My initial documentation of the Skeleton Fork Fern or Whisk Fern (Psilotum nudum) was in January 2015, more than 10 years ago. This fern began appearing spontaneously in my pot in 2011, and I have managed to sustain its population since then. Over time, the fern has spread to several pots; however, its growth has been relatively slow, likely due to competition with larger plants occupying the same containers. In one particular pot, following the demise of the dominant plant, the growth rate of the fern population increased significantly. The fern population became taller and denser (Fig. 1), with sporangia (spore-containing bodies) forming along the stems. It has now established itself as the dominant species in that pot.

Notably, the general appearance of this fern is similar to that of the Firecracker Plant (Russelia equisetiformis), an ornamental flowering plant available in Singapore. This resemblance is particularly evident when the Firecracker Plant lacks leaves or flowers on the entire plant (Fig. 2).

Fig. 1: Skeleton Fork Fern (Psilotum nudum)	Fig. 2: Firecracker Plant (Russelia equisetiformis)

Although the Skeleton Fork Fern is often described as a fern-like plant, it is classified within the fern group (Class Polypodiopsida). The distinction from other ferns begins at the subclass level, where it falls under Adder's-tongues, Fork Ferns, and Allies (Subclass Ophioglossidae). In addition to Psilotum nudum, at least four other species of ferns within Subclass Ophioglossidae have been reported in Singapore, according to the iNaturalist website: Ophioderma pendulum (Ribbon Fern), Ophioglossum petiolatum (Stalked Adder's-Tongue), Ophioglossum reticulatum (Stalked Adder's-Tongue), and Ophioglossum nudicaule (Slender Adder's-Tongue). It should be noted that Ophioderma pendulum is currently considered a synonym of Ophioglossum pendulum.

According to the 2022 Singapore flora checklist, neither Ophioglossum pendulum nor its synonym is listed, despite there being 4 observations of this species in Singapore on the iNaturalist website as of 1 June 2025. Additionally, NParks' Flora & Fauna Web includes this species, although I have yet to encounter one myself. Another species, Psilotum complanatum, is mentioned in the checklist but has been assessed as locally extinct. Of the three Ophioglossum species observed in Singapore on the iNaturalist website, Ophioglossum petiolatum is not included in the checklist, even though there is one observation of it from Singapore.

(a) Young sporangia	(b) Mature sporangia	(c) Empty sporangia
Fig. 3 (a-c): Sporangia (spore-containing bodies) of Skeleton Fork Fern (Psilotum nudum)

The Skeleton Fork Fern is distinguished by its typically leafless, slender, erect green stems, which give it a distinctive appearance. According to my observations, the stems gradually turn yellowish as they age or when exposed to sunlight for extended periods each day. Another notable feature of this fern is its spore-bearing structures, which appear as three-lobed capsule-like objects (Fig. 3) located in the axils along the stem. This contrasts with the more familiar ferns, whose spore-bearing bodies are found beneath their foliage.

Since this fern is quite rare, I will keep a population of it in my pots as long as possible.