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)