Why does wood have a grain?

Wood grain - in a piece of wood is the pattern produced by the annual bands or rings which grow in the trunk of the tree during its lifetime. The tree's rate of growth varies with the seasons. In the spring soft porous wood is needed to carry sap. In the summer, stronger cells of hard wood develop to support the growing weight of new leaves and branches. The number of these alternate bands of soft and hard wood gives the age of the tree. In close-grained wood, which has grown slowly, the annual rings are narrow and packed closely together. If the coarse grained wood has grown more quickly, the rings are broader and spaced more widely apart. Sometimes the rings are irregular, and the grain may be straight, spiral, interlocked or wavy. Skilful sawing is necessary to make the most of the grain and enhance the beauty of furniture made from the wood.