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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.
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.
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
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).
Psilotum complanatum, the flatfork fern, [1] is a rare herbaceous epiphytic fern ally in the genus Psilotum. There is some evidence that it might be a true fern that has lost some typically fern-like characteristics. [ 2 ]
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Leafless, dichotomously branching fossils bearing spines and possessing vascular tissue from the Devonian of Gaspé Peninsula, Canada, were thought by Dawson in 1859 to resemble the modern whiskfern, Psilotum. Accordingly, he named his new genus Psilophyton, the type species being P. princeps.
The stem is either underground or aerial. The leaves may be microphylls or megaphylls . Their other common characteristics include vascular plant apomorphies (e.g., vascular tissue ) and land plant plesiomorphies (e.g., spore dispersal and the absence of seeds ).