “So that’s why your listener has seen all these male flowers forming bundles on the ground. It just reflects the massive production of pollen grains during the reproductive season of oaks.”
To answer the second part of this question: each catkin does not result in an acorn, because the catkin is the male bit. The seeds come from the female flower of the oak tree, which is EVEN LESS obvious than the male part.
And it’s not just that they have to produce many flowers; for wind pollination to work plants have to produce a ton of pollen, too. All of the pollen that you see coating your car and yard furniture during the late spring and early summer is a result of this phenomenon.
(A fun fact: the pollen you most often see is white pine pollen, which is visible because pollen grains are abnormally large. It gets all over everything in part because pine pollens have little air sacs or wing-like structures to help it be carried farther.)
Not only does this super-abundant production strategy result in pollen all over your stuff, it’s also what brings us seasonal allergies: you don’t get substantial amounts of pollen in your nose from plants that are waiting for bees to come along and do the job, the pollen that irritates our immune systems is from all of these wind pollinators.
So, Donna, that’s how this pollination thing works! Those little “acorn tassels” represent a little insight into the fascinating diversity of tree sex, and all of the downstream impacts on the rest of the world!
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