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.)