Psilotum (whisk fern)
Psilotum, commonly known as whisk fern, is a genus of primitive, fern-like plants that exhibit a unique structure, lacking traditional roots and leaves. This group of plants is considered a "living fossil" due to its resemblance to ancient plant forms from the age of dinosaurs, showing minimal evolutionary change over time. Psilotum is classified under the family Psilotaceae in the order Psilotales and includes three primary species: Psilotum nudum, Psilotum complanatum, and the hybrid Psilotum intermedium. Typically found in tropical and subtropical regions, including parts of Asia, Australia, and the southern United States, these plants thrive in various environments and can even be considered weeds in some areas.
The visible structure of Psilotum consists of slender, green, ribbed stems that resemble a whisk broom, with small, scale-like buds instead of true leaves. Instead of roots, the plants utilize rhizomes that anchor them and assist in nutrient absorption, supported by a symbiotic relationship with beneficial fungi. Psilotum is homosporous, producing spores that contain both male and female reproductive parts. The simplicity of its structure, lacking complex vascular systems found in most plants, suggests a closer relationship to ancient plant ancestors rather than modern species. Despite its ancient characteristics, the absence of fossil evidence for Psilotum itself remains a fascinating puzzle for researchers.
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Psilotum (whisk fern)
Psilotum is a genus, or biological classification, of a fern-like type of plant with some unique characteristics. The plants in this category are very primitive and do not include all the systems that most plants have. Psilotum do not have roots or traditional leaves and do not produce seeds. Plants in this genus very closely resemble plants from the age of the dinosaurs and do not show the type of evolution evident in many other plants. Because of this, it is considered by many to be a living fossil. There are three main species in the genus, including Psilotum nudum, Psilotum complanatum, and Psilotum intermedium, which is a hybrid of the others. The best known of these is the Psilotum nudum, which is also known as the whisk fern.
Background
The word psilotum is from Latin and is pronounced zi-lo-tum. It is believed to have come from the Greek word psiloton, which in turn is thought to have derived from the Greek psilon, which means "down" or "soft feather." Some sources cite a similar Greek word, psilos, which means "smooth" or "bald." Either Greek word would fit the description of psilotum, which has a flat structure made up of smooth green tubular protrusions that look like stems without true leaves.
The genus was named by Olof Swartz (1760–1818), a Swedish botanist. His identification and description was published in 1801 in Journal für die Botanik. Psilotum are found in the plant kingdom in the Pteridophyta division. They are classified as Psilotopsida, order of Psilotales, and family of Psilotaceae.
Psilotum grows about eighteen inches (one half meter) in height and is found in tropical and subtropical locations. The plants grow in Asia, the South Pacific Islands, Australia, New Zealand, the southernmost parts of Europe, and several other areas. In the United States, they are found growing between Florida and Texas, as well as in Hawaii. They grow freely enough in all these areas to be considered weeds. However, in Japan horticulturists have grown as many as one hundred variations of psilotum species, some of which are very sought after because of their unique appearance.
The visible part of psilotum consists of very thin, rounded, ribbed stems that are all green in color. Their flat, forked configuration looks somewhat like a skimpy whisk broom, giving the plant one of its nicknames. The plant has no true leaves but has small yellow scale-like buds. In place of roots, psilotum have rhizomes. Rhizomes are stems that grow under the ground and provide some support for the plant. These rhizomes have small hairlike protrusions that help anchor the plant and gather nutrients as roots would do. The rhizomes also attract a fungus called mycorrhiza. This fungus is more efficient at taking in nutrients and water than the psilotum and helps to provide for some of the plant's needs.
Because psilotum has no leaves, the plant's stems are full of chlorophyll. This allows psilotum to absorb the sunlight that is necessary for photosynthesis, the process by which plants generate energy. Psilotum is considered homosporous, which means it produces one kind of spores with both male and female parts for reproduction.
Overview
Despite the fact that it grows readily in a wide range of locations, psilotum occupies a small and unique place in the plant world. It is one of only two plant genera (plural of genus) in the psilotales order; this order has plants considered to be living fossils. It was given this description because it is a very simple plant that seems to not have evolved from prehistoric times.
Besides being rootless and leafless, the structures that support the plant are primitive compared to contemporary plants. They lack the complex vascular system, or system of tubes inside the plant that carry water and nutrients, of most plants. Plants usually have two parts to this system. These include a portion known as the xylem that carries water and the phloem for carrying sugars. Psilotum has the xylem but not the phloem. This is why the stems are green all over; the stems of the plants carry out some of the functions that would normally be done by leaves.
Scientists have been able to determine that psilotum is much more similar to the ancestors of many contemporary plants than it is to modern plants. However, one curious fact puzzles researchers: they have found no fossil evidence of psilotum. Even though it seems like these plants must have predated the plants that grow today, there is no evidence that they existed or that other plants evolved from this form. Their earliest known relatives are the Psilophytales, which appear in the fossil record of the Devonian era, which occurred from 359 to 416 million years ago in the Paleozoic era. It was during the Devonian era that plant life on land began to develop, with Earth seeing the formation of its first forests with large trees, ferns, horsetails, and other plants similar to psilotum.
Although the fossil record does not include any examples of psilotum, contemporary psilotum does resemble in some ways plants that grew in the Silurian era. This period from 416 to 436 million years ago was part of the Paleozoic era that occurred just after the Devonian era. Psilotum resembles plant types from the Cooksonia genus that developed during this time and eventually became extinct. Like psilotum, cooksonia were branched and leafless with small bud-like protrusions. They had a simple vascular system but were not thought to be photosynthetic. Cooksonia also produced spores.
Bibliography
Aiken, Miriam A. "Living Fossils of the Plant Kingdom." University of Victoria, Dec. 1951, nzetc.victoria.ac.nz/tm/scholarly/tei-Bio04Tuat02-t1-body-d2.html. Accessed 26 Nov. 2024.
Bora, Lily. Principles of Paleobotany. International Scientific Publishing Academy, 2010.
"Devonian Period." National Geographic, www.nationalgeographic.com/science/prehistoric-world/devonian/. Accessed 26 Nov. 2024.
"Ferns and Other Seedless Vascular Plants." LibreTexts, 31 Oct. 2023, bio.libretexts.org/Bookshelves/Introductory‗and‗General‗Biology/General‗Biology‗(Boundless)/25%3A‗Seedless‗Plants/25.04%3A‗Seedless‗Vascular‗Plants/25.4D%3A‗Ferns‗and‗Other‗Seedless‗Vascular‗Plants. Accessed 25 Nov. 2024.
"Introduction to the Psilotales." University of California Museum of Paleontology, www.ucmp.berkeley.edu/plants/pterophyta/psilotales.html. Accessed 26 Nov. 2024.
Strickberger, Monroe W. Evolution. Jones and Bartlett Learning, 2000.