Division Sphenophyta (Horse Tails)
This is a group consisting of one extant family, Equisetaceae, and one genus with fifteen species. Morphologically the shoot consists of distinct nodes and internodes. The leaves are whorled and scale-like. In some species whorls of stems develop at each node. The strobilus consists of a central axis on which spore-bearing structures (sporangiophores) are arranged. These are umbrella-like with sporangia attached on the underside around the central stalk. While Equisetum is homosporous, fossil members of the phylum were heterosporous, and Equisetum itself has a tendency to produce unisexual gametophytes. Spores are associated with elators. The gametophytes are photosynthetic and fairly easy to grow on a simple autotrophic medium.
The Sphenophyta is represented in the fossil record from the Devonian; the order Equisitales from the upper Devonian. Fossil members constitute a diverse group of both herbaceous and tree-like forms. There are amazing similarities in stem anatomy between Equisetum and arboreal forms from the Carboniferous. Shared anatomical features include carinal canals associated with the primary xylem for each vascular bundle, and a hollow pith. Aditionally, both Equisetum and extinct members of the Equisetales have elators assiciated with their spores.