Thesis Defense


A Scanning Electron Microscopic Study of the Peduncle, Phyllary

and Palea Trichomes of Encelia (Asteraceae: Heliantheae)


Nancy Anne Charest


Encelia is a genus of woody shrubs and subshrubs which are widespread throughout the southwestern United States and northwestern Mexico. The leaf trichomes have been previously examined using scanning electron microscopy. Although leaf trichome characters are useful in supporting the hypothesis of two distinct clades within Encelia, they are not fully sufficient to elucidate the relationships between taxa within either clade. The purpose of this study is to characterize the trichome complement of three organs associated with the capitulum: the phyllaries, the paleae, and the peduncle, by examining them with the scanning electron microscope; to compare this to the previous study of the leaf trichomes; and to relate the results in terms of a hypothesis of relationships within the genus. As on the leaves, there are three multicellular trichome types: uniseriate hairs, moniliform hairs, and biseriate glandular hairs. Unlike the leaves, however, the distribution among the organs and between taxa of any given trichome type is highly variable. When cladistic analyses include trichome complements as characters, they show a much greater amount of homoplasy and show different branching patterns depending on which characters are used. It appears that characters derived from the trichome complement alone of the paleae, phyllaries and peduncles do not adequately serve to support the existing hypothesis of phylogeny in the genus, nor are they adequate to characterize the differences in gross and microscopic appearances of these organs.



Thesis Committee:

Dr. Stephen H. Bryant, chair

Dr. Lawrence M. Blakely

Dr. David P. Campbell

Dr. Curtis Clark

Thursday, June 2, 1988, 3:30 pm, 3-107