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• W4246012327 abstract "What is Equisetum? Equisetum, a genus whose species are collectively known as horsetails, are the living descendants of giant prehistoric trees that once dominated the Earth. These ancient giants remain with us today in the form of coal. The living species of Equisetum offer biologists a peek into the history and evolution of vascular land plants. Equisetum is the single extant genus of a class of anciently evolved land plants. Fossils of the ancient giants are found dating back to the Devonian, and in the late Devonian and Carboniferous periods, the world's forests were dominated by the calamites — huge tree-like horsetails that reached heights of up to 18 meters. These ancient forests were a far cry from forests in today's world — many large land animals had not evolved by this point, and fossil records indicate that giant insects had the run of the place! The calamite forests were so rich that they eventually formed the world's large coal deposits. As the climate grew drastically drier and colder in the Permian, concomitant with a change in atmospheric oxygen content, the giant calamites eventually died out, leaving only the herbaceous genus Equisetum in modern times. Equisetum species are almost identical to the ancient calamites in morphology, earning Equisetum the label of ‘living fossils’. Horsetails are found in various habitats around the world, though they usually prefer wet, swampy areas. Although there are examples of ‘giant’ horsetails (e.g. one Mexican species has been reported to reach heights of 8 meters), most horsetails are much smaller than their ancestors, and are usually around 1 meter or shorter. How would I know a horsetail if I saw one? Equisetum have a very distinctive appearance, growing in dense clusters as jointed, photosynthetic stems (Figure 1A ). These stems are sometimes branched, with whorls of branches appearing at the joints between segments. Horsetails have very small, scale-like leaves, called microphylls, which are usually non-photosynthetic and are hardly noticeable in some species. The internodes of the stems of horsetails contain a hollow pith, around which smaller hollow canals — called vallecular and carinal canals — are arranged, in close association with the xylem and phloem. One of the most recognizable features of horsetails is their ribbed stems, which are reinforced with silica deposits in the epidermal cells, giving the stem a very rough texture. The stems arise as branches from an underground rhizome. Although the stems tend to die off during winter or in the dry season, the rhizome is perennial. In fact, since removal of horsetails from a particular area requires the removal of the entire rhizome, horsetails can be very hard to eradicate, and are considered pests in some places. How do horsetails reproduce? Horsetails, like their close relatives the ferns, are evolutionarily ancient plants and don't produce seeds to protect and nourish their offspring. The sporophyte, or diploid, phase of the Equisetum life cycle is the dominant one and is represented by the herbaceous stems described above. Structures called strobili at the tips of fertile shoots house the developing haploid spores. In some cases, the fertile shoots look identical to the vegetative shoots (save for the strobili), but in others, the fertile shoots aren't photosynthetic at all and appear as white or brown shoots (Figure 1B). When the spores mature, specialized structures called elaters expand and contract during daily cycles of high and low moisture, helping to fling the spores from the plant. The bright green spores germinate, and the gametophyte, or haploid, phase of the life cycle begins. The photosynthetic gametophytes of Equisetum are small, flat, lobed structures (only a few millimeters wide), with rhizoids attaching them to the substratum. Eggs and sperm are produced on different lobes of the gametophyte. The sperm are flagellated and require water to swim to and fertilize the eggs, making a moist habitat essential for reproduction. Once fertilization is successful, a new diploid sporophyte will develop. What can horsetails tell us about the evolution of vascular plants? Though they are quite anomalous in form — most land plants have highly developed leaves to maximize photosynthetic surface area, while Equisetum leaves are practically vestigial as photosynthetic organs — horsetails, as well as the other seedless vascular plants, evolved at a time when the Earth was quite different. At the time when plants were beginning to colonize land, the climate was quite tropical, and ancient land plants had no need to develop ways to cope with water shortages. As a result, all of the ‘primitive’ vascular plants (seedless plants) have their reproductive cycles intimately dependent on water availability. As discussed in the section above, the gametophytes of these species, and of the other seedless vascular plants, produce flagellated sperm that require water to swim to the eggs. Once the climate turned drier and colder in many parts of the globe, and as atmospheric oxygen gradually rose, these ancient giants died out, and evolution of modern-day vascular seed-bearing plants took off in earnest — along the way developing different modes of reproduction, relying on wind and animal pollinators instead of on water. In the more recently evolved gymnosperms and angiosperms, the gametophyte stage of the life cycle has been reduced to a very small number of cells, which rarely ever even make contact with the ground. The angiosperms also evolved protective seeds, providing nutrients for the young embryo, allowing development and germination of offspring even in otherwise unfavorable conditions. The global climate change, along with concomitant changes in atmospheric content, resulted in a nearly complete overthrow of the seedless vascular plants, which once dominated the planet. The modern-day descendants of these giants, still dependent on water for half of their life cycle, are almost exclusively confined to swampy or tropical habitats. Equisetum, with their highly proliferative and resilient rhizomes, are more versatile than many species and can be found in various environments. How do humans use Equisetum? In many cultures, the rough stems of horsetails are dried and used to scour and polish metal pots and pans, earning them the name ‘scouring rush’ in some parts of the world. In addition, traditional Japanese woodworkers use dried Equisetum as a type of extra-fine sandpaper. Since Equisetum species can be found all over the globe (except Australasia), they have made their way into the traditional medicine practices of various cultures, dating back to ancient Greek and Roman times. The silica in the stems of horsetails is thought to aid in osteoporosis prevention, while extracts from horsetails are used for many different reasons, including as a diuretic, and as a source of natural antioxidants. Very few studies, though, have been performed to weigh the merit of these alternative medical uses for the plant. Interestingly, livestock have been known to become ill and even die after eating large quantities of Equisetum — for this reason, and because most commercial pesticides kill only seed-bearing plants, horsetails are considered to be tricky pests in some areas." @default.
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• W4246012327 date "2012-05-01" @default.
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• W4246012327 title "Equisetum" @default.
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• W4246012327 doi "https://doi.org/10.1016/j.cub.2012.04.005" @default.
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