Flora of North America species comparison

Stephanomeria pauciflora generally grows as an intricately branched, often rounded bush. Occasional plants, usually from Arizona, New Mexico, Texas, and southern Utah, have long, flexuous stems and branches, an architecture that resembles one of the typical forms of S. tenuifolia. Some plants of S. pauciflora have white pappi, also typical of S. tenuifolia. It is not known if these plants represent uncommon and unusual individuals or if they are from populations in which all plants have those traits. It is also not known whether such plants of S. pauciflora grow near populations of S. tenuifolia; if so, they may result from interspecific hybridization. That is a possibility; experimental hybrid plants produced by crossing individuals from the two species were about 20% fertile. Such fertility suggests the species are sufficiently compatible that fully fertile segregants with variously intermediate morphologies could be expected where they hybridize in nature. The experimental crosses were made reciprocally between S. pauciflora from Riverside County, California (L. D. Gottlieb 6653), and S. tenuifolia from Wheeler County, Oregon (L. D. Gottlieb 6692); specimens of the six F1 hybrid plants that were produced are deposited at DAV. Plants of S. pauciflora that are densely tomentose throughout are occasionally found, particularly in the deserts of California and Nevada, and have been named S. cinerea or S. pauciflora var. parishii.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Stephanomeria thurberi has been collected most often in May, June, and early July. These specimens have well developed basal rosettes, stems with nodes 3+ cm apart, relatively short branches, usually only on the distal 30–50%, and scalelike, cauline leaves. That is the form described by Gray. Another form has been collected from late July into early September that is morphologically distinct, as first pointed out by A. S. Tomb on the labels of specimens he collected in 1968 and 1970 (see below). That “summer form” lacks basal rosettes, the stems have more numerous nodes, about 1 per cm, beginning at ground level, and threadlike cauline leaves 3–4 cm. Those specimens usually have relatively few heads. Some specimens collected in July are intermediate, having basal rosettes and relatively long, threadlike cauline leaves, or no rosettes and relatively short, scalelike cauline leaves.

It is not known if the different growth forms represent distinct genotypes that initiate growth at different times, or if the same individual produces aboveground parts with differing appearances during the growth season. It is also not known if the two forms commonly grow together as they do in Coconino County, Arizona (Tomb 280, August 10, 1968, and Tomb 631, June 12, 1970), or if they generally occupy different habitats. The unusual situation calls for study.

(Discussion copyrighted by Flora of North America; reprinted with permission.)