|Tmesipteris elongata at Otira, New|
Zealand. Looks pretty normal, right?
Sometimes, though, diversity isn't exactly what is needed, and plants simplify instead. Tmesipteris is a genus of unusual, "primitive" ferns, found only in a few scattered places in the South Pacific. To a casual eye, it looks as though it has all the usual parts of a fern -- roots, stems, leaves, and even the sort of spore capsules you'd expect on such an ancient lineage. It takes a much closer look, by which I mean microscopes and developmental analysis, to realize that something is odd there.
Those broad green structures along its stem, the ones that do most of its photosynthesizing? They're not leaves. The thready bits at its base, the ones that attach it to the bark where it lives? They're not roots.
Every single part of a Tmesipteris plant is, if you look closely enough, a modified stem.
At some point in the evolutionary past, this plant's ancestors found that leaves, roots, the whole shebang, were all -- for whatever reason -- unnecessary. As I pointed out before, evolution ditches unnecessary things pretty quickly. Why bother with leaves when you can just grow flat bits of stem, and they'll photosynthesize just fine? Why bother with roots when you can just grow thready stem-anchors, and they'll keep you nailed down just as well as you need? Other plants have found plenty of reasons to bother, but for Tmesipteris, there just wasn't a need. It's gotten along very well this way for something like 400 million years, thankyouverymuch, and it has no reason to change now.
Image source: Liefting, Alan [Public domain]. Tmesipteris elongata. Retrieved February 12, 2012, from Wikimedia Commons: <http://commons.wikimedia.org/wiki/File:Tmesipteris_elongata.jpg>