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1.   Introduction
   
2.   Misinformation On Internet
   
3.   Phylogenetic Relationships
   
4.   Evolution & Geologic Time Scale
   
5.   Psilotum nudum On Maui
   
6.   Concluding Statement

In 1959, the famous paleobotanist from UCLA, Daniel Axelrod, wrote an article entitled "The Evolution of the Psilophyte Paleoflora" (Evolution Vol.13). He described the evolutionary significance of the most simple of known vascular plants, notably the "true" psilophytes of the Devonian, Rhynia, Horneophyton, and Psilophyton. This is essentially what I learned in my botany classes at Cal State Univ. Los Angeles, and it was the basic theme of the original version of this article about Psilotum nudum: Modern day relative of ancient land plants. Recently, quite by chance, I came across an on-line reference referring to Psilotum nudum as an imposter. It was broadcast on NPR's "Day to Day" radio program (October 5, 2007). Once thought to be descendents of early vascular plants (the division Psilophyta of the Devonian period), the order Psilotales (including Psilotum) have now been shown by molecular phylogenetics to be members of the fern subclass Ophioglossidae. Although still called a "living fossil" in the literature, the classification of Psilotum as a fern by reduction of characteristics, and its lack of a fossil record, indicates that it may not be as old as previously thought.

According to the University of British Columbia Botanical Garden website, plants of the Psilotales, such as Psilotum nudum, are recognized as the most primitive vascular plants currently living on our planet. Psilotum species are known as whisk ferns, and although they do not appear in the fossil record, they share characteristics with extinct flora like Aglaophyton. The Rhynie Chert in Scotland is a sedimentary rock formation that contains a variety of fossilized plants and animals from the early Devonian period, about 400-412 million years ago. It contains very well-preserved specimens of early vascular plants like Aglaophyton, which, like today’s Psilotum, had no true leaves or roots. It also had underground stems (rhizomes), sporangia, and a dichotomous branching pattern like Psilotum. Additionally, there is evidence that Aglaophyton had a symbiotic relationship with mycorrhizal fungi that helped it absorb nutrients. In fact, fungi often associate with present-day members of the Psilotales in the same way. Because of their similarities to Devonian flora, whisk ferns are uniquely significant for research purposes.

When I started this article I thought Psilotum nudum was a modern-day, living representative of an ancient plant like Aglaophyton; however, appearances can be deceiving, especially with all the recent DNA comparison evidence. The placement of my favorite plant group, the minute duckweeds, into the arum family (Araceae) is proof of how DNA research is changing plant taxonomy. In addition, no fossils of Psilotum nudum have ever been found, at least as of November 2021. This is a very complicated subject, and the close relationship of Psilotales with certain fern groups has made me re-evaluate my original preconceived conclusions. I certainly do not want to perpetuate misinformation on the Internet. All of the latest information about Psilotum nudum has made the subject even more fascinating in my opinion.

Note: This conspicuous (visible) part of the Psilotum life cycle is the diploid sporophyte (spore-producing) generation. Like other primitive vascular plants, including ferns, horsetails (Equisetum), spike-mosses (Selaginella), quillworts (Isoetes), and pillworts (Pillularia), Psilotum has a minute, haploid sexual generation called the gametophyte. Sadly, many of these primitive vascular plants once grew in vernal pools of the San Marcos area, but most have been replaced by shopping malls and housing developments in the name of progress. Like ferns, the Psilotum gametophyte bears sperm-bearing antheridia and egg-bearing archegonia, but unlike ferns, it is nonphotosynthetic. It is minute, only 2 mm in length, and you will probably go through your entire life on this planet without ever seeing one. The sexual phase of Psilotum is beyond the scope of this article; however, I have the following generalized, simplified illustration of a Psilotum gametophyte, and a link to the fern life cycle on Wayne's Word for a comparison. Although the sex organs are similar, the fern gametophyte (prothallus) is heart-shaped and green (photosynthetic).

If we could have wandered about on earth in the early Devonian period, we would probably have seen plants resembling the whisk fern, Psilotum and tree-like versions of club-mosses (lycophytes). In fact, dioramas of that time period often show much taller versions of our modern-day whisk fern. Psilotum has virtually no leaves and no roots. Minute, nonvascular protuberances from the stem are called enations. Above ground, photosynthetic stems branch from the subterranean underground stems (rhizomes). Most references also mention minute rhizoids which apparently function like root hairs. Interestingly, the whisk ferns have developed mycorrhizal associations, perhaps they are necessary in the absence of true roots.

Another group of primitive vascular plants are the lycophytes (division Lycophyta), including our present-day club-mosses Lycopodium and Selaginella. These ancient lineages also date back to the Devonian period, possibly predating the psilophytes. Curiously enough, there are 2 species of Selaginella in the local hills of coastal sage scrub north and east of Palomar College, including Owens Peak. Equisetophytes, tree-like ancestors of our modern day horsetails (Equisetum), were also present in the Devonian flora. At the following link see the beautiful Devonian murals on the website of Smith College Botanical Garden: Plant Life Through The Ages. Imagine yourself walking in these primeval landscapes more than 400 millennia before the present.

Close-up view of the stem of a whisk fern (Psilotum nudum) on the Palomar College campus. It has 3-lobed sporangia in the axils of leaf-like protuberances called enations. Technically, the sporangium is called a synangium formed by the fusion of 3 sporangia. White dots on the green photosynthetic stems are numerous pores called stomata, each pore is flanked by a pair of guard cells. Plants need stomata for gas exchange (oxygen & carbon dioxide). Without guard cells, land plants could not control water loss (transpiration) through their stomata. They would be restricted to moist habitats.

Paired guard cells was a critical step in the evolution of vascular plants. An opening or stoma develops between the inflated (turgid) guard cells due to a differential thickening of their walls. When the guard cells lose water pressure on a hot day, they deflate and push together, thus closing off the stoma and reducing water loss (transpiration) through the leaf or stem. This adaptation has enabled vascular plants to colonize desert habitats with high temperatures and relatively low rainfall.

I saw my first fern in the primitive fern order Ophioglossales at the endangered vernal pools at Miramar Naval Air Station (now Marine Corps Air Station Miramar) when I first moved to San Diego County. Later, botanist James Dillane showed me this seldom-seen fern, called the adder's tongue fern (Ophioglossum californicum), in the ashes following the huge Paradise Fire of October 2003 in Escondido.

A seldom-seen adder's tongue fern (Ophioglossum californicum) following the Paradise Fire in Escondido (October 2003). DNA cladistic studies clearly indicate the order Ophioglossales belongs to a sister clade with the Order Psilotales.   The Huge Paradise Fire   In Escondido Oct. 2003

Cladistic analyses suggest that the hornworts (Anthocerotophyta) originated much earlier in the history of land plants, possibly before the Devonian. Fossil bryophytes are scarce, but some bryophyte-like fossils have been found in Carboniferous deposits (300 million years ago) and older. Hornworts may even be one of the earliest lineages of land plants. Some of their morphological characteristics are similar to sporophytes of the ancient land plant Horneophyton that lacked true vascular tissue. In fact, some paleobotanists have suggested that Horneophyton may be the "missing link" between hornworts and the Rhyniopsida, an extinct class of early vascular plants. Fossils of these ancient land plants are well represented in the early Devonian Rhynie Chert beds in Aberdeenshire in the north of Scotland. This time period is approximately 398-416 million years ago. Hornworts are certainly one of the most unusual and fascinating plant groups on earth. On a road trip from Banning to Idyllwild with Tom Chester and James Dillane, we stopped at a roadside seepage area and, lo and behold, we found a population of hornworts!

After all the research I did for this short article, about the only true statement I can say about Psilotum nudum regarding the evolution of plants on Earth: It is a fascinating little vascular plant with a remarkable resemblance to some of the first land plants to colonize this planet long before the age of dinosaurs!