Psilotaceae is a family of ferns consisting of two genera, Psilotum and Tmesipteris with about a dozen species. It is the only family in the orderPsilotales.
Description
Once thought to be descendants of early vascular plants, Psilotaceae have been shown by molecular phylogenetics to be ferns, and a sister group of the Ophioglossaceae. The family contains two genera, Psilotum and Tmesipteris. The first genus, Psilotum, consists of small shrubby plants of the dry tropics commonly known as "whisk ferns". The other genus, Tmesipteris, is an epiphyte found in Australia, New Zealand, and New Caledonia. All members of Psilotaceae are vascular plants without any true roots. Rather, the plants are anchored by an underground system of rhizomes. The small, stem-like gametophytes of Psilotaceae are located in this rhizome system, and they aid in a plant's nutrient absorption through the soil. This is primarily achieved through saprotrophic feeding on organic soil matter and mycorrhizal interactions. Psilotaceae generally have very small leaves. Some species have leaf-like structures called enations which are without any vascular tissuesexcept for a small bundle at the base. These are almost peg-like, stubby and are generally not considered true leaves. Other species, however, have slightly longer leaves with a simple vascular system consisting of a single midrib running down the center. The sporaniga of Psilotaceae are fused together into small and distinctive yellow balls called synangia. These synangia are located off the stems of the plants. They contain two sporangia each in Tmesipteris species, and three sporangia each in Psilotum species. A thick tapetum nourishes the developing spores, as is typical of eusporangiate ferns like Psilotaceae.
Classification
In the molecular phylogenetic classification of Smith et al. in 2006, Psilotales, containing the single family Psilotaceae comprising Psilotum and Tmesipteris, was placed with the order Ophioglossales in the classPsilotopsida. The linear sequence of Christenhusz et al., intended for compatibility with the classification of Chase and Reveal which placed all land plants in Equisetopsida, made it a member of subclass Ophioglossidae, equivalent to Smith's Psilotopsida. The placement of Psilotales in subclass Ophioglossidae has subsequently been followed in the classifications of Christenhusz and Chase and PPG I. In the pastTmesipteris has been placed in its own family, Tmesipteridaceae, but this has not been maintained in the system of Smith et al. and later classifications.
