  By examining their mass loss history and their distribution in the galaxy I
argue that spherical planetary nebulae (PNe) form a special group among all
planetary nebulae. The smooth surface brightness of most spherical PNe suggests
that their progenitors did not go through a final intensive wind (FIW, also
termed superwind) phase. While 70 per cent of the PNe of all other PNe groups
are closer to the galactic center than the sun is, only 30 per cent of
spherical PNe are. These, plus the well known high scale height above the
galactic plane of spherical PNe, suggest that the progenitors of spherical PNe
are low mass stars having low metallicity. Although many stars have these
properties, only about 10 per cent of all PNe are spherical. By comparing the
galactic distribution of spherical PNe to the metallicity evolution in the
galaxy, I find that the critical metallicity above which no spherical PNe are
formed is [Fe/H] ~ -0.4. I explain this as well as other properties of
spherical PNe in the context of the companion model for shaping PNe, arguing
that spherical PNe are formed from stars which had no close companion, stellar
or substellar, orbiting them.
