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Psilotum is a genus of fern-like vascular plants.It is one of two genera in the family Psilotaceae commonly known as whisk ferns, the other being Tmesipteris.Plants in these two genera were once thought to be descended from the earliest surviving vascular plants, but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales.
Equisetum (/ ˌ ɛ k w ɪ ˈ s iː t əm /; horsetail) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. [2]Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests.
Psilotum has been interpreted as producing sporangia (fused in a synangium) on the terminus of a stem. Equisetum always produce strobili, but the structures bearing sporangia (sporangiophores) have been interpreted as modified stems. The sporangia, despite being recurved are interpreted as terminal.
In the only extant genus Equisetum, these are small leaves (microphylls) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of Equisetum probably arose by the reduction of megaphylls , as evidenced by early fossil forms such as Sphenophyllum , in which the leaves are broad with branching veins.
An interesting case is that of Psilotum, which has a (simple) protostele, and enations devoid of vascular tissue. Some species of Psilotum have a single vascular trace that terminates at the base of the enations. [2] Consequently, Psilotum was long thought to be a "living fossil" closely related to early land plants (rhyniophytes).
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 .
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Psilotum nudum, the whisk fern, [3] is a fernlike plant. Like the other species in the order Psilotales, it lacks roots. [4]Its name, Psilotum nudum, means "bare naked" in Latin, because it lacks (or seems to lack) most of the organs of typical vascular plants, as a result of evolutionary reduction.