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A tracheid is a long and tapered
lignified
lignified
cell in the
xylem Xylem is one of the two types of transport in s, the other being . The basic function of xylem is to transport from roots to stems and leaves, but it also transports . The word ''xylem'' is derived from the word (''xylon''), meaning "wood"; ...

xylem
of
vascular plants Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species) that are defined as land plants with lignified tissue ...

vascular plants
. It is a type of conductive cell called a tracheary element.
Angiosperms Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...
use another type of tracheary element, called
vessel element 300px, SEM image (top) and Transmission Light Microscope image (bottom) of vessel elements in Oak A vessel element or vessel member (also called trachea or xylem vessel) is one of the cell types found in xylem, the water conducting tissue of p ...
s, to transport water through the xylem. The main functions of tracheid cells are to
transport water and inorganic salts
transport water and inorganic salts
, and to provide structural support for trees. There are often pits on the
cell walls Cell most often refers to: * Cell (biology) The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often refer ...
of tracheids, which allows for water flow between cells. Tracheids are dead at functional maturity and do not have a
protoplast Protoplasts of the moss '' Physcomitrella patens'' Protoplast, from ancient Greek (''prōtóplastos'', "first-formed"), is a biology, biological term coined by Johannes von Hanstein, Hanstein in 1880 to refer to the entire cell, excluding the ce ...
. The
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

wood
(
softwood file:Pinus sylvestris wood ray section 1 beentree.jpg, Scots Pine, a typical and well-known softwood Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. Characterist ...
) of
gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also ...
s such as pines and other
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 single ex ...
s is mainly composed of tracheids. Tracheids are also the main conductive cells in the primary xylem of
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

fern
s. The tracheid was first named after the German botanist Carl Gustav Sanio in 1863, from the German ''Tracheide''.


Evolution

Tracheids were the main conductive cells found in early vascular plants. In the first 140-150 million years of vascular plant evolution, tracheids were the only type of conductive cells found in fossils of plant xylem tissues. Ancestral tracheids did not contribute significantly to structural support, as can be seen in extant ferns. The
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

fossil
record shows three different types of tracheid cells found in early plants, which were classified as S-type, G-type and P-type. The first two of them were lignified and had pores to facilitate the transportation of water between cells. The P-type tracheid cells had pits similar to extant plant tracheids. Later, more complex pits appeared, such as bordered pits on many tracheids, which allowed plants to transport water between cells while reducing the risk of cavitation and embolisms in the xylem. As tracheids evolved along with secondary xylem tissues, specialized inter-tracheid pits appeared. Tracheid length and diameter also increased, with tracheid diameter increasing to an average length of 80 μm by the end of the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
period. Tracheids then evolved into the vessel elements and structural fibers that make up angiosperm wood.


References


Further reading

*


External links

*Pictures of softwood tracheid
in cross section
an

both in pine. {{Biological tissue Plant anatomy Plant physiology