§ Evolutionary lines and subgenera Allium is a large genus of monocotyledonous flowering plants with around 1000 accepted species,[4][5] making Allium the largest genus in the family Amaryllidaceae and amongst the largest plant genera in the world.

Some species (such as Welsh onion A. fistulosum and leeks (A. ampeloprasum)) develop thickened leaf-bases rather than forming bulbs as such.

[15][16] Plants of the genus produce chemical compounds, mostly derived from cysteine sulfoxides, that give them a characteristic onion or garlic taste and odor.

The bulbs' outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation.

Many alliums have basal leaves that commonly wither away from the tips downward before or while the plants flower, but some species have persistent foliage.

The leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum.

The flowers have one style and six epipetalous stamens; the anthers and pollen can vary in color depending on the species.

There are several unrelated species that are somewhat similar in appearance to Alliums but are poisonous (e.g. in North America, death camas, Toxicoscordion venenosum), but none of these has the distinctive scent of onions or garlic.

The difficulties arise from the fact that the genus displays considerable polymorphism and has adapted to a wide variety of habitats.

Furthermore, traditional classifications had been based on homoplasious characteristics (the independent evolution of similar features in species of different lineages).

[25] Allium includes a number of taxonomic groupings previously considered separate genera (Caloscordum Herb., Milula Prain and Nectaroscordum Lindl.)

[30] In 2006 Friesen, Fritsch, and Blattner[31] described a new classification with 15 subgenera, 56 sections, and about 780 species based on the nuclear ribosomal gene internal transcribed spacers.

[26] The three evolutionary lineages and 15 subgenera here represent the classification schemes of Friesen et al. (2006)[31] and Li (2010),[26] and subsequent additional species[36][37][38][39] and revisions.

[26] Names from[41] Some sources refer to Greek ἀλέω (aleo, to avoid) due to the odor of garlic.

[15] The majority of Allium species are native to the Northern Hemisphere, being spread throughout the holarctic region, from dry subtropics to the boreal zone,[26] predominantly in Asia.

There are two centres of diversity, a major one from the Mediterranean Basin to Central Asia and Pakistan, while a minor one is found in western North America.

[citation needed] The genus Allium has very large variation between species in their genome size that is not accompanied by changes in ploidy level.

[42] The unusual telomeric sequence of 'Allium cepa' was discovered and cytologically validated to be CTCGGTTATGGG [44] A bioinformatics method for detecting this unique telomere sequence was demonstrated using SERF de novo Genome Analysis [45] Many Allium species have been harvested through human history, but only about a dozen are still economically important today as crops or garden vegetables.

[49] These ornamental onions produce spherical umbels on single stalks in spring and summer, in a wide variety of sizes and colours, ranging from white (Allium 'Mont Blanc'), blue (A. caeruleum), to yellow (A. flavum) and purple (A. giganteum).

[48][50] The following cultivars, of uncertain or mixed parentage, have gained the Royal Horticultural Society's Award of Garden Merit: Dogs and cats are very susceptible to poisoning after the consumption of certain species.

These include onions (A. cepa), French shallots (A. oschaninii), leeks (A. ampeloprasum), garlic (A. sativum), and herbs such as scallions (various Allium species) and chives (A. schoenoprasum).