What if dogs gave birth to kittens, and those kittens grew up to have puppies? That's similar to what some species, such as haircap moss, do. Each alternate generation has a different form and function. Dr Curt Stager and Martha Foley explore the biological oddity "alternation of generations."

Martha Foley: So you’re teaching me a new term from your world of biology: alternation of generations. This is a kind of reproduction, is that right?

Curt Stager: Yes, generation after generation of plants, in this case we’re talking about.

MF: But alternating in some way that is unusual, or at least we haven’t, or I haven’t thought about before.

CS: It’s a very strange thing that basically only biologists talk about, very strange stuff. Except in one case, which, this is common actually among plants, it’s very esoteric, but there’s one situation where you can see it with the naked eye. You come across it if you’re in the woods paying attention to small plants. You’ll see them, you’ll see this happening if you look at mosses in the woods. So you’re out there walking around in the woods. On the ground, often times you’ll see these little bushy ones, they look like little pine trees or something. An inch long, little fluffy guys. And you can find them on the lawn sometimes.

MF: Yeah, lovely moss.

CS: It’s called haircap moss. So, once and awhile, you look at them, and there’s this little slender brown filament coming out of the top of a few, and then there’s this little bud on the end, or a little capsule.

MF: It looks like a little seed to me always.

CS: Looks like a seed or something. And you say, oh they must be reproducing.

MF: They’re blossoming is what I always thought.

CS: They’re blossoming, and in the case of these, they don’t make flowers, they make spores, like a fungus would. So if you look closely at that, they call them haircap moss, this kind, because if you look at that little capsule, it comes up like a little thread, from the top of one of the green moss plants, and it bends down again like a little hook. And then the capsule actually has a sort of a woven, thatch-like little cap on that you can pull off with your fingers.

MF: Ok, so yeah, so big deal?

CS: You open that little capsule with your fingernails and you shake it out in your hand and you’ll see a little bit of green dust. And those are spores. So if you sprinkle them on the ground in the right situation, or in a garden, they could grow into new moss plants. It all looks very simple, but there’s this bizarre twist in the lifecycle of these mosses, which we call alternation of generations. What looks like just a little blossom coming off the bush, or a little branch coming off the tree, this little filament is not a piece of the moss plant under it. It’s an adult plant all in its own right.

MF: Well, how do you know that?

CS: It’s bizarre, because it doesn’t look like a plant, right? Number one it’s attached, and number two it doesn’t have any leaves. It doesn’t look like a plant, it looks like a thread, or something.

MF: It looks like a stick.

CS: It looks like a stick. Alright, now, why is this an adult plant? Well, it turns out, it’s growing on a mother plant. And it turns out the mother plant has eggs, just like animals do, that get fertilized by male plants. So it turns out, if you look at that moss mat, the little green things, some of them are male plants, and some of them are female plants. And they reproduce, in a sense, like an animal does. You know, they fertilize, and this little fertilized egg, rather than doing what you think they’re going to do, which is grow into another moss plant, instead, it stays right there on top of this female, and it sprouts into one of those filaments. And it grows into its own little plant. You think about it; like how can it live?

MF: And then it scatters its spores, which then grow another patch of green furry moss.

CS: Right.

MF: So here’s this intermediate generation.

CS: It’s an intermediate generation.

MF: And is it genetically, can you trace the one to the other?

CS: Yes, so, if this was like a flower on a rosebush, the flower is genetically identical to the rosebush. This is genetically different from the plant it’s sitting on. It’s got half the genes from the mom’s egg, and half the genes from a father plant somewhere in that bunch.

MF: Wow, and yet it doesn’t make the plant.

CS: Right, it’s parasitizing, getting all its nutrition from mom. She’s raising it like the kid that never leaves the house, right? And then it reproduces in a different way, with spores. So it’s first male and female plants give rise to something that makes spores, which gives rise to male and female plants, which makes something that makes spores. As if people got together, they want to raise a family, they give birth to a litter of puppies. And then the puppies grow up, and they want to raise a family, so they have kids and they look like children. And then back and forth, generation by generation, it alternates what they look like. Same thing with the mosses.

MF: You have to wonder why this turned out this way for these plants.

CS: One of the great mysteries of the forest.