Leafless and rootless?
Yes, the little plants known as Whisk Ferns, or more accurately, Psilotum, really do not have any roots or leaves. They grow little green branched stems from small rhizomes that, when dug up, have no roots (see photo below, left). From my days of college botany, I remember being taught that these plants were "primitive" and among the remnant ancestors of vascular plants, or the plants that have vessels within to conduct water and nutrients throughout the plant. Paradoxically, they have survived quite well to this day and seem none the worse for wear. My conviction is that this ability to survive throughout the millions of years while so many other species have gone extinct is a distinctly advanced characteristic.
Psilotum has a life cycle very similar to ferns. They produce spores and have what is known as "alternation of generations", which in other terms is the alternating of a spore-bearing plant (sporophyte) with the gamete, or sex cell, bearing plant (gametophyte). The plant you see in the thumbnail above is the sporophyte, and what look like little seed capsules are actually the spore bearing structures or synangia. These synangia start out green and ripen to a yellowish color. Once ripe, they open and release tiny white spores that go on to germinate and produce a small underground rhizome-like plant whose only function is to produce the sex cells, or gametes, that will come together to produce another sporophyte plant.
Where did they come from?
Those scientists who study plants such as the Whisk Fern are frustrated by the lack of any fossil record relating to the origin of these plants. They seem to have no ancestors at all. Psilotum fossils dated to over 400 million years ago look much the same as the ones found growing today. This is problematical because if they are as primitive as some postulate, one would expect that at some distant time in the past, they were not only more abundant but far more diverse than they are now. This is not the case. So we are left wondering about their unknown origin.
For my part, I'm satisfied to grow them and enjoy them just for their very unusual character. The plants are structured in such a way that even though they look like a group of stems, they could just as well be stem-like leaves with spore cases on them! In fact, these stems do serve that function in the absence of more familiar types of leaves. They even have little pores, called stomata, on them just as familiar leaves do. Looking at them this way, Whisk Ferns could be a particularly adaptable type of fern, considering the apparent great length of time they have been present on this planet.
While we may know nothing about their evolutionary origin, Psilotum nudum is native to the United States and can be found from Arizona and Texas to Louisiana and Florida, and can be found occasionally as far north as South Carolina. Other members of the genus are tropical plants. The spores are small enough to be carried for significant distances, so even if you don't see any in your neighborhood, you might just find that one day you have one coming up in a pot alongside one of your plants. I never pull them up as they cannot hurt my plants. I feel that they have earned the right to grow wherever they happen to spring up.
Oh, just in case you were wondering how these plants grow without roots, the answer is that they have a symbiotic relationship with soil fungi called mycorrhizae. These special fungi serve in the same role as roots do, growing with the rhizomes and providing the plant with what it needs while the plant reciprocates in kind. The hairy protuberances you see on the rhizomes pictured are not true roots, but are called rhizoids.
Photo credit: Wikimedia Commons