Evolution doesn't always tend towards larger, more complex organisms. The dinosaurs' closest living relatives are pigeons and sparrows. Similarly, the tiny, shy aquatic quillworts (Isoetes) descend from "scale trees" that dominated great rainforests 300 million years ago and could've outgrown a 9-story building.

From dinosaur to chicken: Isoetes and its ancestors

Today, the closest living relative of Tyrannosaurus rex is arguably a chicken.

All right, that isn't strictly true: birds are certainly the closest living relatives of dinosaurs (and are in fact dinosaurs themselves, by any reasonable measure), but chickens are no closer to T. rex than are swans, ostriches, or toucans. They're also no more distantly related than any of those, though -- and isn't that remarkable? Goofy, awkward, halfway-flightless domesticated featherbags have as much claim to the tyrannosaur's legacy as anything else alive.

Evolution does this sort of thing all the time, altering species in directions quite different from any human notion of "progress." The plant kingdom has hundreds of examples, and none quite as spectacular as the story of Lepidodendron and Isoetes.

Artist's reconstruction of Lepidodendron.
This one was as tall as a 9-story building.

An extinct genus of giant lycopsids (early ferns) with thick scale-patterned bark and leafy, forking branches, Lepidodendron -- the scale trees -- once dominated great rainforests that stretched around the world. Despite having no true wood to hold them up, they grew up to 30 meters tall with trunks a meter thick. For sixty million years, they flourished worldwide. Their rainforests produced so much plant matter that instead of decaying or fossilizing normally, it compacted into vast beds of coal. Huge scale-tree fossils -- massive trunks, spiky-leaved branches, sporing cones longer than a human hand -- have captured botanists' imagination for as long as miners have been chipping them free of the bedrock. It's no exaggeration to call these vanished giants the Tyrannosaurus of the plant world.

300 million years ago, though, the scale trees went extinct. The Carboniferous ended in a planetary cold snap that brought glaciers creeping down from the poles; practically overnight, the rainforests collapsed. Giant lycopsids all but vanished, and tree ferns took their place in the cold new forests.

A clump of Isoetes tegetiformans with a U.S. penny for scale. 
Not much to look at unless you know what to look for.

Today, Lepidodendron's closest living relatives are the little grass-like quillworts of the genus Isoetes. They're tiny compared to their ancestors: a quillwort 20 cm tall is quite unusual, and the largest species alive only grow to one meter. Their rolled-up, rush-like leaves don't look much like scale-tree branches, and they have nothing at all like bark. They keep their spores tucked close inside the base of the leaves; there is no separate sporing cone. Most of them are aquatic, growing in calm freshwater. The North American species are particularly good at surviving in cold, infertile lakes where little else will grow.

These pointy little proto-ferns look about as much like the great Lepidodendron as a chicken resembles Tyrannosaurus. Just like the chicken, though, there are similarities: the structure of their veins, their different male and female spores, and the history traced in their genes. It's all there, if you can find the right way to see. Inside every quillwort is the shadow of an ancient rainforest.


Image sources:

• Heimans, Eli. "Stigmaria [Lepidodendron] reconstruction." Retrieved 20 Feb 2016 from Wikimedia Commons: <https://en.wikipedia.org/wiki/File:Stigmaria_Heimans.jpg>
• US Fish and Wildlife Service. "Isoetes tegetiformans, on granite outcrop in eastern half of Georgia Piedmont." Retrieved 20 Feb 2016 from Wikimedia Commons: <https://commons.wikimedia.org/wiki/File:Isoetes_tegetiformans.jpg>

Vivipary: a seed within a seed *

Take a good look at the photo below, and tell me: what's wrong with this picture?

If you pointed out the little green shoots, give yourself a pat on the back, because I'm pretty sure that corn isn't supposed to do that. The kernels usually wait until they're planted before they sprout. What we have here is a mutant plant, showing off a phenomenon called vivipary.

Want the explanation? Fruit is not, generally speaking, a particularly good place for a seed to sprout. Take apple trees, for instance. Apple seeds start out buried deep inside a sweet, fleshy ball of fruit: ideally, they shouldn't germinate until the apple has been eaten and its core discarded to rot. If they wait, they need only break through the relatively thin, brittle walls of the core, rather than having to expend lots of energy pushing out through the whole apple. Most importantly, if they wait til the apple is eaten, the animal that ate it has probably carried them a good ways away from the parent tree: they'll be able to grow without having to compete with Mom for sunlight and water.

Since nobody really benefits if a seed jumps the gun, plants have developed ways of keeping seeds quiet until it's time. In its fruits, the parent plant produces a hormone called abscisic acid, which suppresses germination and keeps the seed dormant. As long as the seed is inside the fruit, it gets a steady dose of the stuff and it stays dormant. Once the fruit is off the parent plant, though, and the seed broken out of the fruit, the abscisic acid stops flowing and the seed can sprout.

The trouble is that this system only works when both the parent plant can produce abscisic acid and the new seed is sensitive to it. Sometimes, one end or the other fails, and that's when vivipary happens. Corn sprouting on the cob! Pumpkin vines splitting your jack-o-lantern in two! Dogs and cats sleeping together! It's chaos!

Well, perhaps it's not as bad as that. The phenomenon is called vivipary -- "live-bearing", in the sense of an animal giving birth to live young -- and it's not common in the plant world, because it frankly doesn't work very well for the plants. Viviparous mutant strains are very useful in studying how abscisic acid works, but the naturally-occuring ones tend to die out fairly quickly. A few species, such as garlic and mangroves, have embraced vivipary as a way of getting around some reproductive limitation; mostly, though, it's not the norm.

Should you ever shuck an ear of corn and find a seedling inside, then, you let me know. You'll have a rare oddball on your hands, and I'll want to see it for myself!


Image source:
Hughes, Wayne. Corn4. 15 September 2005. Niches. Retrieved 16 January 2012 from <http://sparkleberrysprings.com/v-web/b2/?p=300>. Used with permission.

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* All Inception jokes should be directed in person to Leonardo DiCaprio.