Pteridium Scop., Fl. Carniol.

Schwartsburd, Pedro B., Navarrete, Hugo, Smith, Alan R. & Kessler, Michael, 2017, Prodromus of a fern flora for Bolivia. XXVI. Dennstaedtiaceae, Phytotaxa 332 (3), pp. 251-268 : 262-263

Pteridium Scop., Fl. Carniol. 169–170. 1760.

Pteridium is monophyletic and shares common ancestry with Hypolepis , Blotiella , Histiopteris , and Paesia ( Der et al. 2009, Zhou et al. 2014, Perrie et al. 2015). It is characterized by having subterranean rhizomes with a dictyopolycyclostele, coriaceous, 3–4-pinnate blades with reflexed pinna margins that generate the pseudo-indusia (also on sterile leaves), and marginal sori along the laminar margins ( Tryon 1941, Ogura 1972, Kramer 1990). Pteridium is often an aggressive weed in habitats disturbed by humans, invading cultivated areas, burnt slopes, and roadsides; but it is usually rare and non-aggressive in natural habitats ( Kessler 1999). Pteridium is also toxic to cattle, horses, and humans, when ingested ( Oliveros-Bastidas & Alonso-Amelot 2010); young croziers are commonly eaten by humans in eastern Asia, and this may be related to a higher incidence of stomach cancer in those areas.

The delimitation of species differs strikingly, depending on authors, with some recognizing only one species (e.g., Tryon 1941) and others up to a dozen ( Perring & Gardner 1976, Mickel & Beitel 1988, Thomson 2000, 2004, Thomson & Alonso-Amelot 2002). There is a more or less stable consensus that ca. 10–20 morphotypes can be recognized within the genus, as species (e.g., Mickel & Beitel 1988), subspecies ( Thomson 2008), or varieties ( Tryon 1941). Genetic papers on Pteridium ( Der et al. 2009, Thomson 2012, Zhou et al. 2014, Wolf et al. 2015) have suggested the recognition of only two widespread diploid species: Pteridium aquilinum in the northern hemisphere and Africa and P. esculentum in the southern hemisphere (except in Africa). In addition to these two diploid species, there are two lineages of allopolyploid hybrids: P. caudatum (= P. aquilinum × P. esculentum subsp. arachnoideum ) in the Neotropics and P. semihastatum (Wall. ex J.Agardh) S.B.Andrews (= P. aquilinum × P. esculentum subsp. esculentum ) in Australasia. Besides considering about a dozen subspecies within P. aquilinum ( Thomson 2008) , Thomson (2012) proposed the recognition of two geographical subspecies of P. esculentum : P. esculentum subsp. esculentum in Australasia and P. esculentum subsp. arachnoideum in the Neotropics. However, Schwartsburd et al. (2014) demonstrated that some morphotypes within neotropical “ Pteridium arachnoideum ” (= P. esculentum subsp. arachnoideum ) have been overlooked. In this work we combine the classification proposed by Thomson (2012) with the recognition of morphotypes suggested by Schwartsburd et al. (2014). Accordingly, we recognize three taxa in Bolivia: P. caudatum , P. esculentum subsp. arachnoideum , and P. esculentum subsp. gryphus .

It is important to notice that there are basically three kinds of hairs on the abaxial surfaces of the blades in Pteridium : 1) Catenate hairs on costae and costules. These hairs are usually reddish to brownish, septate, and ca. 0.5 mm long, and are present in the three Bolivian taxa; 2) Lax hairs on veins; these hairs are subappressed, ca. 0.8 mm long, and give the segments a sericeous appearance. They are present in the subspecies of Pteridium esculentum from Bolivia, but they are mostly absent in P. caudatum ; and 3) Gnarled hairs on laminar tissue between the veins; these hairs are tortuous, thickened, tuberculate, and very short, ca. 0.1 mm long or less. They are not easily seen under magnification of 50× or less, and at lower magnifications, they appear to be an “exudate”, like that found in species of Pityrogramma , Notholaena , and some other genera of Pteridaceae ; such wax-like indument in ferns is commonly called “farinaceous” by some authors (e.g., Tryon 1941), but this term is inappropriate for the gnarled hairs in Pteridium . For more details about these hairs, see Thomson & Martin (1996). The gnarled hairs are especially found in Pteridium esculentum subsp. gryphus . They are absent in subsp. arachnoideum , and they are rarely present in P. caudatum .