Why does wood have a grain?

The 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 the new leaves and branches.The number of these alternate bandsof soft and hard wood gives the age of the tree.(questions and answers)