  The problem of the origin of nitrogen is considered within the framework of
an empirical approach. The oxygen abundances and nitrogen to oxygen abundances
ratios are derived in HII regions of a number of spiral galaxies through the
recently suggested P-method using more than six hundred published spectra. The
N/O - O/H diagram for HII regions in irregular and spiral galaxies is
constructed. It is found that the N/O values in HII regions of spiral galaxies
of early morphological types are higher than those in HII regions with the same
metallicity in spiral galaxies of late morphological types. This suggests a
long-time-delayed contribution to the nitrogen production. The N/O ratio of a
galaxy can then be used as an indicator of the time that has elapsed since the
bulk of star formation occurred, or in other words of the nominal "age" of the
galaxy as suggested by Edmunds and Pagel more than twenty years ago. The
scatter in N/O values at a given O/H can be naturally explained by differences
in star formation histories in galaxies. While low-metallicity dwarf galaxies
with low N/O do not contain an appreciable amount of old stars, low-metallicity
dwarf galaxies with an appreciable fraction of old stars have high N/O.
Consideration of planetary nebulae in the Small Magellanic Cloud and in the
Milky Way Galaxy suggests that the contribution of low-mass stars to the
nitrogen production is significant, confirming the conclusion that there is a
long-time-delayed contribution to the nitrogen production.
