Desmodium tweedyi is an herbaceous flowering plant in the pea family native to northern Texas and southern Oklahoma popularly known as "Tweedy's ticktrefoil" or "tick-clover." The legume or seed pod it produces has given the species its common names from its ability to cling to clothing. Along with other species in the Desmodium genus, D. tweedyi has become a candidate for soil enrichment, suppression of insect pests, mulch and green manure production, and making "good fodder for animals including bobwhite, turkey, grouse, deer, cattle and goats."
Botanical description and etymology Botanical publications describe this plant species as "Desmodium tweedyi Britton," for its scientific name (Desmodium tweedyi), its genus (Desmodium), its specific epithet (tweedyi), and the botanist who first identified it (Nathaniel Lord Britton). Desmodium is derived from the Greek word desmos which means "bond, fetter, or chain," referring to the connected segments of the fruit (legume) giving it a chain-like appearance. "Tweedyi" is a Latinized version of Frank Tweedy's last name – the topographer and botanist in whose honor this species was named, after he collected the first specimen on record.
Plant taxonomy Introducing the kingdom At the top of the taxonomy or scientific classification for Desmodium tweedyi is the kingdom of plants (Plantae), this being one of an estimated 374,000 plant species on earth – vascular plants that conduct water, sap, and nutrients occupying 82% and flowering plants 79% of that total. D. tweedyi's striking flower is pictured here.
A fundamental debate A close look at the taxonomy offered here illustrates a major debate in the field of botany, with an older category in the plant kingdom such as Embryophyte for all land plants on earth, ranked as Subkingdom, followed by the five unranked "clades" or monophyletic groups composed of a single common ancestor with all its lineal descendants. The traditional taxonomy system sometimes referred to as "Linnaean classification" put each species in a hierarchy of formal ranks, each identified using binomial nomenclature – for example, the scientific name Desmodium tweedyi including the genus and epithet. These older groupings are often "polyphyletic," containing taxa descended from more than one ancestor. In contrast, the five unranked clades utilize "phylogenetic nomenclature" or classification based on chromosomal studies, electron microscopy, or molecular biology -- a revolution that began with Charles Darwin -- in which the older ranking and nomenclature has theoretically been superseded.
A working compromise However, given the present state of the science, many botanists recommend a compromise to prevent a "whole-scale change of names of organisms" with the ensuing confusion which that would create. Botanist P. C. van Welzen offered the following advice:
The Angiosperm Phylogeny Group (APG) was formed in 1997 by an international group of botanists to work toward a consensus on the taxonomy of flowering plants, based on clades and a study of phylogenetics. The five (unranked) clades listed here in the Taxonomy for Desmodium tweedyi emerged from the most recent APG classifications -- APG III and APG IV.
Five (unranked) clades Five clades narrow down to the Desmodium tweedyi family and species, first that of vascular plants (Tracheophytes); second, the clade of all flowering plants (Angiosperms); then the clade of flowering plants with two seeds on germination (Eudicots); fourthly, the clade made up of about a quarter of all flowering plants with significant fossil records originating in the Cretaceous period 125 to 100 million years ago (Rosids); and then an important fifth "nitrogen-fixing clade" (Fabids), which emerged and includes four orders of flowering plants that carry this special trait. D. tweedyi is one of the species that has retained it.
Species' order (Fabales) One of those four orders in the Fabid clade is Fabales, and the similar characteristics of this large order of flowering plants was described as early as 1838 by Sir Edward Bromhead (1789-1855), hence the botanical name Fabales Bromhead. These plants are "inclined to a tropical habitat," by his account; they have many flowers clustered on a stem referred to by botanists as "inflorescence," a flowering group or cluster on a stem. And of the plant's fruit he said the following: "Fruit leguminous [seeds in a pod or chain] or rarely drupaceous [as a cherry seed with a hard pit] or samaroid [as in a winged seed vessel]."
Family (Fabaceae) The Fabaceae family is "cosmopolitan," present in arid to tropical, grassland and coastal environments, absent only from Antarctica. As the third largest family of flowering plants -- with 730 genera and 19,500 species -- it is economically important with food sources for beans, peas, peanuts, and soybean. Typical features of the family include nitrogen metabolism in the roots and fruit (legumes) which give the family its alternate name (Leguminosae).
Subfamily (Faboideae) and tribe (Desmodieae) Reflecting change in the science of Botany, the Sixth International Legume Conference was held in South Africa in 2013 with the theme "Towards a New Classification System for Legumes." In 2017 the Legume Phylogeny Working Group formed at that conference announced its taxonomic change in the Fabaceae or legume family from three to six subfamilies, D. tweedyi located in one of the new classifications, Faboideae (or Papilionoideae). Members of this broad subfamily include trees, shrubs, and herbaceous plants. The list of all 503 Faboideae genera includes Desmodium, with nitrogen-fixing root nodules present in many of its 14,000 species. The diagram of a complex tree posted on the website for Faboideae –- and continued on a second website for one of this subfamily's tribes (Desmodieae) -- spans at least six more clade branches, not included here. Phylogenetic research has begun to publish major changes to this part of the Tree of Life.
Genus (Desmodium) The Desmodium genus is made up of 450 species which occur mostly in warm areas of the world, especially in Asia, Brazil, and Mexico, but also in the southwestern U. S. The fruit (legumes) of this genus has generated additional common names such as "Tick-clover, Beggar's-lice, and Beggar's-ticks," because these legumes "fall into 1-seeded segments which stick to hair or clothing, hence the common names." Another Desmodium nick-name is "hitch-hiker." This article includes a link to this genus's list of species, one of which is D. tweedyi.
Species (Desmodium tweedyi) Desmodium tweedyi, of course, is a member of all these ranked groups and unranked clades, carrying most but not all the attributes or characteristics of its "parent" and "ancestor" predecessors. Classifications for the leaves, flowers, and fruit -- the latter forming is distinctive desmos or "chain" of 3 to 5 seeds, closely characterize the family tree. This seed pod or legume has invited agricultural interest in the species, to be outlined in the "history of research" section. With many of the other Desmodium species, this one has retained the important trait of the fifth "nitrogen-fixing clade" of Fabids, which is certainly part of what makes it attractive for farming and conservationists. Meibomia tweedyi is the only species synonym or alternate botanical name, to date. This is part of the history for this species and will be described under the research heading.
An important note The Christenhusz and Bing article inventorying the world's plants estimated that ca 2000 new species are described each year, with some concern that "the rate of discovery is slowing down, due to reduction in financial and scientific support for fundamental natural history studies."
Plant form
The Desmodium tweedyi plant species has been described with stem standing erect up to four feet high, its leaves 1 to 4.5 inches in width -- the widest (ovate) part at the base and rhombic-elliptic to egg-shaped – the leaves with long, soft, strait leaf-hairs (pilose) and raised veins in a spreading (pubescent) network beneath. The plant stalk supporting the blade (petiole) is 1.5 to 3.5 inches long, the stalk (pedicel) supporting individual flowers in the cluster (inflorescence) 0.4 to 0.9 inches long (Picture B). Flower petals (corollas) are white (picture A). Fruit segments 0.2 to 0.3 inches long are distinctly flat, breaking into two to six segments, with hooked hairs characteristic of all species in the Desmodium genus, causing them to stick to hair or clothing. Leaflets grow in clusters of three (tripinnate) distributed along both sides of an elongate axis attached to the main stalk, each leaf distinctly colored with a large conspicuous pale to whitish area along the midvein, as in the picture labeled D. Leaflet clusters attach to the main stalk with a small appendage (stipel). The flower cluster's "inflorescence axis" occurs "with short hooked hairs, petals white with greenish or yellowish tinge, sometimes with lavender base."
In a specimen collected in 1879 by the species' namesake, Frank Tweedy, Nathaniel L. Britton described the fruit pod (legume) as follows: "Joints of the pod rhombic [parallelogram-shape], minutely canescent [glowing], slightly raised in the calyx [enlargement over seed], resembling those of D. canescens, to which species the plant appears to be most nearly related. The only pod on the specimen examined consists of but three joints." Picture C of the D. tweedyi collection here displays four seed segments in the plant legume. The report of Tweedy's specimen now housed in New York Botanical Garden's Steele Herbarium is pictured with this article.
Plant growth and ecology Desmodium tweedyi grows in thickets in limestone areas in central Texas from "Blackland Prairie west to Rolling Plains and Edwards Plateau." Flowering followed by fruit legumes occur early in June to July. Typical habitat consists of calcareous or carbonate sandy loam near creeks and rivers. D. tweedyi has become the subject in a number of research studies and is a candidate in farm plantings for animal fodder and soil enrichment.
Geographical distribution This plant species is found in north-central Texas with a total range extent of from 8,000 up to one million square miles, and thus endemic or limited to one State within one nation, a very small area in terms of the global scale. Reports of the species' presence in southern Oklahoma are doubted by Kartesz and Meacham. However, 25 of the 66 specimens of Desmodium tweedyi held in American herbaria for the period of years from 1872 to 2015 were collected in Oklahoma, confirming this State as part of this species' geographical range. Entering the species scientific name and the collector Frank Tweedy, brings up his find of Desmonium tweedyi in 1879; entering just the scientific name reports all 66 specimen finds, showing those found in Oklahoma versus Texas. The map of the distribution for this species above shows the states of Texas and Oklahoma.