Psilophytes
Order Psilotales
The psilophytes consist of two genera, Psilotum (whisk fern) and Tmesipteris (hanging fork fern). Both are enigmatic plants which appear to be ancestral, but may be advanced and reduced ferns. They appear ancestral since Psilotum is leafless and rootless with dichotomizing stems similar to early land plants. Both have leafless outgrowths along their stems, but they lack vascular tissue, thus making them enations. There is no reliable fossil record from this group until the Cenozoic, but they may be a much more ancient group.
Above: Psilotum nudum, growing on volcanic rock in Hawai'i
Above: Tmesipteris, growing as an epiphyte from a rock outcrop
Ecology and Form
Tropical, herbaceous, epiphytic plants
Sporophyte (=spore-bearing phase)
Vegetative features
Stems
Isotomous or equal branching (Y-shaped)
Upright stems are determinate
Axes dichotomize before elongating internodes
Rhizome present: indeterminate and bifurcating
Vascular tissue present
Actinostele in distal axes
Siphonostele in proximal axes
Leaves
No leaves
Exhibit leaf-like outgrowth (=enations)
Enations do not have vascular tissue
Enations in Tmespteris are larger and much more leaf-like
Roots
This group lacks roots
Rhizomes possess rhizoids
Reproductive features
Spore cases (sporangia)
3 eusporangia (large spore cases with many spores) are fused into one structure called a synangium
Gametophyte (=gamete-bearing phase)
Very small, less than 2 mm
Subterranean and non-photosynthetic
Saprotrophic, absorbing nutrients aided by the presence of symbiotic fungi (mycorrhizae)
Gametophyte possesses a vascular strand, unlike all other pteridophytes, except the extinct rhyniophytes (Holloway 1939)
Above: The sporophyte of Psilotum nudum showing dichotomous branching, synangia, and tiny enations along stem
Above: Up-rooted Psilotum nudum with rootless rhizome exposed
Above: close-up of Psilotum synangia
Above: the gametophyte of Psilotum nudum
Classification
└Tracheophytes
└Euphyllophytes
└Monilophytes
└Psilotales
Diversity
2 genera: Psilotum and Tmesipteris
4 species
Geologic Age
??? (Cenozoic) - present
Possibly more ancient, originating in the Devonian
Additional Resources
Wacky World of Whisk Ferns (In Defense of Plants, Apr 2019)