Where Do Ferns Come From? A Generation Nobody Notices

A fern alternates between two entirely distinct multicellular forms, each an independent living plant. The familiar leafy fern produces spores rather than seeds, released in enormous numbers from structures on the underside of the fronds, and each spore carries one set of chromosomes rather than two. A spore landing somewhere suitable grows into a small flat green structure a few millimetres across, which is the second form, and which lives independently on the soil surface producing eggs and swimming sperm. Fertilisation there produces a cell with two sets of chromosomes, which grows into a new leafy fern. Both forms are the same species and neither is a stage of the other in the way a caterpillar is a stage of a butterfly.

The division of labour between the two is consistent:

• •The large familiar plant produces spores and is the long-lived form
• •The small flat plant produces eggs and sperm and lives for weeks
• •Sperm swim, which means fertilisation requires a film of water
• •That water requirement is why ferns are associated with damp places
• •The small form is easily overlooked and is frequently mistaken for a liverwort
• •Both forms photosynthesise and neither depends on the other for food

Every land plant alternates between these two kinds of generation, and what differs is which one dominates. In mosses and liverworts the spore-producing generation is small and permanently attached to the larger gamete-producing one, which is the reverse of the fern arrangement. In ferns the two are independent and the spore-producing one is dominant. In seed plants including conifers and flowering plants, the gamete-producing generation is reduced almost to nothing and is retained inside the parent, with pollen and the structures inside an ovule being what remains of it, which removes the need for water entirely and is a substantial part of why seed plants dominate dry environments. Seen across the whole group, the trend is a progressive reduction of the generation ferns still display openly.

The production and dispersal of spores is worth describing because the numbers and the mechanism are both remarkable. Spores form in capsules clustered on the underside of fronds, in patterns that are frequently the main character used to identify species, sometimes protected by a flap. A single plant releases many millions in a season. The capsules in many ferns open by a spring mechanism, with a row of thick-walled cells that dry out and contract, bending the capsule back until it snaps forward and flicks the spores away at considerable speed, which has been filmed at high frame rates and is among the fastest movements plants make. Spores are light enough to travel enormous distances on wind, which is why ferns colonise remote islands early and why several species have distributions spanning continents.

How ferns reproduced was genuinely unknown for a very long time and the ignorance left traces. Spores are microscopic, the small generation is inconspicuous, and the connection between them and the visible plant is not obvious, so European tradition held that ferns produced an invisible seed, and folklore attached properties to it, including that anyone possessing fern seed would become invisible, which appears in Shakespeare. The sequence was worked out in the nineteenth century through microscopy, with the alternating generations described in the 1840s and 1850s alongside similar work on mosses and algae. Swimming sperm were observed, which established the water requirement and explained the distribution of the plants directly.

A fern alternates between the familiar leafy spore-producing plant and a separate independent green structure a few millimetres across that produces eggs and swimming sperm. Fertilisation needs a film of water, which is why ferns favour damp places. Seed plants reduced that second generation almost to nothing and retained it inside the parent, which removed the water requirement.