Where is the Exarch Xylem found

The Xylem [ksy’le: m] (Greek: ξυλον, wood) or the Wooden part of the higher plants is a complex, woody conductive tissue that serves to transport water and inorganic salts through the plant, but also takes on supporting functions.

The Xylem is found together with the Phloem in ducts, the so-called vascular bundles, which the shoot axes (in herbaceous Stalk, called the trunk of trees), which run through the petioles and leaves. The roots of dicotyledons have a central xylem core.


Xylem can be found:

  • in vascular bundles, in turn found in non-lignified plants and the non-lignified plant parts of lignified plants
  • as a secondary xylem, formed in woody plants by a cambium between the primary xylem and phloem
  • as part of steles that are not arranged in vascular bundles, in many ferns

In transition phases of plants with secondary growth, the first two are not mutually exclusive, although a vascular bundle will normally only contain primary xylem.

The branches of the xylem follow Murray's law.[1]

Wood (i.e. xylem in the secondary state) is used

  • as a consolidation system
  • as a water pipe system (hydro system)
  • and as a storage system for assimilates.

The different cell types can be assigned to these functions. Tracheids have heavily lignified cell walls, are dead and serve both for consolidation and for water conduction (with a maximum of 0.4 mm / s). Tracheal members are used exclusively for water conduction and they are also dead. Tracheal members have dissolved partitions and form long tube systems, the tracheas. They have a much larger diameter, which results in a significantly lower resistance and thus faster water transport (up to 15 mm / s, in extreme cases 40 mm / s). Wood fibers are similar to tracheids, but have much more thick walls and no pits. Wood fibers are often dead, in which case they are used for strengthening, but not always, in this case they are also used for storage. Tracheids as well as tracheas and wood fibers are elongated, i.e. prosenchymatic. Wood parenchyma cells are alive, they are used to store starch and oil, and they play an important role in the repair of emboli. They are not elongated (but isodiametric). The wood of the gymnosperms consists primarily of tracheids and has a monotonous structure. Trachea are absent and parenchyma only around rays and resin canals. Angiosperm wood has a more complex structure. Here tracheas specialize in water conduction and wood fibers specialize in consolidation. Rays are more extensive and made up of several layers of cells.[2]

The xylem of plants that are very old in evolutionary terms, such as ferns and conifers, consists exclusively of tracheids. In most angiosperms, the xylem also contains well-developed vessels and wood fibers. Since the sequence of the steps in the specialization of all these tissues can be easily observed, the study of the xylem provides important information about the development history of the higher plants.

As Hadrom one describes a xylem without consolidation cells, i.e. without sclerenchymal fibers. The hadrom is very similar to the xylem, but does not occur in cormophytes (vascular plants) like the xylem, but in bryophytes (mosses). The entirety of the water conduction tissue of the bryophytes is called the hadrom.

Cell types


Main article tracheids

Tracheids are elongated cells that live in youth and later die with thick woody cell walls, which are characterized by small, well-defined, thin areas, the so-called Dots. Tracheids are single cells, the transverse walls of which are not completely dissolved, but are "speckled" (court pits).

The function of the court pits with porus and torus (gymnosperms) lies in the water pipe and in a valve function that prevents air from entering (air embolism!). Tracheids occur mostly in gymnosperms, but also in angiosperms. Gymnosperms (e.g., conifers) only have tracheids, not tracheas.


Main article Trachee (plant)

The individual cells of the trachea, the vascular members, are specialized tracheids, the cell walls of which have one or more pores at their ends; Dead vessel members lined up vertically together form a continuous tube called a trachea or vessel. Trachea are mainly found in angiosperms and are used to conduct water and the salts (electrolytes) dissolved in it.

Sclerenchymal fibers

Main article sclerenchyma

Sklerenchyma fibers are specialized tracheids with heavily thickened, lignified cell walls. They serve only to a limited extent as a guide; rather, their task is to mechanically strengthen the xylem. When fully formed, they are dead.

Wood / xylem parenchyma

Are less specialized, living cells that occur in the wooden part. They are usually not elongated, i.e. isodiametric (i.e. the cell diameters are approximately the same in all directions).

Primary and secondary xylem

Primary xylem is formed by the procambium during primary growth in the vegetation cones of the shoot axis and the roots. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before the secondary xylem. Xylem develops according to certain patterns that vary in the respective position of the proto- and metaxylem, e.g. endarch, in which the protoxylem is directed towards the center of the stem or the root, and exarch, in which the metaxylem is directed towards the center.

Secondary xylem is formed by cell division of the cambium, which is located between the xylem and the phloem. The cambium releases cells of the secondary xylem on the inside and cells of the secondary phloem on the outside. Such a cambium, which forms tissue on two sides, is called a dipleuric cambium.

Secondary xylem is found in the Gnetophyta and Ginkgophyta and to a lesser extent also in the Cycadophyta, however the two most important groups are:

  • Conifers: There are some 600 species of conifers. All species have secondary xylem, which is relatively uniform in structure in this group. Many conifers become large trees: the secondary xylem of such trees is sold as softwood.
  • Bedecktsamer (angiosperms): There are between 400,000 to a few quarters of a million species of angiosperms. Secondary xylem can be found in dicots but not in monocots. Secondary xylem may or may not be present in non-monocot angiosperms. It can also vary within a species due to the individual environment of the plant. Given the size of this group, it is not surprising that within angiosperms there are no absolute rules for the structure of the secondary xylem. Many non-monocot angiosperms become trees and the secondary xylem from them is sold as hardwood.

Main function - water transport to the upper areas

The xylem transports water and soluble mineral nutrients from the roots through and into the plant. It is also used to replace the water lost through transpiration and photosynthesis. Xylem juice is mainly made up of water and inorganic ions, although it can contain a number of organic molecules as well. The energy necessary for the transport is not provided by the trachea itself, which consists of dead cells and no longer has any living components. Instead, two phenomena cause water transport in the xylem:

  • Perspiration suction: The main cause of it is the evaporation of water from the surfaces of the mesophyll cells. This perspiration causes millions of tiny menisci in the cell wall of the mesophyll. The resulting surface tension causes negative pressure or tension in the xylem, which pulls the water from the roots into the mesophyll.
  • Root pressure: When the water potential of the root cells is more negative than that of the earth, usually due to high concentrations of solutes, water is moved from the soil into the roots through osmosis. This leads to a positive pressure that forces the xylem sap from the root into the shoot towards the leaves. In some cases, this is how the sap is excreted from the leaves through a hydathode, a phenomenon known as guttation. The root pressure is highest in the morning before the stomata open and perspiration is possible. Different types of plants can have different degrees of root pressure in a similar environment; Examples of this are up to 145 kPa in Vitis riparia, but around zero in Celastrus orbiculatus.[3]

The primary force that causes the upward movement of water in plants is capillary action, based on the adhesion between the water and the surface of the xylem's pathways.[4][5] The capillary action creates a force that creates an equilibrium that counterbalances gravity. When water is removed by transpiration at the top, the upstream current in the xylem is created to restore equilibrium.

The perspiration suction results from the evaporation of water from the surfaces of the cells in the leaves. This evaporation causes the water to flow back into the pores of the cell wall. By capillary action, the water forms concave menisci in the pores. The high surface tension of the water pulls the concavity outwards, the resulting force is great enough to lift water the sometimes necessary hundred meters from the ground to the highest branches.

The aspiration of perspiration requires that the vessels that conduct the water are very small in diameter, otherwise cavitation would destroy the water column. As water evaporates from the leaves, more is drawn up to replace it. When the negative pressure in the xylem reaches an extreme level due to a small amount of water flowing into the roots (e.g. when the soil is dry), the gases in solution in the water come to the fore and form bubbles - an embolism forms. This spreads quickly to neighboring cells, unless there are court pits (these have a torus that closes the opening between neighboring cells in such a case).

Cohesion theory

The cohesion theory is a theory of intermolecular attraction, which describes the rising water in the xylem (against gravity), proven by John Joly and Henry Horatio Dixon. Despite numerous objections, it is the most widely accepted theory for the transport of water through the vascular system of plants, based on the classical research of Dixon-Joly (1894)[6] Askenasy (1895)[7] and Dixon (1914, 1924).[8][9]

Water is a polar molecule. When water molecules interact with each other, hydrogen bonds form. The negatively polarized oxygen atom of one water molecule forms a hydrogen bond with a positively polarized hydrogen atom of another water molecule. This attractive interaction (along with other intermolecular forces) is one of the most important factors in the occurrence of surface tension in liquid water. It allows plants to move water from the root through the xylem and into the leaf.

Water is constantly being lost through transpiration in the leaf. When a water molecule is lost, new water is drawn in through cohesion and adhesion. Perspiration suction, using capillary action and the inherent surface tension of water, is the primary mechanism of movement of water in plants. However, it is not the only mechanism involved. Any consumption of water in the leaves produces forces that suck up water.

Perspiration creates tension (i.e. negative pressure) in the mesophyll cells. Because of this tension, the water is literally drawn from the roots to the leaves, supported by cohesion (the pull between the individual water molecules through hydrogen bonds) and adhesion (the stickiness between water molecules and the hydrophilic cell walls of plants). This mechanism of water flow works through the water potential gradient (water flows from places with high water potential to places with low water potential) and the rules of simple diffusion.[10]

Pressure measurement

Until recently, the negative pressure (suction) of the perspiration suction could only be measured indirectly, by applying an external pressure from a Scholander bomb to compensate for the internal pressure.[11] When techniques were mature enough to take direct measurements, there were discussions about whether the classical theory was correct, as it was sometimes not possible to detect negative pressures. Recent measurements seem to confirm the classical theory for the most part. The xylem transport is generated by a mixture of perspiration suction and root pressure, which makes it difficult to interpret measurements.


  • Rudolf Schubert and Günther Wagner: “Botanical Dictionary. Plant names and botanical terms ”. Stuttgart 2000, p. 595. ISBN: 3-8252-1476-1

Individual evidence

  1. Katherine A. McCulloh, John S. Sperry and Frederick R. Adler: Water transport in plants obeys Murray's law. In: Nature. 421, No. 6926, 2003, pp. 939-942. doi: 10.1038 / nature01444. PMID 12607000.
  2. ↑ A. Bresinsky, Ch.Körner, J. W. Kadereit, G. Neuhaus, U. Sonnewald: Strasburger - Textbook of Botany. 36th edition, Spektrum Akademischer Verlag, Heidelberg 2008. ISBN 978-3-8274-1455-7, pp. 187ff.
  3. Tim J. Tibbetts, Frank W. Ewers: Root pressure and specific conductivity in temperate lianas: exotic Celastrus orbiculatus (Celastraceae) vs. Native Vitis riparia (Vitaceae). In: Botanical Society of America (Ed.): American Journal of Botany. 87, No. 9, 2000, pp. 1272-78. doi: 10.2307 / 2656720. PMID 10991898.
  4. ^ Pierre Cruiziat and Hanno Richter in web site for Plant Physiology at Sinauer Associates.
  5. Editors: Anthony Yeo, Tim Flowers: Plant solute transport. Oxford UK: Blackwell Publishing 2007., ISBN 978-1-4051-3995-3 p 221 Google Books
  6. H Dixon, Joly: On the ascent of sap. In: Ann. Bot.. 8, 1894, pp. 468-470.
  7. E Askenasy: About the rising of the juice. In: Bot. Cent.. 62, 1895, pp. 237-238.
  8. H Dixon: Transpiration and the ascent of sap in plants, P. 216, New York: Macmillian 1914
  9. H Dixon: The transpiration stream, P. 80, London: University of London Press, Ltd 1924
  10. Neil Campbell: Biology, P. 759, San Francisco, CA: Pearson Education, Inc. 2002, ISBN 0-8053-6624-5
  11. ↑ http: //bugs.bio.usyd.edu.au/learning/resources/plant_form_function/external_sites/PAP/Lab08_WaterPot/08Lab_14.html

See also

Web links