A biosystematic study of Hypericum section Spachium in eastern North America

A biosystematic study was conducted on the eastern North American taxa of Hypericum sect. Spachium. Morphological, biochemical, cytological, ecological, and distributional data resulted in the recognition of the following twelve taxa: H. boreale, H. canadense, H. cumulicola, H. denticulatum var. denticulatum, H. denticulatum var. acutifolium, H. drummondii, H. incertum, H. gentianoides, H. gymnanthum, H. mutilum, H. majus, and H. setosum.

Scanning electron microscope studies of the pollen of these species showed the pollen to be trizonocolporate with a reticulate to finely reticulate-foveate exine. Three groups were distinguished on the basis of pollen size and exine ornamentation. Three basic patterns of the seed coat were recognizable, but did not correspond with the groups based on pollen morphology. Seed coat morphology provided a reliable means for the distinction of such closely related taxa as H. incertum and H. denticulatum var. acutifolium as well as some of the less closely related taxa.

Biochemical analysis of flavonoids found in the eastern North American species of sect. Spachium revealed the presence of three major types in two different classes of flavonoid compounds: flavonols, C-glycosyl flavones, and flavones. The C-glycosyl flavones and flavones were only found in H. incertum. All taxa investigated were found to contain quercetin based compounds with sugar substitutions occurring at the 3 position. One species, H. boreale, was found to contain a kaempferol based compound.

Chromosome counts for the taxa showed n = 6 in H. setosum and H. cumulicola, n = 8 in H. majus, H. mutilum, H. canadense, H. boreale, and H. gymnanthum, n = 12 in H. gentianoides, H. drummondii, H. incertum, H. denticulatum var. acutifolium, and some populations of H. denticulatum var. denticulatum, and n = 24 in most populations of H. denticulatum var. denticulatum. The counts of n = 6 are the first reports of this number in the genus Hypericum and possibly represent the base number for the genus. The counts for H. setosum, H. cumulicola, H. gymnanthum, and H. drummondii are the first reports for these species.

Hypericum dissimulatum is recognized as a recurrent hybrid of H. canadense and H. boreale or H. mutilum. The hybrid nature of this taxon is indicated by its intermediate morphology and its reduced pollen stainability. Field observations and a study of herbarium specimens of the taxa of the H. canadense complex support the reports of hybridization by previous workers.

The present taxonomic treatment differs from some relatively recent treatments of the eastern North American taxa of sect. Spachium in the following respects: (1) no subspecific taxa are recognized within H. mutilum and H. canadense, (2) H. dissimulatum is treated as a recurrent hybrid, (3) H. gentianoides and H. cumulicola are viewed as distinct species rather than being conspecific, and (4) the H. denticulatum complex is recognized as being comprised of two species, H. denticulatum and H. incertum, the former with two varieties.