With its large palm-like fronds sometimes reaching over ten feet in length, the Giant Leather Fern is a conspicuous part of the Florida wetland ecosystem.
Leather Fern
As a group, the ferns were some of the earliest plants to leave the ancient sea and invade the land. Fossil ferns have been found dating back to 390 million years ago, and DNA sequencing indicates that they may have first appeared over 430 million years ago. During the Carboniferous period 300 million years ago tree-sized ferns covered much of the landscape, and their buried trunks make up most of the world’s coal deposits. Ferns of various types still dominated terrestrial wetland ecosystems during the age of the dinosaurs. But by the Cretaceous period, the flowering plants (“angioperms”) appeared and quickly dominated the world, and today the ferns have fallen to a secondary role. The roughly 12,000 existing species thrive mostly in warm and wet tropical habitats.
There are three species of ferns in the genus Acrostichum. The Mangrove Fern, Acrostichum speciosum, is found in tropical Asia, ranging down into Australia. The Golden Fern, Acrostichum aureum, is found in tropical areas around the world, but it barely reaches Florida and is rare here. Much more commonly seen in the Sunshine State is the Leather Fern, Acrostichum danaeifolium. The largest fern in the state, it ranges from Florida down through Central America and into Brazil.
Like most ferns, the Leather Fern lives in wetland habitat. It prefers damp low-lying areas, and although it can tolerate periodic flooding it cannot withstand long periods of immersion. Unlike the Golden Fern, which can inhabit coastal mangrove swamps, the Leather Fern is less salt-tolerant and prefers inland areas, usually along riverbanks or pond edges where it can get a lot of sun.
Ferns are best-known for their unusual method of reproduction. Unlike the angiosperm flowering plants, ferns do not produce seeds. Many ferns are able to reproduce asexually by sending out runners that sprout tiny new plants, each a genetic clone of its parent. These sprawling roots, or “rhizomes”, are important in helping to hold soil in place and to prevent riverbanks from collapsing.
But ferns also reproduce sexually, though they have an odd two-stage process for this. Mature ferns produce spores. Unlike seeds, which are fertilized and have a full set of chromosomes, spores are unfertilized and are “haploid”, with just half the number of chromosomes, akin to plant pollen or animal sperm. When the fern is mature, it will send up a number of specialized fronds that carry spore-producing areas on their undersides. In most ferns, these are contained in packets called sori, which look like little brown-red spots on the underside of the leaves. But in the Acrostichum species, the entire underside of the leaflet produces spores, giving them a rusty appearance.
Spores on the underside of a Leather Fern leaf
These spores are released into the wind. If they land in a suitable spot, they grow into a tiny plant known as a gametophyte, which is inconspicuous and bears no resemblance to the large spreading ferns that we are familiar with. Ferns are the only plants which have this odd intermediate life stage.
The gametophyte then takes the genes from the spore that formed it and packages them into two separate sex cells, the gametes. When it rains, the tiny “male” gametes are able to travel along inside the thin film of water to reach “female” gametes from neighboring gametophytes on the forest floor and join with them to form a new fertilized cell, which is “diploid” and has the full number of chromosomes. This then grows into the familiar fern plant that we know, which is called a sporophyte. When mature, the sporophyte sends up fertile fronds to disperse spores and start the process all over again. Because the tiny spores can be carried for long distances by the wind, ferns can reach new areas much more easily than other plants, and are usually the first vegetation to reach distant islands or to recolonize areas that have been denuded by fires or floods.
Most of the mature fern’s fronds, however, are used for photosynthesis. Like all plants, ferns use chlorophyll to manufacture sugars from sunlight and air. In the Leather Fern, these fronds are strong and stiff and can extend as much as 10-12 feet, covering a wide area and allowing the plant to capture a significant amount of sunlight. These sprawling fronds provide shelter for many of Florida’s birds and animals. They also provided shelter for Native Americans who used the dried fronds as thatch.
New fern fronds emerge as tightly coiled “fiddleheads”, and these were boiled by Native Americans and used as a food source.
A newly-emerging “fiddlehead”
