Fungal mycelia epitomize, at the cellular level of organization, the growth and pattern-generating properties of a wide variety of indeterminate (indefinitely expandable) living systems. Some of the more important of these properties arise from the capacity of an initially dendritic system of protoplasm filled, apically extending hyphal tubes to anastomose. This integrational process partly restores the symmetry lost during the proliferation of hyphal branches from a germinating spore and so increases the scope for communication and transfer of resources across the system. Growth and pattern generation then depend critically on processes that affect the degree to which resistances to energy transfer within the system are sustained, bypassed or broken down. We use a system of reaction-diffusion equations augmented with appropriate initial data to model the processes of expansion and pattern formation within growing mycelia. Such an approach is a test of the feasibility of the hypothesis that radical, adaptive shifts in mycelial pattern can be explained by purely contextual, rather than genetic, changes. Thus we demonstrate that phenotype does not necessarily equate solely to genotype-environment interactions, but may include the physical role in self-organization played by the boundary between the two.