Tylosis (botany)

Tyloses (singular: tylosis) are balloon-like outgrowths or protrusions of parenchyma cells into the lumens of xylem vessels in vascular plants. They occur through pits in the vessel walls. Tyloses are a normal response to injury or infection, but can also form during heartwood formation in some species. Their presence blocks or impedes the flow of water and other substances through the xylem, contributing to the plant's defense mechanism by compartmentalizing decay.

The formation of tyloses is initiated by the differentiation of parenchyma cells adjacent to xylem vessels. These parenchyma cells enlarge and protrude through the pits of the xylem vessel walls. The protruding portion then expands into a balloon-like structure that fills the vessel lumen. The tyloses may remain thin-walled or become thickened and lignified, further hindering water flow.

The formation of tyloses is regulated by various factors, including plant hormones and environmental stimuli such as wounding or pathogen attack. Their abundance and distribution vary among different plant species and can be influenced by age, growth conditions, and disease resistance.

In heartwood formation, tyloses contribute to the durability and decay resistance of the wood. They help to seal off the older xylem vessels, preventing fungal and bacterial colonization. This process is crucial for the long-term survival of the tree and the utilization of its wood for various purposes.

While beneficial in many ways, excessive tylosis formation can also be detrimental to the plant. If a large number of vessels become blocked by tyloses, it can significantly reduce water transport, leading to wilting and potentially death.