  Nonmesonic decays of $\Lambda$ in nuclear medium and light hypernuclei are
studied by using the $\Lambda N \to NN$ weak transition potential derived from
the meson exchange mechanism and the direct quark mechanism. The long range
part of the transition potential is described by exchanges of the pseudoscalar
mesons ($\pi$, $K$, $\eta$), while the vector mesons ($\rho$, $\omega$,
$K^\ast$) may be considered as the medium- and short-range part in the meson
exchange picture. We propose the direct quark transition potential as the short
range part, which is derived from the matrix elements of the $\Delta S=1$
effective weak Hamiltonian in the two baryon states. The results indicate that
the direct quark contribution is significantly large and its behavior is
qualitatively different from the vector meson exchanges. We also find that the
decay rate is sensitive to the choice of form factor and that a soft cutoff
must be used for the pion-baryon verteces so that the strong tensor transition
is suppressed. We find that the $\pi + K + DQ$ results are compatible with
experiment although the $n/p$ ratio is still too large. The $\pi^+$ decays of
light hypernuclei are related to the $\DI=3/2$ amplitudes of the nonmesonic
decay. The role of chiral symmetry for the pionic decays are discussed.
