Herbs annual, with taproot. Stems: epidermis often peeling proximally. Leaves opposite in proximal pairs, alternate or fasciculate distally. Flowers actinomorphic; floral-tube without bulbous base or scales inside; petals white to pale rose-purple; pollen shed in tetrads; stigma usually entire, rarely ± 4-lobed. Capsules narrowly cylindrical, splitting to base, central column persistent, pedicellate to subsessile. Seeds numerous, in 1 row per locule, obovoid to oblanceoloid, entire or constricted near micropylar end, coma present.

Distribution

w North America, nw Mexico

Discussion

Species 2 (2 in the flora).

Section Crossostigma consists of two annual species endemic to western North America from California to British Columbia and Montana (Epilobium minutum) or from California to Idaho and Arizona, rarely to northwestern Mexico (E. foliosum). The two species are similar morphologically, but E. foliosum (n = 16) consistently has leaves folded along the midrib and densely clustered (fascicled) on the upper stem, more crowded inflorescences, smaller petals (1.8–3 mm), and smaller (0.6–0.9 mm) papillate seeds compared to E. minutum (n = 13), which has flat leaves, non-clustered open inflorescences, generally larger petals (2–5 mm), and larger (0.9–1.2 mm) reticulate seeds. Despite repeated efforts to cross them, no experimental hybrids resulted (S. R. Seavey et al. 1977b), nor are there any known natural hybrids, although such hybrids might be difficult to detect. In the only molecular analysis that included both species, they consistently formed a well-supported (92% BS) monophyletic group (D. A. Baum et al. 1994).

Among the few annual species of Epilobium, these two species differ from those of sects. Pachydium (n = 9, 10, 19) and Epilobiopsis (n = 15) by having comose seeds, and from E. brachycarpum (sect. Xerolobium, n = 12) by their smaller stature and numerous leaf, flower, and seed characters.

Selected References

None.

Lower Taxa

Epilobium foliosum, Epilobium minutum