  Medium-resolution spectra from 3650 angstroms to 10,000 angstroms are
presented for 96 giant H II regions distributed in 20 spiral galaxies. We have
calculated two separate grids of photoionization models, adopting single-star
atmospheres (Kurucz) and star clusters synthesized with different Initial Mass
Functions (IMFs) as ionizing sources. Additional models were computed with more
recent non-LTE stellar atmospheres. We use the radiation softness parameter
eta' of Vilchez and Pagel to test for a metallicity dependence of the effective
temperatures of the ionizing stars. Our results are consistent with a
significant decrease in mean stellar temperatures of the ionizing stars with
increasing metallicity. The magnitude of the effect, combined with the behavior
of the HeI 5876/Hbeta ratio, suggest a smaller upper mass limit for star
formation at abundances higher than solar, even when considering the effects of
metallicity on stellar evolution and atmospheric line blanketing. However, the
exact magnitudes of the stellar temperature and IMF variations are dependent on
the choice of stellar atmosphere and evolution models used, as well as on
uncertainties in the nebular abundance scale at high metallicities. Our results
also constrain the systematic behavior of the ionization parameter and the N/O
ratio in extragalactic H II regions. The observed spectral sequences are
inconsistent with current stellar evolution models which predict a luminous,
hot W-R stellar population in evolved H II regions older than 2-3 Myr. This
suggests either that the hardness of the emitted Lyman continuum spectrum has
been overestimated in the models, or that some mechanism disrupts the H II
regions before the W-R phases become important.
