Trees in temperate regions have two growing periods each year. At the start of the growing season the cells produced by the inside of the cambium (the growing layer just under the bark) have thin walls and large cavities to assist in the efficient movement of water and food from the roots to the rapidly developing leaves. This is known as 'spring wood' or 'early wood'. During the summer months when the crown has fully developed the need for water is reduced. Cells developing in this period have thicker walls and smaller cavities and provide a more structural function. This is 'summer wood' or 'late wood'. Cells produced by the outer surface of the cambium form bark.
Hardwood species where the change from thin-walled spring wood to thick- walled summer wood is abrupt are known as 'ring-porous', for example oak. This development of cells produces growth rings. Where the seasonal cell change is gradual the wood is known as 'diffuse-porous' as with beech. Grain feature in these species is less prominent, giving a relatively uniform appearance to the finished wood. In most tropical hardwoods growth is not seasonal so many tropical species are diffuse-porous. Coarseness of grain is determined by cell size and influences the use of different species.
In some species cells develop in a spiral direction around the tree forming spiral grain which, if severe, can be detrimental to its structural performance. Some tropical species produce spiral growth in opposite directions in alternate growing periods, giving rise to interlocked grain and the stripe figure as seen in afrormosia. Grain characteristics influence many aspects of wood use from machining to finishing. The way in which a log is sawn influences the surface appearance, especially with ring-porous species. Sections cut across the growth rings, 'quarter sawn', have a parallel grain pattern; 'flat sawn' sections will demonstrate a more lively (flamey) appearance.
Written by The Wood Bureau's John Park and Brian Keyworth, an architect.