Primitive tracheophyte plants represented by the extant genus Psilotum. Along with the sole species of the related genus Tmesipteris and numerous extinct forms that flourished in the Devonian period, they make up the family Psilotaceae. Whisk ferns have rhizoids rather than roots; the stems, which show dichotomous branching, may be naked or bear scalelike leaves. Recent molecular genetic evidence shows that they belong to the clade of monilophytes, which also contains the horsetails and true ferns.