BOT 101 - Plant Taxonomy

Amaranthaceae A.L. de Jussieu
(Amaranth Family)
Families of tracheophytes (other than angiosperms) as classified by Judd et al., 1999.

Usually herbs or suffrutescent shrubs, sometimes succulent; usually with concentric rings of vascular bundles; betalains present; occasionally with C4 photosynthesis; plastids of the sieve elements with a ± peripheral ring of proteinaceous filaments, but without a central protein crystal. Hairs simple to branched. Leaves alternate or opposite, simple, usually entire or undulate, sometimes serrate
or lobed, with pinnate venation, but veins often obscure, sometimes succulent; stipules lacking. Inflorescences determinate, terminal and axillary. Flowers bisexual or, less commonly, unisexual (plants then monoecious to dioecious), radial, associated with fleshy to dry, papery bracts and/or bractlets, and often densely clustered.

Tepals usually 3-5, distinct to slightly connate, green and herbaceous or fleshy, to white (or reddish), dry, and papery, imbricate. Stamens 3-5, opposite tepals; filaments distinct, slightly to strongly connate; anthers 2- or only 1-locular; pollen grains 7-porate to polyporate, with pores scattered over the surface of the grain. Carpels usually 2 or 3, connate; ovary usually superior, with basal placentation; stigmas 1 to 3, elongate to capitate. Ovules 1 to few, usually campylotropous. Nectar disk or glands often present.

Fruit usually an achene, utricle, or a circumscissile capsule (pyxis), usually associated with persistent, fleshy to dry, peri-
anth and/or bractlets; embryo curved to spirally twisted; endosperm ± lacking, replaced by perisperm (Figure 8.47 and Figure 4.47E).

Floral Formula:
*,-3-5-,5, 2-3); achene, utricle, 1-seeded capsule

Distribution and ecology: Cosmopolitan and especially characteristic of disturbed, arid, or saline habitats.

Genera/species: 169/2360. Major genera: Atriplex (300 spp.), Gomphrena (120), Salsola (120), Alternanthera (100), Chenopodium (100), Ptilotus (100), Suaeda (100), Iresine (80), Amaranthus (60), Corispermum (60), and Celosia (50).

Numerous genera have native or naturalized species within the continental United States and/or Canada; some of these include Alternanthera, Amaranthus, Atriplex, Blutaparon, Celosia, Chenopodium, Froelichia, Iresine, Gomphrena, Grayia, Monolepis, Nitrophila, Salicornia, Salsola, and Suaeda

Economic plants and products: The leaves and/or roots of a few species, such as Beta vulgaris (beet, SwisChard), Spinacia oleracea (spinach), Chenopodium spp. (lamb's-
quarters, goosefoot), and Amaranthus spp. (pigweisd), are eaten. The seeds of several South American species of Chenopodium and Amaranthus are used to make flour. The family includes a few ornamentals, including Celosia (cockscomb), Gomphrena (globe amaranth)/ and Iresine (bloodleaf).

Discussion: Amaranthaceae are here broadly defined and include the Chenopodiaceae, which' usually have been maintained as a separate family because of their usually distinct (vs. slightly to completely connate) stamens and greenish and membranous to fleshy (vs. white, white with green stripes, to pink or red, and dry/papery) tepals.

The monophyly of Amaranthaceae, as broadly circumscribed, has been strongly supported
by chloroplast DNA restriction sites, rbcL sequences, ORF2280 sequences, and morphology (Rodman 1990, 1994; Rodman et al. 1984; Downie and Palmer 1994a,b; Manhart and Rettig 1994; Downie et al. 1997). Separation
of Chenopodiaceae from Amaranthaceae is arbitrary and results in a paraphyletic Chenopodiaceae (Downie et al.
1997; Rodman 1990,1994; Manhart and Rettig 1994).

Currently recognized subfamilies and tribes stress
ovule number, embryo shape, number of anther locules, and perianth form. Many of these groups are probably not monophyletic. Genera with papery tepals and bractlets and monadelphous stamens-Celosia, Iresine, Froelichia, Alternanthera, Blutaparon, Gomphrena, and Alternanthera-may form a clade, which probably does not include Amaranthus. Within this group, genera with unilocular anthers-Froelichia, Alternanthera, Gomphrena, and Iresine-constitute a subclade. Greenish, often fleshy tepals or bracts and separate stamens characterize Atriplex, Chenopodium, Salsola, Salicornia, Suaeda, Beta, and Spinacia, a possibly paraphyletic complex. Spirally twisted embryos and reduced perisperm suggest that Salsola, Suaeda, and relatives form a monophyletic group. Hybridization and polyploidy are common in some genera, leading to taxonomic difficulties at the species level.

The usually small, densely clustered flowers of Amaranthaceae are pollinated by wind or by various insects; selfing as well as outcrossing may occur. The small, dry fruits or seeds, which are typically associated with the accrescent and sometimes hairy perianth, usually are dispersed by wind or water. Salsola includes tumbleweeds. A few species form burrlike inflorescence units that are externally transported by animals. In Amaranthus and Celosia the small seeds tend to fall from the parent plant but germinate only when the site is again disturbed. Many seeds are accidently eaten and dispersed by browsing animals.

References: Blackwell 1977; Carolin 1983; Carolin et al. 1975; Cronquist 1981; Downie and Palmer 1994a,b; Downie et al. 1997; Judd and Ferguson 1999; Kuhn et al. 1993; Manhart and Rettig 1994; Robertson 1981; Rodman
1990,1994; Rodman et al. 1984; Townsend 1993.