1. Amaranthus L. (pigweed, amaranth)
Plants annual,
monoecious or dioecious (rarely a few perfect flowers present in monoecious
species). Stems erect to spreading, glabrous or variously pubescent, green to
yellowish white or reddish purple, often with pink to purple longitudinal lines
or ridges. Leaves alternate, short- to long-petiolate. Leaf blades 0.8–20.0 cm
long, variously shaped, usually pubescent with short, curved hairs when young,
usually becoming nearly glabrous (except often along the margins) at maturity.
Inflorescences axillary and/or terminal, dense spikes, spikelike racemes,
panicles (often appearing as spicate branches arranged racemosely along a
central spicate axis), or axillary clusters. Flowers imperfect. Sepals 1–5 or
sometimes absent in pistillate flowers, free, more or less similar or
differentiated into inner and outer series, often with an awnlike extension,
this sometimes becoming somewhat hardened and spinelike at maturity, herbaceous
to hardened or leathery, green, at least centrally, but often with thin, papery
margins. Staminate flowers with the stamens 3–5, the filaments free. Pistillate
flowers with the ovary ovoid. Ovule 1. Style absent or very short, the 2 or 3
stigmas tapered and slender, persistent. Fruits mostly with papery walls, ovoid
or ellipsoid, with 2 or 3 small beaks, indehiscent or more commonly with
irregular or circumscissile dehiscence, 1-seeded. Seeds often somewhat
flattened, circular or nearly so in outline, rounded or angled along the rim,
the surface smooth (sometimes somewhat roughened in A. blitum and A.
viridis), shiny. Sixty to 70 species, nearly worldwide.
The genus Amaranthus
has great economic importance. Several of the Missouri species (the weedy amaranths) are
serious weeds in crop fields. All of the pigweeds produce copious quantities of
potentially allergenic, wind-dispersed pollen that are major causes of hay fever
during some times of the year. On the plus side, however, certain species have
a long history of cultivation for food. Three species, A. caudatus, A.
cruentus, and A. hypochondriacus, have histories of cultivation in
Latin America as pseudo–grain crops that stretch back to long before European
colonization of the Americas, and these cereal amaranths also subsequently
became cultivated in parts of Europe and Asia (Sauer, 1967; Robertson, 1981;
Costea et al., 2001 a, b). Red-pigmented forms of these species and others,
principally A. hybridus, also have been cultivated as garden
ornamentals, and in Latin America they are
used to produce a red dye for religious ceremonies (Robertson, 1981; Heiser,
1985). Yet other species, such as A. tricolor, have been used widely for
food, fresh as greens and boiled as potherbs, and also as a green supplement to
some livestock feeds. That these uses persist today is evidenced by the fact
that all of the species mentioned have been recorded outside of cultivation in Missouri. In fact, grain
amaranths are available to consumers in many grocery and health food stores,
the seeds dried and ground into flour, popped for use in baked goods and
cereals, and powdered as a component of health food beverages (Robertson, 1981).
They are quite nutritious, a good source of protein, and contain high levels of
lysine. Wild food enthusiasts are cautioned, however, that amaranths growing in
the wild occasionally can absorb and accumulate in their foliage toxic
substances present in the soil.
The species of Amaranthus
considered native in Missouri and the central
portion of the United States
occur in a variety of habitats with moist, bare soil. The natural limits of the
distributional ranges of these species are not well understood, as they have
been spread in recent times by agriculture and other human activities as well
as by natural means. These plants are primary colonizers of both habitats prone
to frequent natural disturbance, especially in the floodplains of major rivers
and streams, and also anthropogenically altered sites, like crop fields and
railroad embankments. Distributional limits of these disturbophiles had already
expanded both westward across the Great Plains and eastward to the Atlantic
seaboard as a result of European colonization by the time that botanists began
paying attention to the biogeography of such species (Sauer, 1957, 1967, 1972).
The weedy
amaranths frequently grow in mixed populations, and a number of hybrids have
been reported. These hybrids are sterile, producing inflorescences with many
bracts but with the flowers lacking or vestigial, or in some cases with reduced
flowers having shrunken, misshapen, or absent fruits, so detecting such plants
is not difficult. However, evaluating the presumed parentage of such sterile
individuals with confidence is often not possible. Carl Sauer, a specialist on
the genus, determined one or a few specimens as representing each of the
following hybrid combinations in Missouri: A. hybridus × A. palmeri,
A. hybridus × A. retroflexus, A. hybridus × A. rudis, A. hybridus
× A. tuberculatus, and A. retroflexus × A. rudis (note,
however that A. rudis is reduced to synonymy under A. tuberculatus
in the present treatment). Other combinations are to be expected where two or
more species grow together, but apparently they are rare, even in large mixed
populations.
Identification
of Amaranthus species is difficult, especially when only staminate
flowers are present, not only because of the presence of occasional hybrids but
also because determination depends on observation of details of the small,
morphologically similar flowers and fruits. Within an inflorescence, the
numerous flowers are at various stages of development, and fruits often
continue to mature in the plant press during drying. Taxonomists traditionally
have divided the genus into two main subgroups, the dioecious subgenus Acnida
(L.) Aellen ex K.R. Robertson and the monoecious subgenus Amaranthus.
Recently, a few authors (Costea et al., 2001a) have segregated subgenus Albersia
(Kunth) Gren. & Godr. from subgenus Amaranthus, based on its mostly
axillary clusters of flowers (vs. mostly terminal spikes or panicles),
indehiscent (vs. circumscissile) fruits, and minute differences in leaf and
seed anatomy, but most botanists still treat this group as section Blitopsis
Dumort. of subgenus Amaranthus. In spite of the instinctive appeal of
these groupings, the validity of maintaining such infrageneric taxa is drawn
into question by the existence of various hybrids between them.