Tyloses are biological structures that form within the water-conducting vessels of certain hardwood trees. These growths are balloon-like extensions of living cells that push into the hollow space of the dead vessel elements. They function as natural plugs, permanently obstructing the vessels to stop the movement of water and air. Tyloses play a dual role in tree survival, acting both as a programmed part of the aging process and as a rapid defense mechanism against external threats.
Structure and Formation
Tyloses originate from the adjacent, living parenchyma cells, which are typically found in the wood rays or surrounding the vessels. The formation process begins when these turgid parenchyma cells detect a change in the adjacent vessel, often due to vessel embolism or air-filling. This change triggers an active growth response in the parenchyma cell.
The living cell begins to push its protoplast through small openings in the vessel wall known as pits. These pits normally allow water exchange between the parenchyma cell and the vessel. As the cell material protrudes through the pit membrane, it balloons out into the open lumen of the dead vessel element.
This outgrowth develops a new wall, which can become layered with materials like cellulose and lignin. The resulting structure is a bladder-like protrusion that expands to completely fill and block the vessel’s interior. A single vessel may contain dozens of these structures, effectively creating a cellular dam.
Role in Heartwood Development
Tyloses play an indispensable role in the programmed process of heartwood formation as a tree ages. Sapwood, the outer layer of wood, is responsible for active water transport, but over time, the innermost vessels cease to function. The tree converts this non-functional sapwood into heartwood, a darker, denser, and more durable central core.
Tyloses are produced to permanently seal these aging vessels, preventing the movement of water and gases. This blockage is a precondition for the next step: the deposition of chemical compounds called extractives.
These compounds, such as tannins and resins, are delivered by the parenchyma cells and accumulate in the sealed vessels. The extractives provide natural resistance to decay and insects, while also giving heartwood its characteristic darker color and increased density.
Defense Against Pathogens and Injury
Beyond their role in aging, tyloses are a defense system deployed by the tree in response to injury or infection. When a tree is wounded by pruning, an insect bore, or a fungal attack, the surrounding living cells quickly initiate tylosis formation. This occlusion limits the spread of damage as part of the tree’s wound response.
The swift formation of these vessel plugs prevents the vertical and horizontal spread of foreign invaders like fungal hyphae or bacteria. This immediate sealing action is a form of compartmentalization, effectively walling off the damaged area to protect the rest of the tree.
The formation of tyloses can also be induced by environmental stressors like drought and frost, which cause vessels to fail due to embolism. In these cases, the structures form to plug the cavitated vessels, limiting further water loss from the system.
Impact on Wood Quality
The presence of tyloses significantly influences the physical properties and commercial value of harvested wood. Because tyloses completely fill and seal the vessels, they make the wood non-permeable to liquids and gases. This low permeability also presents a challenge for wood treatment, as it severely hinders the deep penetration of chemical preservatives during pressure treatment processes.
This watertight quality is highly valued in certain applications, such as cooperage. For example, white oak heartwood is rich in tyloses, making it the preferred material for manufacturing barrels and casks used to age wine and whiskey.
The plugged vessels ensure that the liquid remains inside the barrel, while still allowing for beneficial chemical exchange through the wood cell walls. Conversely, wood species like red oak, which generally lack abundant tyloses, are highly permeable and cannot be used for tight cooperage.