We tend to think of ferns as fragile plants, existing in the shaded, humid understories of forests. This could not be farther from the truth. Their lineage arose on this planet some 360 million years ago and has survived countless extinctions. In truth, they exhibit a staggering array of lifestyles, each with its own degree of adaptability. Take the Hawaiian tree fern, Sadleria cyatheoides for example.
Known in Hawai'i as the 'Ama'u, this tree fern is one of the first species to colonize the barren lava flows that make the Big Island so famous. This is an incredibly harsh landscape and many challenges must be overcome in order to persist. This does not seem to be an issue for the 'Ama'u. It is just as much at home in these water-starved habitats as it is in wetter forests. It is easily the most successful species in this genus, having colonized every island in the archipelago.
Much of its success has to due with a part of its life cycle that is much less obvious to us - the gametophyte stage. The tree fern we see is only half of the story. It is the spore-producing phase conveniently referred to as the sporophyte. When a spore finds a suitable site for germination, it grows into the other half of the life cycle, the gametophyte. This minute structure looks like a tiny green heart and it houses the reproductive organs of the plant. When water is present, male gametophytes release their flagellated sperm, which swim around until they find a female gametophyte to fertilize. Once fertilized, the resulting embryos will then grow into a new tree fern and start the cycle anew.
What sets the 'Ama'u apart from its rarer cousins is the fact that its gametophyte appears to be quite capable of both outcrossing and self-fertilization. Outcrossing, of course, promotes genetic diversity, however, the ability to self-fertilize means that a new plant can grow from only a single spore. This is super advantageous when it comes to colonizing new habitats. Its cousins seem to lack this ability to self-fertilize successfully, restricting them to more localized areas. Taken together, I think it's safe to say that the 'Ama'u is one tough cookie.