A vessel element or vessel member (also called a xylem vessel) is one of the cell types found in

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

, the water conducting tissue of plants. Vessel elements are found in most angiosperms (

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s) and in some

gymnosperm The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

s such as cycads and '' Ephedra'', but absent in conifers. Vessel elements are the main feature distinguishing the "

hardwood Hardwood is wood from Flowering plant, angiosperm trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal ecosystem, boreal latitudes they are mostly deciduous, but in tropics and subtropics mostl ...

" of angiosperms from the " softwood" of

conifer Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...

Anatomy

Xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

is the tissue in

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

s that conducts water (and substances dissolved in it) upwards from the roots to the shoots. Two kinds of cell are involved in xylem transport:

tracheid A tracheid is a long and tapered Lignin, lignified cell in the xylem of Tracheophyta, vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms also use another type of conductive cell, called vessel elements, to t ...

s and vessel elements. Vessel elements are the building blocks of vessels, the conducting pathways that constitute the major part of the water transporting system in flowering plants. Vessels form an efficient system for transporting water (including necessary minerals) from the root to the leaves and other parts of the plant. Xylem Stained

In secondary xylem – the xylem that is produced as a stem thickens rather than when it first appears – a vessel element originates from the

vascular cambium The vascular cambium is the main growth tissue in the stems and roots of many plants exhibiting secondary growth, specifically in dicots such as buttercups and oak trees, gymnosperms such as pine trees, as well as in certain other vascular ...

. A long cell, oriented along the axis of the stem, called a "fusiform initial", divides along its length forming new vessel elements. The cell wall of a vessel element becomes strongly "lignified", i.e. it develops reinforcing material made of

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

. The side walls of a vessel element have pits: more or less circular regions in contact with neighbouring cells. Tracheids also have pits, but only vessel elements have openings at both ends that connect individual vessel elements to form a continuous tubular vessel. These end openings are called perforations or perforation plates. They have a variety of shapes: the most common are the simple perforation (a simple opening) and the scalariform perforation (several elongated openings in a ladder-like design). Other types include the foraminate perforation plate (several round openings) and the reticulate perforation plate (a net-like pattern, with many openings). At maturity, the protoplast – the living material of the cell – dies and disappears, but the lignified cell walls persist. A vessel element is then a dead cell, but one that still has a function, and is still being protected by surrounding living cells.

Evolutionary significance

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 their presence in some members of the

Gnetales Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three Relict ...

suggest that this hypothesis must be re-examined; vessel elements in Gnetales may not be homologous with those of angiosperms, or vessel elements that originated in a precursor to the angiosperms may have been subsequently lost in some basal lineages (e.g., Amborellaceae, Trochodendraceae, and Winteraceae), described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of '' Gnetum'' 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'' (bracken fern), '' Marsilea'' and '' Regnellidium'' ( aquatic ferns), and the enigmatic fossil group Gigantopteridales. In these cases, it is generally agreed that the vessels evolved independently. It is possible that vessels may have appeared more than once among the angiosperms as well.

References

Further references

* Niklas, Karl J. (1997) ''The Evolutionary Biology of Plants''. Chicago and London: The University of Chicago Press. . * Schweingruber, F. H. (1990) ''Anatomie europäischer Hölzer - Anatomy of European woods''. Eidgenössische Forschungsanstalt für Wald, Schnee und Landscaft, Birmensdorf (Hrsg.) Haupt, Bern und Stuttgart. * Timonen, Tuuli (2002). ''Introduction to Microscopic Wood Identification''. Finnish Museum of Natural History, University of Helsinki. * {{Biological tissue Plant anatomy Plant stem morphology Wood

Anatomy

Evolutionary significance

See also

References

Further references