A new species of Blechnum L. (Blechnaceae, Polypodiopsida) from central-western Brazil, with an updated key to the species in the country

We describe a new species of Blechnum from Mato Grosso, Brazil: Blechnum rivulorum sp. nov. This new species was discovered as a result of visits to the herbaria BM, K, P, PR, and S, as part of the Reflora Project, by the first author. We also indicate its position in a new Blechnum phylogeny. Additionally, we confirm the occurrence of Blechnum meridense Klotzsch in Brazil and provide an updated key to all species of Blechnum in the country.

The family is organized in three subfamilies and 25 genera (Gasper et al. 2016;Molino et al. 2019), of which Blechnum L., with about 30 spp., is the third largest (Dittrich et al. 2015;PPG 1 2016).Blechnum in the restricted sense treated by us is restricted to the Neotropics and Africa, and can be recognized by its stoloniferous rhizomes, monomorphic to subdimorphic fronds, and finely denticulate pinna margins (Gasper et al. 2016).The genus was taxonomically investigated in the Neotropics by Murillo (1968) and Rolleri et al. (2012), and in Brazil it was studied by Dittrich et al. (2015), who ascribed 11 species to southern and southeastern Brazil, the richest area in the country.There are some recently described endemic species in northern and central-western Brazil (Dittrich et al. 2012).Altogether, 13 species of Blechnum are known from Brazil (Dittrich & Gasper 2022).
Even though many floras give a good account of Blechnum in many countries (e.g., Proctor 1985, for Jamaica;Mickel & Smith 2004, for Mexico;Ramos Giacosa 2016, for Argentina;Smith & Kessler 2018, for Bolivia), new species are still being discovered, and some species complexes (as Blechnum occidentale s.l.) need new study to avoid Linnean shortfalls (Hortal et al. 2015).Mato Grosso is not a particularly important area of endemism for ferns, but recently Matos & Vasco (2022)

Blechnum rivulorum
Comments: Blechnum rivulorum is closely related to Blechnum polypodioides Raddi and B. asplenioides Sw.From the first it can be distinguished by the pinnae more strongly ascending, generally more than 45° (when ascending, in B. polypodioides, generally not more than 30°), distinct spores (perispore with low folds vs. perispore without folds in B. polypodioides) and stipe diameter (filiform, not more than 0.4 mm diam.vs. stouter stipes 0.9-1.1 mm diam. in B. polypodioides).From B. asplenioides it can be distinguished by the blades abruptly reduced proximally to a vestigial pinna pair or to a smaller pinna (gradually reduced proximally in B. asplenioides, generally to semicircular lobes), by the sinuses (wide in B. rivulorum vs. narrow in B. asplenioides) and by the linear, elliptic or oblanceolate blades (vs.linearlanceolate in B. asplenioides).Furthermore, B. asplenioides tends to be a more robust plant (fronds 5.7-30 cm long vs. 2.9-13.4cm long in B. rivulorum) with more pinnae (6-32 pairs) than B. rivulorum ((1)3-7(11) pairs).
Blechnum rivulorum forms a clade with Blechnum asplenioides and Blechnum polypodioides in the maximum likelihood analysis, with high support, in the Blechnum polypodioides clade (Fig. 4, indicated by an asterisk).Despite our goal was not to provide a new phylogeny of Blechnum s.s., we can identify B. rivulorum affinities.This is a well-supported clade composed by small plants (when compared to the Blechnum occidentale clade), with areolate-veined species (Dittrich et al. 2012) as sister to Blechnum rivulorum+ asplenioides+polypodioides.
At least one specimen has some dimidiate pinnae.Those pinnae are very small in this specimen, but this may be result   of their size.Fertile dimidiate pinnae have sori not following any vein, but instead at the base of a few veins.Variation in frond shape is striking, even at the same individual (e.g., Salino 380, UEC).
This species grows on sandstone rocks, generally along rivers or close to them.Their habitat and morphology suggest that they may be rheophytes.The species name (latin rivulorum = from the rivers) indicates their preferred habitat.
Additional specimens examined (paratypes): Brazil, Mato Grosso: Barra do Garças  New record for Brazil: Blechnum meridense Klotzsch.BRAZIL: Rio de Janeiro, Corcovado, 1894, Herter s.n (S).According to Rolleri et al. (2012), this species was previously recorded in Brazil.They cite the voucher Albricht 165 (LP) and state that the specimen on the left is B. meridense.Unfortunately, there is no photo of it, and we were unable to see any image (according to Laura Iharlegui from LP, pers.com., the specimen Albricht 165 could not be found in the collection).This is the only specimen seen by the authors in Brazil, and since this specimen was collected more than 120 years ago amid an urban area that is one of the main centers of research in Botany in the country, we believe it is probably locally extinct.