I fixed that link up for you.
Here's what that doc wrote about wall 4.
Quote:
Wall 4. After a tree is wounded, the cambium begins to form a new protective wall. The wall is both an anatomical and a chemical wall. This wall separates the tissue present at the time of wounding from tissue
that forms after. It is the strongest of the four walls.
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And a copy of the diagram.
So what we do know is wall 4 has to be grown and is not a change in existing cells but new cells.
Carefull with this thinking or statement.
Quote:
Originally Posted by Underwor  Wall 4 is chemical in nature and as I see it keeps the decay from making a flanking move around the edge of the wound into the wood that forms later. Once this wall is established, then woundwood can cover the outside and seal off the face of the injury from further pathogens. |
Whilst it can extend beyond where callus has grown
it only does so where there is wood not where there is fresh air ... like when a car swipes a tree and takes a chunk out including bark the barrier is set up where there's still bark on the outside but not where the callus failed to form.
Where there's no bark it appears the progression is to form callus as well as the wall.
Here's the illustration and take note of the blue arrows and note the thin orange barrier does extend beyond the wound
but only where there is wood.
So whilst you can have one with out the other it appears so only when contained within the woody system of the tree. And that debarking experiment showed how it is formed in the absence of bark (open wound) with callus.