The fossil record indicates that ferns have lived on Earth for more than 350 million years. With thousands of species today, ferns are still highly diverse especially in the tropics.

      Fern leaves, or fronds, undergo acropetal maturation with the oldest cells found near the base of the leaf. The tip of a fern leaf remains meristematic within a spiraled structure known as a fiddlehead or crozier. The uncoiling or maturation of the leaf is termed circinate vernation.

      Some fern leaves may take up to five years to fully expand. Fronds of some ferns, such as Nephrolepis, exhibit intermittent apical growth so that its apical meristematic activity extends over several seasons. As the apical meristem generates new cells, they elongate the leaf causing the fiddlehead to be located further away from the petiole as the leaf matures. The climbing fern, Lygodium, is known to have fronds up to 45 feet in length. The lateral pinnae found on some ferns start out as croziers also and are known as subcroziers.

      Most fern leaves are borne on a compact stem as a rosette or on a rhizome. The rosette types may either be ground-hugging, such as Adiantum capillus-veneris, or unbranched-tree-like, as in Blechnum gibbum, where the rosette is found at the end of a stem. Some ferns (e. g. the walking fern Asplenium) have leaves that act as stolons: where the tip of the fern leaf touches the ground, a new plant is generated by activity of the leaf's apical cell.

      Several ferns exhibit phyllogenous branching. Hemionitus pinnatifida, Pellaea rotundifolia, and other have leaves that give rise to branches.

      Many ferns have leaves that perform both a reproductive (spore-producing) and a vegetative (photosynthetic) function. Other ferns have separate leaves specialized either to bear sporangia or to do photosynthesis. Sporangia are typically grouped together into structures called sori (sorus, singular). The location and form of sori is often taxonomically important.

      Many ferns are epiphytes that clasp onto other plants high in the canopy or into rocky ledges where they are exposed to sunlight. Approximately one third of all tropical fern species exhibit this type of behavior. The staghorn fern discussed in a previous section has clasping leaves that hold on to a tree or ledge acting as a substrate.

      Ferns adapted to aquatic environments will be discussed in the section on aquatic plants.