Whisk ferns (Psilotum) lack many of the characteristics of other vascular plants and based on their morphology were thought to be among the most primitive (ancestral) vascular land plants. We now know, based on DNA evidence, that they are actually really odd ferns. Same goes for Equisetum (horsetails). If you looked at a Biology textbook from 10-15 years ago, this is very different from what it would have told you.