Tree wound healing

Photo credit: Beau Vista orchards 

Tree wound healing is a beautiful display of plant physiology and anatomy. The most common is now pruning! Here is a description with emphasis on the medullary ray cells. The scientific basis for a tree's response to injury is a sophisticated multi pronged defence mechanism. First codified by Dr Alex Sligo in a model he called called the Compartmentalization Of Decay In Trees (CODIT) in 1970s. The science has moved onwith the understanding of phytohormones The process involves a rapid chemical response followed by a slower physical adaptation, forming Alex’s four "walls" of defence.
The tree rapidly plugs the xylem vessels located and below the wound. This is achieved by physically blocking the vessels with structures like tyloses - balloon-like outgrowths of adjacent cells from rest cells in the cambium, and with gums, or resins, as well as producing antifungal compounds. This wall is quick but weak and aims to slow the vertical spread of decay.
Deep to the wound is a wall of dense latewood cells of the existing annual growth rings, which are rich in lignin. This slows the inward spread of decay toward the centre of the trunk.
Key to repair are the medullary rays which are not often mentioned in the school botany curriculum. They are radial in the trunk and contain undifferentiated cells that respond to injury moving outwards to form callus and inwards to distribute tannins resins and phenolic compounds that block decay both physically and biochemically. These ray cells originate from meristematic tissue in the vascular cambium. This is a layer of active, dividing cells (meristem) located between the existing xylem (wood) and phloem (inner bark) and responsible for the growth in girth of the trunk. As the cambium cells divide, most of them differentiate into the vertical systems of the secondary xylem (wood) and secondary phloem (bark). However, certain cells do not transform into these vascular tissues and remain as specialised, radially arranged parenchyma cells that form ribbon-like sheets that extend radially across the growth rings, from the pith towards the bark. They provide the strongest and most crucial defence mechanism.
After the initial injury, the cambium layer forms callus, this new wood is both anatomically and chemically distinct from the wood formed prior to the injury. Cells in this barrier are often enriched with protective substances like suberin (a waxy, hydrophobic material that prevents water loss and pathogen entry) and polyphenolics, creating an impermeable layer. The growth is faster than normal giving a prominent keloid appearance.

Diagram artist unkown