A. The Bark
The bark consists of several regions, but these can only be identified with accuracy by persons
experienced in this field. From outside inwards the regions are as follows:
1) Epidermis – an outer protective layer which prevents to some degree the loss of moisture.
2) Cork- a protective layer of variable thickness, depending on the species of tree and its age.
Gives protection against desiccation, injury and insect attack.
3) Phellogen or cork-cambium – an actively dividing region producing cork cells.
4) Phloem- a continuous layer around the circumference extending to all branches. It conveys
sugars from the leaves to various parts of the tree. The phloem is rich in nutritive food such as minerals and sugars which form the foods of a large number of insect larvae.
If a tree is ringbarked (i.e. only as far as the sapwood) it will receive water and mineral salts up the sapwood and move manufactured foods down to the cut area, and as a result it will live for many months. When a tree is ringbarked and the sapwood is also cut it will die very quickly, as it is not receiving water from the soil.
5) Cambium – a very thin layer of cells which is not visible by ordinary vision. It is responsible for the growth in girth of trees and is located between the phloem and sapwood cells. The cambium produces phloem cells on its outside and sapwood cells on the inside. Insects feeding in the phloem invariably sever the cambium also.
B. The Sapwood
This region carries water and mineral salts from the roots to the leaves where the water is lost by
transpiration, thus cooling the tree. The tiny pores or tubes which convey the water are known as
xylem vessels in hardwoods and tracheids in pines.
As the tree grows the sapwood becomes relegated to heartwood, which is usually the more extensive region. The sapwood contains ray cells which convey sugars to the sapwood where they are converted to starch and stored.
The presence of starch determines susceptibility to the powder post beetles. The sapwood, along with the heartwood, gives support to the tree.
C. The Heartwood
This region is also known as the “truewood”, but it implies that sapwood is inferior, which in many
cases is not so, Both sapwood and heartwood have similar moisture contents, although heartwood
consists of non-functional cells.
The main function of heartwood is support, but if the centre of a tree is eaten out by termites it
can still live, the sapwood giving sufficient strength until more heartwood is produced.
The heartwood cells are darker in colour than those of the sapwood, dead, contain toxic materials
and few, if any, food materials. The lignin content of heartwood is higher than that of the sapwood,
contributing to the darker colour. This can be seen in black bean and many wattles, but in pine woods the difference is not so marked.
D. The Pith
The pith is the centre part of the tree, composed of dead cells and represents the earliest growth
of the tree.