  In this paper, the extinction problem for a class of distylous plant
populations is considered within the framework of certain nonhomogeneous
nearest-neighbor random walks in the positive quadrant. For the latter,
extinction means absorption at one of the axes. Despite connections with some
classical probabilistic models (standard two-type Galton-Watson process,
two-urn model), exact formulae for the probabilities of absorption seem to be
difficult to come by and one must therefore resort to good approximations. In
order to meet this task, we develop potential-theoretic tools and provide
various sub- and super-harmonic functions which, for large initial populations,
provide bounds which in particular improve those that have appeared earlier in
the literature.
