The limit to asymmetry, (ζ^low, [dimensionless]), is defined according to Eq. 5. All radii in CCO-trees are scaled with reference to r_root (radius of the root segment). r_root itself is then set so as to make the resistance of the whole tree yield the total flow required by the boundary conditions. The root segment has bifurcation level 0. Following any path from root to its terminal segment, the level count is increased by 1 at each bifurcation. According to Strahler's terminology terminal segments are assigned Strahler order 0 (Strahler, 1952, 1957). If both daughters of a bifurcation have unequal Strahler orders, the parent segment acquires the larger Strahler order. If both daughters have Strahler order j, the parent segment has Strahler order j + 1. The root segment always has the maximum Strahler order occurring in a tree. Successive segments may have equal Strahler orders, whereas they always have different bifurcation levels.