Every fern lives two lives, and one is a tiny green heart
The leafy fern you know is only half the story. Its spores grow into a separate, free-living plant just millimetres across, shaped like a heart, whose sperm must swim through water to reach an egg. Only then does a new frond unfurl. Ferns have run this double life for over 400 million years.
More than 10,500 fern species survive today, reproducing by spores rather than seeds or flowers. The group's origins are estimated at about 423 million years ago, in the late Silurian, when land plants were first spreading. Yet Polypodiales, which make up 80 percent of living fern diversity, only appeared and diversified in the Cretaceous, alongside the rise of flowering plants.
The cycle begins when a spore lands and grows into a gametophyte called a prothallus, a flat, green, one-cell-thick sheet of 3 to 10 millimetres, anchored by root-like rhizoids. It bears spherical organs releasing swimming sperm and flask-shaped ones holding a single egg. After fertilisation, the embryo grows into the familiar sporophyte, which soon stands on its own while the prothallus withers. That sporophyte eventually releases spores by meiosis from clusters called sori, usually visible as dots beneath its fronds.
New leaves typically unroll from tight coils known as fiddleheads or croziers. Some ferns grow separate fertile and sterile fronds, the fertile ones sometimes so narrow they lack green tissue. Fronds can be simple or divided as many as five times over. Tree ferns build erect trunks, reaching up to 20 metres in species on Norfolk Island and in New Zealand. Plenty of impostors borrow the name, including the asparagus fern, a flowering plant, and the air fern, which is actually a dried relative of jellyfish.
Classification has been reshuffled repeatedly. Linnaeus recognised 15 genera in 1753; the old category Pteridophyta has been abandoned as unnatural, and genetic studies now place horsetails and whisk ferns among the ferns while clubmosses sit on a far older branch. Economically ferns are minor, used as food, medicine and ornaments, though bracken and water fern are serious weeds worldwide and bracken is carcinogenic.
Source: Fern
