What is a tree?

Trees are plants and carry out the life processes that all plants share. However, trees are not actually a scientific group of their own. Trees may be cone-bearing plants (gymnosperms) or flowering plants (angiosperms). Tree ferns are technically not trees as they do not contain wood.

All the groups of plants that include trees are vascular plants. This means they have vascular tissues called xylem and phloem. Xylem and phloem link all parts of the plant, transporting water, minerals and manufactured food around while also forming part of the structural support for plants.

What makes a plant a tree?

All trees are perennial plants. Many tree varieties live for tens or even hundreds of years, and they tend to live longer than most other types of plants. Trees are also different from many other plants in the following ways:

• Trees typically have a permanently woody stem or trunk.
• Most trees grow to a considerable height, usually bearing lateral branches at some distance from the ground.

Shrubs are also woody plants but tend to have several perennial stems and are usually shorter than 4 m. In New Zealand, we don’t tend to distinguish too much between trees and woody shrubs. One reason for this is that many of our plants vary considerably in their growth habits depending on the climate they are growing in.

How do plants grow?

Plants grow bigger because of specialised dividing tissues called meristems. These are found in the shoot tips, root tips and lateral buds and at the tips of any branches or lateral stems. The cells in these meristematic tissues actively divide by mitosis, making the plant grow bigger. This is called primary plant growth.

There are also meristematic cells inside the vascular tissues (xylem and phloem) that run up and down inside the plant stems, roots and leaves.

How do trees make wood?

The major reason trees can grow so big is their ability to create woody tissue as they grow. This process is called secondary plant growth. Wood contains a chemical called lignin. Lignin is a component of most plant cell walls, providing rigidity and shape. It is also an important component of vascular tissues and is present in large amounts in all vascular plants, providing structure and support. The hydrophobic properties of lignin also help the xylem transport water.

In woody plants, the vascular bundles grow sideways around the inside of the plant stem and join up as the plant gets older. They become the bands of vascular tissues that you can see in a stem cross-section.

Each year, the woody stem grows wider due to new xylem and phloem being made by the meristematic tissue. This tissue is called the cambium in a woody stem. Wood is the xylem, and the phloem (outside the cambium) becomes part of the bark.

As new phloem is made, the old phloem is pushed out to become bark on the outside of the stem/trunk.

As new xylem ages, it becomes increasingly lignified. Eventually, the old xylem stops its job of transporting water and becomes the heartwood of a tree. The active xylem in a tree is also called sapwood. The rings in a tree trunk show the yearly growth of xylem, which, by the time it has become wood, is largely just lignin and cellulose.