Four different genomes are present in Hordeum.
The I genomes is found in Hordeum bulbosum and H. vulgare (nomenclature according to the International Triticeae Consortium, many plant breeders call it H).
The Xu genome is found in diploid Hordeum murinum
The Xa genome is found in diploid Hordeum marinum.
Relationships among the polyploid taxa are complex.
Hordeum is the most distinctive genus in the Triticeae. All its wild species have 3 spikelets per node, a rachis that breaks up at maturity. All its spikelets have awn-like glumes and a single floret. In the wild species, only the floret of the central spikelet sets seed; the florets of the lateral spikelets are sterile or staminate.
Plants annual or perennial, forming clumps, sometimes shortly rhizomatous. Culms to 150 cm, erect or geniculate.
Leaf blades 2-14 mm wide, flate or involute, sometimes involute only at the margin.
Inflorescence spikelike, 2-15 cm long, breaking up at maturity (except in cultivated taxa), with 3 spikelets per node, usually only the central spikelet setting seed, usually lateral spikelets pedicellate, central spikelet sessile; disarticulation in the rachis.
Spikelets with 1 floret per spikelet, only that of the central spikelet setting seed (except in cultivated taxa, either all three setting seed or just the lateral spikelets).
Glumes of all spikelets similar in length, awnlike, 5-85 mm long, sometimes thickened or widened near the base. Lemma of the central floret 3-19 mm long, usually awned, awn to 90 mm long, lemmas of the lateral florets usually reduced, unawned or shortly awned, those of the lateral spikelets sometimes reduced and particularly that of the central spikelet. Paleas almost equal to the lemmas. Lodicules broadly lanceolate, ciliate. Anthers 0.6-10 mm.
There are about 35 species of Hordeum.
Eurasia and the Americas. Hordeum grows in moist, often saline habitats.
Fora brief time, Hordeum was divided into two genera with all species other than the two I genome species, H. vulgare and H. bulbosum, being placed in Critesion. Such a treatment was not well accepted.
Bothmer, R. von, N. Jacobsen, R.B. Jørgensen, and I. Linde-Laursen. 1991. An ecogeographical study of the genus Hordeum. Systematic and ecogeographic studies on crop genepools 7. International Board for Crop Genetic Resources, Rome.
Jakob, S.S. and F.R. Blattner. 2006. A chloroplast genealogy of Hordeum (Poaceae): Long-term persisting haplotypes, incomplete lineage sorting, regional extinction, and the consequences for phylogenetic inference. Molecular Biology and Evolutioin 23:1602-1612.