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Sphenophyta (Arthrophyta) A phylum of tracheophyte plants, the only living members of which are the horsetails (Equisetum). Horsetails have a perennial creeping rhizome supporting erect jointed stems bearing whorls of thin leaves. Spores are produced by terminal conelike structures. The group has a fossil record extending back to the Palaeozoic with its greatest development in the Carboniferous period, when giant tree forms were the dominant vegetation with the Lycophyta.