After being inoculated on an agar gel, the slime mold P. polycephalum
grows on the substrates. On a nutrient substrate the slime
mold grows similarly to excitation waves in a non-linear excitable
medium. On a non-nutrient substrate Physarum exhibits localized
growth zones, similar to self-localized traveling excitations, propagating
on the substrate away from the original inoculation site,
see details in . The active growth zones are subjected to internal
instabilities and therefore divide periodically. Thus a growing and
branching tree of protoplasmic tubes emerges from the inoculation
zone (Fig. 1).