Types of Fern
Ferns and their allies are some of the most ancient vascular plants, dating back some 360 million years. The only other vascular plant group that is older are the lycophytes, sometimes called lycopods or clubmosses. The discussion here will include all true ferns, plus a few spikemosses and clubmosses. All of these plants have a true vascular system, though only ferns ( Pteridophytes) have true leaves, known technically as megaphylls. Reproduction is done through spores, not seeds, and these are produced in specialized structures on the leaf surface.
In ferns clusters of spores are called sori and are often covered by a protective tissue while young known as the indusium. After maturing the spores are wind dispersed and germinate. The germinated spore grows into a haploid (containing half the genetic information) generation known as the gametophyte. As this ages it forms a photosynthetic structure known as the prothallus that in turn produces both male and female gametes - sperm and eggs. The eggs remain fixed to the prothallus, but the sperm must swim to an egg to fertilize it. This is one reason why ferns grow in wet areas, or areas that at least have a wet season. If they grew only in a dry place they couldn't reproduce because the sperm wouldn't be able to swim to the egg and fertilize it. Interestingly, this is also how ferns occassionally hybridize. Sperm from one species swims to the egg of a different species and fertilizes it. This need for nearly wet conditions during the gametophyte generation is important when trying to grow ferns from spore. Once the egg is fertilized, a diploid (containing the full complement of DNA) zygote is made, and thus begins the sporophyte generation. This generation is the one that grows large and what we commonly recognize as "a fern". It in turn produces more spores and the cycle goes on. This cycle is called alternation of generations.
Ferns, spikemosses, and clubmosses are found throughout the world, a thing not unexpected from such an ancient group of plants. They can be found on every continent except Antarctica, almost any island (no matter how isolated), and range from the deep tropics to almost the arctic circle. Likewise they inhabit an astonishing range of habitats, from grasslands, to temperate forests, to windswept tundra, tropical and temperate rain forests, swamplands of all latitudes, semi-deserts, and some are even found in lakes, both floating on top of the water and some submerged. We tend to think of ferns as growing in the ground, that is, as a terrestrial plant, but in truth many ferns and fern relatives can be found growing on rocks (lithophytes) or on other plants (epiphytes), typically on trees. Some species can be found throughout much of the world, such as Lycopodium clavatum (found everywhere but Australia) or Asplenium trichomanes which can almost literally be found anywhere - from the subactic regions of Asia to the tropical mountains of Africa, stonewalls in Europe, the Hawaiian Islands, Cuba, and almost anywhere else you can think of. Some species are obligate terrestrials, while others only found as epiphytes or some only as lithophytes. Others still can be lithophytes or epiphtyes, yet others can be all three at the same location. I've seen extensive colonies of the Japanese native fern Microsorium buergerianum growing up trees, on thin twigs of bushes, over rock faces, and in deep rich loamy soils - and all of this in just a few square meters! It is estimated that no less than one third of known fern species are epiphytic - a fact that might startle the layman's idea of what a fern is.
As diverse as ferns are in habitat, they are equally so in habit. A dizzying array of forms exist, even within the main group (known as leptosporangiate ferns). Some grow in clumps, while others grow as vines up trees. Yet others spread by underground stolons or some grow as actual trees complete with a trunk. The basic leaf form tends to be pretty simple, the typical fern leaf known as a frond, but in truth variation is extreme. Each frond is composed of a blade (the leafy looking part) and the stipe (the stem part, called a petiole in many other plant leaves). The stipe is commonly covered in scales, but can also be naked or sometimes hairy and occasionally even glossy or spiked. Depending on the species it can be very long or very short. The blade itself is composed of the rachis (the central stem of the blade) and the pinnae (little leaflets coming off the rachis). The pinnae can in turn be divided up to four times, so you hear the terms once pinnate, twice pinnate, three times pinnate, and four times pinnate to describe different species. If the pinnae are undivided the fern is known as once pinnate. If the pinnae are once divided then the resulting frond is twice pinnate, and so on. The divisions of the pinnae are called pinnules and the central vein of them is called the costa. Some ferns don't have any pinnae at all, and so look like a normal plant leaf, more or less, and are known as simple fronds.
If that weren't enough, some ferns have different frond forms from the typical type. One of the more obvious groups are the Gleicheniaceae, the forked fern family. As the name suggests, the frond grows in a fork shape with two long divisions growing at an angle away from each other. These ferns grow new fronds from both the base of the plant as in more typical ferns, or they can grow a new frond from between the dividing point of the frond's two divisions - a condition known as pseudodichotomous branching. The resulting habit is almost vine-like with plants trailing along the ground.
All ferns have a stem that either trails along the ground, is underground, is vine-like, or forms an actual trunk. This stem is called a rhizome. Off this grows the roots, either into the soil or onto some other substrate. Off this also grows the fronds from a growing point, commonly called the crown. Again, the variation between species in the rhizome and roots is extreme and there is no easy way to generalize about them. Some ferns grow no definite crown at all. In some species, such as the floating water fern, Azolla, the rhizome is underdeveloped. In other species, such as the massive tree fern, Cyathea medullaris, it can grow 20 meters long!
Ferns have long been important for people as a food source, a building material, of course in the horticultural trade, and even in making traditional religious objects. Tree fern fiber is a common additive to many epipyhtic plant composts and that in itself is a rather large local industry where these ferns live. In addition, some lycopods have been researched for possible pharmaceutical drug development. Complex, ancient, and varied, these plants by their very nature are here to stay probably long after humans are gone from this planet.