If you know which fern family your unnamed fern belongs to you can "key it out" at the Flora of North America site. Click on your family, then use the key. Once you have the genus, click on it, and key out your unknown fern to species level. These are technical keys using technical terms, but they are good.
Wild ferns are such pretty and interesting plants that local botanists love to study them and publish books about them. Therefore, there's a good possibility that a field guide to your local ferns exists. You should ask around to see what's available because learning their names and trying to find all the ferns in your area is fun. Moreover, learning your local ferns is fairly easy. Compared to wildflowers, trees, birds, or even mosses, relatively few species of fern exist.
One of my local ferns, which I got a real kick from learning about, is shown at the left. It grows epiphytically on a tree (an epiphyte is a plant growing upon other plants but not necessarily as a parasite). It's the Resurrection Fern, Polypodium polypodioides. Here's why it's called "Resurrection Fern": If it goes a few days without rain, it shrivels up and looks dead, but as soon as it rains it "resurrects," or comes back to life.
Fern "leaves" such as the one at the right are referred to as fronds. That frond belongs to the fern known as the Ebony Spleenwort, Asplenium platyneuron, common throughout most of the US. As is the case with the leaves of flowering plants, the fronds of some fern species are composed of a single blade, while other species have their blades divided into smaller sections, and sometimes even those sections can be divided, and divided again. The frond at the right is once-divided into over 60 subdivisions. Instead of calling those subdivisions "leaflets," as we do when talking about leaves, when dealing with ferns we call them pinnae (pronounced PIN-EE; the singular is pinna, pronounced PIN-uh). In many fern species the pinna is divided into subdivisions known as pinnules. The "midrib" of a frond, conspicuously dark in the spleenwort picture at the right, is referred to as the rachis (pronounced RAY-kiss). The short "stem" at the frond's base is known as the stipe.
Not possessing flowers, ferns reproduce with spores, which are so small you can hardly see them with the naked eye. They are like grains of dust. In many fern species spores are produced in tiny, curious, spotlike items referred to technically as sori (pronounced SOR-eye, singular sorus, pronounced SOR-us). Most non-specialists refer to sori as fruitdots. At the left you see sori on the undersides of pinnae of the Western Sword-Fern, Polystichum munitum. Notice that each sorus is composed of dozens of tiny, spherical items. The spherical things are not spores, but rather stalked, baglike sporangia (singular sporangium) which themselves contain several spores. When the sporangia are mature they'll burst, release their spores into the wind, and the spores will be carried to a new location where, if environmental conditions are just right, they'll germinate to form fern prothalli, from which eventually new ferns will emerge. There's more on prothalli and fern reproduction below.
Some ferns, such as the Oblique Grape-Fern, Botrychium dissecturm var. obliquum at the right (also found in the woods near my home) produce spores on separate, very different structures. In the picture, that tall, slender thing is a cluster of tiny grape-like items in which the spores are produced, called sporangia. Each sporangium contains about 50 to 2,000 spores, depending on the fern species.
In supermarkets you often see a special horticultural variety of Swordfern being sold in hanging pots. Though that fern may have hundreds of pretty fronds cascading over the pot's side, you may not be able to find a single fruitdot. The sellers of Swordferns discovered long ago that the general public considers fruitdots to be ugly, maybe even contagious, so a sterile variety was developed without sori. I've always thought that that was a little sad...
In the spring, fern fronds emerge from the ground in a special way. As the picture below shows, instead of opening like a book or unfolding the way a regular leaf does, fern fronds behave rather like those things kids blow through at parties, to make the paper cylinder shoot across people's faces. In the process of unrolling, fern fronds look like the curled head of a fiddle, and are therefore referred to as fiddleheads.
The picture at the left shows the fiddlehead of a Christmas Fern, Polystichum acrostichoides, growing near my home. The picture is about twice life size. Christmas Fern fiddleheads may uncurl so early in spring that they experience freezing temperatures, and this may account for the shaggy coat of scales and hairs covering the stem, for the scales and hairs provide at least a little insulation.
The ferns' sex life is pretty interesting. Let's begin with the spore. The spore germinates in an environment appropriate for its development. Cells divide like crazy, but the plant body that develops from the spore is not the fern we may be expecting! Rather, the object that results when a fern spore germinates is a tiny prothallus, illustrated below.
The ruler marks at the bottom of the picture on the right are 1/16th of an inch apart -- about 3 mm -- so the objects you see there (which I just collected from a deeply shaded cliff near my home) are tiny. Before explaining what you see there, let's consider the diagram at the left.
The heart-shaped item in the diagram is the fern prothallus -- the thing that forms when a fern spore germinates and the cells divide for a while. A mature fern prothallus has the following:
When fern prothalli are mature and water is present, sperm swim from the antheridia through the water to the archegonia, and when a sperm fertilizes an egg in an archegonium, then a zygote is formed, and this zygote develops into an embryo, which eventually grows into a fern of the type we are familiar with. Ideally, the sperm from one prothallus fertilizes the egg of another prothallus, thus mingling genetic material form two different plants.
Therefore, what we see in the above-right picture are two old prothalli where fertilization already has taken place and the embryo has grown into the first hints of a fern frond. As the frond develops the old prothallus shrivels up and disappears. In the picture, the brown, crumbly looking thing is all that's left of the prothallus.
The next step in the fern life cycle is for the fern fronds to mature to the point that they produce spores, and when those spores are released and one germinates, the entire cycle begins again.
An interesting point to notice is that water is required so that a prothallus's sperm can swim to a receptive egg. If there's no water, there's no fertilization and no fern... This is a real problem for ferns, in the same way that amphibians (frogs, salamanders, etc.) have the problem that to reproduce they must return to water. In both cases, that of the fern and that of the amphibians, this necessity for having water during sexual reproduction is a reflection of the organism-types primitive nature. Both ferns and amphibians evolved early in the history of land life on Earth, and both kinds of organism never did overcome their need to have water handy before they could reproduce. Later-day reptiles (and humans) and later-day flowering plants can indeed enjoy sexual reproduction without having water handy.
Don't expect all ferns to fit your stereotype of what a fern should be. At the right you see a fern that climbs like a vine, the Japanese Climbing Fern. Some ferns float on water!
If you have ferns in your backyard and your backyard is fairly typical, your ferns are more likely to be planted or potted than growing wild. Probably the most commonly purchased real potted fern is Nephrolepis exaltata, the Swordfern, a native of the tropics. In large supermarkets, Swordferns are often sold in hanging baskets displayed next to the fruit section.
The popular potted plant known as Asparagus Fern is not a fern at all. It's an asparagus, of the genus Asparagus, a flowering plant in the Lily Family, and thus not even a spore producer, which all ferns are.
You may enjoy looking over Jim's Field Notes on Ferns.
You can review ferny books at Amazon.com in both the US and the UK by clicking here.