Life cycle of a typical fern

In this illustration, A represents the mature fertile frond of an Adiantum species, Maidenhair fern. B shows the fertile pinnule and the sori along the margin (edge of leaf).  There is a flap of tissue known as an indusium covering the sporangia (where spores are located). C, sporangia and spores, D, first obvious step in the production of a new fern,  this is the gametophyte stage or prothallus. E, prothallus underside bearing rhizoids that anchor the prothallus to soil. F, Antheridia (male) releasing spermatozoides and G, the archegonia (female) containing the egg to be fertilized. H, a new Adiantum (sporophyte) attached to the prothallus with a juvenile frond and developing root system.

The plant that we then see is known as the sporophyte.  Not all ferns will reproduce in the above manner. Some genera such as the Angiopteris and Lycopodium depend on another important step to complete the germination process.  This usually requires the introduction of a specific fungus, known as mychorrizae to invade the gametophyte to help the sporophyte develop.

Pteridophyta

Ferns are considered vascular plants and are some of the oldest living plants on earth.  Currently there are between 10,000 and 12,000 fern species in the world.  The ferns, generally known as pteridophytes, also includes plants that are called the fern allies.  These plants consists of the families of Psilotum, Iosetes, Lycopodium, Equisetum, and Selaginella. The fern allies also include the rarely cultivated Tmesipteris of Australia and New Caledonia.

One of the most interesting of the fern allies is the Lycopodium group.  In the Carboniferous the lycopods dominated the landscape; they were giants among plants.  

Ferns do not produce flowers, nor fruit nor seeds.  The inappropriate name of Artillery fern and Asparagus ferns are misnomers.  Upon careful inspection of these plants they are not ferns because they produce flowers and seeds.

When observing a fern, the leafy section of the fern is typically known as a frond.  The smaller segments that make up the entire frond are called pinnae.  And, an individual section is known as the pinnule.

When observing ferns, people know them to have spores.  However, the spores are usually not apparent because they are microscopic.  What you generally see is the sori.  The sori is the outside protection which encases the sporangia which houses the spores. Sporangia are usually, but not always confined to the underside of the frond. The sori may look like 
fruit dots, or squiggly lines, or even straight lines. While these are not technical descriptions for them, they are never-the-less a necessary component in what a fern is.  Many ferns are identified by the type of sori they have.

Sometimes the sori are located along the margins of the pinnules, or they may follow the veins on the pinnae, or simply may cover the entire underside of the frond.

Dimorphic fronds

Many ferns are very clever in the way they display their fertile fronds.  While the majority of them display their sori underneath the frond, many have interesting adaptations. One genus, Belvisia, has sori along whip that resembles a rat's tail. Another fern Heterogonium pinnatum has a highly contracted fertile frond.  And then on a couple of the Platycerium, there are special spoon shaped lobes bearing the sporangia.

There are many ferns with dimorphic fronds, for instance the Platycerium group is strongly dimorphic. Some of the Tectarias too are dimorphic, as are members of the Aglaomorpha group.

Mother Ferns

Just a brief note about Mother ferns.  This does not include all of the ferns which are known to produce bulbils on their fronds.  There is a much greater list.  

Everyone seems to know at least one mother fern by sight. However, this name applies to a much broader spectrum of ferns that produce bulbils, bulblets, babies, pups, etc.