seed plants, which grow from an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Horsetails bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporangiophores, which bear sporangia at their edges, and in extant horsetails cover the spores externally - like sacs hanging from an umbrella, with its handle embedded in the axis of the cone. In extinct groups, further protection was afforded to the spores by the presence of whorls of bracts - big pointed microphylls protruding from the cone. The extant horsetails are homosporous, but extinct heterosporous species such as Calamostachys casheana appear in the fossil record. The sporangia open by lateral dehiscence to release the spores. The spores bear characteristic elaters, distinctive spring-like attachments which are hygroscopic: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal. Taxonomy Classification The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plants, such as the ferns (Polypodiopsida). Before the advent of modern molecular studies, the relationship of this group to other living and fossil plants was considered problematic. Because of their unclear relationships, the rank botanists assigned to the horsetails varied from order to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Calamophyta, Sphenophyta, or Equisetophyta. Other authors regarded the same group as a class, either within a division consisting of the vascular plants or, more recently, within an expanded fern group. When ranked as a class, the group has been termed the Equisetopsida or Sphenopsida. Modern phylogenetic analysis, back to 2001, demonstrated that horsetails belong firmly within the fern clade of vascular plants. Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, and considered the ferns (monilophytes), to comprise four classes, with the horsetails as class Equisetopsida sensu stricto. (This distinction is necessary because of the alternative usage of Equisetopsida sensu lato as a synonym for all land plants (Embryophyta) with rank of class.) Chase and Reveal (2009) treated the horsetails as subclass Equisetidae of class Equisetopsida sensu lato. The consensus classification produced by the Pteridophyte Phylogeny Group in 2016 also places horsetails in the subclass Equisetidae, but in the class Polypodiopsida (ferns broadly defined). Phylogeny The following diagram shows a likely phylogenic relationship