  We present new J=1-0 12-CO observations along the northern spiral arm of the
grand-design spiral galaxy M100 (NGC 4321), and study the distribution of
molecular hydrogen as derived from these observations, comparing the new data
with a set of data points on the southern arm published previously. We compare
these measurements on both spiral arms and on the interarm regions with
observations of the atomic and ionized hydrogen components. We determine
massive star formation efficiency parameters, defined as the ratio of H alpha
luminosity to total gas mass, along the arms and compare the values to those in
the interarm regions adjacent to the arms. We find that these parameters in the
spiral arms are on average a factor of 3 higher than outside the arms, a clear
indication of triggering of the star formation in the spiral arms. We discuss
possible mechanisms for this triggering, and conclude that a density wave
system is probably responsible for it. We discuss several possible systematical
effects in some detail, and infer that the conclusions on triggering are sound.
We specifically discuss the possible effects of extinction in H alpha, or a
non-standard CO to H_2 conversion factor (X), and find that our conclusions on
the enhancement of the efficiencies in the arms are reinforced rather than
weakened by these considerations. A simple star forming scheme involving
threshold densities for gravitational collapse is discussed for NGC 4321, and
comparison is made with M51. We find that the gas between the arms is generally
stable against gravitational collapse whereas the gas in the arms is not,
possibly leading to the observed arm-interarm differences in efficiency, but
also note that these results, unlike the others obtained in this paper, do
depend critically on the assumed value for the conversion factor.
