Branch initiation by global control

Branch formation is a dominating pattern element of Oliva porphyria (Figure 6.1). Considering that the shell patterns resemble space-time plots, it is obvious that the formation of a branch is based on the sudden initiation of a backward-running wave. In the view of the normal behavior of waves in excitable media, this is a very unusual event. A new wave must spread into a region that should be refractory due to the primary wave.