The horsetails are a class in the Pteridophyta (ferns). They were one of the most important plant groups in the Palaeozoic era. 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" arose because the branched species somewhat resemble a horse's tail.