Tylosis (botany)
Encyclopedia
Tylosis is the physiological process and the result of occlusion in the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 of woody plants as response to injury or as protection from decay in heartwood.
It is an essential process in the Compartmentalization Of Decay In Trees (CODIT) and other woody plants, forming walls one and two of CODIT.

Process

As a tree grows, its cambium
Cambium (botany)
A cambium , in botany, is a tissue layer that provide undifferentiated cells for plant growth. It forms parallel rows of cells, which result in secondary tissues....

 expands, leaving heartwood behind its sapwood. This heartwood is the result of protection from decay as the plant responds by blocking the xylem vessels with gums, resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

s and waxes, the waste products of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. These contain high levels of volatile organic compound
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...

s called terpene
Terpene
Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

s that are toxic to tree pathogens such as bacteria, fungi and insect larvae. The heartwood or injured portions of the plant therefore receive protection from decay as long as the levels of terpenes are maintained high enough to deter or control pathogens.
These waste products are produced by the cambium and transported to the centre of the stem by cellular structures called medullary rays
Medullary ray (botany)
In botany, Medullary rays refer to a characteristic found in woods. In this context the term refers to radial sheets or ribbons extending vertically through the tree across and perpendicular to the growth rings. Also called pith rays or wood rays, these formations of primarily parenchyma cells...

 radiating from the center of the stem. These then enter axial paratracheal parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

 cells which are still alive after the xylem vessel has been left behind by the growth of the cambium. As the wood ages, the contents of the parenchyma cell burst into the dead vessel through a pit connecting the two.The parenchyma vessel then dies as its contents are disgorged into the empty space of the dry vessel.

Anatomy

Observed in section under a microscope, tyloses appear as balloon-like protrusions emanating from axial paratracheal parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

 cells into xylem vessels. In some types, there may be a distinct barrier between the tyloses emanating from the pits into the vessels, while they may be barely distinguishable in other cases.

Role in compartmentalization

As part of wall one, tylosis of broadleaf plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

 vessels counteracts the axial spread of fungal hypha
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

e and other pathogens by slowing down their vertical spread with a physical barrier. A similar process occurs in gymnosperm
Gymnosperm
The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...

s, which block access to tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...

s by closing the pits that join them to each other.

The process is similar in wall two, where the blocked vessels provide a defense against the radial spread of pathogens, limiting their horizontal spread through the plant stem. Protection in wall two is stronger at the boundaries where annual rings
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

meet.
The effectiveness of both walls is affected by the speed at which they are established by tylosis, being typically faster in healthier plants.

External links

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