  The semileptonic and leptonic decays of heavy mesons are studied as a
phenomenological application and exploration of a heavy-quark limit of
Dyson-Schwinger equations. The single form factor, $\xi(w)$, which
characterises the semileptonic decay in this limit, is calculated and compares
well with recent experimental extractions. We obtain a lower bound of 1/3 on
the slope-parameter $\rho^2 \equiv -\xi^\prime(1)$, which, in calculations with
realistic input, is exceeded by a great deal: agreement with experimental data
requiring $\rho^2 \sim 1.2 - 1.6$. The flavour and momentum dependence of the
light-quark propagators has observable consequences.
