  We present a symmetry analysis of electronic bandstructure including
spin-orbit interaction close to the insulating gap edge in monolayer black
phosphorus ('phosphorene'). Expressions for energy dispersion relation and
spin-dependent eigenstates for electrons and holes are found via simplification
of a perturbative expansion in wavevector $k$ away from the zone center using
elementary group theory. Importantly, we expose the underlying symmetries
giving rise to substantial anisotropy in optical absorption, charge and spin
transport properties, and reveal the mechanism responsible for valence band
distortion and possible lack of a true direct gap.
