# Psilotum complanatum
**Psilotum complanatum** Sw. is a flattened-stemmed whisk fern of the family Psilotaceae, widespread in the tropics of the Americas, Asia, and the Pacific. Recent taxonomic work treats it as an interspecific hybrid, **Psilotum × complanatum**, between *P. nudum* and *P. flaccidum*, a status now accepted by Kew's Plants of the World Online[1](https://powo.science.kew.org/taxon/114220-3) • [2](https://link.springer.com/article/10.1007/s12228-024-09800-3). The species epithet comes from Latin *complanatus*, flattened, in reference to the flattened stems[3](https://naturalhistory2.si.edu/botany/hawaiianflora/speciesdescr.cfm?genus=Psilotum&species=complanatum).
| Key fact | Detail |
|---|---|
| Scientific name | *Psilotum complanatum* Sw.; treated as the hybrid *Psilotum × complanatum* by Plants of the World Online[1](https://powo.science.kew.org/taxon/114220-3) |
| Family | Psilotaceae (whisk ferns), order Ophioglossales[2](https://link.springer.com/article/10.1007/s12228-024-09800-3) |
| Hybrid parentage | *P. nudum* and *P. flaccidum*, per a 2024 morpho-anatomical reassessment[2](https://link.springer.com/article/10.1007/s12228-024-09800-3) |
| Diagnostic feature | Flattened ultimate branch forks, versus the more or less terete (round in cross-section) forks of *P. nudum*[4](https://apps.lucidcentral.org/ferns/pdf/entities/psilotum_complanatum.pdf) |
| Distribution | Mexico, Central America, the Caribbean, tropical Asia, Malesia, Australia, and across the Pacific to Hawaii[5](https://www.nparks.gov.sg/florafaunaweb/flora/5/8/5855) |
| Hawaiian names | Moa, moa nahele, pipi, o'o moa[6](https://manoa.hawaii.edu/lifesciences/faculty/carr/psilot.htm) |
| Spores | Aborted in the specimens examined in the 2024 hybrid treatment[2](https://link.springer.com/article/10.1007/s12228-024-09800-3) |
## Taxonomy and the hybrid question
The genus *Psilotum* has long served in botany teaching as an example of the earliest land plants, because it lacks roots and true leaves and has a protostele; molecular work instead places Psilotaceae in the clade Ophioglossidae, sister to the remaining ferns after the divergence of the horsetails ([Equisetales](https://www.edgepedia.org/equisetales))[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). Two species of *Psilotum* are generally recognized, and *P. complanatum* is the one with the largest number of nomenclatural inconsistencies in the world's herbaria[2](https://link.springer.com/article/10.1007/s12228-024-09800-3).
A 2024 taxonomic assessment by J. Daniel Tejero-Díez and colleagues, pteridologists publishing in *Brittonia*, concluded from type specimens, field collections, and anatomical analysis that two species, *P. nudum* and *P. flaccidum*, exist, and that the name *P. × complanatum* belongs to their interspecific hybrid[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). The hybrid has nomenclatural priority over *Psilotum × intermedium* W.H. Wagner, the name Wagner established in 1993 for Hawaiian hybrids of the two parents[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). The authors designated seven lectotypes and an epitype to stabilize application of the name, since the type of *P. complanatum* is sterile and lacks synangia[2](https://link.springer.com/article/10.1007/s12228-024-09800-3).
The hybrid interpretation is supported by spore morphology. In the 2024 study, all 28 specimens of *P. × complanatum* examined had aborted spores, a typical sign of hybrid sterility, although some samples from Hawaii and Mexico showed well-formed spores mixed with irregular ones[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). Other authorities continue to list the taxon as an accepted species: the International Plant Names Index records the name *Psilotum complanatum* Sw. in *Synopsis Filicum*, published 4 October 1867[7](https://www.ipni.org/n/114220-3), and GBIF maintains a species entry alongside the hybrid entry[8](https://www.gbif.org/species/2688175).
## Description
*Psilotum complanatum* is an epiphytic or rock-dwelling plant with dichotomously branching, leafless green stems arising from a rhizome covered with rhizoids[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). Its defining character is in the ultimate forks: *P. nudum* has more or less terete ultimate forks, whereas *P. complanatum* has flattened ones[4](https://apps.lucidcentral.org/ferns/pdf/entities/psilotum_complanatum.pdf).
In the 2024 treatment, the hybrid's habit is scapose and erect to arching or reflexed, with flattened, leathery twigs, and its form varies with microhabitat: epiphytic plants are arched-pendulous with a short foot, while plants on rock resemble *P. nudum* but have flattened, reflexed distal branches[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). The spore-producing organs are bi- to trilocular synangia, 1.5 to 2 mm in diameter[2](https://link.springer.com/article/10.1007/s12228-024-09800-3).
A comparison drawn from the Niue flora work distinguishes the two taxa in the field: *P. complanatum* grows decumbent, trailing, or pendulous, with secondary stems and branchlets 2 to 4 mm wide and flattened or nearly so except for a keeled midrib, while *P. nudum* is upright or slightly sprawling with branchlets 1 to 2 mm wide, subterete, and prominently three-angled[9](https://rsnz.onlinelibrary.wiley.com/doi/10.1002/nzb2.70108).
## Distribution and habitat
The natural distribution recorded by Singapore's National Parks Board spans Mexico, Central America ([Costa Rica](https://www.edgepedia.org/costa-rica), El Salvador, Honduras, Nicaragua), Cuba, the [Dominican Republic](https://www.edgepedia.org/dominican-republic), Jamaica, Colombia, Peru, the [Nicobar Islands](https://www.edgepedia.org/nicobar-islands), Vietnam, Thailand, Peninsular Malaysia, Singapore, parts of Indonesia, the Philippines, New Guinea, the Solomon Islands, Australia (New South Wales, Queensland), many Pacific island groups, and Hawaii[5](https://www.nparks.gov.sg/florafaunaweb/flora/5/8/5855). A 2026 paper describes the species as widespread in tropical Asia, the Pacific, Mexico, and Central America[9](https://rsnz.onlinelibrary.wiley.com/doi/10.1002/nzb2.70108).
The species grows as an epiphyte on trees or on rocks. Historical herbarium tickets from the Calcutta Herbarium record it "growing under a fern on a tree" on the Chanderiang river in Perak, and "growing from seams of rock" in Chil-hua-hua, Mexico[10](https://bsi.gov.in/uploads/userfiles/file/Rare%20Books/Botanical%20Notes%20And%20Papers.pdf).
## Conservation
The [Royal Botanic Garden Edinburgh](https://www.edgepedia.org/royal-botanic-garden-edinburgh)'s fern factsheet for Thailand, Laos, and Cambodia describes the species as widespread and not under any known threat[11](https://websites.rbge.org.uk/thaiferns/factsheets/index.php?q=Psilotum_complanatum.xml). At the edge of its range the picture differs: in Niue, where it was recorded for the first time in a 2026 survey, an estimated 10 to 12 mature plants at two locations led to a regional assessment of Nationally Critical under IUCN criteria[9](https://rsnz.onlinelibrary.wiley.com/doi/10.1002/nzb2.70108).
## Relationship with Psilotum nudum
The two taxa grow together across much of the tropics and readily hybridize where they coexist, which is how the hybrid explanation of *P. complanatum* arose; the discovery of the hybrid provided an explanation of how two independent species of *Psilotum* may coexist in the same environment[2](https://link.springer.com/article/10.1007/s12228-024-09800-3). In Hawaii, both taxa carry Indigenous names: moa or moa nahele, and also pipi and o'o moa[6](https://manoa.hawaii.edu/lifesciences/faculty/carr/psilot.htm). Molecular study has shown genetic differences between *P. nudum* and *P. complanatum*[2](https://link.springer.com/article/10.1007/s12228-024-09800-3).
## References
1. [Psilotum × complanatum Sw., Plants of the World Online, Kew Science](https://powo.science.kew.org/taxon/114220-3)
2. [Tejero-Díez, J.D., Grego-Valencia, D., Prado, J., Aguilar-Rodríguez, S. & Terrazas, T. (2024). Psilotum (Psilotaceae-Ophioglossales): a taxonomic assessment based on morpho-anatomical studies of the species from the Neotropics. Brittonia 76: 198–216.](https://link.springer.com/article/10.1007/s12228-024-09800-3)
3. [Psilotum complanatum, Flora of the Hawaiian Islands, Smithsonian Institution](https://naturalhistory2.si.edu/botany/hawaiianflora/speciesdescr.cfm?genus=Psilotum&species=complanatum)
4. [Psilotum complanatum, Ferns of Thailand, Laos and Cambodia (lucidcentral key entity)](https://apps.lucidcentral.org/ferns/pdf/entities/psilotum_complanatum.pdf)
5. [Psilotum complanatum, NParks Flora & Fauna Web, Singapore](https://www.nparks.gov.sg/florafaunaweb/flora/5/8/5855)
6. [Psilotaceae, Non-Flowering Plant Families, University of Hawaii Botany](https://manoa.hawaii.edu/lifesciences/faculty/carr/psilot.htm)
7. [Psilotum complanatum Sw., International Plant Names Index](https://www.ipni.org/n/114220-3)
8. [Psilotum complanatum Sw., GBIF Backbone Taxonomy](https://www.gbif.org/species/2688175)
9. [Heenan, P.B. (2026). Additions to the Fern Flora of Niue (Polynesia). New Zealand Journal of Botany 64(2).](https://rsnz.onlinelibrary.wiley.com/doi/10.1002/nzb2.70108)
10. [Botanical Notes and Papers, Botanical Survey of India (Calcutta Herbarium notes)](https://bsi.gov.in/uploads/userfiles/file/Rare%20Books/Botanical%20Notes%20And%20Papers.pdf)
11. [Psilotum complanatum, Ferns of Thailand, Laos and Cambodia, RBGE](https://websites.rbge.org.uk/thaiferns/factsheets/index.php?q=Psilotum_complanatum.xml)
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*Topic: Encyclopedia › Life and health › Plants and algae › Ferns and lycophytes › Fern biology and systematics › Fern classification and paleobotany › Whisk ferns (Psilotaceae) › Psilotum complanatum*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license