ORGANOGENESIS PECULIARITIES IN THE STEM APICES OF RUDBECKIA-BICOLOR AND PERILLA-NANKINENSIS DURING TRANSITION TO FLOWERING IN RESPONSE TO PHOTOPERIODIC INDUCTION

Localization and sequence of initiation and development of reproductiv e elements in the stem apices during their transition from a vegetativ e state to flowering were studied with the help of scanning electron m icroscopy. The model plant species Rudbeckia bicolor Nutt. and Perilla nankinensis (Lour.) Decne. underwent photoperiodic induction in respo nse to the increasing number of long days for R. bicolor and short day s for P. nankinensis favorable for flowering. Development of the repro ductive apex in R. bicolor involves successive initiation and differen tiation of ''simple'' and ''block'' meristems possessing different typ es of symmetry. Under influence of long-day induction, reproductive or ganogenesis of a stem apex occurs in two stages. At the first stage, t he apex meristem gives rise to a common primordium that produces bract and flower primordia. At the second stage, bract and flower primordia arise from two solitary bumps. In P. nankinensis during transition to flowering in response to short-day treatment, flower primordia are in itiated separately in the bract primordia axils. These peculiarities o f bract primordia formation in the plant species under study allow us to make conclusions about the different origins and physiological stat uses of the bracts. These peculiarities will serve as the basis for th e study of cell interactions in the stem apices during transition to r eproductive organogenesis.