Trees grow in two ways: they get longer, and they get wider. That doesn’t sound too different from the way an animal grows, but unlike animals, trees only grow longer at their margins. The tip of each stem contains a small cluster of undifferentiated cells called the apical meristem: as these cells divide, the undifferentiated cells are pushed forward and new shoot cells are laid down in their wake. The result? Older cells in a tree’s stem never change position relative to the ground: the part of a 300 foot redwood tree that’s at eye level now was also at eye level when the tree was only 8 feet tall.
So why aren’t trees tall lanky whips of spaghetti-thin twigs? They have another trick, which lets them grow thicker as trunk and branches push out at their tips: the tissues near the outer surface of the stems called secondary meristem. When these tissues divide, they add layers of cells to the outside of the stem, gradually adding girth to the tree, forming rings of new wood over older tissues, and engulfing older branches over time.
Where animals remodel during growth, plants enclose – so every tree contains the shape of its younger self.