All About A Tree

Figure 1 - Cross section of a tree

1. The Pith

The Pith is the centre of the tree and is composed of soft spongy parenchyma cells which is also another name for the pith. In trees the pith is surrounded by xylem which is one of the two types of transport tissues in trees, the xylem is most concentrated in the inner section of the tree. The xylem stores and transports nutrients and root-sap around the tree to the leaves (Edlin, 1978). Phloem is a living tissue that carries organic nutrients and sugar-sap around the tree, which it receives from photosynthesis from the leaves which is transported to parts of the tree including the roots. The phloem is found in the leaves and the inner layer of the bark. The name comes from the Greek word “phloios” meaning “bark”. As the tree grows in diameter it compresses and squeezes the pith, after many years it is surrounded by layers of heartwood.

2. The Heartwood

The heartwood began its life as sap-wood but as the years have gone on the inner section of the stem is closed off from active sap movement. The heartwood is the densest part of the tree, yielding the hardest timber. You can usually distinguish between sapwood and heartwood by their colour; heartwood is usually darker than sapwood, as can be seen in figure 2. Ones the chemical change in the wood takes place it renders the centre of the stem more durable and is therefore only there for support of the trees crown.

3. The Bark

The bark on a tree is a good way of distinguish between different types of trees. The bark on trees is a waterproof layer made up of cork that surround the stem and maintains a constant water supply. The bark also protects the tree from temperature change, insects, fungal diseases and unexpected damage. Bark is a breathable material that allows the stem to get enough air passing through it to carry on functioning (Edlin, 1978). The patterns on treess are formed by the expansion of the wood but because the waterproofness of the tree must stay constant the cambium will produce new layers of cork cells beneath it. This process gives the bark depth and different patterns. Trees split in different ways but follow a trend depending on the tree type.

4. Cambium

Between each cylinder of cells in the stem is a dividing cell called the cambium. During the activate growing seasons the cambium will produce more phloem on the external side which enables it to become larger on the outer side (Edlin, 1978). On the inner side of the cambium produces more xylem. This is the reason for the formation of the annual growth rings in the wood.

5. The Disease

This is a section of the tree that will have been infected with some sort of fungus or disease. They come is a large variety of types and can affect pants and trees in different way.

6. Knot

Knots are imperfections of a tree causing lump or holes in the trunk or inside the tree. They are normally caused by natural growth and will vary in appearance. However, some knots are formed by fungal diseases and can spread to other trees.

7. Annual Growth Rings

Each year a tree will develop more cells causing it to grow. When this happens it produces a ring, this happens every year. The in spring the cambium produces large cells which mean the annual growth rings are wider; these rings are referred to as springwood. Towards the end of summer growth slows down and the cells are much smaller causing the rings to be smaller this is referred to as summerwood (Fallah, Riahifar, Barari & Parsakhoo, 2012). Rings come in all different sizes depending on a number of factors; temperature, sunlight, water, surrounding areas and many others. Narrow rings indicate a lack of sunlight and water.

8. Sapwood

Sapwood is the younger outer part of the tree, at one point all of the wood was originally sapwood. Its main function is to move nutrients around the tree from the leaves to the roots (Fallah, Riahifar, Barari & Parsakhoo, 2012). It also works as a storage area for the nutrients and will give nutrients out at different rates depending on the season. Sapwood will be larger in trees that have space to grow such as trees that grown in the open compared to trees growing in dense forest. Ones sapwood becomes old it starts the process of becoming heartwood.

9. The Growth

Figure 1 shows that the pith of the tree does not have to always be in the centre. In this tree you can see that the pith is off centre and looks to be growing wider on one side. There are a number of factors that can suggest why the pith will be off centre it could be due to the trees location in relation to the sun or whether it was exposed to winds that usually occur in the same direction.

Reference 

Edlin, H.L. (1978). The Tree Key. London: Frederick Warne & Co. 22-42.

Fallah, A. Riahifar, N. Barari, K. & Parsakhoo, A. (2012). Investigating the out-of-roundness and pith-off-centre in stems of three broadleaved species in Hyrcanian forests. Journal of forest science. 58, 513-518.

The National Christmas Tree Association. (2008). Stems and Rings. Available: http://www.realtrees4kids.org/sixeight/stemsrings.htm. Last accessed 6th November 2014.