  The effects of a boundary on reaction systems are examined in the framework
of the general single-species reaction/coalescence process. The boundary
naturally represents the reactants' container, but is applicable to exciton
dynamics in a doped TMMC crystal. We show that a density excess, which extends
into the system diffusively from the boundary, is formed in two dimensions and
below. This implies a surprising result for the magnetisation near a fixed spin
in the coarsening of the one-dimensional critical Ising model. The universal,
dimensionally-dependent functional forms of this density excess are given by an
exact solution and the field-theoretic renormalisation group.
