notholaenids

Introduction

Underside of fertile leaves of Notholaena candida and Notholaena meridionalis. Note the dense coating of farina, and the sporangia confined to vein-tips (and protected by an at-most slightly modified leaf margin). © Carl Rothfels.

The notholaenids are an entirely New World group of ferns, adapted to xeric habitats. The clade includes approximately 38 species and is most diverse in Mexico and the southwest United States, although some species penetrate into the New World tropics, and one, Notholaena galapagensis, occurs on the Galapagos Islands. These are the small ferns that, along with members of Pellaea, Argyrochosma and Cheilanthes, are common in cliff-face crevices and nestled under rocks in the deserts of the US and Mexico. The genus-level treatment for this group is unresolved, pending further data, but it is clearly dominated by taxa traditionally treated in the large genus Notholaena. Most notholaenids are farinose, often bright white or occasionally yellowish beneath, with at-most weakly modified fertile leaf margins (Notholaena sensu Windham, 1993a); only a few taxa lack farina.

Notholaena—the genus that comprises most of the notholaenid clade—has been the focus of much of the historic taxonomic confusion within the cheilanthoids. Treatments for the genus have included taxa now recognized in genera as disparate as Argyrochosma, Astrolepis, and Paragymnopteris (in the pellaeids), Cheilanthes (including species from both the myriopterids and the hemionitids), and Cosentinia (in the pteridoids—not even a cheilanthoid). Alternatively, other authors have chosen to not recognize Notholaena at all, instead moving all its taxa into an expanded Cheilanthes (Copeland, 1947; Mickel, 1979). For a list of names published under Notholaena for species which should be excluded from this genus and their recommended treatment based on the most recent data, see Excluded Species.

The difficulty in establishing a stable circumscription for Notholaena is due to two factors. First, morphology-based generic boundaries in cheilanthoids are notoriously difficult. In response to their desert habitats, these ferns tend to evolve one of a small number of morphological syndromes, and thus similar-looking ferns may not be closely related (Gastony and Rollo, 1998; Rothfels et al., 2008). Only through careful morphological and cytogenetic study (Benham and Windham, 1992; Windham, 1987; Windham and Yatskievych, 2003) have natural groups been recognized, groups that are now supported by DNA data (Gastony and Rollo, 1998; Kirkpatrick, 2007; Rothfels et al., 2008). Secondly, the generic name Notholaena was lectotypified three times, based on three distantly related taxa (one notholaenid, one hemionitid, and one pellaeid; Rothfels et al., 2008; Yatskievych and Smith, 2003). Two of these types (N. marantae—the pellaeid, and N. trichomanoides—the notholaenid) are still in active use, so, depending on the worker, the name Notholaena could be being applied to entirely different clades. Yatskievych and Smith (2003) presented convincing arguments against overturning the first lectotypification (N. trichomanoides), and thus I treat N. trichomanoides and its allies as Notholaena, and treat N. marantae under Paragymnopteris (see Rothfels et al. 2008).

Characteristics

Gametophyte of Notholaena californica ssp. californica. Note the farina "pom-poms." The presence of farina on the gametophytes may be a synapomorphy for the notholaenids. © Carl Rothfels

The notholaenids, centered around Notholaena as circumscribed above, are a diverse and disparate group, containing approximately 38 species, 29 of which occur in Mexico (Mickel and Smith, 2004). The 11 notholaenid species that occur in the USA (Windham, 1993b) contribute to the seemingly unlikely fact that Arizona (which, with all its deserts, doesn’t seem like the most fern-friendly place) has the 5th highest number of fern species of all US states. While notholaenids show some clear trends (presence of farina; absence of false indusia), members of the clade as a whole are united by very few obvious characters. In general, notholaenids are farinose, have (at most) a weakly differentiated leaf margin (false indusium), and a tendency towards pentagonal leaves. They all share a chromosome base number of x=30 (the morphologically similar but distantly related Argyrochosma has a base number of x=27), and the species studied thus far all have farina on their gametophytes. This latter feature—farinose gametophytes—may be a synapomorphy for the clade (Giauque, 1949; Rothfels et al., 2008), but more study is needed.

Included in the notholaenids are a number of morphologically anomalous taxa. The Central American genus Cheiloplecton is strongly supported as a member of this clade, despite being nonfarinose and having a distinctive strongly inrolled false indusium (Rothfels et al., 2008). It has always posed a taxonomic challenge (it has names under both Pellaea and Cheilanthes), but, prior to DNA data, was never considered close to Notholaena. The N. aureolina clade is comprised of four taxa, most of which have only recently been placed in the notholaenids (Rothfels et al., 2008) and given names under Notholaena (Yatskievych and Arbeláez, 2008). Three of them are farinose, but with an unusual bright yellow or orange farina color and well developed false indusia (otherwise very rare in Notholaena). The fourth member of this group—Notholaena brachypus—lacks farina, and is instead densely hairy and scaly. Finally, sister to the rest of the clade is another anomalous species, the nonfarinose Cheilanthes leucopoda. It has unusual sticky-glandular pentagonal leaves; in gross morphology it more closely resembles the myriopterids, where it has traditionally been treated.

Discussion of Phylogenetic Relationships

The molecular phylogenetic study of Rothfels et al. (2008) used plastid DNA sequences to investigate relationships in the notholaenids and revealed five clades that are convenient handles for discussion of this group: Cheilanthes leucopoda; Notholaena standleyi; the N. aureolina clade; Cheiloplecton; and core Notholaena. Each of these clades is well supported by molecular data; however, relationships within them may be less certain, and the position of a number of unsampled taxa is as yet unresolved.

• Cheilanthes leucopoda: A relationship between this species and Notholaena s. s. was first suggested by the molecular phylogeny of Gastony and Rollo (1998), and confirmed by Rothfels et al. (2008).
• Notholaena standleyi: The position of N. standleyi was perhaps the greatest surprise of the Rothfels et al. (2008) study. This species is very similar to Notholaena sulphurea in virtually all features and has even been treated as a variety of that species by some authors (as Notholaena sulphurea var. quinquefidopalmata Farw.). Yet the two taxa are resolved in very different places: N. standleyi is one of the early-diverging notholaenid branches, while N. sulphurea is nested deep within the core Notholaena. The position of N. standleyi renders Notholaena sensu Windham (1993a) paraphyletic by the inclusion of the N. aureolina clade and Cheiloplecton (see below).
• The N. aureolina clade: This clade encompasses four taxa, three of which have been included in phylogenetic studies (Rothfels et al., 2008). The unsampled species (N. jaliscana) is believed to fall here, based on its similarity to Notholaena aureolina (one of the studied species; Yatskievych and Arbeláez, 2008).
• Cheiloplecton: While earlier molecular studies placed Cheiloplecton close to Notholaena (Gastony and Rollo, 1995; Gastony and Rollo, 1998), it was not until N. standleyi was sampled (Rothfels et al., 2008) that it became clear that this genus is nested within Notholaena sensu Windham (1993a).
• Core Notholaena: This is a clade of approximately 26 species, all of which are small, farinose ferns of dry habitats. These species comprise the genus Notholaena sensu Windham (1993a) and Mickel and Smith (2004), with the exception of N. standleyi, which here falls outside of core Notholaena.
  

Additionally, there are three species whose phylogenetic position is uncertain, and whose morphology is sufficiently unusual that I treat them here as incertae cedis: N. aurantiaca, N. nigricans, and N. solitaria.

Among the taxa historically treated within Notholaena, two deserve particular mention: Notholaena nivea and its allies are here segregated as the genus Argyrochosma (Windham, 1987), and Notholaena sinuata and its allies are treated in Astrolepis (Benham and Windham, 1992). Both genera are in the pellaeid clade, and are thus only distantly related to the notholaenids.

Excluded Species

For reasons outlined in the Introduction, many taxa have names in the genus Notholaena, but are not members of the notholaenids (and thus are not closely related to Notholaena as treated here). Below is a list of such names, with a suggested synonym and phylogenetic placement for each (where available).

The above list was formulated around the data from Tropicos, with input from the International Plant Names Index, and Hassler and Swale's Checklist of World Ferns. Much of the information about probable phylogenetic placement is due to data generated by Kathleen Pryer's lab; Layne Huiet generated much of these data, and was particularly helpful in formulating this list.

Other Names for notholaenids

• Vernacular Names: Cloakferns and allies

References

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