Flora of North America species comparison

Epilobium siskiyouense is endemic to the Klamath region in southwestern Oregon (Jackson County) and north-central California in the Salmon, Scott Bar, and Siskiyou mountains of Siskiyou and Trinity counties. As noted by Hoch and Raven, this geographical range and several morphological features appear to be intermediate between those of E. obcordatum and E. rigidum. All three species have unusually large flowers (12–26 mm) with four-lobed stigmas, and as a group are quite distinct from their congeners in the region. Despite these similarities, the three taxa differ substantially in details of floral structure, especially regarding the dimensions of the floral tube. Specifically, E. rigidum has mean petal length 18.2 mm, floral tubes 1–1.6 × 2.5–3.6 mm; E. siskiyouense mean petal length 17.1 mm, floral tubes 2.1–4 × 2.9–5 mm; and E. obcordatum mean petal length 18.6 mm, floral tubes 3.2–5.2 × 2.2–3.6 mm. Thus, in flowers that are similar in overall size and aspect, E. rigidum has a very short, broad floral tube, E. obcordatum has a relatively long, narrow tube, and E. siskiyouense has a tube intermediate in size and shape. In terms of the ratio of tube length to width, the three taxa do not overlap. Although these characters are difficult to include in a key (since they require floral dissection and/or precise measurements), they are diagnostic for these species.

Epilobium siskiyouense has an additional diagnostic floral character that is unique in the genus. Whereas most other species of Epilobium have a simple ring of spreading hairs, sometimes with a low ridge of tissue near the mouth of the floral tube, E. siskiyouense has a relatively broad ring of tissue (0.3–0.6 mm wide), shaped like a washer, from which spreading hairs arise; this feature may provide protection for the nectar.

Epilobium siskiyouense has two distinct patterns of vestiture on the stems. In some specimens, the lower stems are mixed canescent and glandular puberulent and the inflorescence only glandular puberulent. In other specimens, the stems are subglabrous below a sparsely canescent and glandular puberulent inflorescence. There is no obvious correlation of this difference with any other morphological, ecological, or geographical factors.

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

Epilobium luteum is one of the most distinctive species in the genus due to its large creamy flowers, a color otherwise known only in the distantly related E. suffruticosum. It appears that E. luteum is most closely related to species of the Alpinae group, sharing not only similarities in perennating habit and structures, but also the derived CC chromosome arrangement (S. R. Seavey and P. H. Raven 1977, 1978).

Because Epilobium luteum is modally outcrossing and commonly pollinated by bees, it sometimes hybridizes with several other species of Epilobium when they grow sympatrically. One recurrent hybrid combination is E. luteum × E. ciliatum subsp. glandulosum, first described and named E. ×treleasianum H. Léveillé, and later E. luteum var. lilacinum L. F. Henderson. Following a suggestion by P. A. Munz (1965) that E. ×treleasianum was a hybrid of that particular parentage, S. R. Seavey (1993) verified the relationship in a series of crossing experiments. Epilobium ×treleasianum occurs over a wide geographical range in Alaska, British Columbia, and Washington, forming repeatedly when the parental species co-occur, and often backcrossing with one or both parents, forming hybrid swarms with varying morphological combinations. It also grows vigorously vegetatively.

Another less common hybrid of Epilobium luteum was described as E. ×pulchrum Suksdorf, and a minor variant as E. ×pulchrum var. albiflorum Suksdorf (S. R. Seavey 1993). The second parent of these hybrids is less obvious, but based on morphological features, it is most likely E. hornemannii, which also grows frequently in sympatry with E. luteum.

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