For over 100 million years they were varied, and dominated the understorey of late Paleozoic forests. They are seen in the coal measures of the Carboniferous period, and some were trees reaching up 30 metres. The group is now almost extinct, but one genus survives. They are vascular plants that reproduce by spores and not by seeds. The name "horsetail" came because the branched species somewhat look like a horse's tail.
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