- This article is about the fern ally Genus
Equisetum. The flowering plant Mare's tail (Hippuris
vulgaris), unrelated to the Genus Equisetum, is also known as "horsetail".
| Horsetail |
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Vegetative stem with (at each node) a whorl of branches and dark-tipped leaves
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| Scientific classification |
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| Species |
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- Subgenus Equisetum (Horsetails)
- Equisetum arvense - field horsetail or common horsetail
- Equisetum bogotense
- Equisetum diffusum
- Equisetum fluviatile - water horsetail
- Equisetum palustre - marsh horsetail
- Equisetum pratense - shade horsetail
- Equisetum sylvaticum - wood horsetail
- Equisetum telmateia - great horsetail
- Subgenus Hippochaete (Scouring rushes)
- Equisetum giganteum - giant horsetail
- Equisetum myriochaetum - mexican giant horsetail
- Equisetum hyemale - rough horsetail or scouring rush
- Equisetum laevigatum - smooth horsetail
- Equisetum ramosissimum - branched horsetail
- Equisetum scirpoides - dwarf horsetail
- Equisetum variegatum - variegated horsetail
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The horsetails and scouring-rushes comprise 15 species of plants in the Genus
Equisetum. This genus is the only one in the Family
Equisetaceae, which in turn is the only family in the Order Equisetales and the Class Equisetopsida. This class is
sometimes placed as the sole member of a division: Equisetophyta, though
some authorities place it instead in the Division Tracheophyta or
Archeophyta. The plants in the Genus Equisetum are considered fern
allies. Other classes and orders of Equisetophyta are known from fossils, but all are extinct so far as is known.
Vegetative stem: N(ode), I(internode), B(ranch in whorl), L(=fused megaphylls)
The genus is near-cosmopolitan, being absent only from Australasia. They are winter-deciduous (temperate species) or evergreen (some tropical
species), and are mostly 0.2-1.5m tall, though E. telmateia can exceptionally reach 2.5m, and the tropical American
species E. giganteum 5m, and E. myriochaetum 8m.
These are plants without conspicuous leaves, but with hollow, jointed, ascending stems that may or may not have side-branches
radiating out from the nodes, depending on species. The stem is ridged and grooved, with from (3-)6-40 ridges. The leaves are
minute, pointed-triangular, and form in a whorl at each node on the stem; there is one leaf for each ridge on the stem.
Usually, the name horsetail is applied to the branching species, while the name
scouring-rush is applied to the unbranched or sparsely branched species. The name horsetail arose because it was
thought that the stalk resembled a horse's tail; the name Equisetum means "horse hair". The name scouring-rush refers
both to its rush-like appearance and to the fact that the stems accumulate silica and were used for scouring dishes in the
past.
Strobilus terminal on an unbranched stem
The spores are borne in a cone-like structures (strobilus, pl.
strobili) at the tip of some of the stems. These reproductive stems are often unbranched, and in some species are
non-photosynthetic and produced early in spring separately from photosynthetic sterile stems. Horsetails are mostly homosporous, though in E. arvense,
smaller spores give rise to male prothalli. The eusporangia have an annulus that
act as a moisture-sensitive spring, ejecting the spores through a weak spot of the sporangia.
Many plants in this genus prefer sandy soils, though some are aquatic and others adapted to wet clay soils. One horsetail,
E. arvense, can be a nuisance weed, because it readily regrows after being pulled
out, as the stalk-producing rhizome is deep underground and almost impossible to dig
out. It is also unaffected by many herbicides designed to kill seed plants. The foliage is poisonous to grazing animals if eaten in large
quantities.
The horsetails were a far larger and more diverse group in the distant past before the evolution of seed plants. In the Carboniferous period, they
included large trees reaching to 30m, with the Genus Calamites (Family
Calamitaceae) abundant in coal deposits.
External links
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