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Equisetaceae
Fossil range: Late Devonian-Recent
375.0–0 Ma
"Candocks" of the Great Horsetail (Equisetum telmateia telmateia), showing whorls of branches and the tiny dark-tipped leaves
"Candocks" of the Great Horsetail (Equisetum telmateia telmateia), showing whorls of branches and the tiny dark-tipped leaves
Scientific classification
Kingdom: Plantae
Division: Pteridophyta
Class: Equisetopsida
Order: Equisetales
Family: Equisetaceae
Genera

Equisetum
and see text

Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. They are commonly known as horsetails.

In fact, Equisetum is the only surviving genus of the entire class Equisetopsida, which for over one hundred million years was very diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 mverification needed tall; the genus Calamites of family Calamitaceae for example is abundant in coal deposits from the Carboniferous period.

A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally misidentified and misnamed as "horsetail".

Contents

Etymology

Microscopic view of Rough Horsetail, Equisetum hyemale (2-1-0-1-2 is one millimeter with 1/20th graduation).
The small white protuberances are accumulated silicates on cells.

The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble a horse's tail. Similarly, the scientific name Equisetum derives from the Latin equus ("horse") + seta ("bristle").

Other names include candock for branching individuals, and scouring-rush for unbranched or sparsely branched individuals. The latter name refers to the plants' rush-like appearance, and to the fact that the stems are coated with abrasive silicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made of tin. In German, the corresponding name is Zinnkraut ("tin-herb"). Rough Horsetail E. hyemale is still boiled and then dried in Japan, to be used for the final polishing process on woodcraft to produce a smoother finish than any sandpaper.

Distribution and ecology

The genus Equisetum is near-cosmopolitan, being absent only from Australasia and Antarctica. They are perennial plants, either herbaceous and dying back in winter as most temperate species, or evergreen as most tropical species and the temperate species Rough Horsetail (E. hyemale), Branched Horsetail (E. ramosissimum), Dwarf Horsetail (E. scirpoides) and Variegated Horsetail (E. variegatum). They mostly grow 0.2-1.5 m tall, though the "giant horsetail" are recorded to grow as high as 2.5 m (Northern Giant Horsetail, E. telmateia), 5 m (Southern Giant Horsetail, E. giganteum) or 8 m (Mexican Giant Horsetail, E. myriochaetum), and allegedly even more1.

Many plants in this genus prefer wet sandy soils, though some are semi-aquatic and others are adapted to wet clay soils. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. The Field Horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is also unaffected by many herbicides designed to kill seed plants.

Vegetative stem:
B = branch in whorl
I = internode
L = leaves
N = node

If eaten in large quantities, the foliage of some species is poisonous to grazing animals, including (somewhat ironically given its common name) horses2. On the other hand some speciesverification needed are cooked and eaten by humans in Japan.

Anatomy

Strobilus of Northern Giant Horsetail (Equisetum telmatei braunii), terminal on an unbranched stem.

In these plants the leaves are greatly reduced and usually non-photosynthetic. They contain a single, non-branching vascular trace, which is the defining feature of microphylls. However, it has recently been recognised that horsetail microphylls are probably not primitive like in Lycopodiophyta (clubmosses and relatives), but rather advanced adaptations, evolved by the reduction of a megaphyll3. They are therefore sometimes actually referred to as megaphylls to reflect this homology.

The leaves of horsetails grow in whorls fused into nodal sheaths. The stems are green and photosynthetic, and distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6-40 ridges). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except being smaller and more delicate.

Spores

The spores are borne under sporangiophores in strobili, cone-like structures at the tips of some of the stems. In many species the cone-bearing stems are unbranched, and in some (e.g. Field Horsetail, E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic sterile stems. In some other species (e.g. Marsh Horsetail, E. palustre) they are very similar to sterile stems, photosynthetic and with whorls of branches.

Horsetails are mostly homosporous, though in the Field Horsetail smaller spores give rise to male prothalli. The spores have four elaters that act as moisture-sensitive springs, assisting spore dispersal after the sporangia have split open longitudinally.

Evolution and systematics

The Equisetopsida were formerly regarded as a separate division of spore plants and also called 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.4

As mentioned above, all living horsetails are placed in the genus Equisetum. But there are some fossil species that are not assignable to the modern genus:

  • Pseudobornia contains the oldest known Equisetaceaeverification needed; it grew in the late Devonian, about 375 million years ago.
  • Equisetites is a "wastebin taxon" uniting all sorts of large horsetails from the Mesozoic; it is almost certainly paraphyletic and would probably warrant to be 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.5

Species

The living members of the genus Equisetum are divided into two distinct lineages, which are treated as subgenera. Hybridogenic species are common, but such hybridization has only been recorded between members of the same subgenus.6

In addition, there are numerous ill-determined populations. One of them, the Kamchatka Horsetail ("Equisetum camtschatcense"verification needed), is an ornamental forming imposing stands of these archaic plants.

Kamchatka Horsetail in Parc floral de Paris
Subgenus Equisetum
Branched Horsetail (E. ramosissimum)
Subgenus Hippochaete

Named hybrids

Equisetum × moorei (Rough Horsetail × Branched Horsetail)
Hybrids between species in subgenus Equisetum
Hybrids between species in subgenus Hippochaete

See also

Footnotes

  1. ^ Husby (2003)
  2. ^ Israelsen et al. (2006)
  3. ^ Rutishauser (1999)
  4. ^ Pryer et al. (2004), Smith et al. (2006)
  5. ^ Weber (2005)
  6. ^ Pigott (2001)

References

  • Husby, Chad E. (2003): How large are the giant horsetails? Version of 2003-MAR-19. Retrieved 2008-NOV-20.
  • Israelsen, Clark E.; McKendrick, Scott S. & Bagley, Clell V. (2006): Poisonous Plants and Equine. PDF fulltext
  • Pigott, Anthony (2001): National Collection of Equisetum – Summary of Equisetum Taxonomy. Version of 2001-OCT-04. Retrieved 2008-NOV-20.
  • Pryer, K.M.; Schuettpelz, E.; Wolf, P.G.; Schneider, H.; Smith, A.R. & Cranfill, R. (2004): Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. Am. J. Bot. 91(10): 1582-1598. PDF fulltext
  • Rutishauser, Rolf (1999): Polymerous Leaf Whorls in Vascular Plants: Developmental Morphology and Fuzziness of Organ Identities. International Journal of Plant Sciences 160(Supplement 6): 81–103. doi:10.1086/314221 PDF fulltext
  • Smith, Alan R.; Pryer, Kathleen M.; Schuettpelz, Eric; Korall, Petra; Schneider, Harald & Wolf, Paul G. (2006): A classification for extant ferns. Taxon 55(3): 705-731. doi:10.1086/314221 PDF fulltext
  • 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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