INFLORESCENCE AND FLORAL ONTOGENY IN ASTERACEAE - A SYNTHESIS OF HISTORICAL AND CURRENT CONCEPTS

The interpretation that all floral events on the capitulum lake place in an acropetal or centripetal sequence is widely accepted, but many c ontradictory examples have been found. A non-acropetal sequence has lo ng been recognized for species with secondarily or tertiarily condense d heads, but species with uncondensed heterogamous heads also display some degree of departure from the strictly acropetal plan. Ray, or per ipheral, flowers occasionally are initiated after and below/outside of the first series of actinomorphic (disk) flowers. In addition, ray fl owers almost always lag behind in both size and developmental stage, o nly catching up much later, prior to anthesis. Flower primordia occasi onally arise as common primordia together with the subtending receptac ular or involucral bract. The pappus is quite plastic with regard to t he timing of its initiation and to the number and type of primordia pr oduced, although it is seldom the first organ to be initiated.