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Excerpts from Jim Conrad's
Naturalist Newsletter
Entry dated November 7, 2023, from notes taken at Cascadas de La Piedad waterfall 3kms NW of the community of San Pablo, municipality of Almeaco de Bonfil; bedrock of igneous andesite; N20.1008°, W100.0041°, elevation 2360 meters (7750ft); extreme southern Querétaro state, MÉXICO
PERSICARIA LAPATHIFOLIA
Even during the ongoing two-year dry period classified by the North American Drought Monitor as an extreme D3 drought, a small stream of clear, cold water meandered among livestock-grazed meadows and cornfields atop the valley's bed of pinkish, igneous andesite rock. Just before tumbling over a high ledge to create the picturesque Cascada de La Piedad, or Piety Waterfall, the above wildflower was thriving at the water's edge. With its slender, spikelike flowering head, pinkish flowers, simple narrow leaves and moist habitat, it was a classic species of the plant group I grew up calling smartweeds; they were very common weeds in Kentucky, in the US. But many smartweed species are recognized, and their taxonomy isn't well established, so this plant deserved a close look.
At first I thought it was what I call the Mexican Smartweed, a very similar species living near water at lower elevations on Querétaro's semiarid altiplano. However, the inflorescences' stems, or peduncles, on that species are covered with conspicuous stalked glands, while our plant's peduncles bore no stalked glands. Our plant wasn't that, then, so more details needed to be noted.
In inflorescences of certain smartweed species the flowers are so dispersed that broad sections of the flowering stem, or rachis, are plainly visible between flowers, but this species' flowers were densely packed. Also, some species' inflorescences are much longer than what's seen above; often they curve downward so that their tips dangle.
Our plant's flowers tended to develop five tepals -- the term tepals being used when the calyx's sepals and the corolla's petals or lobes are indistinguishable -- and six stamens, as indicated at the right. In some species the styles extend well above the tepals' height, but here they remain short. Among the species, stamen number can range from five to eight, and we seem to have six or seven. As best seen on the opening flower bud at the picture's lower, right, our plant's tepals bore glistening, warty glands on their outer surfaces; many species lack glands.
Smartweeds belong to the Buckwheat-Smartweed-Knotweed Family, the Polygonaceae. In the field, if suspect you may have a member of this family, you look for what's shown above; most species in the family produce something similar. At the base of each leaf's petiole, where on many plants small, leaflike stipules appear -- one on each side of the petiole base -- in this family the stipules usually enlarge, fuse with with another, and form usually-cylindrical sheaths encircling the stems. These structures are called ocreas, and they're typical of the Buckwheat Family.
The Flora del Bajío, which covers our upland central Mexican state of Querétaro, documents five Buckwheat Family genera for the state. In this area, if you have an erect, herbaceous Buckwheat Family member with five tepals, which may be glandular, according to the Flora, you have the genus Polygonum. That's the genus that years ago I called the smartweed genus. However, many species which once were in Polygonum now have been reassigned to other genera. In the 2015 study by Tanja Schuster and others entitled "An updated molecular phylogeny of Polygonoideae (Polygonaceae): Relationships of Oxygonum, Pteroxygonum, and Rumex, and a new circumscription of Koenigia," it's said that the reassigning process is "still ongoing."
In the Flora del Bajío, our plant with its inflorescences' closely packed flowers, and with peduncles lacking glandular hairs, easily is identified as Polygonum lapathifolium. However, our plant is one of those already banished from Polygonum; currently Kew's Plants of the World Online database regards that name as a synonym of PERSICARIA LAPATHIFOLIA.
Persicaria lapathifolia has so many English names that you can take your pick: Pale Smartweed, Pale Persicaria, Pale Knotweed, Pink Knotweed, Curlytop Smartweed, Curlytop Knotweed, Willow Weed, and surely more, plus it has various names in other languages. Local names documented here in Querétaro include chilillo, or "little chili pepper." One reason for so many common names is that, according to Kew's POW database, it's regarded as native to the subarctic and temperate Northern Hemisphere south to western and central Malasia, northern Africa and Australia. Also, it occurs in many moist environments as well as roadsides, floodplains, waste places and cultivated fields, from sea level at least to our elevation of 2360 meters (7750ft). That's a remarkable range of habitats.
I was quite familiar with this species in the US, though I remember its flowers as white, and arranged in much longer, often tip-dangling, inflorescences. Images of the species on the Internet mostly agree with my memories, though our pink-flowered, shorter inflorescence form also is represented. This may be explained by the Flora of North America's statement that "... Persicaria lapathifolia is a morphologically variable complex with more than two-dozen infraspecific taxa described in the New World and Old World." Also, the species manifests as numerous ecotypes -- distinct forms occupying particular habitats. Elsewhere, our plant is described as normally self-fertilizing, so genetically pure lines develop, which sometimes are thought to be distinct species.
The 2021 study by Ahmed M. Abd-ElGawad and others entitled "Persicaria lapathifolia Essential Oil: Chemical Constituents, Antioxidant Activity, and Allelopathic Effect on the Weed Echinochloa colona," reviews the traditional medicinal uses of our plant, which include "... antibacterial, antiviral, anti-inflammatory, astringent, antiseptic, anti-stomach complaint, hepatoprotective, and antifungal uses in addition to its use for the treatment of dysentery, burns, and fevers." In contrast, in the 2014 study by W. Lugton and J. Woolacott entitled "Liver necrosis and photosensitisation in cattle after eating Persicaria lapathifolia (pale knotweed) and Persicaria orientalis (Prince's feather)," it's reported that liver damage caused by feeding on two Persicaria species, of which our plant was one, caused deaths among cattle. Also, the earlier-mentioned study by Abd-ElGawad and others found that essential oils extracted from our plant exhibited "... substantial allelopathic activity against the germination, seedling root, and shoot growth of the weed E. colona." Allelopathy is the chemical inhibition of one organism by another, due to the release into the environment of certain substances. It's clear that one reason our plant is so widespread is that it vigorously wages chemical warfare against its enemies, including damaging the livers of those who eat it.
Still, in D.E. Moerman's 1998 classic, Native American Ethnobotany, it's reported that in the US "The Keres, Navajo, and Potawatomi prepared medicinal infusions with Persicaria lapathifolia, and the Zuni used decoctions made from the plants as cathartic and emetic drugs." Cathartic and emetic drugs often function by irritating the intestinal lining, so that sounds about right. Here in Mexico, the online Atlas de las Plantas de la Medicina Tradicional Mexicana reports that in this area the plant's ground branches are applied to treat hemorrhoids, and the red rash appearing around the anus and often into surrounding areas on the newly born.
In short, this little wildflower along a pretty stream is extraordinarily adaptive to many environments in many places. Its advantages arise from genetic variability and, with its toxins, "playing tough" with its enemies and competitors.