cones. They are also known to have possessed massive underground rhizomes, which allowed for the production of clones of one tree. This is the only group of trees of their period known to have a clonal habit. This type of asexual reproduction would allow them to spread quickly into new territory, and help to anchor them firmly in the unstable ground along rivers and in newly deposited delta sediments. The rhizomes of Calamites look quite similar to the stems in most cases, but have nodes that get progressively closer together as they approach the apical area (the growth tip that spreads outward through the soil). Different forms Calamites come in a variety of different "form genera". One type, Calamites suckowi, is distinguishable from other Calamites forms by its prominent, swollen nodes and relatively wide-spaced longitudinal ribs. Another example, Calamites cisti, has much smaller nodes and the ribs are typically closer together. In addition, the distance between successive node lines on a Calamites suckowi specimen is typically much wider than the diameter. In other forms like Calamites cisti, the opposite is true or the specimen is just slightly wider than the diameter. However, the value of these form taxa is limited. The distance between nodes, for example, is highly variable, and an intercalary meristem means that this distance varied as the organisms grew. Extinction and classification The genus Calamites is placed in the family Calamitaceae in the plant class Equisetopsida (formerly known as Sphenophyta) in the fern allies division Pteridophyta. The Calamitaceae finally became extinct in the Lower Permian, a time which also saw the origin and diversification of the herbaceous genus Equisetum, the only living sphenophyte genus. See also Archaeopteris References How large are the giant horsetails? “Sphenophyta: Fossil Record.” Fossil Record of the Sphenophyta, ucmp.berkeley.edu/plants/sphenophyta/sphenofr.html. Stewart, W. N. Paleobotany and the Evolution of Plants. Cambridge University Press, Cambridge (1983). Davis, Paul and Kenrick, Paul. Fossil Plants. Smithsonian Books (in association with the Natural History Museum of London), Washington, D.C. (2004). Calamites, web.archive.org/web/20120315050635/www.scientific-web.com/en/Biology/Plants/Pteridophyta/Calamites.html. Horsetails Carboniferous plants Permian plants Prehistoric plant genera Carboniferous first appearances Cisuralian genus extinctions Fossils of Georgia (U.S. state) Paleozoic life of New Brunswick Paleozoic life of Newfoundland and Labrador Paleozoic life of Nova Scotia Paleozoic life of Nunavut Paleozoic life of Prince Edward Island Prehistoric plants of North America