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Histological and histochemical examination of floral initiation was conducted to determine the pattern of flowering in Rudbeckia hirta, a long-day (LD) plant. Plants were grown under 8-hour short days (SDS) until they had 14 to 16 expanded leaves. Half of the group of plants was moved to LD conditions consisting of natural daylength plus a 4hour night interruption. Rudbeckia hirta had a pattern of differentiation in flowering similar to that reported in species requiring one inductive day for initiation. Rudbeckia hirta required 8 LDs for evocation and 18 LDs for completion of initiation. Involucral bracts initiated after 18 LDs, after which the receptacle enlarged and was capped by a meristematic mantle of cells signaling the start of development. Floret primordia did not initiate, even after 20 LDs. Increases in pyronin staining were observed in actively dividing cells of the procambium, leaf primordium, and corpus of the vegetative meristems. After 8 LDs, the pith rib meristem stained darkly, a result indicating the arrival of the floral stimulus. An increase in pyronin staining was also observed in the meristematic mantle covering the receptacle after 18 LDs, a result indicating increased RNA levels. All flowering plants share the same stages of floral morphogenesis, although species, and even individuals within a species, can be heterogeneous in their response to floral inductive conditions (Bernier, 1989). Meristems are composed of distinct zones that change during floral initiation and ultimately become unrecognizable (Bernier et al., 1981; Esau, 1977). The typical vegetative meristem is comprised of the tunica, a twoor three-cell layer of anticlinally dividing cells capping the meristem (Esau, 1977), and the corpus, which is comprised of three zones. The central zone of the corpus consists of large, slowly dividing cells (Bernier et al., 1981; Jacqmard, 1970). The peripheral zone is approximately ring shaped and surrounds the central zone with smaller, actively dividing cells from which all leaf primordia and stem tissue, except the central pith, originate. The pith rib meristem, a zone of large, flattened, vacuolated cells, lies immediately below the central zone; periclinal divisions in this region and parts of the peripheral zone give rise to stem elongation in caulescent species. This zone disappears with floral development (Bernier et al., 1981; Nougarède, 1967). According to Bernier et al. (1981), the most significant finding of the histological and histochemical study of meristems is that a prefloral stage, characterized by peak activation and division of cells in the central and peripheral zones, is identifiable during floral transition in almost all species, regardless of inductive requirements. In the Asteraceae, after transition to inductive conditions, cell activity increases in the central zone and cells stain darkly with pyronin, an RNA-specific stain (Bernier et al., 1981; Jacqmard et al., 1972; Nougarede, 1967). This stage is followed by an increase in cell division and a subsequent loss of distinct zonation, and the meristem begins to dome as a result of periclinal divisions and elongation in the pith rib meristem. Involucral bracts develop first and, although leaf-like, they can be distinguished from true leaves as they arise from periclinal divisions in the second tunica layer without prior formation of a foliar buttress (Bernier et al., 1981; Philipson, 1946). The parenchymatous core continues to enlarge, for publication 9 Nov. 1992. Accepted for publication 26 Mar. 1993. The blishing this paper was defrayed in part by the payment of page charges. stal regulations, this paper therefore must be hereby marked advertisely to indicate this fact. assistant.

Anatomical Changes in Rudbeckia hirta L. during Transition to Flowering