Some species of Psilotum have a single vascular trace that terminates at the base of the enations. Consequently, Psilotum was long thought to be a "living fossil" closely related to early land plants (rhyniophytes). However, genetic analysis has shown Psilotum to be a reduced fern. It is not clear whether leaf gaps are a homologous trait of megaphyllous organisms or have evolved more than once. While the simple definitions (microphylls: one vein, macrophylls: more than one) can still be used in modern botany, the evolutionary history is harder to decipher. See also Vegetation classification References Plant physiology Plant anatomy Leaves