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.