Fern life cycle

DID YOU KNOW?

The silver fern is a national symbol of New Zealand; the Maori once used their silvery glow to navigate at night

The life cycle of ferns

How this ancient group of plants has survived for millions of years

About 450 million years ago, the fi rst land plants emerged from the water. They were simple green fl aps, a bit like modern liverworts. Later, slightly more advanced moss-like plants appeared, and these evolved into the more complex form of ferns, horsetails and club mosses. 300 million years ago, ferns and their allies dominated a world that was much wetter than today. As a result, they only needed a simple system to move water to their stems and leaves.

These ancient plants had a two-stage life cycle. The mosses and liverworts that we typically fi nd on rocks and in damp woods are all gametophytes – the sexual stage in the moss’s life cycle, which produce the equivalent of eggs and sperm, called gametes. In wet weather, the male gametes swim over the moist surface of moss leaves to fertilise the eggs. These then develop into a stalk topped by a cap. This second stage, which relies entirely on the gametophyte for nourishment, is called the sporophyte, because it produces spores like simple seeds. These are shaken from pores in the cap and blown to a new site, where they grow into new gametophytes, but these can only survive in wet conditions.

Ferns have evolved so that the sporophyte has become the dominant stage in their life. Ferns, horsetails and club mosses are all sporophytes that live freely, even in quite dry conditions. The spores they release develop fi rst into gametophytes and these reproduce sexually to create the next generation.

From spore to fern…

The life of ferns is an echo of their evolution from the fi rst simple green land plants

4. Green heart

If the spore lands somewhere damp, it begins to grow into a heart-shaped fl ap of green tissue, called a prothallus.

6. Swimming males

Male gametes, produced in structures called antheridia, move through water by waving their many whip-like fl agella.

5. Prothallus

The gametophyte generation of the fern. Different structures on its surface produce male and female gametes (sex cells).

Ferns, fossils and fuels

300 million years ago, ferns and their relations formed the main vegetation of our planet. Flowering plants did not appear for another 175 million or so years. In this Carboniferous period, the continental plates carrying North America and Europe were situated close to the equator in wet, tropical conditions. In the steamy swamps, club mosses over 30 metres (98 feet) tall grew in dense forests. The remains of these club mosses and horsetails formed much of today’s coal, and many impressions of fern fronds are found as fossils in ancient rocks.

7. Fertilisation

The male gametes swim into the neck of the archegonium, which produces the female cell, and fertilise this ‘egg’.

3. Spore release

When the spores are fully developed, the sori open. The spores are released and blow away in the wind.

8. New fern

A new fern plant (sporophyte) grows from the fertilised ‘egg’. This can also spread using creeping underground stems.

2. Spore makers

On the underside of fern fronds, brown swellings – sori – enclose bodies called sporangia on which tiny spores are formed.

1. ‘Adult’ fern

The plants we call ferns are sporophytes – the spore-producing stage of the fern’s life cycle.