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Published In: Journal de Botanique, Appliquée à l'Agriculture, à la Pharmacie, à la Médecine et aux Arts 1(3): 122, pl. 5, f. 15. 1813. (J. Bot. Agric.) Name publication detailView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 8/29/2017)
Acceptance : Accepted
 

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Desmodium Desv. (beggar’s lice, tick treefoil, stick-tights) Contributed by Jay A. Raveill

Plants perennial herbs, with a short, often woody rootstock, often also taprooted, at least when young. Stems 1 to several, erect or ascending to less commonly spreading (not twining), often unbranched below the inflorescence, unarmed, glabrous or variously pubescent with glandular or nonglandular hairs, the nonglandular hairs often multicellular or with hooked tips. Leaves alternate, pinnately trifoliate or the lowermost (or rarely nearly all) leaves occasionally simple, short- to long-petiolate, with pubescence similar to that of the stem. Stipules shed as the leaves mature or persistent to various degrees, linear to broadly ovate, green and herbaceous when young, becoming brown or less commonly dark red and papery at maturity, the margins entire, often minutely hairy along the margins and on the outer surfaces, the venation prominent; stipels present, minute, usually persistent. Leaflets linear to broadly ovate or nearly circular, the lateral leaflets often slightly smaller and more rounded than the terminal one, rounded or angled to a usually well-developed (rarely nearly absent) stalk at the base, rounded to sharply pointed at the tip, sometimes with an abrupt, minute sharp point at the very tip, the margins entire, both surfaces glabrous or more commonly minutely to prominently hairy, pinnately veined. Inflorescences erect to ascending or arched (lateral inflorescences sometimes spreading), racemes (often grouped into panicles), rarely reduced to loose clusters, terminal and/or axillary, the axis with hooked hairs and sometimes also with straight or glandular hairs, the flowers paired or less commonly appearing as small clusters at each node, the bract(s) subtending each pair or cluster of flowers small, with prominent veins, shed as the flowers mature, the bractlet subtending each flower minute, linear, inconspicuous. Calyces 5-lobed but appearing 4-lobed, the short tube slightly inflated, 2-lipped, the upper 2 lobes fused except for a small notch at the tip, the lowermost lobe narrower and slightly longer than the others, hairy, sometimes inconspicuously so, usually inconspicuously nerved except for the midrib of each sepal, persistent at fruiting but not becoming enlarged. Corollas papilionaceous, glabrous, pale to dark pink, rarely white or cream-colored, often with a pair of contrasting purple, green, and/or yellow markings near the base of the banner petal (darker-colored corollas turning dull greenish blue or purplish with age, pale flowers turning cream-colored with age), the petals tapered or abruptly rounded (occasionally cordate) to a short, stalklike base, the banner obovate to oblong-obovate, the wings oblong, about as long as and usually somewhat fused to the keel basally, the keel oblanceolate and curved in outline, boat-shaped. Stamens 10, 9 of the filaments fused and 1 free nearly to the base, the anthers small, attached near the midpoint, all similar in size. Ovary linear to narrowly oblong-ellipsoid, sessile or short-stalked, the style slender, incurved, usually glabrous, more or less persistent at fruiting, the stigma small and terminal. Fruits loments, flattened, tapered abruptly to a short, often inconspicuous stalklike base 1.0–6.5 mm long, mostly 2- to several-seeded, indehiscent, divided into 1-seeded segments that separate at maturity, the segments with well-defined margins and a conspicuous network of nerves on each face, the connections between the segments nearly symmetrical or the lower margin somewhat more deeply lobed between the segments than the upper margin, the faces and sometimes also margins with hooked hairs. Seeds slightly kidney-shaped to nearly circular in outline, somewhat flattened, the surface smooth, green or pale yellow to dark brown, often turning black when dried. About 275 species, nearly worldwide but absent from portions of western North America and Europe.

The hooked hairs that are characteristic of the genus aid in dispersal of the fruit segments by adhesion to the fur of various passing mammals. A walk through an area with a dense population of Desmodium in the autumn is a testament to the efficacy of this dispersal mechanism, with socks and trousers becoming densely coated. The segments of the loments can be difficult to remove from fur or clothing when fresh; the process becomes easier once the segments have been allowed to dry. Steyermark (1963) noted that the fruits of some species are eaten by birds such as turkeys and quail and that the herbage is sometimes browsed by deer. He also noted that livestock graze on some of the species. Some of the Old World species are cultivated as green manure and fodder (Ohashi, 2005).

Identifications of species within this genus can present significant challenges. The taxon-specific differences can be subtle and/or subjective with a very small percentage of specimens displaying aberrant morphologies. Numerous varieties and forms have been named to account for extreme morphologies, but in the present work such infraspecific taxa are not considered sufficiently distinct to warrant formal recognition. The most obvious and frequently encountered variants are mentioned. Positive identification in Desmodium often requires the simultaneous examination of multiple characters including fruit, flower, and vegetative traits. The most significant difficulties in determinations involve species with similar fruit shape but contrasting pubescence and/or leaflet shape. Occasionally collectors have distributed several species under the same collection number, so caution should be used when making identifications based on duplicate collections (and in gathering samples from multiple plants in the field).

Some brief comments may aid in identifications. Desmodium flowers open in the morning and usually wilt by mid-afternoon, and often several consecutive nodes along the branches bloom on the same day. Although the flowers generally appear paired at each node, there often are one or more additional flower buds visible between the first pair; these buds only occasionally develop and flower later in the season. The number of segments in the fruit is dependent on the number of seeds that mature. Because in many fruits at least some ovules abort, the maximum number of segments should be considered the true number for identification purposes. The stalklike base of the fruit (stipe), can be difficult to measure due to the constriction of the lower part of the fruit, especially when the lowermost ovule is abortive. A change in texture and pubescence is used as the point of demarcation.

Herbarium specimens of other genera of legumes lacking reproductive structures are often misidentified as Desmodium. One distinguishing feature is that most Desmodium species have a pair of small stipels (stipulelike outgrowth at the base of each leaflet), which are not present in most other Missouri legume genera. Another helpful feature is that none of the Missouri Desmodium species are vines, so any evidence of climbing excludes this genus.

Descriptions of species are based on herbarium specimens collected from Missouri when possible. Some features that are often not well preserved in herbarium specimens, especially traits of the calyx, corolla, and bracts, also include data recorded from living plants observed in the field or grown in the greenhouse of the University of Central Missouri (Warrensburg). Corolla length, a critical trait in identification, is given for dried specimens, as the flowers usually shrink slightly upon drying.

Three of the Missouri species traditionally included in Desmodium (D. glutinosum, D. nudiflorum, and D. pauciflorum) are segregated into the genus Hylodesmum in the present work. For further discussion, see the treatment of that genus.

 

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1 Stems prostrate or mostly spreading (2)
+ Stems erect or ascending to slightly arched (4)
2 (1) Stipules lanceolate to elongate-triangular, the base asymmetrically rounded but not clasping, semipersistent or shed early Desmodium × humifusum
+ Stipules ovate, with a short-tapered tip, the base slightly clasping, persistent (3)
3 (2) Central leaflet ovate, the undersurface with a conspicuous network of raised veins; corollas white or cream-colored, drying pale yellow; fruits often twisted between the segments, with hooked hairs confined mostly to the margins of the segments Desmodium ochroleucum
+ Central leaflet circular, broadly ovate, broadly obovate, or broadly rhombic, the undersurface with an inconspicuous to evident network of raised veins; corollas pink (rarely white), drying greenish blue to bluish purple; fruits flat, not twisted between the segments, with hooked hairs on both the margins and faces of the segments Desmodium rotundifolium
4 (1) Stipules large and conspicuous, mostly 7–20 mm long, broadly ovate to triangular-lanceolate, persistent or semipersistent (5)
+ Stipules small, mostly 2–7 mm long, narrowly lanceolate to narrowly lanceolate-triangular, shed early or semipersistent (8)
5 (4) Leaflets with the undersurface having a conspicuous network of raised veins, with hooked hairs along the veins throughout (6)
+ Leaflets with the undersurface having a faint to more evident network of at most slightly raised veins, lacking hooked hairs or these sparse and confined to larger veins near the leaflet base (7)
6 (5) Inflorescences terminal as well as from the upper leaf axils, much-branched, the axis with dense, hooked and multicellular glandular hairs, as well as straight, spreading, nonglandular hairs; corollas 8–11 mm long; fruit segments 5–14 mm long, rhombic and angular with deeper indentation below than above Desmodium canescens
+ Inflorescences terminal, not or few-branched, the axis with dense, hooked and multicellular glandular hairs, but lacking straight, nonglandular hairs; corollas 6–8 mm long; fruit segments 4–6 mm long, rounded, nearly equally indented above and below Desmodium illinoense
7 (5) Leaflets angled or slightly tapered to a bluntly or sharply pointed tip; stipules (5–)7–10 mm long; stipels 1.5–4.0 mm long; petioles 1–3 cm long; fruit segments 3–7 mm long, 3.5–5.0 mm wide Desmodium canadense
+ Leaflets tapered to a sharply pointed tip; stipules 10–20 mm long; stipels 5–9 mm long; petioles 4–10 cm long; fruit segments 7–10 mm long, 4–6 wide Desmodium cuspidatum
8 (4) Fruit segments bluntly angled on the lower margin (9)
+ Fruit segments rounded on the lower margin (13)
9 (8) Leaflets densely velvety-hairy on the undersurface, the central leaflet ovate-triangular, ovate, or rhombic; stipules semipersistent, often dark red and reflexed at maturity Desmodium viridiflorum
+ Leaflets glabrous to densely hairy but not velvety to the touch, the central leaflet linear, oblong, lanceolate, ovate, or rhombic; stipules shed early or semipersistent, generally brown and appressed or slightly spreading at maturity (10)
10 (9) Stems and leaves glabrous or nearly so, glaucous (this sometimes not apparent in dried specimens); flower stalks (8–)10–20 mm long; fruits with the stalklike base 4.5–6.5 mm long; corollas 8–10 mm long; stipules shed early Desmodium laevigatum
+ Stems and leaves more or less hairy, not or only slightly glaucous; flower stalks 3–10(–17) mm long; fruits with the stalklike base 2–5 mm long; corollas 6–9 mm long; stipules generally semipersistent with at least some present in fruiting specimens (11)
11 (10) Stems and leaves with scattered appressed and straight and/or spreading and hooked hairs; at least the central leaflet lanceolate or narrowly oblong, generally 3–8 times as long as wide Desmodium paniculatum
+ Stems and leaves with moderate to dense, spreading, straight and/or hooked hairs; all leaflets ovate to broadly lanceolate or nearly rhombic, 1.5–4.0 times as long as wide (species difficult to separate) (12)
12 (11) Median portion of the stem with primarily hooked hairs, sometimes with a few straight hairs, all or mostly at the nodes; leaflets broadest at or slightly below the midpoint, the tip rounded or bluntly pointed, sometimes inconspicuously notched, the undersurface with a more or less obvious network of raised veins Desmodium glabellum
+ Median portion of the stem with relatively long, spreading, straight hairs and often with shorter, hooked hairs; leaflets widest well below the midpoint, the tip bluntly to sharply pointed, the undersurface usually with a faint network of raised veins Desmodium perplexum
13 (8) Fruits with the segments 4 or 5 (rarely fewer due to abortion), positioned in a more or less straight line; corollas 5–13 mm long (14)
+ Fruits with the segments 1–3 (sometimes 4 in D. nuttallii), appearing curved or arched downward in outline; corollas 3–5 mm long (to 7 mm in D. nuttallii) (15)
14 (13) Petioles of upper leaves 1–8 mm long, lower petioles to 30 mm long; flowers 8–13 mm long; primary bracts conspicuous before flowering, 5–10 mm long; stalklike base 1–2 mm long and hidden by the persistent calyx; stipules 5–10 mm long Desmodium canadense
+ Petioles of upper leaves 10–50 mm long; flowers 5–8 mm long; primary bracts inconspicuous before flowering, 1.5–3.0 mm long; stalklike base 2–5 mm long and extending above the persistent calyx; stipules 2–6 mm long Desmodium paniculatum
15 (13) Leaflets relatively narrow, 4–10 times as long as wide, linear, narrowly lanceolate, or narrowly oblong (16)
+ Leaflets relatively broad, 1–4 times as long as wide, oblong, lanceolate, ovate, deltoid, rhombic, or nearly circular (17)
16 (15) Petioles nearly absent, 1–3 mm long; upper margin of each fruit segment convex (broadly rounded) Desmodium sessilifolium
+ Petioles 5–20 mm long; upper margin of each fruit segment straight to slightly concave; leaflets appearing strongly folded longitudinally Desmodium strictum
17 (15) Leaflets densely velvety-hairy on the undersurface; corollas 4–7 mm long; fruit segments 2–4; stipules often dark red and reflexed at maturity Desmodium nuttallii
+ Leaflets glabrous to densely hairy but not velvety to the touch; corollas 3–5 mm long; fruit segments 1–3; stipules brown at maturity, appressed-ascending or slightly spreading (18)
18 (17) Stems (below the inflorescence) and leaves glabrous or nearly so; leaflets with a barely visible network of raised veins; petioles of median leaves 5–30 mm long Desmodium marilandicum
+ Stems and leaves obviously hairy; leaflets with a conspicuous network of raised veins; petioles of median leaves 1–20 mm long (19)
19 (18) Leaves with the central leaflet 0.5–3.0 cm long, usually less than twice as long as wide; petioles 1–10 mm long Desmodium ciliare
+ Leaves with the central leaflet 3–7 cm long, at least twice as long as wide; petioles of the median leaves 5–20 mm long Desmodium obtusum
 
 
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