In woody plants, a tylosis (plural: tyloses) is a bladder-like distension of a parenchyma cell into the lumen of adjacent vessels. The term tylosis summarises the physiological process and the resulting occlusion in the

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

of woody plants as response to injury or as protection from decay in

heartwood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

. It is a key process in wall one of the compartmentalization of decay in trees (CODIT) and other woody plants. Carpinus betulus1 cross beentree

Anatomy

Observed in section under a microscope, tyloses appear as balloon-like protrusions emanating from axial paratracheal parenchyma cells into xylem vessels through pits linking the two. 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.

Tyloses

Tyloses are outgrowths on

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

cells of

vessels of secondary heartwood. When the plant is stressed by

drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...

infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

, tyloses will fall from the sides of the cells and "dam" up the vascular tissue to prevent further damage to the plant. Tyloses can aid in the process of making

sapwood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin that ...

into

in some hardwood trees, especially in trees with larger vessels. These blockages can be used in addition to gum plugs as soon as vessels become filled with air bubbles, and they help to form a stronger heartwood by slowing the progress of rot.

Role in compartmentalization

Tylosis in the vessels of

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s counteracts the axial spread of fungal

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e and other pathogens by slowing down their vertical spread with a physical barrier. A similar process occurs in

gymnosperm The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

s, which block access to

tracheid A tracheid is a long and tapered Lignin, lignified cell in the xylem of Tracheophyta, vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms also use another type of conductive cell, called vessel elements, to t ...

s by closing the pits that join them to each other. The blocked vessels also provide a defense against the radial spread of pathogens, limiting their horizontal spread through the plant stem. Protection is stronger at the boundaries where

annual rings Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, the study of climate ...

meet. The effectiveness of both vertical and horizontal barriers is affected by the speed at which they are established by tyloses, being typically faster in healthier plants.

Role in heartwood formation

As a tree grows, its

cambium A cambium (: cambiums or cambia), in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. A cambium can also be defined as a cellular plant tissue from whic ...

adds an annual increment (or ring) of new wood, and in many species of trees the older wood towards the centre of the tree becomes less important for physiological processes like water and nutrient transportation, and this is converted by the tree to heartwood. This heartwood has no active defenses against infection, but is protected from infection and decay by the blockage of the xylem vessels with tyloses and various substances such as gums,

resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...

s and waxes containing high concentrations of

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...

s such as

terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...

s that are toxic to insect larvae and tree pathogens such as bacteria and fungi. These products are produced by the cambium and transported to the centre of the stem by cellular structures called medullary rays radiating from the center of the stem and then enter living axial paratracheal parenchyma cells. As the wood ages, the contents of the parenchyma cell burst into the dead vessel through the pit linking the two. The parenchyma cell then dies as its contents are disgorged into the empty space of the dry vessel and highly effective decay-inhibiting substances, notably tannin, are formed and absorbed by the adjacent vessel walls.

External links

Pictures of tylosis in ''Ulmus'' (Elm), with detailed description of the process

Sources

{{Authority control Plant anatomy Wood Trees Forestry