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Tree Growth

Trees are the tallest free-standing organisms in the world.

They live longer and grow larger than any other living organism on earth.

The world's tallest known living tree is a redwood tree called the Hyperion. It measures 115.61 metres (379.3 ft) in height, making it 22metres taller than Big Bens tower.

The cells from which all plants grow are called meristem. They are found at the tips of buds and roots. In most woody plants there is also a layer of cells called the cambium. These wrap around every stem and limb of the plant like a glove. The cambium allows trees and shrubs to grow outwards as well as upwards in each growing season, allowing them to become much larger than plants without a cambium.

Inside a living tree trunk there are several layers of cells.

The outer layer is the bark. Bark is tough and waterproof and protects the tree from the elements, insects, pests and fungal diseases. Bark also helps the tree to retain its moisture. As the trunk grows fatter, the bark spreads and cracks, often becoming gnarled in appearance.

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Underneath the bark, there is a layer of tissue called the phloem. This transports sugary sap made during photosynthesis from the leaves to all the other parts of the tree.

The next layer is the cambium which is responsible for the outward growth of the tree.

Beneath the cambium, are a number of layers of woody tissue called xylem.

The first few layers of xylem are known as sapwood. They are responsible for transporting water from the roots to the rest of the tree.

The innermost layers of xylem are right in the centre of the tree. These layers are known as heartwood. The heartwood is dense and strong and provides stability as the tree grows.

 

One growth cycle

Places like the UK have a seasonal climate. This means that plants do most of their growing in spring and summer. In every growing season the cells of the cambium divide to produce layers of new cells:

  • On the outer side of the cambium, new cells are added to the phloem.
  • On the inner side of the cambium, new cells are added to the xylem.

The xylem cells made during spring are wide with thin walls.These wide cells help the tree transport large volumes of water from the roots to the trunk and branches. The water from the roots supports the growth of new leaves and flowers.

Towards the end of the growing season, the cambium produces xylem cells that are narrow, with thicker walls.

The tree needs less water at this time of the year because growth activity is slowing down. Instead, the tree needs to produce strong, dense wood to help support its new growth. For this reason, the cells continue to narrow and thicken until the autumn.

All the xylem cells produced by the cambium during the year contribute to the outward growth of the tree.

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Oak tree in the growing season  

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Ash tree in the dormant season