Do Trees Grow From Top or Bottom?

Do trees grow from the top, the bottom or both?

 —Lynne C., San Diego, Calif.

Both, in a way. Readers may be surprised to learn that only a tiny portion—say, 1 percent—of a tree’s mass is alive and capable of growing. (Wood is nonliving material.) Twigs contain living cells, which cause branches to lengthen and the tree to grow taller as it assumes the height and form characteristic of its species. Under the bark is a thin layer of cells that cause the tree to increase in girth, so it does continue to grow “from the bottom,” though not in height. If a rubber band is wrapped around the trunk of a mature tree, the band will not move upward over time: It will stretch outward.