Pteridophytes

The first plants with pipes

Walk along a shady stream in the hills and you will see ferns, with their long feathery leaves. Ferns, horsetails and their relatives are pteridophytes.

Pteridophytes were the first land plants with vascular tissue: xylem, which carries water, and phloem, which carries food. With these pipes, a plant can carry water up a tall body. So pteridophytes can grow much larger than bryophytes.

They grow in cool, damp and shady places, though some grow in sandy soil. People use them as medicines, as soil binders and as ornamental plants.

The plant body is the sporophyte

In pteridophytes, the main plant body is the sporophyte, which is diploid.

• It has a true root, stem and leaves, with well-formed vascular tissue.
• The leaves may be small, called microphylls, as in Selaginella.
• They may be large, called macrophylls, as in ferns.

Spores

• The sporophyte bears sporangia. Each sporangium grows from a leaf-like part called a sporophyll.
• In some plants, the sporophylls are packed together into compact strobili, or cones, as in Selaginella and Equisetum.
• Inside each sporangium, spore mother cells divide by meiosis. Each makes four haploid spores.

In ferns, the sporangia sit in small clusters called sori on the lower side of the leaflets, the pinnae. In many ferns a sporangium releases $64$ spores: $16$ spore mother cells, each giving $4$.

The prothallus

A spore grows into a gametophyte called the prothallus.

• It is small and easy to miss, but it is multicellular and free-living.
• It is usually green, so it makes its own food.
• It needs a cool, damp, shady place to grow.
• It bears the sex organs: antheridia and archegonia.

Water is needed to carry the antherozoids from the antheridia to the mouth of the archegonium. One of them fuses with the egg, and a zygote forms. The zygote grows into a new, well-formed sporophyte, the plant that we know as a fern.

The prothallus needs damp shade, and fertilisation needs water. So living pteridophytes grow only in a few narrow kinds of places.

Four classes

Lycopodium is called the club moss, and Adiantum the maidenhair fern. Though named a moss, the club moss is a pteridophyte: it has vascular tissue.

You should find that $20$ leaflets, $6$ sori on each and $30$ sporangia in each sorus give $3600$ sporangia and $230\,400$ spores. Only a few of them land in a damp, shaded place and grow.

Try it in Python

The program counts the sporangia and the spores on one frond.

You should see $5760$ sporangia and $368\,640$ spores. With $48$ spores in each sporangium, the number falls by a quarter.

Practise with new numbers

Common mistakes

• Thinking the fern plant is the gametophyte. The fern that you see is the diploid sporophyte. The gametophyte is the small prothallus.
• Thinking ferns make seeds. They spread by spores. The brown dots under a frond are sori, not seeds.
• Forgetting water. Pteridophytes still need water for fertilisation, like bryophytes.
• Calling every cone a gymnosperm. Equisetum and Selaginella also bear cones, called strobili.

Remember these

• Pteridophytes are the first land plants with xylem and phloem.
• The main plant body is the diploid sporophyte, with true roots, stems and leaves.
• Sporangia grow on sporophylls; in some plants these form strobili.
• Spores grow into a free-living prothallus with antheridia and archegonia.
• Classes: Psilopsida, Lycopsida, Sphenopsida, Pteropsida.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What made it possible for pteridophytes to grow taller than bryophytes?

2. Where do you find the sori of a fern?

3. In which class would you place Equisetum?

4. Give two reasons why ferns are found in only a few kinds of places.

5. One sporangium has $16$ spore mother cells. How many spores does it make?