Equisetidae is one of the four
subclasses of
Polypodiopsida
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...
(ferns), a group of
vascular plant
Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
s with a fossil record going back to the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
. They are commonly known as horsetails.
[ They typically grow in wet areas, with whorls of needle-like branches radiating at regular intervals from a single vertical stem.
The Equisetidae were formerly regarded as a separate division of spore plants and called Equisetophyta, Arthrophyta, Calamophyta or Sphenophyta. When treated as a class, the names Equisetopsida s.s. and Sphenopsida have also been used. They are now recognized as rather close relatives of the ]fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...
s (Polypodiopsida) of which they form a specialized lineage.[ However, the division between the horsetails and the other ferns is so ancient that many botanists, especially paleobotanists, still regard this group as fundamentally separate at the higher level.
]
Description
The horsetails comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of leaves
A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
. In the only extant genus ''Equisetum'', these are small leaves (microphyll
In plant anatomy and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of ...
s) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of ''Equisetum'' probably arose by the reduction of megaphylls, as evidenced by early fossil forms such as ''Sphenophyllum
''Sphenophyllum'' is a genus in the order Sphenophyllales
Sphenophyllales is an extinct order of articulate land plants and a sister group to the present-day Equisetales (horsetails). They are fossils dating from the Devonian to the Triassi ...
'', in which the leaves are broad with branching veins.[
The ]vascular bundle
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will inc ...
s trifurcate at the nodes, with the central branch becoming the vein of a microphyll, and the other two moving left and right to merge with the new branches of their neighbours.[ The vascular system itself resembles that of the vascular plants' eustele, which evolved independently and convergently.][ Very rapid internode elongation results in the formation of a pith cavity and a ring of carinal canals formed by disruption of the primary ]xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
. Similar spaces, the vallecular canals are formed in the cortex.[ Due to the softer nature of the phloem, these are very rarely seen in fossil instances. In the ]Calamitaceae
Calamitaceae is an extinct family of plants related to the modern horsetails. Some members of this family attained tree-like stature during the Carboniferous Period (around ) and in Permian Period, reaching heights of up to 20 meters. The famil ...
, secondary xylem (but not secondary phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
) was secreted as the cambium grew outwards, producing a woody stem, and allowing the plants to grow as high as 10m. All extant species of ''Equisetum
''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds.
''Equisetum'' is a "living fossil", the only living genus of the entire subclass ...
'' are herbaceous, and have lost the ability to produce secondary growth.[
The underground parts of the plants consist of jointed ]rhizome
In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow ...
s, from which roots and aerial axes emerge. The plants have intercalary meristems in each segment of the stem and rhizome that grow as the plant gets taller. This contrasts with most seed plants, which grow from an apical meristem - i.e. new growth comes only from growing tips (and widening of stems).
Horsetails bear cones (technically '' strobili'', sing. strobilus
A strobilus (plural: strobili) is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but some botanists restrict the use of the term cone to th ...
) at the tips of some stems. These cones comprise spirally arranged