Morphology vs. physical properties: some comments and questions
  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. 
Romano L.M. Corradi (Instituto de Astrofisica de Canarias, La Laguna,   Tenerife, Spain)