A propagating flame front in a two dimensional fuel (paper in this case) develops a fingering instability due to oxygen deficiency. Beyond a critical oxygen input velocity the front will tend to coalesce and the fingers will disappear into an unbroken front.
The fingers tend to tip split but strictly avoid each other. The distance between fingers depends on the oxygen input velocity, while the width of the finger is determined by a number of factors such as the size of the gap through which the oxygen is supplied, and the actual gas composition.
The parameters in this example are extremely close to extinction of the flame.
Fingering instability in combustion: An extended view
Zik O. & Moses E. (1999) Physical Review E. 60, 1, p. 518-531 PDF