Three groups of spore plants with pipes
Clubmosses, horsetails and ferns belong to the group called pteridophytes. They are different from mosses because they have true roots, stems and leaves and pipes (xylem carries water up, phloem carries food around). That is why they can grow taller than mosses. But like mosses they make spores, not seeds, and they live best in damp places.
- Clubmosses (lycopods): low plants with tiny scale-like leaves. Spores are made in small cones on top of the shoots.
- Horsetails: hollow, jointed stems. At each joint there is a ring of thin branches. The stems feel rough because they contain silica (sand-like material). Spores are made in a cone at the tip.
- Ferns: large, feathery leaves called fronds.
Body of a fern
The main stem of a fern is usually underground and grows sideways. It is called a rhizome. Thin roots grow from it. The leaves, called fronds, are divided into many small leaflets. A young frond is rolled up like a spring, a fiddlehead, and opens slowly as it grows.
Inside the stalk are pipes: xylem (water up) and phloem (food around). The big green fern you see is the sporophyte (2n).
Fern life cycle step by step
- On the underside of fronds are brown dots called sori (one is a sorus). Each sorus is a cluster of spore cases (sporangia).
- When dry, the cases burst and release thousands of tiny spores (n) that fly on the wind.
- A spore that lands on damp soil grows into a small, flat, heart-shaped green plant called a prothallus (n). It is about 1 cm wide and has thin rhizoids below.
- The prothallus makes sperm in male parts and eggs in female parts on its underside.
- In a film of water the sperm swim to the egg. The fertilised egg becomes a zygote (2n).
- The zygote grows into a young fern (sporophyte, 2n) that grows out of the prothallus. The prothallus then withers.
So a fern also has alternation of generations. Unlike a moss, the big plant you see is the sporophyte (2n), and the gametophyte (n) is the tiny prothallus. Water is still needed for the sperm to swim.
Ancient ferns and coal
About 360 to 300 million years ago, the Earth was warm and wet with huge swampy forests. Many plants were giant relatives of today's ferns, horsetails and clubmosses. Some were as tall as trees, 20 m or more.
- The plants died and fell into the swamp water.
- There was little oxygen, so they did not rot away fully.
- Mud and sand covered them in layers. The pressure and heat grew.
- Over millions of years the buried plant matter was squeezed, lost water and became rich in carbon. It turned into coal.
Coal is a fossil fuel. It gives energy when burned, but burning also releases carbon dioxide. It cannot be replaced in a human lifetime. Peat (from mosses) is like young coal that has not yet been buried and squeezed for long. Fossil leaf prints in coal show the shapes of these ancient ferns.
Why these plants matter
They hold soil, shelter small animals, and many ferns are garden plants. Some water ferns such as Azolla live with helpful bacteria and are used as green manure in rice fields. Horsetails were once used to scrub pots because of their rough stems.
Try it: spot the sori
Find a fern in a garden or park. Gently turn over a mature frond and look for brown dots under the leaflets. Tap the frond over a white paper. A fine dust may fall: those are spores. Draw one leaflet and mark the sori. In the 3D, slide the time bar to see how a forest becomes coal.
Key formulas and definitions
- Key term: pteridophyte = plant with pipes that reproduces by spores
- Key terms: rhizome (underground stem), frond (leaf), sorus (cluster of spore cases), prothallus (tiny gametophyte)
- Big fern = sporophyte (2n); prothallus = gametophyte (n)
- Order: spore โ prothallus โ sperm + egg โ zygote โ young fern โ sori โ spores
- Coal: dead plants โ buried โ pressure and heat over millions of years โ coal
Worked examples
1. How is a fern different from a moss?
A fern has true roots, stems, leaves and pipes (xylem and phloem), so it can grow taller. The big plant you see is the sporophyte (2n). In a moss there are no pipes and the main plant is the gametophyte (n).
2. A student finds a leaf with brown dots in neat rows under the leaflets. What is it and what are the dots?
It is a fern frond. The dots are sori, clusters of sporangia that hold spores.
3. Why do ferns still need damp places even though they have pipes?
The prothallus is tiny and the sperm must swim in a film of water to reach the egg. So fertilisation needs water.
4. Explain in four steps how ancient ferns became coal.
1) Plants died and fell into swamps. 2) Little oxygen, so they did not rot fully. 3) Mud buried them. 4) Pressure and heat over millions of years turned them into coal.
Common mistakes
- Saying ferns have seeds or flowers. They make spores.
- Thinking the prothallus is the big fern. The prothallus is tiny (about 1 cm) and is the n stage.
- Calling sori eggs or insects. Sori are clusters of spore cases.
- Thinking coal formed in a few years. It takes millions of years.