A bifurcation in an optimal wiring cost tree has to lie on a plane and consecutive planes are independent of each other.

<p><b>A.</b> Projecting the red bifurcation point () onto the blue bifurcation plane reduces the costs of all three edges involved, and therefore the total cost of the entire tree. Note that in this example the bifurcation plane is defined by neighboring bifurcation points (), but some or all of these could be replaced by fixed or terminal points. <b>B.</b> Most of the distributions of angles between consecutive bifurcation planes approximate the uniform distribution, except for Purkinje cells, L5 PFC pyramidal cells and <i>M. mirabilis</i>. Data from the other 6 neuron types and 3 coral species were statistically consistent with a uniform distribution (p>0.05, one-sample Kolmogorov-Smirnov test, considering only basal dendrites for L2/3 pyramidal neurons). <b>C.</b> Example of bifurcation planes in a L2/3 somatosensory cortex pyramidal cell. The bifurcation planes are color coded according to their cone angles.