  Contrary to empirical observations, lowest-order $k\cdot\hat{p}$ theory
predicts that monolayer black phosphorus ("phosphorene") is completely immune
to zigzag-polarized optical excitation at the bandgap energy. Using symmetry
arguments, we derive a $2\times2$ Hamiltonian under the $k\cdot\hat{p}$
formalism including higher-order corrections, which is used to show that the
experimentally-measured band-gap transition with zigzag polarization is
dominated by the third order $k\cdot\hat{p}$ perturbation in the interband
optical matrix element, whereas the effects of spin-orbit interaction are
negligible in this material, consistent with a trivial orbital diamagnetic
contribution to the $\textit{g}$-factor.
