Fern Life Cycle – Complex Process of Alternation of Generations

Ferns are ancient plants that never make flowers or seeds. Instead, they spread using tiny spores, and their life cycle switches back and forth between two completely different-looking plants: the leafy fern we know, and a tiny heart-shaped plant most people never notice. This back-and-forth pattern is called alternation of generations.

Ferns: Older Than the Dinosaurs

Plants first moved onto land roughly 470 million years ago. Later, the first vascular plants appeared. These plants have special tubes inside them that carry water and food, which allowed them to grow much taller than mosses. Ferns came along not long afterward, and fern-like plants are common in fossils from around 360 million years ago.

That means ferns were around long before the dinosaurs. Today there are about 10,000 kinds of fern, ranging from tiny floating water ferns and the delicate kidney fern to tree ferns that can grow up to about 20 meters (65 feet) tall.

Parts of a Fern

A fern’s leaves are called fronds. Each frond has a stalk and a leafy part called the blade, or lamina. In many ferns the blade is divided into smaller leaflets called pinnae, giving it a feathery look.

New fronds grow from an underground or creeping stem called a rhizome. They start out tightly curled into a spiral called a fiddlehead (or koru in New Zealand) and slowly unroll as they grow. Fronds make food from sunlight, and many of them also make spores.

If you turn over a mature fern frond, you may see rows of brown or orange dots. These dots are called sori. Each one is a cluster of tiny capsules called sporangia, and each sporangium is packed with spores. Fronds that carry sporangia are called fertile fronds. When the spores are ripe, the sporangia burst open and fling the spores into the air.

Two Generations, Two Different Plants

A fern’s life cycle needs two generations to be complete:

• The sporophyte is the big, leafy fern plant you see in woods and gardens. Its cells are diploid, which means they have two full sets of chromosomes (one from each parent). The sporophyte makes spores.
• The gametophyte is a tiny, flat, green, heart-shaped plant, usually less than 1 cm across. Its cells are haploid, which means they have only one set of chromosomes. The gametophyte makes sperm and eggs.

Spores are not seeds. A seed holds a baby plant and a food supply, but a fern spore is just a single cell. When it lands somewhere damp, it grows into a gametophyte, not straight into a new leafy fern.

Stages of the Fern Life Cycle

Before scientists understood genetics, fern reproduction was a real puzzle. Nobody could find fern seeds, and the tiny gametophytes looked nothing like the adult ferns. Here is how the cycle actually works, step by step.

Stage 1: Making Spores

Inside the sporangia, cells of the diploid sporophyte divide by a special kind of cell division called meiosis. Meiosis halves the number of chromosomes, so it produces haploid spores. Animals use meiosis to make their eggs and sperm, but ferns use it to make spores.

Stage 2: The Gametophyte Grows

A spore that lands on damp, shady soil starts to divide by ordinary cell division, called mitosis. It grows into a green prothallus, which is another name for the gametophyte. The prothallus has no true roots, stems or leaves. Instead, tiny hair-like rhizoids anchor it and soak up water.

On its underside, the prothallus develops two kinds of structures. Antheridia make sperm, and archegonia each hold a single egg. Often the same prothallus makes both.

Stage 3: Fertilization

Fern sperm have tiny whip-like tails and must swim to the egg, so fertilization can only happen when there is a film of water, such as dew or rain. The sperm swim into an archegonium and one of them joins with the egg. This creates a diploid cell called a zygote, which is the very first cell of a new sporophyte.

Because sperm and eggs can be made on the same gametophyte, a fern can self-fertilize. This is handy when there are no other ferns nearby. Cross-fertilization between different gametophytes mixes genes from two parents, which can give the young fern useful new traits.

Stage 4: A New Fern Grows

The zygote grows into an embryo, still attached to the prothallus, which feeds it at first. Soon a tiny first leaf and root appear. Once the young fern can make its own food, the prothallus withers away.

Baby ferns are delicate. Their tiny stems and leaves dry out easily, so long periods of strong, direct sunlight can kill them. As the fronds grow bigger and the plant develops a proper root system and veins, it can carry water from the soil to its leaves and cope much better. When the fern is mature, it starts making sori and spores, and the cycle begins again.

Other Ways Ferns Reproduce

The life cycle above is sexual reproduction, because it involves sperm and eggs. Ferns can also make new plants without them, which is called asexual reproduction.

• Rhizomes: many ferns spread by creeping rhizomes that send up new fronds along the way, forming large patches.
• Plantlets: some ferns grow baby plants on their fronds. In the walking fern, the frond tip droops to the ground and roots, starting a new plant. These plantlets have the same genetic makeup as the parent.
• Apogamy: in some ferns, a new sporophyte grows straight out of the gametophyte without any fertilization. This is common in ferns from dry places, where water for swimming sperm is hard to find.

Where Ferns Live

Ferns grow almost everywhere, from the tropics to cold mountain slopes, but most kinds live in warm, wet tropical forests. Because their sperm need water to swim, damp places suit them best. Gametophytes are usually found on moist, shaded soil, rotting logs, stream banks and in sheltered cracks in rocks.

Some ferns grow on rocks and cliffs, tucked into cracks in walls and boulders. Many prefer acidic rocks like granite and sandstone, while others grow only on limestone. In the tropics, a large share of ferns are epiphytes, plants that grow on tree trunks and branches without harming the tree.

Ferns that live in dry or sunny spots often have features similar to desert plants, such as thick, leathery fronds, a waxy coating, and lots of hairs or scales to hold in moisture. Some can even shrivel up in a drought and turn green again after rain.

At the other extreme are water ferns, such as water clovers and the tiny floating mosquito fern. Some of these grow in seasonal pools that fill with rain and race through their life cycle before the water dries up.

Why the Fern Life Cycle Matters

The fern life cycle shows how plants lived before seeds and flowers evolved. Ferns still rely on water for their sperm to swim, which is why they love damp places. Next time you see brown dots under a fern frond, you will know you are looking at the start of a whole new generation.

Related: Moss Life Cycle – Complicated and Impressive Aspects You Should Know