Ontogeny and histochemistry of the vegetative apex of Cosmos bipinnatus "Sensation"

Abstract

Plants of Cosmos bipinnatus Cav. "Sensation" (a quantitative short-day plant) were grown under continuous conditions of long days to maintain the vegetative state. The vegetative apex shows a cytohistological zonation pattern superimposed upon a tunica–corpus organization. The biseriate tunica is separated into two zones: axial and lateral; while the corpus is separated into three zones: the peripheral, central, and rib meristem. Based on mitotic activity (both frequency and index) there is neither a "méristème d'attente" nor an "anneau initial" present. A double plastochron cycle is present because of the decussate leaf arrangement and the presence of prominent leaf bases. Cytohistological zonation, nucleolar volume, RNA content, and mitotic index vary in a cyclic manner with the phase of plastochron. Although the degree of zonation decreases just after leaf buttress initiation, zonation does not disappear at any time during a plastochron.