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C. Ellstrand and Kristina A. Schierenbeck (29) note that invasions typically involve long lag periods before they become successful and require multiple introductions. The authors ' explanation is that hybridization between the invaders and resident populations is a stimulus often required for successful invasion. Hybrid progenies may enjoy genetic advantages over their progenitors. Ellstrand and Schierenbeck show that, as predicted by their model, invasiveness can evolve.

Stebbins devoted two chapters to polyploidy. Pamela S. Soltis and Douglas E. Soltis (30) set forth the genetic attributes that account for the great success of polyploid plants: about 50% of all angiosperm species and nearly 95% of all ferns. Polyploids maintain higher levels of genetic variation and heterozygosity, and exhibit lesser inbreeding depression, than diploids, possibly because most polyploid species have arisen more than once, from genetically different diploid parents, in addition to the presence of more than two homologues. Genome rearrangement seems to be a common attribute of polyploids and many plant species may be ancient polyploids (see maize in ref.24). Soltis and Soltis conclude that, the advances of the last 50 years notwithstanding, much remains unknown about polyploid plant species, including their general mode of formation.

We are grateful to the National Academy of Sciences for the generous grant that financed the colloquium, to the staff of the Arnold and Mabel Beckman Center for their skill and generous assistance, and to Mrs. Denise Chilcote, who performed the administrative functions of the colloquium with skill and dedication.

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