Systematic studies of the genus Isoetes in the southeastern United States

Systematic studies of the genus Isoetes in the southeastern United States were undertaken. Various experimental techniques, herbarium and field studies were employed to produce a new treatment for the genus in this region.

A review of the relevant literature was helpful in placing the taxonomic problems into an historical perspective. Since the present thesis was not a formal revision involving a study of type specimens, the only intention was to test the validity of previously published taxonomic treatments.

Over thirty populations of Isoetes representing eight taxa were surveyed for flavonoids by means of two-dimensional paper chromatography; seven compounds were detected, all of which were determined to be flavones. Although sufficient leaf material for statistical comparisons was not available, it became obvious that intraspecific variation of flavone constituents was greater than consistent interspecific differences. Species-specific flavonoid profiles could not be resolved with the techniques employed in this study. A composite chromatogram for the flavonoids of the southeastern United States quillworts was prepared.

Scanning electron microscopy was very valuable in allowing for the delimitation of the taxa on the basis of spore exine ornamentation. An examination of spores from over fifty specimens served as the basis for representative photographs and descriptions. The megaspores were found to show three basic patterns: (1) reticulate, (2) cristate, and (3) tuberculate. The microspores showed four general patterns: (1) levigate, (2) echinate, (3) papillate, and (4) verrucate.

An attempt was made to quantify the phenetic relationships among the various taxa. This was accomplished by computing paired affinity indices and group affinity values based upon twenty, twostate characters. Additionally, the paired affinity indices were depicted graphically by polygonal diagrams.

Previous reports of experimental hybridizations in Isoetes are unknown in the literature. Breeding experiments were undertaken to test for genetic compatibility among four selected taxa in three sections of the genus. All of the potential crossing combinations were successful, and the possibility was raised that apogamy occurs in at least one taxon. Natural hybrids have been recognized in the course of field and herbarium work and were treated as such. Among these are collections currently referred to as Isoetes louisianensis Thieret which, in this study, are determined to be I. engelmannii X I. melanopoda. The implications of these intersectional hybrids for species concepts in the genus are discussed.

Finally, a synopsis of Isoetes for the southeastern United States was undertaken. The morphology and habitat preferences for each species is discussed and distribution maps based on over 400 annotated herbarium specimens are presented. A general taxonomic key as well as a key based only on spore characters for southeastern United States quillworts is included.