  Some of the correlations between morphological and other physical properties
of planetary nebulae (PNe) are reviewed. In particular, the finding that
bipolar (b) PNe have more massive progenitors than the other morphological
classes is discussed in detail. Earlier results are confirmed; including all
the various sources of uncertainty, the Galactic distribution of objects
indicate that b PNe are formed by stars with initial masses >1.3 solar masses,
while elliptical (e) PNe by progenitors with masses <1.3 solar masses. Recent
results for the chemical abundances of b and e PNe and their orientation within
the Galaxy are also presented. Finally, the key role in the discussion of the
formation of b PNe played by detached binary systems such as symbiotic stars is
briefly discussed.
