A node in a directed graph is called a root if it has indegree zero, i.e., there are no edges pointing to it. Moreover, it is usually thought that a rooted tree or a rooted graph has only one root (cf. origin). A counterexample would be a text that grew out of two texts, each of which had its own original (being its root). The presence of (at least) one root is required for a tree to be called a stemma.
