Flora of North America species comparison

Rumex acetosella in the broad sense is an extremely variable and taxonomically complicated polyploid complex, which includes diploids, tetraploids, hexaploids, and octoploids. This complex (excluding more distantly related arctic-montane R. graminifolius and its allies) probably originated and developed mostly in southern Europe and southwestern Asia. Some races of R. acetosella now are distributed almost worldwide as introduced and often completely naturalized aliens.

Á. Löve (1941, 1983) assumed that in this group chromosome numbers are strictly correlated with morphology. In his opinion, every chromosome race represents a distinct species: diploid Rumex angiocarpus Murbeck [= Acetosella angiocarpa (Murbeck) Á. Löve]; tetraploid R. multifidus Linnaeus [= R. tenuifolius (Wallroth) Á. Löve = Acetosella multifida (Linnaeus) Á. Löve]; hexaploid R. acetosella in the narrow sense [= A. vulgaris (W. D. J. Koch) Fourreau, with gymnocarpous A. vulgaris subsp. vulgaris and angiocarpous A. vulgaris subsp. pyrenaica (Pourret ex Lapeyrouse) Á. Löve]; hexaploid R. graminifolius Rudolph ex Lambert [= A. graminifolia (Rudolph ex Lambert) Á. Löve]. However, the distribution given by Löve for these taxa seems unnatural. Studies by J. C. M. den Nijs and collaborators (den Nijs 1974, 1976, 1984; den Nijs and T. Panhorst 1980; den Nijs et al. 1980, 1985; see also W. Harris 1969, 1973) indicate that the situation is more complicated. They postulated the development of two major evolutionary lines into two ploidy complexes: a primary western Mediterranean one and a secondary eastern Mediterranean one. According to this scheme, polyploid races independently and spontaneously emerged (and still are emerging) within different ancestral populations.

The most widespread, almost cosmopolitan race, presumably native to the southwestern Mediterranean region, including southwestern and Atlantic Europe, which is common in North America, is characterized by a hexaploid chromosome set (2n = 42), nonmultifid lateral lobes of basal leaves, and angiocarpy (fruits are not easily separable from accrescent inner tepals). It was commonly and erroneously referred to as Rumex angiocarpus Murbeck, or R. acetosella subsp. angiocarpus (Murbeck) Murbeck. According to J. R. Akeroyd (1991), who in general followed the taxonomic revision of the group by J. C. M. den Nijs (1984), the correct name for this taxon is R. acetosella subsp. pyrenaicus (Pourret ex Lapeyrouse) Akeroyd (=Acetosella vulgaris subsp. pyrenaica (Pourret ex Lapeyrouse) Á. Löve). Gymnocarpous nonmultifid and multifid forms (R. acetosella subsp. acetosella and R. acetosella subsp. acetoselloides (Balansa) den Nijs, respectively) also occur in North America, but evidently rarely. The distributions of subspecies of R. acetosella in North America are poorly known. Keys and detailed descriptions for the subspecies were provided by den Nijs and Akeroyd. However, the tempting simplicity of the keys is somewhat suspicious. The alternative point of view (and an alternative key) may be found in Á. Löve (1983).

Rumex acetosella subsp. arenicola Mäkinen ex Elven was recently described from Greenland and reported for Scandinavia and arctic Russia (R. Elven et al. 2000). This entity seems to be morphologically transitional toward Rumex graminifolius (see discussion under that species below). According to Elven et al., it differs from other infraspecific entities of R. acetosella in having the following characters: leaves usually without basal lobes (as in R. graminifolius), with revolute margins; inflorescence sparsely branched; tepals and pedicels densely covered with red papillae (as in R. graminifolius). From R. graminifolius and related taxa (R. beringensis and R. krausei) it can be distinguished by narrower inner tepals (similar in size to those in other subspecies of R. acetosella). The distribution of subsp. arenicola and its relations to other taxa are in need of further study.

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

Rumex persicarioides often has been treated by American botanists as a variety or synonym of R. maritimus (see R. S. Mitchell 1978). It and R. fueginus differ from Eurasian R. maritimus in many respects and are as distinct as many widely recognized Eurasian taxa of this aggregate (e.g., R. palustris, R. rossicus Murbeck, R. ucranicus Fischer ex Sprengel, R. marschallianus Reichenbach, R. amurensis Fr. Schmidt ex Maximowicz, R. evenkiensis Elisarjeva). When submerging R. persicarioides as a variety of R. maritimus, Mitchell noted: “Taxonomic treatment of the group from a Eurasian point of view would undoubtedly shed light on the minor problems which we face in North and South America.” However, from a Eurasian point of view (see e.g., K. H. Rechinger 1937, 1949; J. E. Lousley and D. H. Kent 1981; N. N. Tzvelev 1989b), all North American native taxa of subsect. Maritimi are evidently specifically different from any native Eurasian ones (with the only possible exception of Pacific plants, which are discussed below).

Plants similar to Rumex persicarioides, but with bigger tubercles and occuring along the Pacific coast from northern California to British Columbia, are, in my opinion, closer to R. fueginus in their habit and vegetative characters. K. H. Rechinger (1937) provisionally determined such specimens as R. persicarioides. J. E. Dawson (1979) noted that the Pacific plants differ from Atlantic ones in having bigger tubercles (more than 1.9 × 0.7–1 mm in western plants; less than 1.9 × 0.7 mm in eastern R. persicarioides in the narrow sense), and described these large-tubercled plants as a distinct variety, “R. maritimus var. pacificus���, unfortunately, an invalid name. However, that taxon seems to be extremely closely related to or possibly conspecific with the northeastern Asian species, R. ochotskius Rechinger f., which is known in eastern Asia from northern Japan to the Okhotsk Sea region of Russian Far East (especially Sakhalin and Kuril islands). The latter species also has large (to 2–2.5 mm) botuliform tubercles with obtuse apices. In the original description Rechinger stated: “…foliorum forma R. maritimo simillimus…,” but N. N. Tzvelev (1989b) in his recent treatment of the genus in the Russian Far East noted that most of the specimens of R. ochotskius seen by him had leaf blades rotundate-truncate or broadly cuneate at the base. The R. persicarioides-like plants from the Pacific coast of the United States and Canada (as well as their most probable allies from eastern Asia) need additional study. At present I prefer to place them provisionally into R. persicarioides, following Rechinger’s treatment.

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