Wood structure

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Wood grain
© Forestry Commission

Gymnosperms and angiosperms

Wood is an extremely versatile material and is ingeniously arranged to provide a living structure that combines both strength and flexibility.

Building a woody trunk and branches is expensive for a plant in energy terms but cheap to maintain because most of the cells are dead. Wood is a strong, long-lasting material and trees are long lived so they can justify the investment.

The two major groupings of trees; the Gymnosperms or conifers and the Angiosperms or broadleaved trees have different types of wood structure. In the timber industry they are often referred to as ‘softwoods’ and ‘hardwoods’ respectively.

  • In gymnosperms, the wood is made up of many thin tubes called tracheids; each between 0.1mm and 10mm long and 1.2 thousandths of a millimetre wide. These are lined up longitudinally in the wood. Each tracheid is a closed unit which joins to its neighbours by tiny holes. Water has to make a slow steady journey through hundreds and thousands of tracheids to get from the roots to the leaves.
  • In angiosperms, the tubes making up the wood are wider in diameter and the cells are open-ended, abutting each other to form vessels that may be several metres long. This system can transport large volumes of water very quickly from roots to leaves.

These two distinct wood cell structures have evolved to assist the tree in transporting water under different climatic conditions.

When leaves lose water through their stomata on warm days, the leaf cells become drier. The dry cells pull water from the next driest cell and this continues until the pull reaches a leaf vein. This exerts suction on the water in the xylem tissues and the tension is passed down the tree eventually pulling a whole column of water up from the roots.

But suction can only work if there is no air in the system and air can arise if there is too much tension, if the tree is damaged or if the water freezes.

In very hot weather leaves can lose water faster than the tree can transport it leading to an increase in tension in the xylem tissue. In some cases, the tension is so great that individual columns of water can break or cavitate, allowing air to enter the system.

Similarly, in very cold weather, the water in the xylem tissue cools and freezes and the gas dissolved in it appears as air bubbles. In the spring these bubbles must be re-dissolved into the water so the tubes can function.

In gymnosperms (conifer species) the water-carrying tubes are narrow and each tube only carries a small amount of water at a slow speed. This means the columns of water are less likely to cavitate during hot weather and any air bubbles forming in the winter remain small and quickly dissolve in spring. This enables the gymnosperms to cope with more extreme climatic conditions.

In the angiosperms, the wide, long water-carrying tubes are at a greater risk of cavitation when weather is hot. During the winter, if conditions freeze, the tubes may also develop large air bubbles that take several weeks to re-dissolve in spring; making the tubes useless at a critical time.

Fortunately, the angiosperms have a back-up policy to help them in either eventuality. The wide xylem tubes are usually formed early in the growth season by the cambium. But each year the cambium also produces narrower, thicker vessels later in the growing season. If extremes of temperature do occur, and the wide vessels fill with air and stop transporting water, then the suction from the leaves becomes so great that water will begin to move up these narrower, thicker xylem tubes formed from previous years of growth.

Ultimately, the wood structure of angiosperms (broadleaved trees) has evolved to suit temperate climates, where extremes weather conditions are rare.

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Oak trees grow in temperate climates

Broadleaves are more suited to temperate climates
© Forestry Commission

Conifers are suited to extreme temperatures
Conifers suit extreme temperatures   
© Forestry Commission

Sapwood and heartwood

Within both soft and hardwood trees sapwood and heartwood are formed. The sapwood is the xylem tissue. It contains living cells that transport water around the tree, as described above. Over time, the innermost sapwood is gradually converted to heartwood through a specific and deliberate process.

Tree core

As the heartwood is formed, the water-carrying tubes within it are blocked and filled with resins and gums which help stiffen the stem to build strength and provide protection from pests, disease and rot.

Logs showing sapwood and heartwood
Logs showing darker heartwood © Forestry Commission

Production of new sapwood and expansion of dead heartwood are ongoing processes in the growth cycle of a living tree.

If a tree is felled, the sap will slowly evaporate, shrinking the timber which may cause it to split. When this process, called seasoning, is finished the timber is much more stable and can be used for joinery.

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These are lines of living cells which radiate out from the centre of the tree (heartwood) out through the xylem and cambium to the phloem tissue just below the bark. The primary role of the ray cells is to store excess glucose produced through photosynthesis. This excess glucose is transported from the leaves through the phloem cells and into the ray stores where it is converted to either starch or fat. Trees maintain stores to help flower and fruit production in the spring and as an insurance policy against bad times when food production might be low; such as storm damage to the canopy.

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Most cells in the xylem or wood are orientated vertically up and down the line of the trunk. This is known as the grain. If you were to cut the tree at right angles to the trunk you would cut across the grain.

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