Adiantum L.

Terrestrial or rupestral ferns. Rhizomes erect to long-creeping, scaly; roots rarely bearing small tubers. Rhizome scales non-clathrate, acicular to narrowly ovate or narrowly triangular, attached at base, concolorous, yellow-brown to red-brown or dark brown. Fronds monomorphic. Stipes and rachises red- or dark brown, highly polished. Laminae 1–5-pinnate or rarely helicoid, herbaceous or coriaceous, glabrous or hairy or rarely scaly (not NZ), rarely glaucous or farinose (not NZ) abaxially. Pinnae not articulated to rachis; ultimate segments rarely articulated to costae and deciduous (not NZ). Veins free or rarely reticulate (not NZ). Sori borne mainly on acroscopic margins of lamina segments, on the underside of orbicular to elongate, modified, reflexed, marginal, lamina flaps (“indusia”); paraphyses absent or rarely present (not NZ). Spores trilete, lacking chlorophyll; perispores scabrate, rugose or tuberculate, without an equatorial flange.

A genus of c. 225 species, included in the subfamily Vittarioideae (PPG 1 2016).

Adiantum is now considered to be monophyletic and sister to the vittarioid ferns (Pryer et al. 2016), although this relationship was regarded as uncertain (e.g. Bouma et al. 2010) until adequate taxon sampling had been achieved. Even now, there is insufficient sampling across the whole genus to determine how many clades are present. Bouma et al. (2010) were able to identify three major clades, with all of the indigenous New Zealand species present in clade C along with other species from different southern hemisphere continents. However, their sampling outside New Zealand was very limited. Lu et al. (2011) sampled 86 taxa from a wider geographical range using five plastid markers and found nine major clades, six of them within Chinese Adiantum. Nevertheless, the phylogeny of the genus as a whole still requires further investigation. Tryon & Tryon (1982) recognised eight major groups on morphological grounds, but there has been no modern monograph of the genus.

Within New Zealand, Allan (1961) recognised six indigenous species – A. aethiopicum, A. cunninghamii, A. diaphanum, A. formosum, A. fulvum and A. hispidulum. Parris & Croxall (1974) subsequently reinstated A. viridescens, Parris (1980) suggested that A. pubescens should be recognised in addition to A. hispidulum, and Brownsey (in Webb et al. 1988) added two naturalised species to the New Zealand flora. However, Large & Braggins (1993) found that A. hispidulum and A. pubescens could only be distinguished on the basis of their hair characters, and even then with a high degree of variability. They treated A. pubescens as a variety of A. hispidulum. Brownsey et al. (2019) investigated the presence or absence of hairs on the abaxial lamina surfaces of all indigenous New Zealand Adiantum species, and concluded that most are dimorphic in this regard, and that such variation should be treated as variation within the bounds of individual species distinguished on the basis of other morphological characters, a conclusion also reached for Australian and Malesian plants by Bostock (1992). As a consequence, Brownsey et al. (2019) reduced A. viridescens to synonymy with A. fulvum.

Species of Adiantum are widely cultivated throughout the world. Many of the cultivars and species in cultivation have been described and illustrated by Goudey (1985) and Hoshizaki & Moran (2001).

In New Zealand, species of Adiantum can be recognised by their 1–5-pinnate fronds, polished dark brown stipes, flabellate or oblong ultimate lamina segments, and sori protected by reflexed lamina segments that are usually reniform or sometimes oblong. The spores are trilete and scabrate (Large & Braggins 1991).

The base chromosome number in Adiantum is x = 29 or 30, but there is also at least one aneuploid with x = 57 and an extensive polyploid series with diploids through to decaploids recorded (Tryon 1990; Tindale & Roy 2002).