| Psilotaceae | |
|---|---|
 | |
| Closeup ofPsilotum nudum | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Division: | Polypodiophyta |
| Class: | Polypodiopsida |
| Subclass: | Ophioglossidae |
| Order: | Psilotales Prantl |
| Family: | Psilotaceae J.W. Griff. & Henfr. |
| Genera | |
Psilotaceae is a family offerns (class Polypodiopsida) consisting of two genera,Psilotum andTmesipteris with about a dozen species.[1] It is the only family in theorderPsilotales.[2]
Once thought to be descendants of early vascular plants (thePsilophyta of theDevonian period),Psilotaceae have been shown by molecular phylogenetics to be ferns (Polypodiopsida), and a sister group of theOphioglossaceae. The family contains two genera,Psilotum andTmesipteris. The first genus,Psilotum, consists of small shrubby plants of the dry tropics commonly known as "whisk ferns". The other genus,Tmesipteris, is anepiphyte found inAustralia,New Zealand, andNew Caledonia.
All members of Psilotaceae arevascular plants without any true roots. Rather, the plants are anchored by an underground system ofrhizomes. The small, stem-likegametophytes of Psilotaceae are located in this rhizome system, and they aid in a plant's nutrient absorption through the soil. This is primarily achieved throughsaprotrophic feeding on organic soil matter andmycorrhizal interactions.
Psilotaceae do not have leaves. Some species have leaf-like structures calledenations which have no vascular tissue except for a small bundle at the base. These are almost peg-like, stubby and are generally not considered true leaves, though they likely evolved from them. Members ofTmesipteris may appear to have leaves, but these are reallyphylloclades, or flattened stems.
Thesporangia of Psilotaceae are fused together into small and distinctive yellow balls called synangia (shown in the picture ofP. nudum above). These synangia are located off the stems of the plants. They contain two sporangia each inTmesipteris species, and three sporangia each inPsilotum species. A thicktapetum nourishes the developingspores, as is typical ofeusporangiate ferns like Psilotaceae.
In the molecular phylogenetic classification of Smith et al. in 2006, Psilotales, containing the single family Psilotaceae comprisingPsilotum andTmesipteris, was placed with the orderOphioglossales in the classPsilotopsida.[3] The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009)[4] which placed all land plants in Equisetopsida,[5] made it a member of subclassOphioglossidae, equivalent to Smith's Psilotopsida.[4] The placement of Psilotales in subclass Ophioglossidae has subsequently been followed in the classifications of Christenhusz and Chase (2014)[6] and PPG I (2016).[2]
In the pastTmesipteris has been placed in its own family, Tmesipteridaceae,[7] but this has not been maintained in the system of Smith et al. and later classifications.
