Temporal range: Late Devonian to Recent
Equisetopsida, or Sphenopsida, is a class of plants with a fossil record going back to the Devonian. They are commonly known as horsetails. Living species typically grow in wet areas, with needle-like leaves radiating at regular intervals from a single vertical stem.
The Equisetopsida were formerly regarded as a separate division of spore plants and also called Equisetophyta, Arthrophyta or Sphenophyta; today they have been recognized as rather close relatives of the typical ferns (Pteridopsida) and form a specialized lineage of the Pteridophyta.
The Sphenophytes comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of leaves. In the only extant genus Equisetum, these are small leaves (microphylls) with a singular vascular trace. However, sphenophyte leaves probably arose by the reduction of a megaphyll, as evidenced by early fossil forms such as Sphenophyllum, in which the leaves are broad with branching veins. The plumbing of these leaves is interesting: the vascular traces trifurcate at the junctions, with one thread going to the microphyll, and the other two moving left and right to merge with the new branches of their neighbours. The vascular system itself curiously resembles that of the vascular plants' eustele, which evolved convergently. A primary xylem contains carinal canals; in the Calamitaceae, secondary xylem (but not secondary phloem) can be secreted as the cambium grows outwards, producing a woody stem, and allowing the plants to grow as high as 10m. The cortex itself contains valecular canals; due to the softer nature of the phloem, these are very rarely seen in fossil instances.
The plant does not bear a coherent root system but underground rhizomes, from which roots and aerial axes emerge.
The plant contains an intercalary meristem: that is to say, each segment of the stem grows as the plant gets taller. This contrasts with the seed plants, which contain an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Growth was determinate - i.e. the plants' phenotype dictated a maximum height, which the plant would grow to then get no higher.
Sphenophytes bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporophores, which bear spores in four clusters, and in extant sphenophytes cover the spores externally - like four sacs hanging from an umbrella, with its handle embedded in the central cone body. In extinct groups, further protection was afforded to the spores by the presence of whorls of bracts - big pointy microphylls protruding from the cone.
The spores themselves 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. Dispersal is aided in the first instance by laterally dehiscing sporangia, which pop open and scatter spores.
The extant horsetails represent a tiny fraction of Sphenophyte diversity in the past. There were three orders of Equisetopsid; the Pseudoborniales, which first appeared in the late Devonian. Second, the Sphenophyllales which were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian respectively. The Equisetales existed alongside the Sphenophyllales, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genus Equisetum.
The organisms first appear in the fossil record during the late Devonian, a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See Evolutionary history of plants) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to .
The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plants. 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 have assigned to the horsetails varies from order to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Sphenophyta, or Equisetophyta. Other authors have 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.
Recent phylogenetic analysis has produced evidence that this group of plants belongs firmly within the fern clade of vascular plants. A 2006 classification by Smith et al. places the class Equisetopsida within an unranked clade of broadly defined ferns, as a sister to two classes more traditionally called ferns, Marattiopsida and Polypodiopsida.
The probable relationships within Equisetopsida are shown in the cladogram below:
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