The horsetails comprise 15 species of plants in the genus Equisetum. This genus is the only one in the family Equisetaceae, which in turn is the only family in the order Equisetales and the class Equisetopsida. This class is now usually placed as the sole member of the Division Equisetophyta, though some authorities place it instead in the Division Tracheophyta or Archeophyta. The plants in the genus Equisetum are considered fern allies. ...