  A detailed comparison of HERA data at low Bjorken-$x$ and low
four-momentum-transfer squared, $Q^2$, with predictions based on $\ln{Q^2}$
evolution (DGLAP) in perturbative Quantum Chromo Dynamics suggests inadequacies
of this framework. The standard DGLAP evolution was augmented by including an
additional higher-twist term in the description of the longitudinal structure
function, $F_{\rm L}$. This additional term, $F_{\rm L}~A_{\rm L}^{\rm
HT}/Q^2$, improves the description of the reduced cross sections significantly.
The resulting predictions for $F_{\rm L}$ suggest that further corrections are
required for $Q^2$ less than about 2 GeV$^2$.
