Even people who grow them as air-purifying houseplants may not give all that much thought to they way ferns reproduce. The process is a truly strange one. However, according to a paper in Biosciences, it's not the strange one described in biology textbooks.

Ferns were the original forest giants, emerging 300 million years ago in the Carboniferous era. They remain the second most diverse group of vascular plants, with 12,000 known species. They reproduce by producing tiny plantlets called gametophytes that contain both sperm and eggs. These can then combine to make a new plant that is genetically identical to the parent.

According to Professor Christopher Haufler of the University of Kansas, this fact has led to misunderstandings so widespread they have infected almost all biology textbooks that discuss fern reproduction.

“Everybody has been hung up on the idea that ferns reproduce themselves, that they use genetically identical eggs and sperm to produce a new individual. That capacity has superseded logic in thinking through how they might actually reproduce in nature,” Haufler said in a statement.

Glorying under the title “Sex and the Single Gametophyte”, Haufler's paper points out that research in the 1980s revealed this self-replication process is a rarely used back-up to the more common form of fern sex. Sperm from one plant and eggs from another usually combine to produce offspring that shuffle their parents' DNA, just as most plants do.

There's a good reason for this. Asexual reproduction has advantages, avoiding the difficulties and frequent dangers of finding a mate. However, most branches of the tree of life have found these to be offset by the opportunity sexual reproduction provides to bring the most suitable DNA to the fore.

If all ferns were identical to their parents they would be very vulnerable to disease, parasites, and changing conditions. Moreover, plants reproducing this way would be unlikely to have diversified so much.

In fact, ferns are about as promiscuous as it is possible for an immobile life form to be, exchanging genes not only with species from which they have long separated, but also with hornwarts, relatives of moss.

By comparing the genetics of individual ferns with those around them, botanists have shown just how rare it is for wild ferns to be clones of a single parent.

Instead, Haufler says, most fern reproduction takes place in waterborne orgies where spores carrying sperm and eggs from many plants are swept together after heavy rains to breed.

The gametophytes that result are so small – just a few millimeters (fractions of an inch) wide – that they are seldom spotted. On the other hand, gametophytes produced by isolated plants in laboratories or domesticated environments are much more likely to be spotted and treated as typical.

Few textbooks reflect this knowledge, however, and Haufler has started a campaign to bring them up to date, although he acknowledges that ferns' capacity for solo sex can be useful when colonizing new locations where they lack partners to breed with.

^{The sporophyte stage of Polypodium appalachianum, which produces the spores that germinate to produce the gametophyte at the top. Haufler named this species. Professor Christopher Haufler}