The xylem [ksy'le: m] ( Greek ξύλον xýlon 'wood') or the wood part of the higher plants ( vascular plants ) is a complex, woody conductive tissue that transports water and inorganic salts through the plant, but also takes on supporting functions .
The xylem found together with the phloem in pathways, the so-called vascular bundles , the stems (with herbaceous plants stems , in trees strain called), the petioles and leaves through them. The roots of dicotyledons have a central xylem core.
Primary and secondary xylem
Xylem can be found:
- as the primary xylem in vascular bundles of non-lignified plants as well as in the non-lignified plant parts of lignified plants
- as a secondary xylem in woody plants, formed by a cambium between the primary xylem and phloem
- as part of steles that are not arranged in vascular bundles, in many ferns
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, which vary in the respective position of the proto- and metaxylem, e.g. B. endarch, in which the protoxylem is directed towards the center of the stem or 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 in the Cycadophyta, but the two most important groups are:
- Coniferous trees (conifers): There are about 600 species of conifers. All species have secondary xylem, which in this group is relatively uniform in structure. Many conifers become large trees; the secondary xylem of such trees is sold as softwood.
- Bedecktsamer (angiosperms): There are over 400,000 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 plant's individual environment. 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.
In the transitional phases of plants with secondary growth, the primary and secondary xylem are not mutually exclusive, with a vascular bundle normally containing only primary xylem.
The branches of the xylem follow Murray's law.
Cell types and their functions
Wood (i.e. xylem in the secondary state) serves as a consolidation system, as a water pipe system and as a storage system for assimilates. The different cell types can be assigned to these functions.
Tracheids are elongated, living, later dead cells with thick, heavily lignified cell walls . The transverse walls of the individual cells are not completely dissolved, but are characterized by small, well-defined thin areas, the so-called pits (courtyard pits).
The stippled tracheids serve both for consolidation and for water conduction (with a maximum of 0.4 mm / s). The pits also serve as a water pipe. They also have a valve function by preventing air from entering (air embolism!).
Tracheal or vascular members are specialized tracheids, the cell walls of which have one or more pores at their ends . Lined up vertically, individual dead cells form long tube systems, the tracheas or vessels.
The trachea have a much larger diameter, which results in less resistance and thus faster water transport (up to 15 mm / s, in extreme cases 40 mm / s).
Trachea are mainly found in angiosperms and are used to conduct water and the salts ( electrolytes ) dissolved in it .
Wood fibers (sclerenchyma fibers) are also specialized tracheids. However, they have much more thick walls and no pits.
Their task is to mechanically strengthen the xylem. Some wood fibers are still alive, in this case they also serve to store a small amount.
The cells of the wood or xylem parenchyma are less specialized living cells in the wood part. Unlike the previously mentioned cell types, they are not elongated. Their cell diameters are approximately the same in all directions.
Xylem in different groups of plants
- The wood of the gymnosperms consists primarily of tracheids and has a monotonous structure. Tracheas are absent and parenchyma only around rays and resin canals .
- Angiosperm wood has a more complex structure. Here tracheas specialize in water pipes and wood fibers specialize in consolidation. Rays are more extensive and made up of several layers of cells.
- The xylem of plants that are very old in evolutionary terms, such as ferns and conifers , consists exclusively of tracheids. In most angiosperms (angiosperms) the xylem also contains well-developed vessels and wood fibers . Since the sequence of 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.
- The entirety of the water- conducting tissue of the moss (Bryophyta) is called the hadrom . The hadroma is a xylem without consolidation cells, i.e. without sclerenchymal fibers. It is similar to the xylem.
The water transport
The xylem transports water and dissolved mineral salts from the roots through and into the plant. It is also used to replace the water lost through transpiration and photosynthesis. Xylem juice is mostly made up of water and inorganic ions, although it can contain a number of organic molecules as well.
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, together with other intermolecular forces, leads to the cohesion of the water particles with one another and to the occurrence of surface tension in liquid water.
It enables plants to move water from the root through the xylem into the leaf against gravity.
The plant loses water through transpiration from the surface, consumption of growth water or metabolism. When water molecules leave the xylem through evaporation, capillary force causes the molecules immediately following to refill the xylem. This mechanism is called perspiration suction . Perspiration creates tension (negative pressure) in the mesophyll cells .
Up to a certain height of rise, the atmospheric pressure also allows all other molecules in the water column to move up (the capillary force only acts at the interface between water, wall and air). From a rise of about 3 meters, the atmospheric pressure is no longer sufficient to balance the weight of the water and the flow resistance of the xylem. A vacuum or boiling bubbles would form in the water column . The water transport in taller plants is therefore explained by the cohesion of the water molecules with one another, which counteract the formation of a vacuum or the boiling of the water in the xylem. The root pressure also has a supportive effect .
Until recently, the pressure difference of the perspiration suction could only be measured indirectly, by applying an external pressure through a Scholander bomb to equalize the internal pressure. When techniques were mature enough to be able 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 largely confirm the classical theory. The xylem transport is generated by a mixture of perspiration suction and root pressure, which makes it difficult to interpret measurements.
- Katherine A. McCulloh, John S. Sperry, Frederick R. Adler: Water transport in plants obeys Murray's law . In: Nature . tape 421 , no. 6926 , 2003, p. 939-942 , doi : 10.1038 / nature01444 , PMID 12607000 .
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