26.29   ZEA L.
Hugh H. Iltis

Plants annual or perennial; monoecious, inflorescences unisexual or bisexual with the pistillate spikelets basal and the staminate spikelets distal. Culms (0.2)0.5-6 m tall, 1-5 cm thick, solitary or several to many together, monopodial, often branching (branches frequently highly reduced and hidden within the subtending leaf sheath), usually succulent when young, becoming woody with age; lower nodes with prop roots; internodes pith-filled. Leaves not aromatic, cauline, distichous; sheaths open; auricles sometimes present; ligules membranous, shortly ciliate; blades 2-12 cm wide, flat. Pistillate or partially pistillate inflorescences terminal on axillary branches; staminate inflorescences (tassels) paniculate, of 1-many branches or rames, sometimes with secondary and tertiary branching. Wild taxa: Pistillate inflorescences solitary, distichous rames (ears), these often tightly clustered in false panicles, each usually wholly or partially enclosed by a thin prophyll and an equally thin bladeless leaf sheath; rames composed of 5-15 spikelets in 2 ranks; disarticulation in the rame axes, dispersal units (fruitcases) consisting of an indurate, shiny rame segment and its embedded spikelet. Pistillate spikelets solitary, sessile, with 1 floret; pedicels and pedicellate spikelets suppressed; lower glumes exceeding the floret, indurate on the central, exposed portion, hyaline on the margins, concealing the caryopses at maturity. Domesticated taxon: Pistillate inflorescences solitary, polystichous spikes (ears) terminating reduced branches, each spike surrounded by several to many, often bladeless leaf sheaths and a prophyll (husks), with 60-1000+ spikelets in 8-24 rows, neither spikes nor spikelets disarticulating at maturity. Pistillate spikelets in subsessile pairs, each spikelet with 1 functional floret; glumes shorter than the spikelets, indurate basally, hyaline distally; lower florets suppressed. All taxa: lemmas and paleas hyaline, unawned; lodicules absent; ovaries glabrous; styles (silks) 2, appearing solitary by being fused except at the very tip, filamentous, sides stigmatic. Caryopses subspherical to dorsally compressed; hila round; embryos about 2/3 as long as the caryopses. Wild taxa: Staminate panicles terminal on the culms and primary branches, sometimes also on the secondary branches and pistillate inflorescences; rames distichous, similar in thickness and structure, axes disarticulating below the sessile spikelets after pollination, abscission layers evident. Domesticated taxon: Staminate panicles terminal on the culms, central axes always much thicker than the lateral branches and irregularly polystichous, lateral branches distichous to more or less polystichous, not disarticulating, without abscission layers below the sessile spikelets. All taxa: Staminate spikelets in sessile-pedicellate pairs, each with 2 staminate florets; glumes membranous to chartaceous, stiff to flexible, sometimes with a pair of winged keels, 5-14(28)-veined, acute; lemmas and paleas hyaline; lodicules 2; anthers 3. x = 10. Name from the Greek zea or zeia, a kind of grain.

Zea is an American genus of five species, four of which are native to montane Mexico and Central America. The fifth species, Z. nicaraguensis H.H. Iltis & B.F. Benz, is said to have been ubiquitous at one time in coastal Pacific Nicaragua, but is now known from only four or five small populations near sea level in seasonally flooded savannahs and riverine forests inland from the Bay of Fonseca, Nicaragua.

The often weedy, wild taxa, known as teosinte, are used in plant breeding as well as in developmental and evolutionary studies. The genus has also been the focus of physiological and genetic research, mostly involving the domesticated taxon, Zea mays subsp. mays. Examples of such work include Barbara McClintock's Nobel prize-winning discovery that genes can jump from one chromosome to another, and recent work on the evolution of tassel morphology (e.g., Westerbergh and Doebley 2002).

Zea mays subsp. mays, the most widespread taxon in the genus, was first domesticated about 7,000 years ago and soon became widely planted in the Americas. It is now grown in all warmer parts of the world and is the worlds third most important crop plant. No other American grass has such agricultural importance.

In the Flora region, Zea mays subsp. mays is widely grown commercially; Z. luxurians is sometimes grown for forage; while Z. diploperennis and Z. perennis, and the other subspecies of Z. mays, are almost completely confined to research plantings.

SELECTED REFERENCES Doebley, J.F. 1990. Molecular systematics of Zea (Gramineae). Maydica 35:143-150; Doebley, J.F. and H.H. Iltis. 1980. Taxonomy of Zea (Gramineae). I. A subgeneric classification with a key to taxa. Amer. J. Bot. 67:982-993; Doebley, J.F., A. Stec, and C. Justus. 1995. Teosinte branched1 and the origin of maize: Evidence for epistatis and the evolution of dominance. Genetics 141:333-346; Eubanks, M.W. 2001. The mysterious origin of maize. Econ. Bot. 55:492-514; Hitchcock, A.S. 1951 [title page 1950]. Manual of the Grasses of the United States, ed. 2, rev. A. Chase. U.S.D.A. Miscellaneous Publication No. 200. U.S. Government Printing Office, Washington, D.C., U.S.A. 1051 pp.; Iltis, H.H. 2000. Homeotic sexual translocations and the origin of maize (Zea mays, Poaceae): A new look at an old problem. Econ. Bot. 54:7-42; Iltis, H.H. and J.F. Doebley. 1980. Taxonomy of Zea (Gramineae). II. Subspecific categories in the Zea mays complex and a generic synopsis. Amer. J. Bot. 67:996-1004; McVaugh, R. 1983. Flora Novo-Galiciana: A Descriptive Account of the Vascular Plants of Western Mexico, vol. 14: Gramineae (series ed. W.R. Anderson). University of Michigan Press, Ann Arbor, Michigan, U.S.A. 436 pp.; Westerbergh A. and J. F. Doebley.2002. Morphological traits defining species differences in wild relatives of maize are controlled by multiple quantitative trait loci. Evolution 56:273-283.

Pistillate inflorescences terete, with 2+ rows of paired spikelets, each spikelet with a functional floret, hence the spikelets in 4+ rank; staminate spikelets of wild taxa only slightly imbricate; lower glumes of the staminate spikelets flexible and translucent, loosely enclosing the upper glumes before anthesis, rounded on the back, the lateral veins not more prominent than those between, never forming winged keels; plants annual (sect. Zea) ..... 4. Z. mays
Pistillate inflorescences somewhat flattened, with 2 rows of solitary spikelets, hence the spikelets appearing 2-ranked; staminate inflorescences with densely imbricate spikelets; lower glumes of the staminate spikelets stiff, not translucent, strongly enclosing the upper glumes before anthesis, with more or less flat backs, the lateral veins evidently more prominent than those between, keeled and winged distally; plants annual or perennial (sect. Luxuriantes) (2)
Plants annual; lower glumes of the staminate spikelets narrowly winged; pistillate inflorescence units 1-many per node, usually enclosed by their subtending leaf sheaths, occasionally with 1-2 rames on naked peduncles that exceed the subtending leaf sheaths ..... 3. Z. luxurians
Plants perennial, rhizomatous; lower glumes of the staminate spikelets strongly winged; pistillate inflorescence units 1-4(5) per node, almost always exceeding the subtending leaf sheaths (3)
Rhizomes to 15 cm long, with internodes 0.2-0.6 cm long, often forming scaly, tuberous short shoots; culms to 3.5 m tall and 3 cm thick; leaf blades to 40 cm long and 4-5.5 cm wide ..... 1. Z. diploperennis
Rhizomes to 40 cm long or more, with internodes 1-6 cm long, lacking tuberlike shoots; culms to 2.5 m tall and 1.5-2 cm thick; leaf blades often to 65(80) cm long and 2-4.5 cm wide ..... 2. Z. perennis

1.   Zea diploperennis H.H. Iltis, Doebley & R. Guzmán

Plants perennial; rhizomatous, rhizomes to 15 cm, internodes 0.2-0.6 cm, often forming scaly, tuberous short shoots 1-2 cm thick. Culms 1-3.5 m tall, (1)2-3 cm thick, solitary or in large clumps. Blades usually to 40 cm long, 4-5.5 cm wide, linear-lanceolate. Pistillate peduncles (1)2-4(5) per node, 5-25(52) cm, the 3-5 longer peduncles extending far beyond the subtending leaf sheaths, each peduncle with 1(2) pistillate rames; pistillate rames 5-10 cm long, 4-5 mm thick, distichous, with 5-10 solitary spikelets, frequently not enclosed in a leaf sheath; fruitcases trapezoidal in side view, 6-9 mm on the long side, 2.5-4.5 mm on the short side, 4-5 mm in diameter. Caryopses concealed by the lower glumes. Terminal staminate panicles 6-18 cm, with 2-15 branches; branches 6-15 cm, erect to divergent, internodes 2-6 mm; spikelets 8.5-11.5 mm long, about 3 mm wide, densely imbricate; lower glumes flat dorsally, stiff, not translucent, margins tightly enclosing the upper glumes, the 2 principal sublateral veins prominently keeled and apically winged. 2n = 20.

Zea diploperennis, although locally abundant, is rare in the wild, being known only from a few populations in the Sierra de Manantlán, Jalisco, Mexico. It grows at elevations of 1400-2400 m, sometimes forming large clones or extensive colonies in old maize fields and on the edges of oak-pine cloud forests. It is grown for genetic research and plant breeding in many countries and occasionally as an ornamental plant in warmer parts of the contiguous United States. It hybridizes infrequently with Z. mays subsp. mays in its native range.

2.   Zea perennis (Hitchc.) Reeves & Mangelsd.
Perennial Teosinte

Plants perennial; rhizomatous, rhizomes to 40 cm or longer, internodes 1-6 cm, not forming tuberous short shoots. Culms 1-2.5 m tall, 1.5-2 cm thick, loosely clumped, usually branched above. Blades 20-65(80) cm long, 2-4.5 cm wide, linear. Pistillate peduncles 1-2(3) per node, 10-25 cm, at least 1 and sometimes 2 extending far beyond the terminal leaf sheaths; pistillate rames 4-8 cm long, 4-6 mm thick, distichous, with 5-10 solitary spikelets, distal portions often staminate; fruitcases trapezoidal in side view, 6-9 mm on the long side, 2.5-4.5 mm on the short side, 4-5 mm in diameter. Caryopses concealed by the lower glumes. Terminal staminate panicles 12-20 cm, with 2-8 branches; branches 9-15 cm, erect to nodding, internodes 2.4-6.2 mm; spikelets 8.5-11 mm long, 2-2.5(3.2) mm wide, densely imbricate; lower glumes flat dorsally, stiff, not translucent, margins tightly enclosing the upper glumes, lateral veins prominent, strongly winged distally. 2n = 40.

Zea perennis is parapatric to Z. diploperennis, being native to the northern base of the Volcán de Colima, Jalisco, Mexico, at elevations of 1520-2200 m. It is rare, although locally abundant, in and around maize fields and orchards in former open oak and pine forests and savannahs. Zea perennis crosses infrequently with Z. mays subsp. mays. The hybrids, being triploid, are sterile. It has also been cultivated at research stations in the United States for many years and Hitchcock (1951) reported that it was established at James Island, South Carolina. It is not known if the population has persisted.

3.   Zea luxurians (Durieu & Asch.) R.M. Bird
Guatemala Teosinte, Florida Teosinte

Plants annual. Culms 1-3(4) m tall, 1-4 cm thick, unbranched in dense stands, abundantly branched in open areas. Blades 20-80 cm long, 3-8 cm wide, glabrous. Pistillate inflorescences 1-many per node, usually in dense, sheathed, axillary clusters; peduncles usually (0)1-8 cm, slender and not exceeding the leaf sheaths, occasionally 1(2) peduncles as long as 23 cm and exceeding the leaf sheaths; pistillate rames distichous, 6-9 cm, subtended by a sheath, with 5-9 solitary spikelets; fruitcases trapezoidal in side view, 7-11.5 mm on the long side, 3.7-6.5 mm on the short side, 3-5 mm in diameter. Caryopses concealed. Terminal staminate panicles 12-24 cm, with (4)10-28 stiffly ascending branches; branches 7-16(21) cm, internodes 3-6 mm; pedicels 3-5 mm; spikelets 4.6-12 mm, densely imbricate; lower glumes flat dorsally, stiff, not translucent, margins tightly enclosing the upper glumes, (9)12-20(28)-veined, the 2 sublateral veins prominent, keeled, ciliate, narrowly winged distally. 2n = 20.

Zea luxurians is endemic to Central America, growing from Guatemala to Honduras, at elevations of 600-1200 m, and may extend into Oaxaca, Mexico. It was frequently grown for forage about a century ago, and is still sometimes grown for this purpose in the southern United States. It hardly ever tillers in the wild, but forms as many as 50 tillers in favorable agricultural settings and longer day lengths than in its native range. Although it can hybridize with Z. mays subsp. mays, Z. luxurians rarely does so in the wild.

4.   Zea mays L.

Plants annual. Culms (0.5)1-3(6) m tall, (0.5)1-5 cm thick. Blades mostly 30-90 cm long, 2.5-12 cm wide. Pistillate inflorescences rames or spikes, usually shortly pedunculate (sometimes sessile), solitary, 4-30(40) cm long, (0.5)1-10 cm thick, with 2 or more rows of paired spikelets, hence the spikelets 4 or more ranked, rarely terminating in an unbranched staminate inflorescence. Caryopses concealed in fruitcases (wild taxa) or exposed (domesticated taxon); fruitcases of wild taxa distichous, triangular in side view; domesticated taxon without fruitcases, glumes reduced and shallow or collapsed and embedded in the rachis. Staminate panicles 10-25+ cm, with 1-60(235) branches, internodes 1.5-8.2 mm; spikelets 9-14 mm long, 2.5-5 mm wide; lower glumes rounded dorsally, flexible, translucent, papery, loosely enclosing the upper glumes, the 2 lateral veins subequal to the others, not winged. 2n = 20.

Of the five subspecies of Zea mays, only the domesticated subspecies, Z. mays subsp. mays, is widely grown outside of research programs. Three wild subspecies are treated here, albeit briefly, because of their importance as genetic resources for Z. mays subsp. mays.

Pistillate inflorescences cylindrical spikes, 2-5(10) cm thick, with 8-24+ rows of spikelets pairs, each inflorescence tightly and permanently enclosed by several leaf sheaths and a large prophyll, not disarticulating at maturity; caryopses 60-1000+, not concealed by the glumes; staminate panicle branches not disarticulating below the sessile spikelets, lacking abscission layers; central axis of the staminate panicles polystichous, much thicker than the lateral branches; obligate domesticate ..... subsp. mays
Pistillate inflorescences cylindrical, distichous rames, less than 1 cm thick, with 2 rows of spikelet pairs, each rame usually enclosed by a single leaf sheath and a prophyll, disarticulating at maturity into fruitcases; caryopses 4-15, each one concealed within a fruitcase; staminate panicles composed of rames that disarticulate below the sessile spikelets and have evident abscission layers; central axis of the staminate panicles similar in width to the rames; in the Flora region, wild taxa are known only from research plantings (2)
Staminate spikelets (6.6)7.5-10.5 mm long; fruitcases 6-10 mm long, 4-6 mm wide; staminate panicles with 1-35+ ascending to divergent, rather stiff branches ..... subsp. mexicana
Staminate spikelets 4.6-7.2(7.9) mm long; fruitcases 5-8 mm long, 3-5 mm wide; staminate panicles usually with 10-100(235) divergent to nodding branches (3)
Leaves pubescent; staminate panicles usually with (2)10-100(235) branches ..... subsp. parviglumis
Leaves glabrous or almost so; staminate panicles usually with fewer than 40 branches ..... subsp. huehuetenangensis

Zea mays subsp. huehuetenangensis (H.H. Iltis & Doebley) Doebley
Huehuetenango Teosinte

Zea mays subsp. huehuetenangensis is morphologically similar to subsp. parviglumis (see below), but it often grows more than 5 m tall, and has essentially glabrous leaves, and smaller staminate panicles with fewer (less than 40), firmer branches, and different ecological, phenological, and molecular characteristics.

It is endemic to the province of Huehuetenango, Guatemala, where it grows as a common weed on the edges of, and in, maize fields, and in seasonally moist oak cloud and tropical deciduous forests, at elevations from 900-1650 m. In its native range, it commonly hybridizes with subsp. mays, both subspecies flowering from mid-December to mid-January, at the end of the wet season.

Zea mays L. subsp. mays
Corn, Indian Corn, Maize, Mais

Culms (1)2-4(6) m tall, (1)2-5 cm thick. Blades 50-90 cm long, 3-12 cm wide. Pistillate inflorescences spikes, 15-25(40) cm long, 2-5(10) cm thick, cylindrical, tightly and permanently enclosed in several to many leaf sheaths and a large prophyll, with 8-24 or more rows of paired spikelets on a thickened, strongly vascularized, tough rachis (cob), not disarticulating at maturity; fruitcases not developed, rachis internodes fused into the extra-vascular cylinder, glumes reduced, shallow. Caryopses 60-1000+, exposed and naked. Staminate panicles with a polystichous central axis and non-disarticulating branches; central axes usually much denser and thicker than the usually distichous lateral branches, these lacking abscission layers. 2n = 20.

Zea mays subsp. mays is the familiar domesticated corn (or maize), from which around 400 indigenous races and many different kinds of cultivars have been developed. It is an obligate cultigen, unable to persist outside of cultivation because the caryopses are permanently attached to the rachis and enclosed by the subtending leaf sheaths. Supersweet cultivars have a double recessive gene that delays the conversion of sugar to starch; flint corns have unusually hard endosperm; and waxy cultivars have endosperm with an unusually high level of proteins and oils. Popcorns have a core of soft, relatively moist endosperm surrounded by hard endosperm. The grains pop when heat causes the moisture of the inner endosperm to vaporize.

Zea mays subsp. mexicana (Schrad.) H.H. Iltis
Chalco Teosinte, Central-Plateau Teosinte, Nobogame Teosinte

Culms (0.5)1-3 m tall, (0.5)1-2(3) cm thick, unbranched when growing in corn fields to branched and with staminate inflorescences terminating the many branches when growing in the open. Blades 30-85 cm long, 3-8 cm wide. Pistillate inflorescences 5-8(10) cm long, 0.6-0.8 cm thick, distichous, with 2 rows of spikelets embedded in the rachis; rachises disarticulating at maturity; fruitcases (6)9-12(15), 6-10 mm long, 4-6 mm wide, triangular in side view, with pointed apices. Caryopses concealed in the fruitcases. Staminate panicles with 1-35+ ascending to divergent, somewhat stiff, disarticulating branches; central axes of staminate panicles as slender as the lateral branches. Staminate spikelets (6.6)7.5-10.5 mm. 2n = 20.

Zea mays subsp. mexicana is a weedy taxon, native to upland Mexico. It is most abundant in the Meseta Central of the Mexican neo-volcanic plateau, at elevations of 1700-2500 m. It grows almost entirely in and around cornfields, and readily hybridizes with subsp. mays. The long-day tolerant 'Northern Teosinte' is the result of a series of backcrosses between such hybrids and the northernmost population of subsp. mexicana, 'Nobogame Teosinte'.

Zea mays subsp. parviglumis H.H. Iltis & Doebley
Balsas Teosinte, Guerrero Teosinte

Culms (0.5)2-4 m, unbranched or branched above the middle, thinner than in subsp. mexicana. Leaves pubescent. Fruitcases 5-8 mm long, 3-5 mm wide. Caryopses concealed. Staminate panicles with (2)10-100(235) slender, often drooping branches; spikelets 4.6-7.2 mm, distant. 2n = 20.

Zea mays subsp. parviglumis, which has the smallest fruitcases of all the wild taxa, is endemic to the Pacific slope of southern Mexico, from Oaxaca to Jalisco, being most abundant in the Balsas River drainage. It grows in highly seasonal, sunny thorn scrub, and open tropical deciduous forests and savannahs, at elevations of (450)600-1400(1950) m. One of its higher elevation populations appears to be the ancestor of subsp. mays. In the southern United States, Z. mays subsp. parviglumis is grown as part of breeding programs. In its native habitat, it tends to be seasonally isolated from subsp. mays, flowering a few weeks later, but the two sometimes form abundant hybrids in local areas.