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How many living horsetail species belong to the genus Equisetum?
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Equisetopsida
Giant horsetail of Europe (Equisetum telmateia).
There are 15 living species of horsetails that belong to the genus Equisetum. These plants are the sole surviving members of the class Equisetopsida, a group of primitive, spore-bearing vascular plants that were once much more diverse and included tree-sized species during the Carboniferous Period.1