The Trunk

A number of complex processes take place in and beneath the bark of your bonsai and to understand how your tree grows, you will need to understand them.

There are six layers forming the outer skin of a tree. In this section we will look at what each layer does.

The outer layer of the Bark is the Epidermis. This forms a waxy, waterproof skin, helping the plant to retain moisture. This skin is also present on the leaves.

The epidermis is peppered with small holes, called stomata. These holes are used, both to breath and vent excess moisture into the atmosphere.

The Bark is the next layer in. Bark is formed by the bark cambium. The cells produced, are designed to be impervious to both moisture, and gasses. They soon harden and die.

The expansion of the living tree beneath the bark stretches and cracks the dead layers, causing the bark to fissure. This gives the familiar textures to the bark of our trees.

The Bark Cambium is the next layer in. The bark is formed by its own layer of cambium, fed by the sugars in the phloem.

The Phloem comes next. It is the pathway in which the sugar bearing sap is transported around the tree, powering its growth. It is the region of the tree responsible for the development that takes place in the two cambium layers that sandwich it.

The Cambium layers are where most of the growth takes place.

In periods of growth, it is the cambiums which use most of the energy (sugars) contained in the sap, powering the development of the layers on either side of the cambium, of new shoots, roots and the repair of any parts of the tree that become damaged.

Cells produced in the cambium layer move either inward, forming the Xylem, or outwards as the Phloem and the layers on either side of them.

The Xylem is the part of the tree that carries the nutrient rich water to the top of the tree.

As the new xylem is created by the cambium, older cells forming the tubes in which the water moves die, forming the wood.

The cells of the xylem contain large amounts of a substance called lignin and it is that lignin which forms the wood.

Sap Flow

We are all aware of sap, and understand that it moves up and down the tree, performing a similar function to our own blood.

The roots absorb both water and nutrients. This cocktail travels up the tree through the tubes in the xylem. As it reaches the top of the tree the mixture finds its way out through the branches and shoots to the leaves.

Having passed through the leaves and the process of photosynthesis, the sugar rich sap moves throughout the tree in the tubes of the phloem.

The Wood

Let's start by looking at what wood is.

The wood is formed as the inner layers of the xylem are replaced by new growth.

The cells that form the tubes of the xylem, as with all cells in plants have a membrane of cellulose. The cells of trees and shrubs however have an extra component, lignin. Lignin is what gives woody plants their strength.

To the left we see a section through a twenty-four year old Scots pine, felled for timber in Norfolk, England. Counting the trees growth rings gives its age.

Close inspection of the distances between the rings shows larger gaps in the centre and hence faster growth when the tree was young.

Commercial timber is classified as two types, Hardwoods and Softwoods and this classification parallels the division of Gymnosperms, the conifers, producing softwood and the Angiosperms, the hardwood trees, the flowering broadleaved trees.

Hardwoods are much stronger than their Softwood cousins. The reason for this increased strength is that the tubes in a hardwood are generally smaller than softwoods and there is more wood between the tubes.

The tubes were the vascular pathways through which the sap used to flow. These tubes became redundant as the plant grew and the bark moved away from them.

A hardwood cross section

A Softwood cross section

Allen. C. Roffey 02:08 14/09/2008