A very young sapling is a small twig. The growth centre is wrapped around the outside of it and lays down new cells, one layer at a time. Once the tree has filled out, the original twig is locked inside the rest of the wood. It is quite dark in colour and is known as the pith.
The wood growth cells are in a thin layer between the bark and the sapwood. This thin layer is called the cambium. It lays down new bark on the outside, and new wood cells on the inside. As the tree grows, each layer of wood cells remains in its original position except the cambium. It is continually moving outwards.
The bark offers protection for the cambium and for the new wood. However as the tree’s girth is continually increasing, the bark must grow to accommodate it. It sheds continually under the pressure from the new bark being created underneath.
Sapwood is the newest wood in a tree. When the wood cells are first laid down, they have very thin walls and large hollow voids in the middle. The outermost wood carry sap from the leaves to the cambium where it is used to manufacture new cells. The sap is very high in starch, and remains in the sapwood even after the wood has been milled, dried and processed.
The starch makes sapwood very attractive to fungal and insect attack. If the growing tree is injured or if insects can get to the sapwood, the tree reacts by floating the area with gum and laying down much more wood and bark to heal the damage.
As the sapwood sees service over a number of seasons, the cell walls thicken up, making them stronger, but for softwoods, less useful as conduits. Eventually they are blocked completely and in all species, the remaining void is used to store waste products from the tree growth. These waste products originate in the leaves as by-products of photosynthesis or in the cambium as by-products of cell formation. Once the cells are blocked off, they are free of starch and the waste products they now contain are known as extractives.
