  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.
