Hemidictyum C.Presl, Tent. Pterid.

Kessler, Michael & Smith, Alan R., 2018, Prodromus of a fern flora for Bolivia. XXIX. Aspleniaceae, Phytotaxa 344 (3), pp. 259-280 : 259-280

Hemidictyum C.Presl, Tent. Pterid. 110. 1836.

A rather isolated and distinct, monospecific genus, widespread in the Neotropics. Hemidictyum was long regarded as related to the diplazioid ferns ( Athyriaceae ), and even belonging in the genus Diplazium because of its long-linear sori and indusia, but differences from that family were pointed out by Mickel & Smith (2004, citing unpublished molecular data by R. Cranfill), and also include the different chromosome base number of x = 31 (x = 40, 41 in Athyriaceae ) and inframarginally areolate venation (veins free in most Diplazium species, or anastomosing in much different patterns in a few Diplazium spp. , e.g., subgroup Callipteris ). Molecular phylogenetic studies indicate that Hemidictyum is sister to the clade of Asplenium + Hymenasplenium ( Rothfels et al. 2012a, Mynssen et al. 2016). Hemidictyum contrasts with these two genera in many features, but especially in the non-clathrate stipe base scales, very large blades with opposite to subopposite pinnae and cordate-clasping pinna bases, and the unique venation with inframarginal, pentagonal or hexagonal areolate venation near the margin ending in a continuous, greatly thickened, blackish, raised commisural vein. Between these commisural veins and the pinna margins are narrow green bands, bordered by conspicuous, membranaceous, hyaline pinna margins. Veins on both sides of the blade are prominently raised (contrasting with most Aspleniaceae ), and do not end in distinct hydathodes adaxially (variable in Aspleniaceae ). Additional unique features distinguishing Hemidictyum from its sister clade are the base chromosome number of x = 31 (vs. usually x = 36, occasionally 35, in Asplenium , x = 38 or 39 in Hymenasplenium ). Hemidictyum also differs in the thickened, opaque indusia (thinner, more translucent in Aspleniaceae ), and the sporangia dehisce such that the lateral walls form a “bucket” on the distal end (vs. two separate distal flaps in Aspleniaceae ). Spores of Hemidictyum are similar in ornamentation to those of some other Aspleniaceae (prominently ridged) but differ in their golden brown color (contrasting with generally dark brown to blackish spores of Aspleniaceae ). Based on these differences, Hemidictyum has been placed in a family of its own ( Rothfels et al. 2012b), but based on overall similarities, we prefer its inclusion in an expanded concept of Aspleniaceae .