Flower structure in ten Asteraceae species: considerations about the importance of morpho-anatomical features at species and tribal level

Abstract

Flowers of Asteraceae have conspicuous features that are important at the tribal level, such as the branches of the style, pappus, shape of the corolla, and anthers. The floral ontogeny of ten Brazilian weed species of Asteraceae was performed for Conyza bonariensis L., Cosmos sulphureus Cav., Eclipta alba L., Elephantopus mollis Kunth, Emilia sonchifolia L. DC., Galinsoga quadriradiata Ruiz and Pav., Erechtites valerianifolius (Link ex Spreng) DC., Parthenium hysterophorus L. Praxelis clematidea R. M. King and H. Rob, and Sigesbeckia orientalis L. in order to describe the morpho-anatomy, ontogenetic steps, and identify features that may support the tribeal classification. Flowers and buds were fixed in glutaraldehyde, embedded in historesin, and sectioned with a rotary microtome. Sections were stained with Toluidine Blue. The inflorescence can be heterogamous or homogamous; the pappus is absent or persistent; the corolla is tubular or ligulate; anthers have an apical appendage which is obtuse or acute; and some anatomical features are the ovary usually trapezoidal-shaped; the pappus either vascularized or not; the corolla with possible secretory structures, only two species do not show twin hair on the outer ovary epidermis;and ovarian mesophyll usually composed of two or three regions. In general, the species analyzed show several similar features, such as the anther structure, pappus, and stigma. A distinct feature was observed among the species: the absence of pappus, and twin hairs in three species: E. alba (Heliantheae tribe), P. hysterophorus (Heliantheae tribe), and S. orientalis (Millerieae tribe), all belonging to “Helliantheae Alliance” clade.