What are gametophyte ferns?

 The plant below is a gametophyte fern. 

It looks strikingly like an early stage Arabidopsis thaliana embryo. 

What is a gametophyte fern? 

It is less than 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 alternation of generation, there are two separate generations in the life cycle of the plant, each one giving rise to the other. 

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 independently of one another.

What are these sporophytes and gametophytes?

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. 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 to either the egg cells of that same plant, or possibly even to those of an adjacent plant. When it finds an egg cell, it 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 plants, which take turns in living their lives.

Some sporophyte ferns are large trees. So 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.

Here are some more photos of gametophyte ferns.

If you would like to know more about how these photos were taken, please visit http://chlorophyllosophyimages.blogspot.com/2017/12/focus-stacking-setup.html

If you would like to grow your own fern gametophytes, please visit this page: