The darkness of sunspots has been attributed by many authors (Biermann 1941; Danielson 1961) to the inhibition of the normal solar convective processes by the presence of strong magnetic fields. Observations of the solar photospheric granulation pattern have also shown that a weak longitudinal field exists outside the activity regions. Although these observations have not revealed any close association between the magnetic field and individual granules, nor the exact reasons for the darker cell boundaries, it must be accepted that, overall, the role of the magnetic field must be such as to influence the cell structure and reduce the normal heat transfer by convection.