Three hundred million years ago, a walk through a lowland forest or swamp, or along a riverbank or floodplain, would have revealed a great variety of trees, even though the familiar conifers and flowering plants of today were not present. Prominent among the trees of the time, and also common in the shrubbery, were plants in the Sphenophyta, distinguished by their straight stems with branches or leaves arranged in regular whorls. Some Paleozoic sphenophytes grew up to thirty meters tall (nearly 100 feet).
Today, the sphenophytes consist of only one genus, Equisetum, with about thirty living species known worldwide. A few species, like the tropical Mexican species pictured above, may reach ten feet in height, but most living species are small, weedy plants -- remnants of former glory. Equisetum are known as horsetails, foxtails, or scouring rushes -- this last name is derived from the fact that Equisetum stores granules of silica within its cells, making it an effective tool for scrubbing pots and polishing wood. These plants are sometimes pestiferous weeds, and are somewhat toxic to livestock, but they also have medicinal value: Equisetum was used in traditional native American and Ukrainian medicine to stop bleeding, and recent research has shown that Equisetum is also effective as a diuretic.
See images of living sphenophytes at the University of Wisconsin.