  Cord-forming fungi form extensive networks that continuously adapt to
maintain an efficient transport system. As osmotically driven water uptake is
often distal from the tips, and aqueous fluids are incompressible, we propose
that growth induces mass flows across the mycelium, whether or not there are
intrahyphal concentration gradients. We imaged the temporal evolution of
networks formed by Phanerochaete velutina, and at each stage calculated the
unique set of currents that account for the observed changes in cord volume,
while minimising the work required to overcome viscous drag. Predicted speeds
were in reasonable agreement with experimental data, and the pressure gradients
needed to produce these flows are small. Furthermore, cords that were predicted
to carry fast-moving or large currents were significantly more likely to
increase in size than cords with slow-moving or small currents. The
incompressibility of the fluids within fungi means there is a rapid global
response to local fluid movements. Hence velocity of fluid flow is a local
signal that conveys quasi-global information about the role of a cord within
the mycelium. We suggest that fluid incompressibility and the coupling of
growth and mass flow are critical physical features that enable the development
of efficient, adaptive, biological transport networks.
