Wednesday, February 15, 2006

Growing ferns

I'm going to start this honestly: I have no idea how to grow ferns from spores. This probably explains why I'm crazy enough to try to figure out how to do it with my class.

Why would I want to bother rearing ferns in class? Well, ferns (like all other non-seed plants like mosses and liverworts) don't reproduce via seeds; instead, they produce haploid spores that fall to the ground and grow into little haploid plants called gametophytes (termed prothalli in ferns). These haploid gametophytes produce gametes (sperm and eggs), and just like in animals, the sperm swim to the egg and fertilize it. Once the egg is fertilized (and thus diploid), it grows into a sporophyte, the leafy part of a fern most people recognize.

As a quick side note, most folks (reading this blog) probably know that animal gametes (sperm and eggs) are produced by meiosis, a process wherein a diploid cell divides and becomes four haploid cells. However, fern gametes are produced by mitosis, not meiosis. The reason fern gametes are produced by mitosis is that they're created by gametophytes, which themselves are haploid (have only one copy of each chromosome), so going through meiosis would be impossible for the cells in the gametophyte. In ferns, meiosis is used by the diploid sporophyte to produce haploid spores (which grow into the gametophyte that eventually creates the sperm and eggs).

Non-seed plant life cycles are pretty cool, but unfortunately teaching about them is generally dull. I show my students a few figures, they look at a few slides of cross-sections of plant reproductive organs, see the sori (which contain sporangia that produce spores) on the underside of fern fronds, observe a little preserved gametophyte, and we all agree that we've learned how non-seed plants reproduce. But that's always seemed somehow insufficient.

So, what I'd like to do is have the students actually rear ferns. It seems as though it's at least possible; these three sites have good summaries of how to rear ferns in layman's terms, and from what I can tell it primarily requires spores, sterile soil, and lots of time (anywhere from two months to a year). Assuming we could go from spore to small sporophyte in two months, I could fit that into a single semester; the students could plant the spores at the beginning of the semester and then, if we're lucky, see mature sporophytes popping up at the end of the course. Maybe the students could even take home their little sporophytes at the end of the semester. Watching the gametophytes grow wouldn't be quite as dramatic as watching the caterpillar Manduca sexta going from a 1-mg egg to a 5-g pupa in three weeks (see my series of posts on that here), but it'd still be cool.

The American Fern Society has a spore exchange that supplies spores to people, and even has its own detailed set of rearing instructions. Unfortunately, however, I haven't been able to find any estimates of the time it takes specific species to go from spore to gametophyte to sporophyte, and thus don't know what species would be best to use. Looks like I've got some research to do.

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