True Ferns
Subclass Polypodiidae
The true ferns or leptosporangiate ferns consist of over 12,000 species of spore-bearing, vascular plants that have complex vasculatures (siphonostele), and frequently compound leaves, called fronds. Almost all ferns have circinnate vernation, which means that they create new leaves as fiddleheads that uncurl during maturation. Sporangia are produced on the lower (abaxial) surface of leaves in clusters called sori. They are the most successful (living) spore-bearing plant group, showing a diverse range of habits and forms: herbaceous forms, floating forms, tree-like habit, even vines.
Above: (left) cross-section through a fern rhizome showing the siphonostele; (left-middle) sori on the bottom side of a fern frond; (center) new fern fronds called fiddleheads emerging in spring; (right-middle) fertile fronds of a regal fern, Osmunda regalis; (right) compound leaf of Pteridium
Diversity
Represented by 7 extant orders, 44 families, ~250 genera, and ~12,000 species
Classification
└Tracheophytes
└Euphyllophytes
└Ferns (Polypodiopsida)
└Polypodiidae
Geologic Range
Early Carboniferous - present (Galtier and Scott 1985; Galtier and Phillips 1996);
By the end of the Carboniferous, six families were present.
In subsequent major filicalean radiations in the Permian, Triassic, and Jurassic, several families (e.g., Osmundaceae, Schizaeaceae, Matoniaceae, and Dipteridaceae) with extant representatives replaced these Carboniferous families (Rothwell 1987).
Sporophyte (spore-bearing phase)
Vegetative features
Stems
Usually underground rhizomes, although upright stems are found in tree ferns
Water ferns have highly modified stems
Leaves
Most true ferns have large compound leaves, called fronds
Leaves can be once to several times pinnate
Some palmately compound and simple leaf (e.g. Asplenium) species exist
New leaves form as fiddle heads or croziers
This is called circinate vernation ("vernation" is the pattern of leaf growing or unfolding in plants)
Reproductive features
Sporangia grow on underside (=abaxial) surface of leaf
Sporangia are clustered into a sorus (plural = sori)
Each sporangium has a thickened "mohawk" structure to disperse spores, called an annulus
True ferns are leptosporangiate: tiny sporangia, with 12-128 spores, and an annulus, which develops from a single initial cell
Gametophyte (gamete-bearing phase)
Gametophytes are small, but multicellular and free-living
Most ferns have photosynthetic gametophytes
Monoecious and dioecious forms exist
Above: Large fiddleheads of a tree fern in the NYBG Conservatory
Above: Gametophyte of a fern
Below: Gametophyte with an emerging sporophyte
Additional Reading
Cracking the Case of the Giant Fern Genome (12Sep2022, NY Times)
Fossilized nuclei reveal Jurassic genome in ferns (Paleoplant Blog 12Dec2014)
YouTube tutorial about fern lifecycle (YouTube 27Oct2010)