Pteridaceae E.D.M. Kirchn. 1831

Introduction

Pteridaceae are one of the largest fern families, comprising over 1000 species (~10% of extant ferns).  But even more notable than their species richness is the remarkable morphological and ecological diversity of the family, which includes obligate epiphytes, free-floating aquatics, brackish-swamp dwellers, several clades specialized for colonizing rocky deserts, and many generalist understory species. While some of these life forms do appear in other fern lineages (many other families, for example, contain epiphytes), most are rare and no other fern family contains such a wide range of ecological specializations.

Pteridaceae have been the subject of much taxonomic disagreement—their members have been distributed among at least 20 different families: Acrostichaceae; Actiniopteridaceae; Adiantaceae; Anopteraceae; Antrophyaceae; Bommeriaceae; Ceratopteridaceae; Cheilanthaceae; Coniogrammaceae; Crytopgrammaceae; Hemionitidaceae; Llaveaceae; Negripteridaceae; Notholaenaceae; Parkeriaceae; Platyzomataceae; Pteridaceae; Sinopteridaceae; Taenitidaceae; and Vittariaceae (Smith et al. 1996; Hassler and Swale, 2003). Some of these former family designations (e.g., Llaveaceae, Parkeriaceae, and Vittariaceae) correspond to monophyletic groups now known to be nested within the broader Pteridaceae, while others (e.g., Taenitidaceae) are assemblages of more distantly related species (Schuettpelz et al., 2007).

Characteristics

Perhaps due to the group's ecological breadth, Pteridaceae lack clear, unique, morphological features that unite their constituent species. One of the best characteristics is that members of this family never have true indusia—outgrowths of the lower leaf surface that protect the sporangia—although they frequently have modified leaf edges that serve the same function. These recurved leaf margins are sometimes strongly differentiated from the rest of the leaf tissue, and are thus referred to as “false indusia.”

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Left: Abaxial leaf surfaces of Adiantum poiretii, showing sori and false indusia. © 2008 Carl Rothfels. Center: Fertile sporophyte of Coniogramme sp., showing sporangia distributed along the veins on the underside of the leaf. © 2007 . Right: Naked sporangia arranged along the veins of the leaf of Hemionitis palmata. © 2004

Instead of the round or elongate sori (clusters of sporangia) that are typical of many ferns, members of the Pteridaceae usually have their sporangia distributed along the veins on the underside of the leaf, often in marginal coenosori (a more-or-less contiguous line of sporangia around the margin of the leaf). More rarely, the sporangia may be attached to the false indusium itself (Adiantum), or spread evenly across the undersurface of the leaf (Acrostichum).  In addition to these macromorphological characteristics, members of the Pteridaceae tend to share a chromosome base number of x=30 (occasionally x=27, 28, or 29).

Discussion of Phylogenetic Relationships

Plastid DNA sequence data suggest that the Pteridaceae comprise five (mostly) well-supported clades (Schuettpelz et al., 2007). The cheilanthoids are sister to the adiantoids (which include both Adiantum and the group formerly referred to as the Vittariaceae); these two clades are together sister to the pteridoids plus the ceratopteroids; and the cryptogrammoids are, in turn, sister to this large clade. Of these five clades, only the cheilanthoids lack strong support, due to some uncertainty about the phylogenetic position of their earliest-diverging branch (the Doryopteris ludens clade—see the cheilanthoid page). These five clades, none of which correspond perfectly with previously recognized families or subfamilies, do show some ecological differentiation. The ceratopteroids contain the aquatic taxa; the pteridoids contain most of the generalist forest understory species; and the cheilanthoids contain most of the xeric habitat specialists.

This phylogenetic structure differs from the most recent formal taxonomic treatments of the family (Kramer, 1990; Tryon et al., 1990), which recognized two families (Pteridaceae and Vittariaceae), and divided the Pteridaceae into six subfamilies: Adiantoideae, Ceratopteridoideae, Cheilanthoideae, Platyzomatoideae, Pteridoideae, and Taenitidoideae. Molecular data demonstrate that the Vittariaceae are nested within Pteridaceae (specifically within the adiantoid clade), and, while the circumscriptions may be close in some cases, none of the formally described subfamilies of Tryon and Tryon (1990; Adiantoideae, Cheilanthoideae, etc.) corresponds perfectly to the informal clades recognized here (adiantoids, cheilanthoids, etc.). A formal revision of Pteridaceae awaits further data.

"Pteroids" vs. "Pteridoids"

Note that the term “pteroid” is colloquial shorthand to refer to members of the Pteridaceae, whereas “pteridoid” refers to members of a specific Pteridaceae subclade.

Other Names for Pteridaceae E.D.M. Kirchn. 1831

• Adiantaceae
• Sinopteridaceae
• Hemionitidaceae
• Vernacular Names: pteroids

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