Flora of North America species comparison

The occasional occurrence of sterile pollen has been noted in this as well as in other species of Micranthes (as Saxifraga, K. I. Beamish 1961). Some populations exhibit gynodioecism (P. E. Elvander 1982).

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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.

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