Friday, 3 August 2012

Gametophytes

You might not believe it if you haven't studied botany (you might not believe it even if you have), but these are mature, fully grown, reproductive ferns.
Fern gametophytes.  I guess these might be Pneumatopteris pennigera, because that's what they were growing under.
All land plants have two multicellular stages in their life cycles, but in seed plants, one of the stages is so small that we can't see it without a microscope.  Ferns are good for demonstrating this alternation of generations because both life cycle stages are big enough to see with the naked eye.

The little heart-shaped plants above are fern gametophytes.  A familiar, big, leafy fern plant (it's called the sporophyte phase) produces spores in thousands of little structures—sporangia—underneath its leaves, and the spores don't grow into new leafy ferns, but into these little gametophytes.  Gametophytes in turn produce gametes (eggs and sperms), and when a sperm fertilizes an egg a new sporophyte fern is born.
Fern gametophyte, lower surface showing the notch and the tuft of rhizoids.
Fern gametophytes are delicate, thin sheets of plant tissue that are confined to damp places.  New growth arises in the notch of heart-shaped gametophytes like this one, where tiny cells are actively dividing.
The small actively dividing cells of the notch form the growth point of the gametophyte.
The gametophyte is anchored to the ground by a tuft of rhizoids (they're not roots: they don't take up water and they have a very simple structure).  Just back of the notch and among the rhizoids, the sexual structures form.  Some of these—archegonia—are female and each makes an egg.  Others—antheridia—are male, and each makes numerous sperms.  The sperms swim through a film of water, and find eggs following a chemical (often a sugar like malose) that diffuses from the archegonia.
Archegonia (circled).  They're very hard to photograph with just a dissecting microscope and a hand held camera.
One of the joys of teaching botany is you get to see (over and over again) some of the strange and wonderful aspects of the plant world.  Before I taught, I'd always thought gametophytes would be almost impossible to find—after all, the seedless plants are called cryptogams, which refers to their hidden sex lives.  But gametophytes aren't hard to find.  Look for a damp bank under overhanging fern sporophytes.  The gametophytes are often among mosses and can be hard to see at first.  It's easiest to look for tiny fern sporophytes that are just emerging between the lobes of the gametophytes (there's one overlapping the edge of the coin below); when these are small enough, the gametophytes they grew from haven't yet shriveled. Once you get used to what you're looking for, you'll find they're pretty common.

Occasionally you can strike it lucky.  This freshly-cut bank (below) was almost entirely covered by fern gametophytes, in many the eggs had been fertilized and young sporophytes were just emerging.
Fern gametophytes, those on the right with young sporophytes emerging.
You can grow your own too.  You'll need some peat in a pot, and it's not a bad idea to sterilize it first (I guess a microwave would be a simple way to do this at home).  Collect a leaf from a fern you want to grow (check it's got sporangia underneath), and let it dry out in a paper bag so the spores are shed.  Scatter the spores (fine brown or yellow dust) onto the surface of the thoroughly moistened peat, and cover the pot with a sheet of glass or a plastic bag, to keep the moisture in and other spores out.  Exposed to sunlight (not too strong), the spores should germinate to produce a dense crop of gametophytes, and if you wait long enough and keep them moist, they should produce little sporophytes that you can plant out.

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