Monday, 5 January 2015

The complex romantic life of the fern.

A gametophyte generation plant of the species Dicksonia antarctica.

The plant above is a gametophyte fern, which is, beyond doubt, the finest thing that ever existed in nature.

It is only about half a centimetre across, and it will never get very much bigger than this. What is it though, and how does it relate to the big ferns (like the one below) that we are more familiar with from our gardens?

A sporophyte generation plant of the species Dryopteris wallichiana.

Ferns are not quite like people in their mode of their reproduction. They have a system called "alternation of generation". In this system, a tiny gametophyte plant (top photo above) reproduces to make a new large sporophyte plant (second photo above). Then the large sporophyte plant reproduces to make a new tiny gametophyte plant. And round they go again, alternating between sporophyte and gametophyte for ever.

The two plants are both individuals in their own right, and they can live entirely independently of one another.

What are these sporophytes and gametophytes though?

A sporophyte fern is a large plant that produces spores. We can easily see the spores clustered on the underside of the leaves of ferns that we meet in the garden. The spores each have only one set of chromosomes. When they fall off the leaves of the large sporophyte fern, they land on the ground and develop into tiny gametophyte plants.

The tiny gametophyte plants have only one set of chromosomes in each of their cells. The plants never grow much larger than a centimetre across and at maturity (a few weeks) they produce egg and sperm cells which also only have one set of chromosomes. The sperm cell swims across the plant's surface and possibly even onto an adjacent plant, until it finds an egg cell and fertilises it. This fertilisation event produces a cell with two sets of chromosomes, which then goes on to develop into a large sporophyte fern plant.

It always astonishes me that something as deceptively simple as a fern has such a complicated life cycle. It actually includes two completely different kinds of plants, which take turns in living their lives.

Since some sporophyte ferns are large trees, this alternation of generation is the size equivalent of a skyscraper giving birth to a shoe, which then gives birth to a new skyscraper. It's pretty odd, and pretty amazing. What's more, the process has been going on quietly and successfully for many many more years than human reproduction, in spite of its complexity.

If you would like to know more about the gametophyte fern, or even grow some yourself, then you can find the instructions in my post called Growing Fern Gametophytes.