The state (Pennslyvanis) foresters just produced a booklet on trees. Common Trees of Pennsylvania. It is very informative. However there is a couple of half truths I think are worthy to mention. The booklet points to the center of the tree and says trees form heartwood. Yes, heartwood is a protection wood, however, all trees do not form heartwood. Maple (Acer) trees are sapwood trees and they do not form heartwood.
Second they state that the cambium zone produces wood to the inside. Not so, the cambium forms xylem to the inside which is not lignified. Later the cells differentiate and they take on lignin. Then and only then is it correctly called wood. Wood is a substance made up of lignin, cellulose and hemicelluloses. In the spring, when it appears that the bark slips, its not the bark that slips its the xylem slipping. At that time if you take your finger nail and scrape the material and you will get a jelly like substance on your finger nail. That's xylem!
So, some trees form heartwood e.g., oak, pine. Some don't e.g., maple, ash. maple is actually a sapwood tree often forming discolored or color altered wood after a wound or injury. Remember heartwood gets its color from an aging process and not from a wound or injury. Ash on the other hand forms what we call false heartwood. Tulip poplar trees form false heartwood that is thought to be the result of the death of branches.
And, the cambial zone produces xylem to the inside and phloem (inner bark) to the outside. The cambial zone does not produce wood. Just outside the phloem is the phellogen which is sometimes called the bark cambium. It produces the outer corky bark (phellem). It is interesting that all parts of the tree are born alive. The outer corky bark is born alive but shortly dies as it uses up its energy in forming suberin.
It also states that the soil bacteria and fungi make soil nutrients available for the tree when actually it is essential elements that are altered by soil microorganisms for absorption by the tree.
They claim the inner bark carries "FOOD" . This is true. I remain optimistic.