Meristem

Meristems are regions of actively dividing, undifferentiated cells that serve as the growth engines of plants. All new tissues in a tree originate from meristematic regions. There are two primary types: apical meristems at the tips of shoots and roots (responsible for elongation), and lateral meristems like the cambium and cork cambium (responsible for increases in diameter).

Apical meristems drive the upward and outward expansion of the tree's canopy and the downward and outward growth of the root system. Each shoot tip and root tip contains a dome of meristematic cells that continuously produce new cells, which then differentiate into the various tissue types needed. The growth hormones produced by shoot apical meristems, particularly auxin, influence the entire tree's growth patterns, including apical dominance (the suppression of lateral buds by the terminal bud).

Understanding meristematic growth is practically important for tree management. Pruning manipulates meristem activity: removing the terminal bud releases lateral buds from suppression, stimulating branching. Heading cuts that remove the tip of a branch stimulate dense regrowth from multiple buds, while thinning cuts that remove an entire branch redirect growth to remaining branches. Herbicide damage to meristems can cause distorted growth, and many tree growth regulators work by modifying meristematic activity.