Jana Cluster, age 11, of Edwardsville, Ill., for her question;
What causes knots in wood?
A tree grows slowly, year by year. Its trunk and branches are made of woody little cells, fitted together like stacks of neatly packed boxes. New layers are added season by season. When the wood is sliced, the boxy layers can tell us the whole history of the tree. They cause the rippling grain and the lumpy knots that appear in the wood.
The furry green Christmas tree has the answer to this knotty question. It is a conifer or cone bearing tree, and these are the trees that give us knotty wood. They are evergreen stuffed with sticky gurus and tangy resins. These tacky substances are in the leaves and twigs, in the trunks and branches. They seal out the cold and protect the hardy evergreens from attacking insects.
The seed for a new Christmas tree develops inside a cone for more than two years. The baby tree is a slender spike and at the age of three are only a few feet tall. It has a trunk, and its branches spread forth at the proper angles. As the seasons go by, it grows five feet, 10 feet, 15 feet tall.
The dense branches at the bottom of the young tree spread over the ground. The trunk is hidden by shorter branches, tapering to a point at the top of the tree. The conifer is a Christmas tree for about 20 years. Then its trunk begins to lift toward the sky, and the lower branches are dropped. For a few years, the tree goes through an untidy stage.
The fallen branches leave deep wounds in the trunk. But the tree seals its wounds with oozing gobs of resin. Season by season, new layers of woody cells are added around the trunk, and the old scars become burled deep inside. Each one is a gob of hardened resin, which marks the spot where a lower branch once grew and later fell to the ground.
A great pine tree may lift its top branches 150 feet above the ground. At last it is cut down, and its tall round trunk is sliced into flat boards. The pale wood is marked with rows of grooves and ridges. These are the layers of woody cells added year by year around the trunk. Here and there a dark island of hard breaks this graceful design and knotty wood. This is the old scar from a fallen branch, now sealed with hardened resin.
Your Christmas tree is too young to have knots buried inside its slender trunk. It may be a pine or a fir, destined to lose its lower branches and reach for the sky. The scars left by the fallen branches will leave knots inside the woody trunk. Your tree may be a spruce or, juniper that will keep its low, brushy branches. This tree may have a few wounds and seal them with resin. There will be only a few hard knots in its woody trunk.