As a graph, a tree is represented by a set of labelled nodes connected by edges. Since most statistics describing a neuron‘s branching relate to the root (e.g. branch order, which increases after each branch point on the way from the tree root to all terminal nodes) it makes sense to attribute a directionality to the edges and to define the root as the node with the index 1. All edges lead away from the root. That defines their directionality uniquely.

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License