Herbsperennial, rarely suffrutescent, with basal rosettes, turions, soboles, stolons, sometimes tipped with turions, or rarely bulblets (gemmae) in leaf-axils, sometimes cespitose. Stems not woody, epidermis not peeling. Leaves opposite proximal to inflorescence or throughout, usually alternate distally. Flowers actinomorphic; floral-tube not bulbous, without scales inside; petals usually rose-purple to pink or white, very rarely cream-yellow (E. luteum); pollen in tetrads; stigma entire or 4-lobed. Capsules narrowly subcylindric to narrowly clavate, splitting to base, central column persistent, pedicellate or sessile. Seeds many, in 1 row per locule, narrowly obovoid or fusiform to narrowly ellipsoid, [rarely with inflated rim around perimeter on adaxial side]; coma usually present, very rarely absent.

Distribution

North America, Mexico, West Indies (Hispaniola), Central America, South America, Eurasia, Africa, Pacific Islands, Australasia

Discussion

Species ca. 150 (27 in the flora).

Section Epilobium is the largest in the genus, comprising about 150 species (168 taxa) dis­tributed in cool montane, alpine, boreal, or arctic habitats on all continents except Antarc­tica, and extending to high elevations in mountains of the tropics. No other section of Epilobium occurs native outside of North America except for one species each of sects. Pachydium and Epilobiopsis in South America; E. brachycarpum (sect. Xerolobium) is adventive in South America and Europe. All species are diploid (n = 18) perennials, and the flowers in most species are protandrous and shed pollen directly onto the stigma when it becomes receptive, sometimes in bud. In all autogamous species the stigmas are undivided. Some species, 13 in North America, have deeply four-lobed stigmas that are often exserted beyond the anthers, the stigma lobes spreading after the pollen has begun to be shed; as a result, these plants are predominantly outcrossed. However, almost all are self-compatible and capable of self-pollination, exceptE. obcordatum, which appears to be self-incompatible (S. R. Seavey and K. S. Bawa 1986; Seavey and S. K. Carter 1994, 1996). Primary pollinators of this group are bees, flies, and butterflies.

Perennating structures are diverse in sect. Epilobium and are important diagnostic characters for many species. As noted elsewhere, soboles from ± woody caudices may be the most generalized type of structure, from which have evolved several other major types (R. C. Keating et al. 1982), defined as follows.

Stolons, either hypogeous or epigeous, are found in many species of sect. Epilobium. Stolons may be filiform, with small decussate leaves or scales and a fleshy or leafy bud at the tip, or thicker and ropelike with larger leaves, or tough and wiry with scales. Species that produce stolons often grow in damp habitats, including boggy areas, wet scree slopes, and damp swales. New stems arise from the tip of the stolons, in clumps with ascending bases, or some distance from the parental stem.

Rosettes, clusters of leaves scarcely distinguishable from cauline leaves, are common in sect. Epilobium, notably including the very widespread E. ciliatum. In general, plants producing rosettes grow in less harsh habitats.

The other common type of perennating structure in sect. Epilobium is the sessile turion, which generally forms underground (to 5 cm), with tightly decussate, fleshy scales, a distinct round or strobiloid form, and little or no internode elongation. Turions may also form at the tips of stolons (as in E. palustre and relatives). A rare variant of these overwintering buds are bulblets (gemmae) that form in distal leaf axils. Stems arising from turions or rosettes are not or only loosely clumped, and generally strict, not ascending as found in most soboliferous or stoloniferous plants.

Species of sect. Epilobium have solid endexine in the distal pollen walls, unlike the rest of the genus, which have large endexine channels in the distal walls. The pollen viscin threads of species in this section are tightly compound (J. Praglowski et al. 1994), unlike those of other sections. These characters suggest that sect. Epilobium is sister to the remaining sections, as supported by recent molecular studies (D. A. Baum et al. 1994; R. A. Levin et al. 2004).

Based on extensive crossing studies, it appears that virtually all species of sect. Epilobium can hybridize with most or all other species, resulting in more or less fertile offspring (S. R. Seavey and P. H. Raven 1977, 1977b, 1977c, 1978). Natural hybridization occurs fairly frequently where two or more species occur sympatrically in nature (Raven and T. E. Raven 1976). Analysis of experimental hybrids revealed the presence of reciprocal chromosome transloca­­tion differences within this section; species or groups of species have been found to differ from one another by one or more reciprocal translocations, resulting in rings or chains of chromosomes, rather than bivalents, in hybrids between the groups (Raven and Raven; Seavey and Raven 1977, 1977b, 1977d, 1978). Most or all North American species tested belong to one of three groups, designated AA, BB, and CC by Seavey and Raven; there are additional arrangements that do not occur in North America. The BB arrangement is found in many species in North America, including all of those with most generalized morphology and distribution, most species in South America and Eurasia, and all species in Australasia, and is the presumed original arrangement. It differs from the AA (primarily including E. ciliatum and relatives) and CC (mainly the north temperate so-called Alpinae) groups by one reciprocal translocation each, and AA differs by two translocations from CC. These and other studies found no evidence that the type of permanent translocation heterozygosity found in Onagreae occurs in any species of Epilobium. As treated here, species are arranged according to their known or presumed chromosome group designa­tion. Because the BB group includes not only the majority of species of sect. Epilobium world­wide, but also several North American species that appear to be more generalized than any others, the sequence of groups here is BB, CC, then AA.

Selected References

None.

Lower Taxa

Epilobium anagallidifolium, Epilobium arcticum, Epilobium ciliatum, Epilobium clavatum, Epilobium coloratum, Epilobium davuricum, Epilobium densum, Epilobium glaberrimum, Epilobium hallianum, Epilobium hirsutum, Epilobium hornemannii, Epilobium howellii, Epilobium lactiflorum, Epilobium leptocarpum, Epilobium leptophyllum, Epilobium luteum, Epilobium mirabile, Epilobium montanum, Epilobium obcordatum, Epilobium obscurum, Epilobium oreganum, Epilobium oregonense, Epilobium palustre, Epilobium parviflorum, Epilobium saximontanum, Epilobium siskiyouense, Epilobium smithii