Basionym: Blechnum spicant var. fallax Lange (1880: 11–12).
Lectotype (designated by Wasowicz et al. 2017):— ISLAND. Tunguhver, varme Kilde, Ch. GrØnlund s.n. (C! barcode C 10021769, left hand specimen, ”no. 2” and its separate parts).
Notes:— Plants classified to the var. fallax are very small, with monomorphic leaves 2–5 cm long. All leaves in this taxon are fertile and in shape similar to the sterile ones in var. spicant. Variety fallax was first discovered by Danish botanist Christian Grønlund during his excursion to Iceland in 1876 (Grønlund 1881). He found the plant growing on the cone of a large hot spring called Deildartunguhver in western Iceland. Grønlund passed his herbarium material to Lange (Löve & Löve 1966), who described var. fallax on the basis of these collections (Lange 1880). During summer 2016 all locations of plants previously classified as var. fallax were visited by the authors. This study proved that var. fallax is still present only in one locality (locus classicus) where it forms a large population of several hundred individuals that are morphologically uniform and conform to the description of var. fallax given by Lange. In the vicinity of all other hot springs the presence of typical, heterophyllous var. spicant was confirmed. No transition forms to var. spicant were observed.