  From a total sample of 45 Abell clusters observed by the Einstein X-ray
observatory, we present the results on the galaxy luminosity function (LF) for
a group of seven clusters that were identified by the morphology of their LFs.
The LFs were derived using photometric data to a completeness limit ~5.5
magnitudes below M*. We found that a single Schechter function with an average
$\alpha \approx -1.0$ gives a good fit to these individual LFs within the
magnitude range. These seven clusters have common properties, which indicate
they form a homogeneous class of dynamically evolved clusters that can be
characterized by the presence of a dominant cD galaxy, high richness,
symmetrical single-peaked X-ray emission, and high gas mass. On the other hand,
steep faint-end slopes (-2.0 < \alpha < -1.4) are usually detected in poorer
clusters. Our result gives a direct indication that the faint-end slope of the
galaxy LF is subject to environmental effects. We propose that the flatness of
the faint-end slope in these clusters results from the disruption of a large
fraction of dwarf galaxies during the early stages of cluster evolution. The
stars and gas from the disrupted galaxies are redistributed throughout the
cluster potential during violent relaxation. This heuristic scenario can
explain the origin of the luminous haloes of cD galaxies and a large fraction
of the gas content in the intracluster medium as a by-product. The correlation
between the cluster gas mass determined from the modeling of the X-ray emission
and the cD halo optical luminosity is presented to support the proposed model.
