AMARANTHACEAE (Amaranth Family)
Plants annual or
perennial herbs (sometimes woody elsewhere), often monoecious or dioecious, not
or only slightly succulent, often with a taproot, glabrous or hairy, often
tinged with pink to purple pigmentation. Stems spreading to erect. Leaves
alternate or opposite, simple, the margins entire or sometimes somewhat wavy
(occasionally minutely sharply toothed in Iresine). Stipules absent
(paired stipulelike axillary spines present in Amaranthus spinosus).
Inflorescences axillary and/or terminal; dense spikes, spikelike racemes, or
panicles, sometimes reduced to small, axillary clusters (globose heads or solitary
flowers elsewhere), the main axis occasionally broadened and flattened
(fasciated) with flowers across the surface. Flowers sessile or very
short-stalked, with 1–3 small, papery to scalelike or hardened (sometimes
appearing spine-tipped) bracts (1 bract and usually 2 additional bracteoles),
imperfect or perfect, hypogynous. Calyx absent or more commonly of (2–)4 or 5
sepals, these free or fused either most of their length or only at the base,
green and often somewhat hardened (sometimes appearing spine-tipped) or white,
yellow, pink, red, or purple and papery, persistent at fruiting. Petals absent.
Stamens (1–)4 or 5, absent or reduced to minute staminodes in pistillate
flowers, the filaments sometimes fused, at least toward the base, the anthers attached
toward their midpoints, usually yellow. Pistil 1 per flower (absent in
staminate flowers), the ovary superior, consisting of 2 or 3 fused carpels,
with 1 locule, the placentation usually basal. Style absent or 1, often very
short, the stigmas 1–3, slender or capitate, occasionally lobed. Ovules 1
(several in Celosia). Fruits mostly capsules (occasionally indehiscent
and achenelike), not winged at the tip, sometimes beaked, indehiscent or more
commonly with irregular or circumscissile dehiscence. Seeds 1 (2–6 in Celosia),
minute, often somewhat flattened, circular in outline or nearly so (the embryo
appearing curved or coiled but not always easily observed). Sixty-five to 69
genera, 900–1,000 species, nearly worldwide, but most diverse in tropical and
subtropical regions.
The
Amaranthaceae are here treated in the traditional sense as a family separate
from the Chenopodiaceae. However, a number of morphological and molecular
phylogenetic studies (Rodman, 1990, 1993; Manhart and Rettig, 1994; Downie et
al., 1997; see also Judd et al., 2002) have presented evidence to suggest that
the Chenopodiaceae as thus circumscribed are paraphyletic; that is, that the
genera of Amaranthaceae represent a specialized subgroup within the lineage of
Chenopodiaceae rather than a separate sister clade. Because some of the
conclusions of these papers are contradictory and a few relationships among
genera are yet controversial (such as the placement of Spinacia L.), it
seems premature to combine these families in a floristic treatment until more
detailed studies can be completed. The morphological features that generally
separate the Amaranthaceae from Chenopodiaceae include their stamens with the
filaments fused basally (vs. free) and papery (vs. herbaceous) perianth and
bracts, but numerous exceptions exist.
Members of the
Amaranthaceae are nearly all wind-pollinated. Pollen grains of most
Amaranthaceae and Chenopodiaceae are virtually indistinguishable
morphologically, and the two families are usually lumped into a single pollen
class in projects that monitor airborne spores and pollen for air quality and
hay fever reports. In addition to members of the genera included below, some
species of Gomphrena L., globe amaranth, are cultivated in gardens for
their ornamental foliage and inflorescences.
In the keys and
descriptions below, measurements of bract length refer to the bract, which
often is larger and more conspicuous than the two bracteoles that usually are
also present, although sometimes the three structures are essentially
indistinguishable.