This volume contains the presentations of the principal speakers at the NATO Advanced Study Institute held at Porto Portese, Italy,23 August - 2 September, 1982. This meeting was the third in a series devoted to the molecular biology of plants. The initial meeting was held in Strasbourg, France in 1976 (J. Weil and L. Bogorad, organizers), and the second in Edinburgh, Scotland in 1979 (C. Leaver, organizer). As in these previous meetings, we have attempted to cover the major topics of plant molecular biology so as to promote the integration of information emerging at an accelerating rate from the various sub- disciplines of the field. In