9 A vertex with no children is called a leaf or an externa/ vertex Two branches of a tree are adjacent if the lower vertex of the first branch is the upper vertex of the second. a sequence of branches in which each is adjacent to its successor is called a path. o The height of a tree is the number of vertices on a longest possible path from the root to a leaf. (Hence a tree containing only one vertex has height 1.) The depth or level of a vertex is the number of branches on a path from the root to the vertex. A vertex with no children is called a leaf or an external vertex.  Two branches of a tree are adjacent if the lower vertex of the first branch is the upper vertex of the second. A sequence of branches in which each is adjacent to its successor is called a path. The height of a tree is the number of vertices on a longest possible path from the root to a leaf. (Hence a tree containing only one vertex has height 1.) The depth or level of a vertex is the number of branches on a path from the root to the vertex