A vessel element or vessel member is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in flowering plants but absent from most gymnosperms such as conifers. Vessel elements are the main feature distinguishing the "hardwood" of angiosperms from the "softwood" of conifers.
Morphology
is the tissue in vascular plants which conducts water upwards in a plant. There are two kinds of cell which are involved in the actual transport: tracheids and vessel elements. Vessel elements are the building blocks of vessels, which constitute the major part of the water transporting system in those plants in which they occur. Vessels form an efficient system for transporting water from the root to the leaves and other parts of the plant. In secondary xylem – the xylem which is produced as a stem thickens rather than when it first appears – a vessel element originates from the vascular cambium. 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. 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 and the scalariform perforation. Other types include the foraminate perforation plate and the reticulate perforation plate. At maturity the protoplast – the living material of the cell – dies and disappears, but the lignifiedcell 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 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, 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, Selaginella, Pteridium aquilinum, Marsilea and Regnellidium, and the enigmatic fossil groupGigantopteridales. 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.
Schweingruber, F.H. Anatomie europäischer Hölzer - Anatomy of European woods. Eidgenössische Forschungsanstalt für Wald, Schnee und Landscaft, Birmensdorf. Haupt, Bern und Stuttgart.
