Stratifications of cellular patterns

Geometrically, foams or covalent graphs can be decomposed into successive layers or strata. Disorder of the underlying structure imposes a characteristic roughening of the layers. Our main results are hysteresis and convergence in the layer sequences. 1) If the direction of construction is reversed, the layers are different in the up and down sequences (irreversibility); nevertheless, under suitable but non-restrictive conditions, the layers come back, exactly, to the initial profile, a hysteresis phenomenon. 2) Layer sequences based on different initial conditions (e.g. different starting cells) converge, at least in the cylindrical geometry. Jogs in layers may be represented as pairs of opposite dislocations, moving erratically due to the disorder of the underlying structure and ending up annihilating when colliding.