Family: Poaceae |
Flora of North America Plants annual or perennial, with or without rhizomes, Culms annual, not woody, sometimes clump-forming. usually erect, not branching above the base; internodes hollow or solid. Leaves: sheaths usually open, those of the basal leaves sometimes closed; collars without tufts of hair on the sides; auricles often present, particularly on lower leaves; ligules membranous or scarious, those of the lower and upper leaves similar; edges sometimes ciliolate, pseudopetioles absent; blades linear to narrowly lanceolate, venation parallel, cross-veins not evident; 2-celled microhairs absent, cross-sectional anatomy non-Kranz; arm and fusoid cells absent, Inflorescences usually spikes or spikelike racemes (but usually called spikes), occasionally panicles, sometimes with morphologically distinct sterile and bisexual spikelets within an inflorescence; nodes of spikes with 1–5 sessile or subsessile spikelets, pedicels absent or to 4 mm; disarticulation usually above the glumes and beneath the florets, sometimes in the rachis, sometimes at the inflorescence base. Spikelets usually laterally compressed, sometimes terete, with 1–16 bisexual florets, all florets usually bisexual or the distal floret sometimes sterile; rachillas sometimes prolonged beyond the base of the distal floret. Glumes unequal to equal, shorter than to longer than the adjacent florets, absent or vestigial in some species, subulate, lanceolate, rectangular, ovate, or obovate, veins 1–15, florets laterally compressed to terete; lemmas lanceolate to rectangular, membranous to coriaceous, sometimes keeled, callus glabrous or hairy; lemma body5(7)-veined, veins not converging distally, inconspicuous, lemma tip entire, lobed, or toothed, unawned or awned, awn(s) terminal, unbranched, lemma-awn junction not evident; palea usually subequal to the lemmas, sometimes considerably shorter or slightly longer; lodicules 2, without venation, usually ciliate; anthers 3; ovary with a hairy, lobed top; styles 2, bases free; stigmas plumose. Caryopses ovoid to fusiform, longitudinally grooved, not beaked, pericarp thin; hilum linear; embryo about 1/3 as long as the caryopsis. Chromosomes: large, base number 7. Because of the economic value of the food species in the tribe, the ease with which its members can be hybridized, and the size of the chromosomes, there have been extensive genetic investigation of the Hordeeae. These have helped illuminate the mechanisms and consequences of polyploidy. Distribution. Members of the Hordeeae are most abundant in temperate to cool regions of the northern hemisphere but are also present in Australasia and South America. All the Australasian species, and only the Australasian species, incorporate the W genome. The annual genera are native only in western Asia. Comments: There are about 400-500 species in the Hordeeae, among which are several important cereal, forage, and range species. Their generic treatment is contentious. Linnaeus (1753) recognized five genera among the species now included in the tribe; Hordeum and Secale are the only two that still have his circumscription. The lack of agreement concerning the generic treatment of the tribe reflects the prevalence of natural hybridization, introgression, polyploidy, and reticulate relationships among its species as wella s differing taxonomic traditions. These factors preclude the circumscription of monophyletic groups and make the delineation of morphologically coherent groups difficult. Tzvelev (1975) argued that these same factors contributed to the tribe's success by maintaining a "generalist" genome. The major disagreement in the treatment of the annual genera concerns Triticum and Aegilops. Some (e.g., Kimber and Feldman 1987) advocate treating them as a single genus in recognition of their close genetic similarity; others argue for maintaining them as separate genera (e.g., van Slageren 1994). Löve (1984) divided them among 14 genera. The two genera are accepted here in their traditional senses, despite the strong arguments for their combination, largely in deference to the wealth of literature, reports, and genetic resources that have been accumulated under these two names. Spontaneous hybridization and introgression between the two are common, and most species of Triticum are derived from hybrids between the two genera. Nevertheless, the two genera are ecologically and morphologically distinct. Treatment of the perennial species is more contentious. Wide acceptance of a single treatment is hampered by the existence of differing taxonomic traditions, and by the lack of a coordinated international examination of morphological variation among the tribe’s species. The prevalence of polyploids and the ease of forming hybrids in the Hordeeae enabled cytogeneticists to build up a picture of the genomic constitution of its members. This led to the discovery that there is a strong, but not perfect, correlation between morphology and genomic constitution. Molecular tools reveal a pattern that is, in general, consistent with the cytogenetic data, but they often reveal an underlying complexity that cannot be discerned using only classical cytogenetic techniques. |