Diplazium Sw., J. Bot. (Schrader) 1800

Kessler, Michael & Smith, Alan R., 2018, Prodromus of a fern flora for Bolivia. XXXII. Athyriaceae, Phytotaxa 334 (2), pp. 141-151 : 142-143

Diplazium Sw., J. Bot. (Schrader) 1800 (2): 4, 61, 61. 1801.

Diplazium comprises about 350–400 species, mainly in tropical regions, with probably over 150 species in the Neotropics; thirty species are known from Bolivia. The genus is characterized by its elongate, usually indusiate sori that are usually placed back-to-back (at least in larger or basal portions of ultimate segments) along the same vein (diplazioid sori), coarse roots, and two, laterally arranged, ribbon-like petiolar vascular bundles. A species group with rhizome scales bearing black, bifid, marginal teeth was segregated in the genus Callipteris ( Pacheco & Moran 1999) , but molecular studies show that this clade falls within Diplazium ( Wang et al. 2002, Wei et al. 2013, 2015).

Diplazium is often confused with Hymenasplenium ( Aspleniaceae ), a few species of which may have diplazioid sori ( Smith 1976, Kessler & Smith 2017 b), but Diplazium seldom has clathrate rhizome scales. Additionally, there are difficult-to-observe characteristics (number of annular cells, number of sporangial stalk cells, position of vascular bundles) separating these two genera. The monotypic genus Hemidictyum (treated by us in Aspleniaceae ; Kessler & Smith 2017 b) has historically sometimes been included in Diplazium because of its linear, indusiate sori, but differs in having ±parallel veins that form a reticulate network of pentagonal or hexagonal areoles in the marginal half of the pinnae, also in having continuous intramarginal veins in each pinna; it also has a very different chromosome base number, x = 31. One other Bolivian species has also most often been regarded as belonging in Diplazium — Desmophlebium lechleri (Mett.) Mynssen et al. ; this, and a related species, Diplazium longisorum (Baker) C.Chr. from Colombia, are now placed in a family Desmophlebiaceae by Mynssen et al. (2016), but we prefer to subsume Desmophlebium in more broadly defined family Aspleniaceae (see Kessler & Smith 2017 b).

Because of its many species, large, often incompletely sampled fronds in herbaria, absence of modern monographic work, and lack of globally based phylogenetic studies that might provide a solid framework for regional floristic treatments, Diplazium is still relatively poorly known, compared to many other fern genera. New species continue to be described (e.g., Adams 1992, Kessler & Smith 2007, Mynssen & Sylvestre 2013), and several species complexes would greatly benefit from thorough biosystematic study. Within Diplazium , blade dissection is generally useful for species identification, but it is not necessarily an indicator of species relationships; juvenile, sterile specimens are often less dissected than adult, fertile forms. Affinities of species are more reliably assessed by characters of venation, indument, pinna and pinnule symmetry, and soriation ( Wei et al. 2013). Judging from chromosome counts of neotropical species, which range from diploid to octoploid ( Smith & Mickel 1977, Smith & Foster 1984), and from frequent intermediate specimens with malformed spores, hybridization may be common between neotropical species of Diplazium (see, e.g., Stolze et al. 1994, Mickel & Smith 2004). In addition, agamospory has been documented in a few species ( Dopp 1932, Walker 1985).

In tropical America, Diplazium is a common genus on the forest floor, mainly at low- to mid-elevations ( Mickel & Smith 2004, Kessler et al. 2014). However, its evolutionary origin appears to lie in the northern hemisphere while this still had tropical climatic conditions in the Eocene ( Wei et al. 2015). It then migrated from Eurasia to North America over land bridges as a member of the boreotropical flora. In addition, there appears to have been long-distance dispersal events between Eurasia and tropical America. In Asia, species of Diplazium , in particular the aptly named D. esculentum (Retz.) Sw. , are commonly eaten as cooked vegetables.

Relatively modern floristic treatments of Diplazium in several neotropical countries and regions are available for Jamaica ( Proctor 1985), Puerto Rico ( Proctor 1989), and the Lesser Antilles ( Proctor 1977), Mexico ( Mickel & Smith 2004); Mesoamerica (Adams 1995); Ecuador ( Stolze et al. 1994), and Peru (Tryon & Stolze 1992).