Q.What is prothallus?

General Characteristics of Pteridophytes

Imagine the first plants that dared to stand tall on land. Before them, the world was ruled by algae and bryophytes — mosses and liverworts that hugged the ground, never growing more than a few centimetres tall because they had no plumbing system to move water upward. Then came the pteridophytes: ferns, horsetails, and club mosses. These were the first plants to evolve true vascular tissue — xylem and phloem — which allowed them to grow upright and reach for sunlight. They are the bridge between the primitive, water-dependent bryophytes and the seed-bearing gymnosperms.

The Intuition: A Plant That Walks on Two Legs

Think of pteridophytes as plants that live a double life. They have a dominant, visible generation — the sporophyte — that looks like a typical fern or horsetail, with roots, a stem, and leaves. This is the plant you see in a forest or a garden. But hidden beneath the soil, or sometimes on the underside of a leaf, lives a tiny, independent generation — the gametophyte — that is no bigger than a fingernail. This small plant must have water to complete its life cycle, because its sperm swim through a film of water to reach the egg. That is why pteridophytes are still tied to moist environments, even though they have conquered land in every other way.

The Precise Statement

Pteridophytes are the first true vascular land plants. They possess a well-developed sporophyte (the dominant, independent phase) that is differentiated into true roots, stems, and leaves. They reproduce via spores, which are produced in sporangia that are often clustered on specialised leaves called sporophylls. The gametophyte is small, free-living, and requires water for fertilisation. They do not produce seeds or flowers.

Key Characteristics in Detail

1. Vascular Tissue

Pteridophytes have true xylem and phloem. Xylem conducts water and minerals from roots upward; phloem distributes food. This plumbing allowed them to grow tall — some tree ferns reach 20 metres. The vascular tissue is arranged in bundles called steles, which vary in complexity across groups.

2. True Roots, Stems, and Leaves

Unlike bryophytes, pteridophytes have genuine roots that anchor the plant and absorb water. The stem is often a rhizome (an underground horizontal stem) in ferns, or an erect aerial stem in horsetails. Leaves are of two types: microphylls (small, single-veined leaves, as in club mosses) and megaphylls (large, multi-veined leaves, as in ferns).

3. Sporophylls and Sporangia

Spores are produced in sporangia. These sporangia are often borne on the underside of ordinary leaves (as in ferns) or on specialised leaves called sporophylls. In some groups, sporophylls are clustered into a cone-like structure called a strobilus (e.g., in Selaginella and Equisetum).

4. Homosporous vs Heterosporous

Most pteridophytes are homosporous — they produce only one type of spore, which grows into a bisexual gametophyte. A few, like Selaginella and Salvinia, are heterosporous — they produce two types of spores: microspores (male) and megaspores (female). Heterospory is a major evolutionary step that later led to seed formation.

5. Dependence on Water for Fertilisation

The sperm of pteridophytes are flagellated and must swim through a film of water to reach the egg in the archegonium. This is why pteridophytes are restricted to damp, shady habitats. Even the largest tree fern still needs water for reproduction.

6. Alternation of Generations

The life cycle alternates between two multicellular generations:

• Sporophyte (2n): dominant, independent, produces spores by meiosis in sporangia.
• Gametophyte (n): small, independent, called a prothallus in ferns, produces antheridia (male) and archegonia (female). …