  We report a new mechanism of pattern formation in growing bistable systems
coupled indirectly. A modified Fujita et. al. model is studied as an example of
a reaction-diffusion system of nondiffusive activator and inhibitor molecules
immersed in the medium of a fast diffusive agent. Here we show that, as the
system grows, a new domain nucleates spontaneously in the area where the local
level of the agent becomes critical. Newly nucleated domains are stable and the
pattern formation is different from Turing's mechanism in monostable systems.
Domains are spatially confined by the agent even if the activator and inhibitor
molecules diffuse. With the spatial extension of the system, a larger domain
may undergo a wavenumber instability and the concentrations of active molecules
within the neighboring elements of a domain can become sharply different. The
new mechanism reported in this work can be generic for pattern formation
systems involving multistability, growth, and indirect coupling.
