MYSTERY PLANT: This plant reproduces by green spores, not seeds

  • Posted: Sunday, April 14, 2013 12:12 a.m.
Photo by Linda Lee
Spores on this week’s fern are green, not brown or black.
Photo by Linda Lee Spores on this week’s fern are green, not brown or black.

Gadshill: “We have the receipt of fern-seed, we walk invisible.”

Chamberlain. “Now, by my faith, I think you are more beholding to the night than to fern-seed for your walking invisible.

– Shakespeare, Henry IV Part 1: Act 2, Scene 1

Well, ferns don’t make seeds. Do they?

Back in the Middle Ages, and well into the 16th century, there was considerable confusion regarding the way plants worked, along with everything else. Plants were thought to reproduce by seeds, and all of the everyday, conspicuous plants known did just that, but then there were those ferns. The ferns never did seem to make any seeds in any form.

But since plants reproduced by seeds, and since ferns are plants, that meant that fern’s seeds were invisible. And that having some fern seeds in your pocket would make YOU invisible.

Can you imagine the scams that might have lead to? “’ello governor, how ‘bout a pouch of fern seed? Only 12 sovereigns for a man such as yourself!” But no fern makes seeds, visible or invisible. Not even the one pictured here.

Ferns reproduce by producing tiny spores, which may effectively be considered as acting like seeds, at least as dispersal units. Their spores are produced in tiny structures called sporangia, and these sporangia are clustered into little patches, each one called a sorus. Depending on the fern species, there may be a lot of sori (plural) or maybe not so many, but the spores themselves are quite tiny and usually produced in great abundance. For most common, every-day ferns, the sori occur in little bumps on the back side of a regular fern leaf. But our mystery fern is a bit different.

This species, and its close relatives (some of which were as big as trees), were common about 200 million years ago, sometimes getting stepped on by dinosaurs. As a kind of modern “fossil,” it produces “regular” fronds but without any sporangia. These non-reproductive, or “sterile” fronds, are quite handsome, arising in a clump, bright green and divided into a number of pointed segments, the whole leaf up to 5 feet tall on really happy plants. There is another kind of frond produced, though, what we conveniently call a “fertile” frond, and this is where the sporangia are.

In fact, the fertile fronds, which are usually a bit shorter than the regular sterile ones, end up with a look all their own: fuzzy, due to thousands of curly hairs, and cinnamon brown at maturity. The sporangia occur on the branches of each fertile frond, but instead of being in recognizable sori, they collectively form a wooly layer on the surface of the fertile frond. Each sporangium will release plenty of extremely tiny spores, when ready. And these spores are green. (Most ferns’ spores are brown or black.)

Our beautiful Mystery Fern is common all over eastern North America. You’ll find it in shady swamps and wetlands along creeks and streams. And it makes an excellent addition to a shady wet garden.

John Nelson is the curator of the A.C. Moore Herbarium at the University of South Carolina, in the Department of Biological Sciences, Columbia. As a public service, the Herbarium offers free plant identifications. For more information, visit www.herbarium.org or call 803-777-8196, or email nelson@sc.edu.

[Answer: “Cinnamon fern,” Osmundastrum cinnamomeum]

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