Osmundaceae

The Osmundaceae (royal fern family) is a family of four to six extant genera and 18–25 known species. It is the only fern family of the order Osmundales an order in the class Polypodiopsida (ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient (known from the Upper Permian) and fairly isolated group that is often known as the 'flowering ferns' because of the striking aspect of the ripe sporangia in Claytosmunda, Osmunda, Osmundastrum, and Plensium (subtribe Osmundinae). In these genera the sporangia are borne naked on non-laminar pinnules, while Todea and Leptopteris (subtribe Todinae) bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns. The stems of Osmundaceae contain vascular tissue arranged as an ectophloic siphonostele; that is, a ring of phloem occurs on the outside only of a ring of xylem, which surrounds pith (and no other vascular tissue).Stipules can be discerned at the leaf bases of these ferns. The hardened leaf bases are persistent and overlap to form a hardened layer surrounding the stem. The mantle of sclerenchymatous leaf bases and intermixed roots can form a woody trunk when the stem emerges above ground, up to 1 meter (3.3 ft) in Todea barbara. Extinct members of the family, which flourished during the Mesozoic, could reach the stature of trees and be termed tree ferns. The leaves are either holodimorphic, with separate fertile and sterile fronds assuming an entirely different structure, or have fertile and sterile portions of the frond very distinct in structure. Sporangia in the Osmundaceae are large, and open at a slit on the top; the annulus that drives the sporangium opening is on the side. 128 to 512 spores are typically present. The spores are green, nearly round, and trilete. The spores germinate into gametophytes, which are green (photosynthetic) and grow at the surface. They are large and heart-shaped. The base chromosome number for members of the order is 22. Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, creating four classes of ferns (Polypodiopsida). At that time they used the term Polypodiopsida sensu stricto to apply to the largest of these. Later the term Polypodiopsida sensu lato was used to refer to all four subclasses, and the large subclass renamed Polypodiidae. This is also referred to informally as the leptosporangiate ferns. The Polypodiidae contain seven orders whose phylogenic relationship is shown in the following cladogram, where Osmundales is seen as a sister to all other members of the subclass. In the Pteridophyte Phylogeny Group classification (2016) Osmundales consists of the single family Osmundaceae, six genera, and an estimated 18 species (Christenhusz and Byng give 25 species). The three genera Osmunda, Leptopteris, and Todea were recognized as members of Osmundaceae by Smith et al. (2006) Of these, the largest genus, Osmunda, had traditionally been treated as three subgenera, Osmunda (3 species), Osmundastrum (2 species), and Plenasium (3–4 species). However, there was suspicion that the genus was not monophyletic. The first molecular phylogeny showed that Osmunda as traditionally circumscribed was paraphyletic and that Osmunda cinnamomea, despite its morphological similarity to Osmunda claytoniana, was sister to the rest of the family. This was later confirmed by a detailed species-level phylogeny of the family by Metzgar et al. (2008) leading to the resurrection of the segregate genus Osmundastrum, by elevating it from subgenus, to contain it and render Osmunda monophyletic. Todea and Leptopteris are consistently resolved as sister groups, and Osmunda was found to contain three separate subclades corresponding to subgenera (now genera) Osmunda, Plenasium, and the recently described Claytosmunda with the single species, Osmunda claytoniana.