39. Amaranthaceae Jussieu
Kenneth R. Robertson & Steven E. Clemants
Herbs, rarely subshrubs, annual or perennial; trichomes simple (branched in Tidestromia). Stems without nodal spines (Amaranthus spinosus sometimes with paired nodal spines). Leaves alternate or opposite, exstipulate, usually petiolate; blade margins entire (entire or serrulate in Iresine; entire, crispate, or erose in Amaranthus). Inflorescences cymules arranged in spikes, panicles, thyrses, heads, glomerules, clusters, or racemes; each flower subtended by 1 bract and 2 bracteoles (latter sometimes 1 or absent in Amaranthus). Flowers bisexual or unisexual (plants then monoecious or dioecious), hypogynous, generally small or minute; tepals mostly (1-)4-5 or absent, distinct or connate into cups or tubes, scarious, chartaceous, membranaceous, or indurate; stamens 2-5, filaments basally connate into cups or tubes, rarely distinct, alternating with pseudostaminodes (appendages on staminal tubes) or not, anthers 2-locular with 1 line of dehiscence or 4-locular with 2 lines of dehiscence; ovary superior, 1-locular; ovules 1 or, rarely, 2-many; style 1 or absent; stigmas 1-3(-5). Fruits utricles, dry, dehiscent or not. Seeds black, reddish brown, or brown, lenticular, subglobose or globose (rarely cylindric), usually small; embryo peripheral, surrounding mealy perisperm.
Genera ca. 65, species ca. 900 (12 genera, 80 species in the flora): nearly worldwide, most abundant in tropics, subtropics, and warm-temperate regions, evidently absent from alpine and arctic regions.
Centers of diversity for Amaranthaceae are southwestern North America, Central America, South America, and Africa south of the Sahara Desert. Generic limits are not well defined in some groups; fewer than 60 or more than 70 genera could be recognized.
Some species occur in severe habitats such as sandy, calcareous, gypseous, saline, or serpentine soils in deserts, semideserts, and seashores. Some species are weedy, including the major agricultural weeds in Amaranthus. Some species are cultivated as ornamentals, particularly Amaranthus caudatus (love-lies-bleeding), A. hypochondriacus (prince’s-feather), A. tricolor (Joseph’s-coat), Celosia cristata (cockscomb), and Gomphrena globosa (globe-amaranth). Native Americans domesticated white-seeded grain amaranths (A. caudatus, A. cruentus, and A. hypochondriacus) for use as cereal grains. Some species of Amaranthus and Celosia are potherbs.
Amaranthaceae are usually divided into subfamilies Amaranthoideae (anthers 4-locular with two lines of dehiscence) and Gomphrenoideae Schinz (anthers 2-locular with one line of dehiscence). Amaranthaceae and Chenopodiaceae have long been recognized as allied families that share a number of features: generally small flowers, one perianth whorl, a syncarpous gynoecium with a superior ovary and often only one ovule, basal or free-central placentation, pollen characteristics, centrospermous embryo development, betalain pigments, and P-type form (c) sieve-element plastids.
Carolin, R. C. 1983. The trichomes of the Chenopodiaceae and Amaranthaceae. Bot. Jahrb. Syst. 103: 451-466. Eliasson, U. H. 1988. Floral morphology and taxonomic relations among the genera of Amaranthaceae in the New World and the Hawaiian Islands. Bot. J. Linn. Soc. 96: 235-283. Robertson, K. R. 1981. The genera of Amaranthaceae in the southeastern United States. J. Arnold Arbor. 62: 267-314. Standley, P. C. 1915. The North American tribes and genera of Amaranthaceae. J. Wash. Acad. Sci. 5: 391-396. Standley, P. C. 1917b. Amaranthaceae. In: N. L. Britton et al., eds. 1905+. North American Flora.... 47+ vols. New York. Vol. 21, pp. 95-169.