2. Chamaecrista (L.) Moench
Plants annual
(perennial herbs or woody elsewhere), unarmed, with 1 to few stems; root
nodules present. Leaves short- to long-petiolate, the petiole with a relatively
large, sessile or stalked, more or less cup-shaped gland. Stipules well
developed, with several prominent, more or less parallel, raised veins,
persistent. Leaf blades evenly 1 time pinnately compound. Leaflets numerous,
opposite, oblong to narrowly oblong or oblong-elliptic, asymmetrical at the
base, the midvein usually somewhat off-center, the margins entire.
Inflorescences axillary or lateral from just above the leaf axil, appearing as
solitary flowers or small, loose clusters (actually short racemes), the flower
stalks with 2 small bracts near or above middle, often twisted 180° so that the
flower is resupinate. Flowers perfect, perigynous, somewhat asymmetrical.
Hypanthium short, saucer-shaped. Calyces of 5 free sepals. Corollas of 5 free
petals, these dissimilar in size and shape, the largest one appearing lowermost
in the flower, yellow (rarely white), sometimes reddish-tinged toward the base.
Stamens 5 or 10, usually all fertile, the filaments short, not fused, the
anthers of different lengths, attached at the base, dehiscing by apical slits
or pores. Styles curved. Fruits legumes, narrowly oblong, straight to curved,
not twisted, strongly flattened, with 4–20 seeds, elastically dehiscent with
coiling valves. Seeds nearly square to trapezoidal in outline, flattened, light
to dark brown; pleurogram absent. About 265 species, widespread, primarily in
the American tropics and temperate regions.
Although long
recognized as a natural group, the generic status of Chamaecrista has
been debated for over 200 years. Chamaecrista and Senna often
have been included in the large genus Cassia, which in its broadest sense
contains more than 600 species and is the largest genus in Caesalpinioideae
(Isely, 1975; Irwin and Barneby, 1976). Bentham (1871) recognized 3 subgenera
that correspond more or less to Cassia in the narrow sense, Senna,
and Chamaecrista. Irwin and Barneby (1981, 1982) provided a discrete set
of characters for recognizing these at the generic level and made the required
nomenclatural changes. In this narrow sense, Cassia consists of only 30
species of trees and shrubs that are confined to the tropics, lack extrafloral
nectaries, have very long, sigmoidally curved filaments, and have anthers that
dehisce by slits or basal pores. In contrast, both Chamaecrista and Senna
are large genera, each with over 250 species of trees, shrubs, and herbs. Their
stamens have very short filaments and dimorphic or trimorphic anthers that
dehisce by terminal slits or pores. Chamaecrista is distinguished from Senna
by the large, persistent stipules, 2 small bracts on the flower stalk,
elastically dehiscent legumes, stamens with usually 2 sizes of anthers, and
seeds lacking a pleurogram. Senna lacks bracts on the flower stalk, has
tardily dehiscent, nonelastic legumes, a graded series of anthers, and seeds
with a distinctive pleurogram appearing as a depressed area. Furthermore, root
nodules with nitrogen-fixing bacteria are found in Chamaecrista, but not
in Senna or the majority of other Caesalpinioideae.
The leaflets of Chamaecrista
are somewhat sensitive to the touch, although not as sensitive as those of
certain taxa of Mimosoideae. Uprooting the plant also causes the leaflets to
close, and herbarium specimens almost always have the leaves in the closed
position. On their own, the leaves close and pull upward at night into a
so-called sleeping position, a phenomenon known as nyctinasty, common to many
Fabaceae. This movement is thought to control water loss or afford protection
from herbivores.
The seed
dispersal mechanism is also interesting. When the seeds are fully mature and
the legume is dry, the two valves separate suddenly, flinging the seeds a meter
or more away (Lee, 1984).
The two Missouri species of Chamaecrista,
particularly the more abundant C. fasciculata, are important wildlife
food plants and sometimes are planted for this purpose. The foliage is
nutritious for deer and livestock, although it contains anthraquinone compounds
that can cause irritation of the digestive tract if eaten in large quantities
(Burrows and Tyrl, 2001). Birds (especially quail and turkey) are fond of the
seeds.