Flora of North America species comparison

Species ca. 350 (18 in the flora).

Euphrasia is distributed throughout temperate regions of the northern and southern hemispheres, with one transtropical connection across the high mountains of Oceania. All Euphrasia species in the flora area belong to sect. Euphrasia, the largest of 15 sections in the genus (G. Gussarova et al. 2008). The section is noted for its taxonomic complexity. Major factors explaining the complex patterns of variation in sect. Euphrasia include interspecific hybridization, recent radiations, parasitism, polyploidization, and breeding system transitions (T. Karlsson 1974, 1986; P. F. Yeo 1978; G. C. French et al. 2008; R. A. Ennos et al. 2012). All North American species are tetraploids except, probably, E. disjuncta, E. farlowii, E. oakesii, E. randii, and E. subarctica. Of 18 species in the flora, nine are considered endemic. To facilitate identification of the species, the aggregate species concept is sometimes used to show relationships among the so-called more conventional, discrete taxa (Gussarova 2005). Following that concept, four aggregate species are represented in the flora area and their associated species treated here are: E. borealis (F. Townsend) Wettstein agg. (species 3 and 4), E. nemorosa agg. (species 6–10), E. oakesii agg. (species 13–15), and E. minima Jacquin ex de Candolle agg. (species 16 and 17).

The key makes no provision for hybrids, which often have intermediate features or a mixture of character states typical of their putative parental species. For a reliable identification, multiple representative plants should be collected and examined from a population to account for individual variation in key characters. The bracts referred to in the key are proximal floral leaves, which are most representative of the variation along the stem. The teeth of the bracts become progressively more acute up the stem as well as on any one leaf from apex to base. To characterize teeth shape, the key uses the most basal ones. Sessile glands are often present on the abaxial surface of bracts; they have no taxonomic value. Glandular (on short versus long stalks) and eglandular hairs are used in characterizing indumentum of different species.

Two infraspecific levels are used in this treatment. This is consistent with the usage in other Euphrasia treatments (for example, P. F. Yeo 1978). Subspecific rank is used for geographic or ecological integrity; varietal rank is used to indicate transitions between the extreme forms without any geographic or ecological distinction.

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

Genera ca. 100, species ca. 2000 (27 genera, 292 species in the flora).

Orobanchaceae are now defined to include both the holoparasitic members traditionally included in the family (A. Cronquist 1981) and the hemiparasitic genera formerly included in Scrophulariaceae. Although multiple research groups focus on members of the Orobanchaceae, a widely accepted infrafamilial classification of the family in the sense of Angiosperm Phylogeny Group (2016) has not yet appeared.

The classification by J. R. McNeal et al. (2013), who found that Orobanchaceae comprise six clades, is followed herein (their named clades are roughly equivalent to tribes). The autotrophic Lindenbergia Lehmann (12 species in the Old World) corresponds to the basal clade sister to the rest of the clades. Species in our region are distributed among the remaining five clades: Cymbarieae D. Don (genus 1), Orobancheae Lamarck & de Candolle (genera 2–6), Rhinantheae Lamarck & de Candolle (genera 7–12), Buchnereae Bentham (genera 13 and 14), and Pedicularideae Duby (genera 15–27). Within the family, genera are arranged alphabetically within tribes, or within Pedicularideae, in subgroups within the tribe.

Parasitic plants attach to their hosts via haustoria (L. J. Irving and D. D. Cameron 2009). Haustoria are produced by both hemiparasitic and holoparasitic Orobanchaceae (E. Fischer 2004). In hemiparasitic taxa, haustoria usually tap their host’s xylem, mostly taking up water, mineral nutrients, and nitrogen from their host, and sometimes also carbon. Holoparasitic taxa derive all of their growth requirements predominantly from the host’s phloem (Irving and Cameron).

Parasitism has evolved once in the family (N. D. Young et al. 1999; J. R. McNeal et al. 2013); holoparasitism has arisen independently three times from the hemiparasitic condition (J. R. Bennett and S. Mathews 2006; McNeal et al.).

Some Orobanchaceae are serious pests, primarily on legume and grain crops in warmer and drier areas, especially in sub-Saharan Africa. Striga is a particularly serious pest that parasitizes mostly monocots; S. gesnerioides attacks eudicots (K. I. Mohamed et al. 2006). Orobanche parasitizes eudicot crops primarily in temperate parts of the world (E. S. Teryokhin 1997). All Striga species and non-native species of Orobanche in the flora area are listed on the Federal Noxious Weed List (http://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/weedlist.pdf) in the United States.

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