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Haircap moss, showing both "generations." Photo: <a href="">Kristian Peters</a>, CC <a href="">some rights reserved</a>
Haircap moss, showing both "generations." Photo: Kristian Peters, CC some rights reserved

Natural Selections: Alternation of generations

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What if people gave birth to puppies, and those puppies in turn gave birth to people? 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."

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Alternation of Generations is an unusual way by which some plants reproduce.

“It’s a very strange thing that basically only biologists talk about,” said Stager, “except in one case. There is one situation where you can see it with the naked eye—if you look at mosses in the woods.”

Haircap moss, specifically. Stager described it as looking like little inch-long pine trees that are bristly and soft. Looking closer at the moss there is a slender brown filament coming out of the top of a few, with a little bud on the end, meaning they are blossoming.

“In the case of these, they don’t make flowers, they make spores,” Stager explained. He said, “if you look at the capsule, it comes up from the top of the moss, like a little thread, then bends down like a little hook.

“The capsule actually has sort of a woven thatch-like little cap on it that you can pull off with your fingers.” Within that capsule are little bits of green dust, that if you sprinkle them on the ground or garden they could grow into new moss plants.

“This little filament is not a piece of the moss plant under it. Its an adult plant all in its own right.”

That is a little bizarre, because it is attached to the other plant, has no leaves, and doesn’t look like a plant. How could this be?

“It turns out, it’s growing on its mother plant. And it turns out the mother plants have eggs just like animals do, that get fertilized by male plants,” Stager said. “Looking at the moss mat, some are male plants and some are female plants, and they reproduce like an animal does, he explained.”

“This little fertilized egg, rather than doing what you think it would do—which is grow into another leafy moss plant—instead, it stays right there on top of this female, and its sprouts into one of those filaments.”

“It has half the genes of the female plant and half the genes from a male plant somewhere in the moss grouping.” said Stager. “It’s parasitizing, getting all its nutrition from mom—and then it reproduces in a different way with spores.”

It is an on-going process, where male and female plants give rise to something that makes spores, which gives rise to male and female plants and so on.

Stager described it further, “[It’s] as if people got together, wanted to raise a family, [and] 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.”

“One of the great mysteries of the forest,” Stager proclaimed.

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