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Optical integrated spectra of bulges have been obtained for 28 spiralgalaxies. By applying an appropriate aperture size for each galaxy, theunavoidable contamination of disk starlight has been carefully minimized andset to $\sim$ 1/7 of the total stellar light. The sample covers a wide range inbulge luminosity and morphology. The Mg$_2$ index shows a tight correlationwith the bulge intrinsic luminosity, similar - and extended at faintermagnitudes - to the relationship known for ellipticals. Other features such asCaIIK, CN, G-band, and TiO show similar trend with the bulge luminosity. On thecontrary, the Fe5270 and Fe5335 iron lines show a constant equivalent width -within some dispersion - for all bulges. A simple application of a stellarpopulation synthesis model suggests that magnesium should be most enhanced withrespect to iron in the brightest bulges. Concerning the structural parameters,bulges do occupy the same region in the fundamental plane as ellipticals. Inconclusion, the present study favors the idea that the bulk of stars in bulgeshave formed with only moderate or negligible influence of the disk material,likely by very similar processes as those driving the formation of ellipticalgalaxies.

The Luminosity-Metallicity Relation for Bulges of Spiral Galaxies