As trees grow they produce cells, which arrange themselves as fibers and water-conducting vessels. Some run vertically, others form radial structures outward from the center of the tree. Each year’s cells create a cylinder of fibers—a growth ring—around the tree, from bottom to top. We learned that the closer together the annual rings are, and the higher the percentage of summerwood, the greater the density and the greater the strength of the wood. Growth rings vary in thickness from year to year, branches grow and create knots. Even the way the sawmill operator cuts a log into lumber directly affects a board’s grain.