Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Shirley C. Tucker

Abstract

Floral and inflorescence development of 39 species of Asteraceae and four additional species representing possible outgroups were examined with the scanning electron microscope (SEM). Taxon choice was designed to include representatives of 12 of the 12 to 18 tribes recognized in the large, morphologically diverse family. Although floral ontogeny has provided phylogenetically useful characters in other plant groups, no similar studies have been applied to solve phylogenetic problems in the Asteraceae. The necessary live material was collected in several ways: from natural populations throughout North America, from the worldwide collections of several botanical gardens, grown from seed, or from cultivated specimens purchased from nurseries. For each species examined, several individual plants were sampled from each population, and in some cases, more than one population was sampled. Inflorescence and floral development was studied to document the range of variation possible among the developmental pathways of each species. Ontogenetic events were relatively stable within each species, when several individuals from either a single population or from different populations were sampled. Heterogamous heads consistently display suppression of peripheral (ray) flower initiation and development as compared to that of the disk flowers of the same head. Disk flowers and ray flowers of the same head are inherently dissimilar from initiation onwards, differing in size and shape of primordia, and in ensuing development. Several ontogenetic pathways were documented for the formation of ray flowers, receptacular bracts, and pappus; they appear to have phylogenetic significance. The corolla ring meristem was found throughout taxa of the subfamily Asteroideae but was absent from the Lactucoideae. Ligulate, bilabiate, and disk flowers throughout the Asteraceae display as high degree of similarity during early stages of development, but are dissimilar in late stages of corolla expansion. The pappus is initiated either during organogenesis with the other organ whorls, or substantially later during floral expansion and differentiation as an achene outgrowth; the two modes of initiation are considered nonhomologous. If initiated during organogenesis, the pappus may form in one of three general patterns.

Pages

356

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