The whisk ferns, traditionally thought not to be true ferns, are odd, "primitive" vascular plants that reproduce solely by spores, without seeds. Recent evidence has suggested that they may in fact be ferns that have lost a number of pteridophytic characteristics, but their status is still uncertain.
There are two species, as listed in the taxobox, with a hybrid known: Psilotum x intermediumW. H. Wagner.
The distribution of Psilotum is tropical and subtropical, in the New World, Asia, and the Pacific. The highest latitudes known are in South Carolina and southern Japan for P. nudum.
Fern ally is a general term covering a somewhat diverse group of vascular plants that are not flowering plants and not true ferns.
Several groups of plants were considered "fern allies": the clubmosses, spikemosses, and quillworts in the Lycopodiophyta, the whiskferns in Psilotaceae, and the horsetails in the Equisetaceae.
In fact, the whiskferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade.
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