Psilotum At Palomar College

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Psilotum On The Palomar College Campus
Latest Update W.P. Armstrong, 25 November 2021
A Little Plant Resembling The First Land Plants On Earth
I met a lot of interesting plants during my lifetime career in botany. Whisk fern (Psilotum nudum) is truly one of the most fascinating. It is a remarkably similar to ancient, extinct plants that lived more than 400 million years ago, long before the age of dinosaurs. New evidence from DNA comparisons and some serious HTML syntax errors have inspired me to revise this article several times.

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

1. Introduction

On a recent walk around the Palomar College campus (an official botanical garden), Grounds Services Supervisor and horticulturist extraordinaire Tony Rangel pointed out a small whisk fern (Psilotum nudum) along a walkway at the north end of campus, probably introduced with nursery stock. I was very surprised at this discovery because I had only seen this plant growing near the base of palms in Tahiti, on the Hawaiian island of Maui, and in southern Florida. It actually has a worldwide distribution in tropical and subtropical regions. I never expected to find one on the campus of Palomar College in San Marcos, CA! It is one of the most interesting plants I studied during my botany career. It has been called a living fossil because of its resemblance to a ancient line of primitive vascular plants dating back to the Devonian period, 400 million years ago. It belongs to the plant division Psilophyta, order Psilotales, family Psilotaceae. The evolution of water-conducting (vascular) tissue in primitive, non-flowering plants of the Devonian period has resulted in the colonization of Earth's continents with an astonishing variety of herbs, grasses, shrubs and trees.

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.

2. Danger Of Posting Misinformation About The Age Or Ancestry Of Psilotum nudum

Case In Point About Posting Misinformation
I was once misled by 2 books with false information that I thought was true and posted on Wayne's Word about the existence of coconut pearls. I later concluded that these so-called "pearls" were a hoax: Palms do not produce pearls of any kind. My point here is that once you post incorrect information on the World Wide Web, it is very difficult to retract it.

  The Authenticity Of Coconut Pearls  

Psilotum nudum at Palomar College. The green, leafless stems bear small, spherical, yellowish sporangia. There are no flowers, fruits or seeds. Upright, branched stems develop from underground stems called rhizomes. There are no true roots. The rhizomes absorb water and nutrients from the soil in association with the intricate mycelium of vital mycorrhizal fungi. When I first saw this plant I couldn't believe my eyes: Is this a modern-day remnant of an ancient plant group that predates most gymnosperms (cone-bearing trees) and dinosaurs? (See Section On Phylogenetic Relationships Of Psilophytes.)
It was growing with red flowering yucca (Hesperaloe parviflora, also spelled Hesperoaloe), native to Chihuahuan Desert of Texas and extending south into Mexico. It is not a true yucca. It is now placed in the asparagus family (Asparagaceae) along with the genus Aloe; however, many older references place it in the agave family (Agavaceae).

Another Psilotum nudum discovered at Palomar College near the Polynesian Garden. This plant often grows epiphytically at the base of other plants and was probably introduced with its Ti plant host. In this case it is growing on a Cordyline fruticosa. The genus Cordyline has been placed in several different plant families. It currently resides in the asparagus family (Asparagaceae). It is also known as the Ti plant. Polynesian people had many uses for the fibrous leaves of this plant.

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).

  Compare With Gametophyte & Sporophyte Generations In Fern Life Cycle  

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.

Image Reconstruction An Early Tree-Like Lycophyte

Petrified bark of an ancient scale tree (division Lycophyta) that lived in swampy lowlands during the Carboniferous period, approximately 390 million years ago. The vertical rows of leaf scars are similar to those of Sigillaria. Trees of this genus reached 100 feet (30 m) tall with a trunk diameter of 6.5 feet (2 m). Another genus of tree lycophytes (Cordaites) may have included the ancestors of modern conifers.

  Plant Life Through The Ages: Devonian Murals  

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.

3. Phylogenetic Relationships Of Psilophytes

Computer generated phylogenetic trees (cladograms) based on DNA sequencing have resulted in some fascinating discoveries on the evolution of life on Earth. Modern cladograms of animal and plant groupings show all taxa descending from a common ancestor. For example, molecular studies clearly indicate that the order Psilotales are positioned on a sister clade (branch) with the Ophioglossales, a primitive order of ferns. Although they have been considered “primitive,” recent developmental and molecular evidence suggests that the Psilotales may actually be reduced from fern-like ancestors. There is apparently some disagreement on the exact evolutionary position of Psilotales; however, DNA research has resulted in drastic changes in plant taxonomy. This latest revelation may change the evolutionary position of the Psilotales. Besides being a glimpse into the past, this little plant closely resembles early plants that lived long before the age of dinosaurs. It is a valuable research organism for students of plant evolution, especially using the modern tools of molecular biology.

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

4. Evolution & Geologic Time Scale For Psilophytes

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!

  Hornworts On The Road To Idyllwild!  

If you are wondering how far back in time is the Devonian period (400 million years ago), see the following geologic time scale that I hand-coded in html. Hornworts are mentioned in red because they greatly resemble some of the earliest land plants, such as Horneophyton & Rhynia. In fact, according to some authorities, Rhynia has characteristics similar to Psilotum: "Psilotum and Rhynia are anatomically quite similar, despite the latter being dead for 400 million years, and the former being very much alive today. They both have a horizontal stem called a rhizome, from which upright stems grow."

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Dominican Republic Amber (Miocene)   W. Indian Locust & Copal
Baltic Amber (Shores Of The Baltic Sea)
Avian Dinosaurs Survived K-T Mass Extinction
K-T Boundary--Giant Asteroid Hit Earth: Mass Extinction Of Dinosaurs
Hell Creek Formation, Mont.: Ficus ceratops = Spinifructus antiquus
Sauropod Dinosaur Dung In India With Phytoliths!
Metasequoia Fossils Resembling Present-Day "Dawn Redwood"
Ancient Flowering Plants: Nymphaeales (Water Lilies) & Magnoliales
Ants  Oldest Ant Fossils (Evolved From Stinging Wasps 110-130 mya)
Araucariads & Southern Supercontinent Called Gondwanaland
Origin Of Flowering Plants: Darwin's "Abominable Mystery"
Santiago Peak Metavolcanics: Owens Peak
Jet: Carbonized Remains Of Ancient "Araucariad" Forests
Gingko: Ancient Gymnosperm   Plants Of Jurassic Park
Pangea: A Giant Supercontinent   Continental Drift (Plate Tectonics)
Petrified Forest NP: Once At Latitude Of Cen. Amer.  Taxonomy Issue
Osmunda Fern Fossils Resembling Present-Day Species In Canada
Cycads & Conifers Flourish
300-360: Dominant Ferns, Horsetails, Club Mosses (Lycopods), Scale
Trees (cf. Sigillaria), Primitive Gymnosperms, Early Cycads, etc. Time
Of Extensive Swamp Forests & Massive Coal Formation.
Rhynie Chert, Scotland: Horneophyton & Rhynia--Earliest Land Plants
(Primitive, Leafless, Spore-Bearing Plants). Very Similar In Appearance
To Present-Day Hornworts (Division Anthocerotophyta: Anthoceros);

   [ Ancestors Of Primitive Vascular Plants Such As The Lycophytes ]
First Known Land Plants
Marine Algae Dominant
Algae & Invertebrates In Seas: Trilobites Dominant

5. Psilotum On The Hawaiian Island Of Maui

Aerial view of the west coast of Maui near Kahana.

Close-up view of the stem of a whisk fern (Psilotum nudum), a vascular plant with primitive characteristics resembling some of the early land plants on Earth. The sporangia were originally described as 3-lobed, but modern technical references consider them composed of 3 fused sporangia. Plants such as this (including related treelike forms according to my course in historical geology) were abundant in ancient swamps 300 to 400 million years ago.

6. Concluding Statement About Psilotum nudum

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!