Herbs annual, with taproot. Stems: epidermis peeling proximally. Leaves opposite and early deciduous in proximalmost pairs, alternate and sometimes fasciculate distally. Flowers actinomorphic; floral tube without bulbous base or scales inside; petals rose-purple to pink or white; pollen usually shed singly, rarely in tetrads; stigma entire and clavate to deeply 4-lobed. Capsules narrowly cylindrical or fusiform, splitting to base, central column persistent, pedi­cel­late. Seeds numerous, in 1 row per locule, obovoid to broadly obovoid, prominently constricted near micropylar end, coma present.

Distribution

w North America, introduced in sw South America, Europe.

Discussion

Species 1.

Section Xerolobium consists only of the annual species Epilobium brachycarpum, with n = 12, a unique chromosome number apparently derived by aneuploidy from the tribal base number of × = 18. It differs from the other annual taxa in sects. Epilobiopsis (n = 15) and Pachydium (n = 9, 10, 19) not only in chromosome number, but also by flowering in summer, whereas the other taxa are primarily spring-flowering.

Section Xerolobium shares with sects. Cordylophorum and Zauschneria the apomorphic features of the so-called incised compound pollen viscin threads (J. J. Skvarla et al. 1978; also found in sect. Pachydium and in Epilobium minutum of sect. Crossostigma) and in having seeds prominently constricted at the micropylar end (S. R. Seavey et al. 1977; also found in E. rigidum of sect. Macrocarpa and E. foliosum of sect. Crossostigma). In addition to its autapomorphic chromosome number, E. brachycarpum is marked as phylogenetically distinct by virtue of shedding its pollen in monads (Skvarla et al. 1975). All but a few exceptionally large-flowered populations in a small area of northern California (sometimes designated as E. jucundum) have monads, a feature otherwise found in Epilobieae only in Chamaenerion. The feature of shedding pollen as monads was clearly derived independently within E. brachycarpum.

Selected References

None.

Lower Taxa

Epilobium brachycarpum