Tyloses are blockages that form within a tree’s vascular system. These structures develop inside the xylem, the network of tubes that transports water from the roots to the leaves. While these vessels are normally clear, a tree can deliberately plug them in response to certain triggers.
The Biological Formation of Tyloses
The formation of tyloses is a cellular process involving the water-conducting xylem vessels and adjacent living cells called parenchyma. While xylem vessels are hollow, non-living tubes, they are surrounded by active parenchyma cells. The walls between them are perforated by small openings known as pits, which allow for exchange between the two cell types.
This process is initiated by several triggers. Physical injury, like a broken branch, can expose the internal xylem to air and pathogens. Drought stress prompts their formation to prevent air bubbles (embolisms) from spreading, and the presence of pathogens causes the tree to create tyloses to wall off the infection.
In response to a trigger, the protoplast (the living contents of the parenchyma cell) pushes through the pits into the adjacent vessel. This outgrowth expands like a balloon, filling a portion of the vessel’s interior. Multiple tyloses can grow into a single vessel, eventually blocking it completely. Tyloses also form naturally as wood ages and sapwood transitions into heartwood, plugging older vessels.
Consequences for Wood Characteristics
The presence of tyloses transforms wood from a porous material into a solid, impermeable one. By sealing the xylem vessels, these blockages make the wood highly resistant to the passage of liquids and gases. This structural change also results in wood that is significantly denser and heavier.
A classic example is the difference between white oak and red oak. White oak wood has abundant tyloses that plug its vessels, making the wood watertight and ideal for barrels used in aging wine and whiskey. In contrast, red oak has few or no tyloses, leaving its vessels open. A barrel made from red oak would leak because liquid can pass through the unblocked vessels.
This blocking of internal pathways also enhances the wood’s natural durability. By sealing the xylem, tyloses eliminate the routes that moisture, fungi, and insects use to penetrate and degrade the wood. Wood with high concentrations of tyloses, like the heartwood of black locust and Osage orange, is exceptionally resistant to decay.
Function in Tree Health and Defense
The ability to form tyloses is a defense mechanism that allows a living tree to respond to injury and disease. The process is part of compartmentalization, the method trees use to seal off damaged or infected areas. When a pathogen like a vascular wilt fungus enters the xylem, the tree rapidly forms tyloses in surrounding vessels to create a barrier. This containment strategy isolates the pathogen and stops its movement.
This response is a targeted process, as the blockages form specifically in vessels adjacent to the point of injury or infection. By plugging these conduits, the tree sacrifices a small part of its water-conducting capacity to protect the entire organism. The effectiveness of this defense often determines a tree’s survival. Trees that are quick and efficient at producing tyloses are more resistant to certain diseases, as the blockages serve as an internal barricade against invaders.