Raffi at 05:19, 6 April 2007

2007-04-06T05:19:03Z

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In [[vascular plant]]s, '''xylem''' is one of the two types of transport tissue in plants, [[phloem]] being the other one. The word "xylem" is derived from classical [[Greek language|Greek]] ξυλον (''xylon''), "wood", and indeed the best known xylem tissue is [[wood]], though it is found throughout the plant.

==Physiology of xylem==

The xylem transports sap from the root up the plant. The xylem is mainly responsible for the transportation of water and mineral nutrients throughout the plant. Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well. This transport is not powered by energy spent by the tracheary elements themselves, which are dead at maturity and no longer have living contents. Two phenomena cause xylem sap to flow:
* '''[[Transpirational pull]]''': the most important cause of xylem sap flow, is caused by the [[evaporation]] of [[water]] from the surface [[mesophyll]] cells to the atmosphere. This [[transpiration]] causes millions of minute [[menisci]] to form in the cell wall of the mesophyll. The resulting [[surface tension]] causes a negative pressure in the xylem that pulls the water from the roots and soil.
* '''[[Root pressure]]''': If the [[water potential]] of the root cells is more negative than the [[soil]], usually due to high concentrations of [[solute]], water can move by [[osmosis]] into the root. This may cause a positive pressure that will force sap up the xylem towards the leaves. In extreme circumstances the sap will be forced from the leaf through a [[hydathode]] in a phenomenon known as [[guttation]]. Root pressure is most common in the morning before the stomata open and cause transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in ''Vitis riparia'' but around zero in ''Celastrus orbiculatus'' [http://www.amjbot.org/cgi/content/full/87/9/1272].

==Anatomy of xylem==

Xylem can be found:
* in [[vascular bundle]]s, present in non-woody plants and non-woody plant parts
* in ''[[secondary xylem]]'', laid down by a [[meristem]] called the [[vascular cambium]]
* as part of a [[stele (biology)|stelar arrangement]] not divided into bundles, as in many [[fern]]s.

Note that, in transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain ''primary xylem'' only.

The most distinctive [[cell (biology)|cells]] found in xylem are the tracheary elements: [[tracheid]]s and [[vessel element]]s. However, the xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells, such as parenchyma, in addition to those that serve to transport water.

==Primary and Secondary xylem==
'''Primary xylem''', also known as ''Protoxylem'', is the xylem that is formed during primary growth from [[procambium]].

'''Secondary xylem''', also known as ''Metaxylem'', is the xylem that is formed during secondary growth from [[vascular cambium]]. Secondary xylem is also found in members of the "[[gymnosperm]]" groups [[Gnetophyta]] and [[Ginkgophyta]] and to a lesser extent in members of the [[Cycadophyta]]. The two main groups in which secondary xylem can be found are:
#[[conifers]] (''[[Coniferae]]''): there are some six hundred species of conifers. All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is marketed as '''[[softwood]]'''.
#[[angiosperms]] (''[[Angiospermae]]''): there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem has not been found in the [[monocots]]. In the remainder of the angiosperms this secondary xylem may or may not be present, this may vary even within a species, depending on growing circumstances. In view of the size of this group it will be no surprise that no absolutes apply to the structure of seco
ndary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as '''[[hardwood]]'''.

==Evolution of xylem==
[[Image:ficusxylem.jpg|thumb|300px|Photos showing xylem elements in the shoot of a fig tree (''Ficus alba''): crushed in hydrochloric acid, between slides and cover slips.]]
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the [[Silurian]] (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier [[Ordovician]] rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the ''protoxylem'' (first-formed xylem) of all living groups of plants. Several groups of plants later developed pitted tracheid cells, apparently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the ''metaxylem'' (following the ''protoxylem'').

In most plants, pitted tracheids function as the primary transport cells. The other type of tracheary element, besides the tracheid, is the [[vessel element]]. Vessel elements are joined by perforations into vessels. In vessels, water travels by ''bulk flow'', like in a pipe, rather than by diffusion through cell membranes. The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms. However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., [[Amborellaceae]], [[Tetracentraceae]], [[Trochodendraceae]], and [[Winteraceae]]), and their secondary xylem is described by [[Arthur Cronquist]] as "primitively vesselless". Cronquist considered the vessels of ''[[Gnetum]]'' to be convergent with those of angiosperms. Whether the absence of vessels in basal angiosperms is a [[cladistics#Definitions|primitive]] condition is contested, the alternative hypothesis being that vessel elements originated in a precursor to the angiosperms and were subsequently lost.

==See also==
* [[cohesion-tension theory]]
* [[secondary growth]]
* [[transpirational pull]]
* [[vascular tissue]]
* [[vascular bundle]]

==References==
* Campbell, Neil A. and Jane B. Reece. 2002. ''Biology'', 6th ed., published by Benjamin Cummings.
* Carlquist, S. and E.L. Schneider. 2002. The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences. ''American Journal of Botany'' 89:185-195 (link to abstract [http://www.amjbot.org/cgi/content/abstract/89/2/185 here]).
* Cronquist, A. 1988. The Evolution and Classification of Flowering Plants. New York, New York: The New York Botanical Garden.
* Kenrick, Paul & Crane, Peter R. (1997). ''The Origin and Early Diversification of Land Plants: A Cladistic Study''. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
*Muhammad, A.F. and R. Sattler. 1982. Vessel Structure of ''Gnetum'' and the Origin of Angiosperms. ''American Journal of Botany'' 69: 1004-1021 (available online [http://links.jstor.org/sici?sici=0002-9122%28198207%2969%3A6%3C1004%3AVSOGAT%3E2.0.CO%3B2-P here]).
* Tyree, Melvin T. Tyree and Martin H. Zimmermann. ''Xylem Structure and the Ascent of Sap, 2nd ed.'' (ISBN 3-540-43354-6) recent update of the classic book on xylem transport by the late Martin Zimmermann

[[Category:Plant anatomy]]
[[Category:Plant physiology]]
[[Category:Plant cells]]

Xylem - Revision history

Raffi at 05:19, 6 April 2007