Polarity, calcium and abscission: molecular bases for developmental plasticity in plants.

This article considers novel aspects to plant development. It starts by outlining the concept of plastic development. It then discusses polarity in plants arguing for the vascular tissue and surrounding cells as a major internal polar axis. The requirement for a change in polarity in this system when regeneration occurs is discussed and molecular mechanisms for specification and changing of polarity are outlined. The process of abscission is then discussed as an example of plastic development and it is concluded that many factors control abscission. Molecular changes in abscission zone cellulase are summarized and the notion of two phases in abscission are developed and generalized to other systems of development in plants.