Evolution and Systematics
The elaters of horsetail plants aid spore dispersal by coiling and uncoiling in response to humidity.
"Besides their distinctive stems and leaves, horsetails and calamites bear spores in cones produced at the tips of the stems or branches. Each cone is composed of many tightly fitting, polygonal scales attached to a central axis (Figure 66). On the inner surface of the scale are several oblong yellowish sporangia filled with green, photosynthetic spores. When the spores are ripe, the central axis of the cone elongates, separating the scales and exposing the sporangia to the air. Upon drying, the sporangia split lengthwise and release the spores, allowing them to be carried away by air currents. The spores are helped on their journey by four strap-like structures called elaters that catch the wind. The elaters coil and uncoil in response to changes in humidity. When the air is dry they extend outward and create wind resistance so that the spores float (Figure 67). When the air is humid the elaters coil around the spore so that buoyancy decreases and the spore drops--with luck onto moist soil where it can germinate. Elaters occur only in horsetails and calamites and are evidence of the close relationship between these plants." (Moran 2004:115)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:328
Specimens with Barcodes:319
Species With Barcodes:26
Evolution and systematics
Equisetaceae is the only surviving family of the Equisetales, a group with many fossils of large tree-like plants that possessed ribbed stems similar to modern horsetails. Pseudobornia is the oldest known relative of Equisetum; it grew in the late Devonian, about 375 million years ago and is assigned to its own order.
All living horsetails are placed in the genus Equisetum. But there are some fossil species that are not assignable to the modern genus. Equisetites is a "wastebin taxon" uniting all sorts of large horsetails from the Mesozoic; it is almost certainly paraphyletic and would probably warrant being subsumed in Equisetum. But while some of the species placed there are likely to be ancestral to the modern horsetails, there have been reports of secondary growth in other Equisetites, and these probably represent a distinct and now-extinct horsetail lineage. Equicalastrobus is the name given to fossil horsetail strobili, which probably mostly or completely belong to the (sterile) plants placed in Equisetites.
- Channing, A.; Zamuner, A.; Edwards, D.; Guido, D. (2011). "Equisetum thermale sp. nov. (Equisetales) from the Jurassic San Agustin hot spring deposit, Patagonia: Anatomy, paleoecology, and inferred paleoecophysiology". American Journal of Botany 98 (4): 680–697. doi:10.3732/ajb.1000211. PMID 21613167.
- Weber, Reinhard (2005): Equisetites aequecaliginosus sp. nov., ein Riesenschachtelhalm aus der spättriassischen Formation Santa Clara, Sonora, Mexiko [Equisetites aequecaliginosus sp. nov., a tall horsetail from the Late Triassic Santa Clara Formation, Sonora, Mexico]. Revue de Paléobiologie 24(1): 331-364 [German with English abstract]. PDf fulltext
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