Adiantum is a large genus with 150-200 species, more or less pan(sub-)tropically distributed but with the greatest diversity in South America. With its delicate, billowing foliage Adiantum is very popular in cultivation. Page & Bennell (1986) provide a key for 16 species and cite several additional ones. According to Hoshizaki (1970) 20 species of Adiantum (including one nothospecies) are in cultivation. As a rule they are very frost-sensitive but at least two species are nevertheless increasingly found as escapes from cultivation in Belgium.
1 Mature frond pedately branched, all divisions arising from a central point === Adiantum aleuticum
Mature frond pinnately branched === 2
2 Nerves of the segments of sterile fronds ending in the teeth of the segment margin; ultimate segments large, usually over 15 mm wide. Pseudoindusium oblong === A. capillus-veneris
Nerves of the segments of sterile fronds ending at the sinuses of the segment margin; ultimate segments small, usually less than 10 mm wide. Pseudoindusium reniform === A. raddianum
Literature:
Guillot Ortiz D., Mateo Sanz G. & Rosselló Picornell A. (2006) Claves para la pteridoflora ornamental de la Comunidad Valenciana. Bouteloua 1: 25-33. [available online at: https://dialnet.unirioja.es/servlet/articulo?codigo=2191829]
Hoshizaki B.J. (1970) The genus Adiantum in cultivation (Polypodiaceae). Baileya 17: 97-191.
Page C.N. & Bennell F.M. (1986) Adiantum. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 20-23. Cambridge University Press, Cambridge.
Pryer K.M., Huiet L., Li F.W., Rothfels C.J. & Schuettpelz E. (2016) Maidenhair Ferns, Adiantum, are Indeed Monophyletic and Sister to Shoestring Ferns, Vittarioids. Syst. Bot. 41(1): 17-23. [available online at: https://pryerlab.biology.duke.edu/uploads/media_items/maidenhair-ferns-a...
Schelpe E.A.C.L.E. & Anthony N.C. (1986) Pteridophyta. In: Leistner O.A. (ed.), Flora of Southern Africa. Botanical Research Institute: XV + 292 p.
Verloove F., van der Ham R. & Denters T. (2007) Exotische muurvarens in België en Nederland. Dumortiera 92: 1-16. [available online at: http://alienplantsbelgium.be/sites/alienplantsbelgium.be/files/dum92p1.pdf]