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HEAD HEAD
1. TALLEST
FERN RIVALS
Norfolk Island tree fern
2. SMALLEST
3. COMMONEST
Water fern
Bracken
Water ferns (eg Azolla species) have stems shorter than 5cm (2in) long and tiny fronds. You’ll find them floating on ponds and in ditches.
Tree ferns like this Pacific species can grow to over 20m (66ft) tall. Their trunks are made up from annual dead leaf bases.
Bracken is a widespread fern, found in every continent on Earth except Antarctica. It is so invasive that it can spread to take over entire hillsides.
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 first land plants emerged from the water. They were simple green flaps, 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 find 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 first 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 first simple green land plants
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.
6. Swimming males Male gametes, produced in structures called antheridia, move through water by waving their many whip-like flagella.
4. Green heart If the spore lands somewhere damp, it begins to grow into a heart-shaped flap of green tissue, called a prothallus.
7. Fertilisation The male gametes swim into the neck of the archegonium, which produces the female cell, and fertilise this ‘egg’.
5. Prothallus The gametophyte generation of the fern. Different structures on its surface produce male and female gametes (sex cells).
8. New fern A new fern plant (sporophyte) grows from the fertilised ‘egg’. This can also spread using creeping underground stems.
3. Spore release When the spores are fully developed, the sori open. The spores are released and blow away in the wind.
1. ‘Adult’ fern © Thinkstock
2. Spore makers On the underside of fern fronds, brown swellings – sori – enclose bodies called sporangia on which tiny spores are formed.
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The plants we call ferns are sporophytes – the spore-producing stage of the fern’s life cycle.
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