Fern is a common name for the cryptogamous (spore-producing) plants belonging to the division Filicophyta (also called Filicinophyta). These are primitive vascular plants with true roots, stems, and complex leaves, comprising about 150 genera and as many as 15,000 species.

View a high magnification image of fern sori.

Most ferns reproduce through the alternation of generations, alternating successive generations of sexual and asexual forms. The sexual form, called the gametophyte or prothallia, is a tiny kidney-shaped plant and difficult to find in the wild. The asexual form, or sporophyte, is represented by the fern plant as it is commonly known.

Sporophyte ferns have two methods of asexual reproduction. One is by vegetative cloning, branching off of the root-like underground stem, or rhizome, often forming large, genetically uniform colonies or clones. The second form of asexual reproduction occurs by distribution of spores. These form on the undersides of the leaves in clusters of spore cases called sporangia, or sori (singular, sorus). Sori appear as brown spots and may or may not be present on all leaves. Some species have sori on all the leaves, while others have specialized leaves that bear the sori. When the sporangia dry out, they break open, releasing the spores into the wind. Germination begins when a spore falls in a place with proper conditions of heat and moisture.