  Despite many studies of the star formation in spiral galaxies, a complete and
coherent understanding of the physical processes that regulate the birth of
stars has not yet been achieved, nor a unanimous consent was reached, despite
the many attempts, on the effects of the environment on the star formation in
galaxies member of rich clusters. We focus on the local and global Schmidt law
and we investigate how cluster galaxies have their star formation activity
perturbed. We collect multifrequency imaging for a sample of spiral galaxies,
member of the Virgo cluster and of the local field; we compute the surface
density profiles for the young and for the bulk of the stellar components, for
the molecular and for the atomic gas. Our analysis shows that the bulk of the
star formation correlates with the molecular gas, but the atomic gas is
important or even crucial in supporting the star formation activity in the
outer part of the disks. Moreover, we show that cluster members which suffer
from a moderate HI removal have their molecular component and their SFR
quenched, while highly perturbed galaxies show an additional truncation in
their star forming disks. Our results are consistent with a model in which the
atomic hydrogen is the fundamental fuel for the star formation, either directly
or indirectly through the molecular phase; therefore galaxies whose HI
reservoirs have been depleted suffer from starvation or even from truncation of
their star formation activity.
