A vessel element is one of the cell types found in xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperm

Gymnosperm

The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...

s such as the conifers.

Vessel elements are the building blocks of vessels, which constitute the major part of the water transporting system in the plants where they occur. Vessels form an efficient system for transporting water (including necessary minerals) from the root to the leaves and other parts of the plant.

In secondary xylem, a vessel element originates from a fusiform initial in the cambium

Cambium (botany)

A cambium , in botany, is a tissue layer that provide undifferentiated cells for plant growth. It forms parallel rows of cells, which result in secondary tissues....

, at maturity the protoplast dies and disappears, but the lignified cell walls persist. It may be seen as a dead cell, which still has a function, and is still being protected by surrounding living cells.

The cell wall is strongly lignified. At both ends there are openings that connect the individual vessel elements. These are called perforations or perforation plates. These perforations may have a variety of shapes: the most common are the simple perforation (a simple opening) and the scalariform perforation (several elongated openings on top of each other in a ladder-like design). Other types include the foraminate perforation plate (several round openings) and reticulate perforation plate (net-like pattern, with many openings). The side walls will have pits, and may have spiral thickenings.

The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the flowering plants. It was once thought that vessel elements were an evolutionary innovation of flowering plants, but their absence from some basal angiosperms and presence in some members of the Gnetales suggest that this hypothesis must be re-examined; vessel elements in Gnetales may not be homologous with those of angiosperms, or vessel elements may have originated in a precursor to the angiosperms and were subsequently lost in some archaic or "basal" lineages (e.g., Amborellaceae

Amborellaceae

Amborella is a genus of rare understory shrubs or small trees endemic to the island of New Caledonia. The genus consists of only a single species, Amborella trichopoda, and is the only member of the family Amborellaceae. Wood of Amborella lacks the vessels characteristic of most flowering plants...

, Tetracentraceae, Trochodendraceae

Trochodendraceae

Trochodendraceae is a family of flowering plants with two living genera found in southeast Asia. The two living species share the feature of secondary xylem without vessels, which is quite rare in angiosperms...

, and Winteraceae

Winteraceae

The Winteraceae are a family of flowering plants. The family includes 120 species of trees and shrubs in 9 genera.The Winteraceae are a mostly southern-hemisphere family associated with the Antarctic flora, found in tropical to temperate climate regions of Malesia, Oceania, eastern Australia, New...

), described by Arthur Cronquist

Arthur Cronquist

Arthur John Cronquist was a North American botanist and a specialist on Compositae. He is considered one of the most influential botanists of the 20th century, largely due to his formulation of the Cronquist system. Two plant genera in the aster family have been named in his honor...

 as "primitively vesselless". Cronquist considered the vessels of Gnetum

Gnetum

Gnetum is a genus of about 30-35 species of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem...

to be convergent with those of angiosperms.

Vessel-like cells have also been found in the xylem of Equisetum (horsetails), Selaginella (spike-mosses), Pteridium aquilinum

Pteridium aquilinum

Pteridium aquilinum is a species of fern occurring in temperate and subtropical regions throughout much of the northern hemisphere....

(bracken fern), Marsilea

Marsilea

Marsilea is a genus of approximately 65 species of aquatic ferns of the family Marsileaceae.These small plants are of unusual appearance and do not resemble common ferns...

and Regnellidium

Regnellidium

Regnellidium is a monotypic genus of ferns of family Marsileaceae.The single living species, Regnellidium diphyllum, Two-leaf Water Fern, is native to South Eastern Brazil and adjacent regions of Argentina. It resembles its relatives from the genus Marsilea, but has 2-lobed leaves . This fern is...

(aquatic ferns), and the enigmatic fossil group Gigantopteridales. In these cases, it is generally agreed that the vessels evolved independently. It is therefore not a stretch to believe that vessels may have appeared more than once among the angiosperms as well.