Yeah, I've used that link already.
OK, I'm reading this from :-
Developmental Stages and Fine Structure of Surface Callus Formed after Debarking of Living Lime Trees (Tilia sp.) -- STOBBE et al. 89 (6): 773 -- Annals of Botany Quote:
During callus growth, remarkable changes take place in the tissue 3?4 weeks after wounding: the outer cells form suberized walls and phenolic substances are deposited in vacuoles. The subsequent callus tissue on the inside is altered into a tangentially oriented belt of flattened cells: the initiation of a phellogen. After 8 weeks, a fully functional wound periderm has formed, consisting of a complete phellogen forming phellem externally and phelloderm internally. The phellem is clearly suberized and contains phenolic substances in its vacuoles. Brown and Sax (1962) observed a similar reorganization in poplar after 2?3 weeks.
The restructuring of the outer surface callus cells can be compared with the formation of a ligno-suberized layer in the bark formed after wounding (Oven et al., 1999). A necrophylactic periderm develops (Mullick, 1977; Biggs, 1985), which later merges into the original periderm at the rim of the wound. According to Oven et al. (1999), the formation of such a ligno-suberized layer in the bark is a precondition for the growth of a wound periderm. Therefore, restructuring of the outer surface callus as well as the formation of a wound periderm are necessary for continuing surface callus development.
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So the way I understand it is that new cells grow and the cells which are right up next to the wound are the boundary (wall 4 we are discussing), however they are also part of the callus. The callus is originally undifferentiated ... like the cells are neutral, not phloem/xylem/root/etc then they change for their roles over a period.