Adiantum raddianum

Adiantum raddianum, the delta maidenhair fern, grows wild across tropical America from Mexico and the Caribbean south to northern Argentina and Uruguay, on forest floors, in rock crevices, and along stream banks and coastal cliffs. Herbarium collections span sea level to about 3,000 m, with most between roughly 500 and 2,100 m. Its triangular fronds, up to about 45 cm long, are divided three or four times into small, fan-shaped leaflets held on wiry, shiny black stalks — the hair-like stalks behind the name maidenhair. New fronds open pale green and darken as they age. The genus name comes from the Greek adiantos, 'unwetted', because water runs straight off the leaflets, and the spores form in small clusters tucked under their folded-over edges rather than on the open surface. It is one of the most popular ferns grown indoors and a famously demanding one: it does best in bright, indirect light, steady high humidity and 18–24 °C, in neutral to slightly alkaline soil that never dries out. Missouri Botanical Garden notes that the fronds die back quickly if the roots dry, scorch in direct sun and lose their vigour in deep shade. A closed terrarium supplies the constant humidity that open rooms lack, though a mature clump spreads 30–60 cm and needs a large vessel. It has naturalised in Hawaii, French Polynesia and the London area.