In contrast to primary growth, which makes a plant’s stem grow taller and its roots grow longer, secondary growth makes a plant grow wider. Primary growth occurs only through a plant’s shoot apical meristems, located at the tip of each branch, and root apical meristems, located at the tip of each root. By contrast, secondary growth occurs in the lateral meristems, which are found in the tree's trunk and branches.
Without secondary growth, plants would eventually topple over, as their thin stems would not be able to support the tall trunks. However, by growing laterally as well as vertically, plants can become quite strong over time. Many long-lived tree species grow exceptionally thick trunks to support their immense bulk. Secondary growth is responsible for the ring patterns found the cross-sections of trees. This happens because the rate of secondary growth changes with the seasons.
Additionally, secondary growth allows trees to live longer. As a perennial plant or tree grows, it replaces the old, dead cells with new, live cells on the outside. After several years, the cells in the middle of the plant die. From this point forward, the middle layers only provide a structural function. Many old trees feature completely hollow interiors, having succumbed to rot.