A Petri Net approach to the study of persistence in chemical reaction   networks
  Persistency is the property, for differential equations in $\R^n$, that solutions starting in the positive orthant do not approach the boundary. For chemical reactions and population models, this translates into the non-extinction property: provided that every species is present at the start of the reaction, no species will tend to be eliminated in the course of the reaction. This paper provides checkable conditions for persistence of chemical species in reaction networks, using concepts and tools from Petri net theory. Nontrivial examples are provided to illustrate the theory. 
David Angeli, Patrick De Leenheer, and Eduardo Sontag