presence (Hexapogon) or absence (Iris) of arils on the seeds, referred to as arilate or nonarilate. Taylor (1976) provides arguments for not including all arilate species in Hexapogon. In general, modern classifications usually recognise six subgenera, of which five are restricted to the Old World; the sixth (subgenus Limniris) has a Holarctic distribution. The two largest subgenera are further divided into sections. The Iris subgenus has been divided into six sections; bearded irises (or pogon irises), Psammiris, Oncocyclus, Regelia, Hexapogon and Pseudoregelia. Iris subg. Limniris has been divided into 2 sections; Lophiris (or 'Evansias' or crested iris) and Limniris which was further divided into 16 series. Evolution The concept of introgressive hybridization (or "introgression") was first invented to describe the pattern of interspecific hybridization followed by backcrossing to the parentals that is common in this genus. Subgeneric division Subgenera Iris (Bearded rhizomatous irises) Limniris (Beardless rhizomatous irises) Xiphium (Smooth-bulbed bulbous irises: Formerly genus Xiphion) Nepalensis (Bulbous irises: Formerly genus Junopsis) Scorpiris (Smooth-bulbed bulbous irises: Formerly genus Juno) Hermodactyloides (Reticulate-bulbed bulbous irises: Formerly genus Iridodictyum) Sections, series and species Distribution and habitat Nearly all species are found in temperate Northern Hemisphere zones, from Europe to Asia and across North America. Although diverse in ecology, Iris is predominantly found in dry, semi-desert, or colder rocky mountainous areas. Other habitats include grassy slopes, meadowlands, woodland, bogs and riverbanks. Some irises like Iris setosa can tolerate damp (bogs) or dry sites (meadows), and Iris foetidissima can be found in woodland, hedge banks and scrub areas. Cultivation Iris is extensively grown as ornamental plant in home and botanical gardens. Presby Memorial Iris Gardens in New Jersey, for example, is a living iris museum with over 10,000 plants, while in Europe the most famous iris garden is arguably the Giardino dell'Iris in Florence (Italy) which every year hosts a well attended iris breeders' competition. Irises, especially the multitude of bearded types, feature regularly in shows such as the Chelsea Flower Show. For garden cultivation, iris classification differs from taxonomic classification. Garden iris are classed as either bulb iris or rhizome iris (called rhizomatous) with a number of further subdivisions. Due to a wide variety of geographic origins, and thus great genetic diversity, cultivation needs of iris vary greatly. Iris grow well in most any garden soil types providing they are well-drained. The earliest to bloom are species like I. junonia and I. reichenbachii, which flower as