Struthiopteris fallax S. Molino, Gabriel y Galan & Wasowicz

Struthiopteris fallax (Lange) S. Molino, Gabriel y Galán & Wasowicz (Figure 9E; Figure 14C‒D; Figure 16). Type:— ICELAND. Tunguhver, varme Kilde, no date, Ch. GrØnlund s.n. (C 10021769! photo, left hand specimen, i.e., plant no. 2 and its separate parts, lectotype, designated in Wasowicz et al. 2017a). Synonyms:— ≡ Blechnum spicant (L.) Roth var. fallax Lange (1880: 50 [2983]). Struthiopteris spicant (L.) Weiss var. fallax (Lange) Wasowicz & Gabriel y Galán in Wasowicz et al. (2017b: 198). Plants terrestrial, rhizomes short, erect, with lanceolate, concolorous, chestnut-coloured scales, margin entire, 0.3‒0.6 x 1.5‒2 mm; fronds 1‒5 cm, monomorphic, forming a rosette above the ground, shortly petiolate, stipes 0.1‒0.4 cm, brown, black or purplish, laminae lanceolate, 0.4‒0.7 x 1.3‒5 cm, pinnatifid, pinnae tapering towards the base, the middle ones 0.1‒0.2 x 0.2‒0.6 cm, rachises dark basally, green towards the apex, with uniseriate multicellular hairs topped by a red gland; veins free, simple or 1-furcate, catadromous, ending in adaxial hydathodes; sori linear, continuous, forming coenosori on both sides of costa, occupying the entire length of the pinna; indusia linear, continuous; sporangia with 13‒21 arc cells; spores monolete, 20‒40 x 30‒60 µm, perisporium with marked narrow folds. Habitat and distribution:— Endemic to northwestern Iceland, at Deildartunguhver hot spring, the biggest hot spring in Europe. Slope in hot spring, whose water is at a temperature of 100° C, and the soil at a constant temperature of 30‒40ºC (Figure 3) (Wasowicz 2021). Chromosome number:— n =68, tetraploid (Löve & Löve 1966). Tómansson (2016) suggests that this species could be a diploid instead of a tetraploid. Etymology:— The epithet “ fallax ”, which means deceptive, was given probably because of its deceptive difference from the typical plant (S. spicant) (Lange 1880; Löve & Löve 1966). Vernacular names:— Tunguskollakambur (Icelandic) (Flora of Iceland 2024). Taxonomic notes:— This species was collected by Grønlund (1881) and cultivated by him in Copenhagen, where it grew with the same characteristics as in nature, which supports its treatment as a distinct species from Struthiopteris spicant (Hallgrímsson 1968). This further demonstrated its ability to grow in soils not at the high temperatures of Deildartunguhver, and in fact it has also been grown in the Botanical Garden at Akureyri (northern Iceland) in unheated soil, and in pots in Boulder, Colorado, again growing with the same characteristics as in nature despite differences in climatic conditions (Löve & Löve 1966). Despite its proven ability to grow in colder soils, no other populations have been found elsewhere in Iceland. We do not know the cause of this, but the relatively high proportion of aberrant and aborted spores suggest a lower dispersal power. In the phylogenetic analyses where it has been included, the species has not been separated from S. spicant (Molino et al. 2019b, c). However, these were phylogenies based on Sanger sequencing of three chloroplast markers, and in the dating of the genus the divergence between these two species has been shown to be very recent, approximately 0.5 million years, so these analyses may not be sufficient to discriminate both species phylogenetically (Molino et al. 2019b). Furthermore, Struthiopteris fallax and S. spicant are radically different morphologically, anatomically, palynologically, and ecologically, which led to the separation of these two taxa as distinct species (Molino et al. 2019c). Struthiopteris fallax is a plant with monomorphic fronds, these approximately 5 cm long, whereas S. spicant is dimorphic or subdimorphic, with fronds usually 20 cm or longer. Differences have also been found in the shape of the epidermal cells, the cross section of the fertile pinnae and the size of the spores (Molino et al. 2019c). They also have a clearly differentiated niche, and it has been shown, as discussed above, that when S. fallax spores are grown in environments other than their natural habitat, they grow with the same characteristics. No hybrids have been found between Struthiopteris fallax and S. spicant, probably because, although both species occur in Iceland, the populations are far apart (Löve & Löve 1966). This plant is the only endemic fern in Iceland, and its area is estimated to cover only about 100 m 2 with 100‒200 individuals (Löve & Löve 1966, Wasowicz et al. 2017a, b, 2021), which makes it a very special element and may be particularly endangered due to the few individuals and the very specific area it occupies. Material reviewed:— ICELAND. Deildartunguhver, P. Wasowicz & J. M. Gabriel y Galán s.n. (MACB 109359!).