Apical dominance in higher plants is due to

Apical dominance is the phenomenon whereby the main central stem of the plant is dominant over (i.e. grows more strongly than) other side stems; on a branch, the main stem of the branch is further dominant over its own side branchlets. It occurs in higher plants due to hormones auxin, which is responsible for apical dominance, and cytokinin, which is responsible for breaking of apical dominance.