Hymenophyllum Revolutum – Filmy Fern

Hymenophyllum revolutum is a small, delicate filmy fern with very thin, translucent fronds that are typically only one or two cells thick, giving it a “filmy” appearance. It grows as either an epiphyte or terrestrially, forming dense, turf‑like patches over time from long‑creeping, wiry rhizomes that branch and intertwine.

Detailed description

The fronds are light green, narrow to broadly ovate, and usually 2‑ to 3‑pinnate, with the final segments being linear to oblong and often forked, with deeply toothed margins. The stalks (stipes) are short (a few millimeters up to a couple of centimeters), thin, and brittle, with a narrowly winged rachis in the upper part. The sori are borne on short, winged side branches near the main axis, typically one per primary pinna, sitting along the acroscopic (upper) margin, and are protected by inrolled leaf margins that act as a hood‑like indusium.

Care and cultivation

This species is highly sensitive to drying out and needs consistently high humidity and a constantly moist but not waterlogged substrate. It thrives best in a very airy, moisture‑retentive medium such as live or semi‑dried sphagnum moss, fern fiber, or on a damp plaque of tree fern or cork, where the rhizomes can creep over the surface. Gentle, pure water (rain, distilled, or reverse‑osmosis) is preferred because it lacks mineral buildup that can harm the thin leaf tissue.

Light should be low to moderate, diffused, and without strong sun; bright shade or indirect light in a terrarium or glasshouse works well, but hot, direct rays will quickly scorch and desiccate the fronds. Temperature‑wise it prefers cool‑to‑warm conditions, avoiding extremes, and is happiest in a sheltered, stable microclimate rather than exposed, drafty spots. Fertilizer, if used, should be extremely dilute and applied very infrequently, as its natural habitats are low‑nutrient and the plant is adapted to absorb nutrients slowly from thin films of water and moss.

Adaptability

Filmy ferns in this genus are adapted to life in very humid, sheltered habitats where moisture is nearly constant, such as moss‑covered branches, rock overhangs, and damp forest floors. Their thin, acellular‑thick leaves maximize gas exchange and water absorption but make them extremely vulnerable to desiccation, so they cannot tolerate dry air or intermittent drying. Over time, Hymenophyllum revolutum can spread locally by extending its creeping rhizomes and forming small mats, but it is generally slow‑growing and takes a long time to establish in cultivation.

Growing conditions

In cultivation, think of a microenvironment similar to a shaded, dripping mossy niche: high humidity (ideally 80% or higher), cool‑to‑moderate temperatures, and constant moisture. It does well in sealed or semi‑sealed terrariums, or on shaded walls and rafters with a fine mist system or nearby water feature that keeps the air and surfaces damp. Good air film movement (not direct wind) is important to prevent fungal growth, while the rhizomes and substrate base must never fully dry out. Avoid heavy fertilization, compacted potting mixes, and hot or dry “room” conditions, as these will cause the delicate fronds to brown and die back rapidly.