(Psilotum is not a fern. It belongs to an informal group known as “fern allies.”)
Psilotum nudum (sigh-LOW-tum NEW-dum)
Today John and George continued exploring Seabranch State Park near Hobe Sound, Florida. Our shoes got wet in the eastern margin of the park in a low, wet, swampy, shadowy, ferny, prehistoric forest. My unscientific rule of thumb is: the swampier the woods, the more primitive it be. The swampy lands in Seabranch look like a museum dinosaur diorama with ferns taller than a person, tree trunks festooned with dangly mossy things, and a lush carpet of primordial ooze only a salamander might love. Now I do want to push the primitive thing too hard, because this is an imagination venture (please no condescending e-mails explaining profoundly that the local swamp is not actually primitive). Still, a swamp is a venue to see plants we think of as primitive, if you define “primitive” as representing a particularly ancient plant group.
The Seabranch swamp is a wonderland of liverworts, which are about as primitive as a land plant can be, but we will defer those until another day. Mosses are likewise ancient, ferns are too, bald cypress is no evolutionary spring chicken, and you get the idea. We are glancing back to plant groups in rough terms four times older than flowering plants and 400 times older than humans.
Today’s encounters take the imagination back 400 million years to the early Devonian Period. Looking down from space a modern alien wouldn’t recognize the Devonian Earth. Europe and Asia were a single continent. So were Africa, India, Australia, and Antarctica. And, creeping forth from the ancient seas, plants had only recently invaded the land.
Despite the passage of almost half a billion years, you might be surprised how much we know about the early land plants thanks to exquisite fossils preserving anatomical details over the eons. The most famous Devonian plant fossil site is called the Rhynie chert near Aberdeen, Scotland where an entire swamp is so perfectly mineralized it looks like a stone-wizard waved his magic wand just yesterday.
Those ancient plants had distinctive evenly forked Y-shaped branching, in contrast with the uneven branching patterns characteristic of modern stems. They had no roots, although underground rhizomes were fashionable. As remains true of modern ferns and the so-called fern allies, the ancient land plants reproduced by spores rather than by flowers, fruits, and seeds. The spore cases were either on the tips are on the sides of the stems. (A spore differs from a seed in many ways, principally by being merely a single cell.)
Below are a couple of examples of plants we know from the Devonian fossils:
Wouldn’t it be amazing if a plant like that walked the earth in 2015? A botanical pre-dinosaur! Well, you know where I’m going with this: it lives in the Seabranch State Park “Rhynie forest.”
Some textbooks call the plant whisk-fern, actual people call it Psilotum. It would’ve been comfortably at home in the Rhynie chert. Psilotum has no flowers, fruits, or seeds. The branching is evenly Y-shaped in the prehistoric fashion, and the spores occupy little pillboxes along the side of the stem. It looks nothing like a modern plant; instead, it looks like the ancient fossils.
Life is never straightforward. DNA and other studies undermine the idea of Psilotum as a diehard from Devonian times. DNA places it among the ferns so in contemporary classifications it is one. So why a plant that looks like it’s straight out of Rhynie doesn’t show exactly the expected relationships is a wee conundrum for future botanists. In the meantime, in my mind it jumped right out of the rocks.
If you’re wondering about spores, by the way, as in ferns and other fern allies, and certainly also in the prehistoric land plants, they don’t germinate like a seed and replicate the parent plant. In Psilotum the spores germinate into a tiny completely underground generation living off of subterranean fungal symbiosis. The baby Psilotum, technically called the gametophyte (gam-EET-oh-fight), looks like a root without the rest of the plant. You’ll never see one. But it’s not so hard to find the parent Psilotums hanging out of the broken leaf bases on cabbage palm, or occasionally on moist hummock in the swamp. And there is even an easier way to visit Jurassic Park. These plants are often weeds and plant nursery flowerpots.