Flora of North America species comparison

Species ca. 37 (32 in the flora).

Heuchera, the largest herbaceous genus exclusively in North America and Mexico in the Saxifragaceae, is notoriously difficult taxonomically because of morphological intergradation among species, possibly following hybridization, and because character expression, especially in flowers and fruits, varies with the stage of development. Heuchera species usually remain distinct in nature due to geographic isolation and differences in flowering phenology, but climatic fluctuations have caused range extensions and contact among species, resulting in hybridization and persistent hybrid populations (C. O. Rosendahl et al. 1936; J. A. Calder and D. B. O. Savile 1959; S. A. Spongberg 1972; C. K. Wilkins and B. A. Bohm 1976; E. F. Wells 1979, 1984; B. G. Shipes and Wells 1996; D. E. Soltis et al. 1991). Multiple origins of autotetraploid lineages from diploid ancestors have been documented in some species, sometimes associated with increased flower size relative to diploid progenitors, causing further taxonomic difficulties (Soltis et al. 1989; K. A. Segraves and J. N. Thompson 1999). Molecular techniques [particularly rbcL, a chloroplast gene, sequence divergence and cpDNA (chloroplast DNA) restriction site analyses] have been used to help assess relationships within the genus (Soltis et al. 1991) and have suggested that Heuchera is paraphyletic. One group of species, scattered throughout sects. Rhodoheuchera Rosendahl, Butters & Lakela, Holochloa Nuttall ex Torrey & A. Gray, and Heucherella Torrey & A. Gray, appears to be sister to the remainder of the genus, including other species scattered throughout sects. Heuchera, Heucherella, and Holochloa (Rosendahl et al.), and other genera, including Elmera, Tellima, Tiarella, and Conimitella, and most species of Mitella (Soltis et al. 1991), adding confusion at the sectional and generic levels. Soltis et al. (1991) suggested that hybridization among individuals of distantly related species of Heuchera followed by subsequent backcrossing among hybrid offspring and individuals belonging to parental species might have introduced maternally inherited chloroplasts into hybrid progeny that resembled pollen (paternal) parents. The suggested capture of chloroplasts from one species by another following hybridization could explain the complex pattern of cpDNA relationships within the genus.

Some species of Heuchera are cultivated as perennial ornamentals in cooler parts of North America. The most commonly available cultivated species include H. americana, H. cylindrica, H. grossulariifolia, H. micrantha, H. richardsonii, H. rubescens, H. sanguinea (coralbells), and H. villosa. Cultivated alum-roots often exhibit white-variegated or bronze-red leaves. The sale and cultivation of these native species as landscaping ornamentals sometimes is implicated in distribution occurrences far beyond their natural ranges. Crosses between Heuchera and Tiarella, known as ×Heucherella, are also cultivated.

The flower in Heuchera has a hypanthium, or floral tube, adnate to the unilocular ovary proximally and free from the ovary distally for 0.1–7 mm. Some species have virtually no hypanthium beyond that adnate to the ovary. The hypanthium bears at its rim usually five, rarely six, sepals alternating with usually five, rarely six, petals, which are borne in the sinuses between the sepals or inserted inside the hypanthium distal to the ovary, alternating with the sepals. Stamens are attached inside the hypanthium opposite the sepals. The portion of hypanthium distal to the ovary varies from radial to bilateral in different species, and, additionally, the sepals may counter or exaggerate the effect, by being equal or unequal in length on either side of a flower. In some species, the distal portion of the hypanthium tube is radially symmetric and the flower is radial. In other species, the adaxial sepals and the adaxial side of the distal portion of the hypanthium tube are longer than the abaxial sepals and abaxial side of the hypanthium tube, and the flower is bilateral. In some species, the adaxial sepals are shorter than the abaxial, and the flower appears to be radially symmetric until examined closely.

Whereas the inferior portion of the ovary is adnate to the hypanthium, the superior portion of the ovary arises at the base of the free hypanthium tube and is not adnate to the tube. The superior portion of the ovary consists of two, rarely three, pointed hollow beaks each tapering to a solid style and terminating in a capitate stigma; the beaks of the ovary are often surrounded by yellow, rarely orange, nectariferous tissue proximally.

Floral measurements in the key are given at anthesis. Stamens and styles continue to elongate during flowering, and hypanthium, ovary, and fruit dimensions change over time. The length of the hypanthium free from the ovary is measured between the distal point of adnation to the ovary and the bases of the sinuses on the adaxial side of the flower (the longer side of the hypanthium).

In Heuchera, stamens and stigmas may be deeply included, or included to slightly exserted, or exserted 1–6 mm; the length of exsertion is measured from the distal lip of the connate portion of the hypanthium.

Virtually all hairs in Heuchera end in multicellular glands. Surfaces are described as short, medium, or long stipitate-glandular.

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

Genera ca. 38, species ca. 600 (23 genera, 158 species in the flora).

Classification of Saxifragaceae has been varied and controversial (e.g., A. Cronquist 1981; H. G. A. Engler 1930; J. Hutchinson 1973; G. K. W. Schulze-Menz 1964b; A. L. Takhtajan 1997; R. F. Thorne 1992). Molecular phylogenetic data (D. R. Morgan and D. E. Soltis 1993; Soltis et al. 1993, 2001; Angiosperm Phylogeny Group 1998, 2003) reveal that genera of Saxifragaceae in the broad sense are allied with at least ten separate, often distantly related families of flowering plants. These data also suggest that Saxifragaceae in the narrow sense as treated here consists of about 38 genera worldwide, equivalent to subfamily Saxifragoideae, one of the 15 subfamilies recognized by Engler and one of the 17 recognized by Schulze-Menz of the broadly defined Saxifragaceae. Molecular phylogenetic data (Soltis et al. 2001) show that the narrowly defined Saxifragaceae fall into two major groups: Saxifraga, and the heucheroid clade encompassing all other genera. Molecular data further show that Saxifraga, as traditionally understood, is polyphyletic, comprising two distinct lineages (treated here as Saxifraga and Micranthes) and the monospecific North American Cascadia. The major split between Saxifraga and the heucheroid clade is supported not only by molecular data from six DNA regions but by differences in patterns of floral morphology. Saxifraga has a relatively uniform floral morphology (radially symmetric flowers, with bilateral symmetry restricted to one Asian group of species, which consistently have the same number of sepals, petals, stamens, and carpels). Almost all of the variation in the family in numbers of sepals, petals, stamens, and carpels occurs in the heucheroid clade. Radially symmetric flowers predominate there, but some bilateral flowers are found in Bensoniella, Micranthes, Tolmiea, and some species of Heuchera.

Penthorum, the only genus in Penthoroideae of Saxifragaceae (H. G. A. Engler 1930), is morphologically anomalous in the Saxifragaceae and has often been included in Crassulaceae or, as treated here, its own family, Penthoraceae (Angiosperm Phylogeny Group 1998). Itea and Ribes, sole members of Iteoideae and Ribesoideae, respectively, are treated here as separate families, Iteaceae and Grossulariaceae. Crassulaceae, Grossulariaceae, Iteaceae, Paeoniaceae, Penthoraceae, and Saxifragaceae belong to the Saxifragales, as treated here, as well as Altingiaceae, Cercidiphyllaceae, Daphniphyllaceae, Haloragaceae (which includes Penthoraceae and Tetracarpaeaceae), Hamamelidaceae, Peridiscaceae, and Pterostemonaceae (Soltis et al. 2005).

Carpenteria, Decumaria, Deutzia, Fendlera, Fendlerella, Hydrangea, Jamesia, Philadelphus, and Whipplea once belonged to Hydrangeoideae of Saxifragaceae (H. G. A. Engler 1930) and are treated in the flora as Hydrangeaceae. Escallonia, the sole genus of Escallonioideae (Engler), is treated in the flora as Escalloniaceae. Molecular, morphological, and chemical data (C. R. Bensel and B. F. Palser 1975, 1975b, 1975c, 1975d; B. A. Bohm et al. 1988; M. L. Haskins and W. J. Hayden 1987; L. Hufford and W. C. Dickison 1992; D. R. Morgan and D. E. Soltis 1993; Soltis and B. A. Bohm 1982; Soltis et al. 1993; Soltis and P. S. Soltis 1997; Angiosperm Phylogeny Group 1998, 2003) indicate that Hydrangeaceae and Escalloniaceae are more likely to be related to the asterids, within which Hydrangeaceae appears to be close to Cornales.

Parnassia and Lepuropetalon have been included in Saxifragaceae as the subfamily Parnassioideae in the past (A. Cronquist 1981; H. G. A. Engler 1930; J. Hutchinson 1973; S. A. Spongberg 1972). Based on molecular, morphological, and chemical data, these genera appear to be only distantly related to other genera of Saxifragaceae and are here moved to Parnassiaceae (C. R. Bensel and B. F. Palser 1975, 1975b, 1975c, 1975d; B. A. Bohm et al. 1986; D. R. Morgan and D. E. Soltis 1993; Soltis et al. 2001; A. L. Takhtajan 1969; R. F. Thorne 1992). Molecular phylogenetic analyses (M. W. Chase et al. 1993; Zhang L. B. and M. P. Simmons 2006) have aligned Parnassiaceae with Celastraceae, either as a sister family or as a basal member of Celastraceae.

Molecular systematic studies have revealed that the approximately 38 remaining genera form a strongly supported clade (D. E. Soltis et al. 1993, 2000; Soltis and P. S. Soltis 1997; S. B. Hoot et al. 1999; V. Savolainen et al. 2000) that corresponds to the Saxifragoideae of Engler and is identical to the family circumscriptions of A. L. Takhtajan (1969, 1997) and R. F. Thorne (1992).

H. G. A. Engler’s (1930) subfamily Saxifragoideae contained one tribe, Saxifrageae, which consisted of four subtribes: Astilbinae, Leptarrheninae, Saxifraginae, and Vahliinae. Vahliinae is now known to be very distantly related to the Saxifragaceae. G. K. W. Schulze-Menz (1964b) elevated three of Engler’s subtribes to tribes: Astilbeae, Leptarrheneae, and Saxifrageae. K. Klopfer (1973) later recognized two large groups, one centered around Heuchera having parietal placentation, another centered around Saxifraga having axile placentation. Recent molecular phylogenetic and systematic studies indicate the presence of six well-marked clades, informally recognized as the Astilbe, Boykinia, Chrysosplenium, Darmera, Heuchera, and Leptarrhena groups (reviewed in Soltis et al. 2001). Relationships within some of these groups (e.g., the Boykinia and Heuchera groups) have also been studied in detail from a phylogenetic standpoint (e.g., Soltis and R. K. Kuzoff 1995; Soltis et al. 1997).

During fruit development in some genera (especially Saxifraga and Lithophragma), partially to mostly inferior ovaries swell to become increasingly superior due to allometric shifts (R. K. Kuzoff et al. 2001). Ovary position in keys and descriptions here refers to flowers during or shortly after anthesis. Reports of variation in ovary position in some genera and species may be due to observations of flowers at different developmental stages. D. E. Soltis et al. (2005) reported that most genera of Saxifragaceae display appendicular epigyny in floral development, which begins with a minute convex floral apex. During or after perianth initiation, the floral apex becomes concave, giving rise to an inferior ovary. Ovaries in Saxifragaceae often appear to be nearly superior or one-half to three-fourths inferior, but with their appendicular epigynous ground plan, they are not truly superior or homologous to superior ovaries. Instead, they are “superior mimics” with “pseudosuperior” ovaries.

Species in some North American genera (particularly Boykinia, Darmera, Heuchera, Micranthes, Saxifraga, Tellima, Tiarella, and Tolmiea) are popular horticultural subjects.

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