  We look at a graph property called reducibility which is closely related to a
condition developed by Brown to evaluate Feynman integrals. We show for graphs
with a fixed number of external momenta, that reducibility with respect to both
Symanzik polynomials is graph minor closed. We also survey the known forbidden
minors and the known structural results. This gives some structural information
on those Feynman diagrams which are reducible.
