Historically, the Equisetales included a small number of taxa that were thought to be related to one another on the basis of their presumed herbaceous habit and lack of secondary tissues. The order Calamitales was used for those genera that were arborescent and produced secondary tissues.
Comprehensive studies of several equisetophyte genera, however, have suggested that such features as habit, spore morphology, bracteate versus nonbracteate cones, and the degree of fusion of the leaves can no longer be used to separate the two groups (Good, 1975). Accordingly, all the genera in these two groups are now included within a single order, the Equisetales. This combination is supported by the presence of similar anatomy and morphology in the two groups. Both calamites and equisetaleans produce elaters on their spores and possess similar primary vascular structure and meristematic organization of the primary axes.
The Equisetales are characterized by the presence of distinct nodes, internodes, and longitudinal internodal ridges on their stems. Branches and leaves are borne in distinct whorls; leaves are connate or free and vascularized by a single, undivided vein. The underground portion of the plant usually consists of a rhizome bearing numerous adventitious roots. Apical meristems contain a single, large apical cell and intercalary meristems are present at the base of each internode which also produces growth in length. Vascular tissue is organized into bundles that alternate at successive nodes. Each bundle includes a protoxylem canal in the internode. Sporangia are recurved and organized into strobili that may or may not contain sterile bracts. Spores possess superficially attached elaters. The geologic history of the Equisetales can be traced from the Devonian to the recent, with the most conspicuous components of the flora present during the Carboniferous (Jongmans, 1911). In this chapter, the order is separated into four families: the Calamitaceae, Tchernoviaceae, Gondwanostachyaceae, and the Equisetaceae.
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