2.1- Overview Allium is one of about fifty-seven genera of flowering plants with more than 500 species. It is by far the largest genus in the Amaryllidaceae, and also in the Alliaceae in classificationsystems in which that family is recognized as separate. Allium species occur in temperate climates of the northern hemisphere, except for a few species occurring in Chile (such as A. juncifolium), Brazil (A. sellovianum), and tropical Africa (A. spathaceum). They vary in height between 5 cm and 150 cm. The flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from small (around 2-3 mm in diameter) to rather large (8-10 cm). Some species (such as Welsh onion A. fistulosum) develop thickened leaf-bases rather than forming bulbs as such. Plants of the Allium genus produce chemical compounds (mostly derived fromcysteine sulfoxides) that give them a characteristic (alliaceous) onion or garlic taste and odor. Many are used as food plants, though not all members of the genus are equally flavorous. In most cases, both bulb and leaves are edible and the taste may be strong or weak, depending on the species and on ground sulfur (usually as sulfate) content. In the rare occurrence of sulfur-free growth conditions, all Allium species lose their usual pungency altogether. In the APG III classification system, Allium is placed in the familyAmaryllidaceae, subfamily Allioideae (formerly the family Alliaceae). In some of the olderclassification systems, Allium was placed in Liliaceae. Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic.
2.2-Taxonomy The taxonomy of Allium is poorly understood, with incorrect descriptions being widespread. Allium spicatum has been treated by many authors as Milula spicata, the only species in the monospecific genus Milula. In 2000, it was shown to be embedded in Allium. Subsequent molecular phylogenetic studies have shown the 2006 classification is a considerable improvement over previous classifications, but some of its subgenera and sections are probably not monophyletic. One of these studies focused on the subgenus Amerallium, which is strongly supported as monophyletic. Another study focused on Allium ampeloprasum and its relatives within the section Allium of subgenus Allium. Sampling in this study was not sufficient to test the monophyly of section Allium. In 2006, a phylogeny of Allium was published based on the nuclearribosomalgeneITS. The authors of this study divided Allium into 15subgenera and 72sections. They defined the subgenus Rhizirideum in a much narrower sense than in previous classifications. Alliumis the onion genus, with 600-920 species, making it one of thelargest plant generain the world. As of March 2014, theWorld Checklist of Selected Plant Familiesaccepts 920 species: See more information here => List of Allium species
2.3-Description Allium species are herbaceous perennials with flowers produced on scapes. They grow from solitary or clustered tunicate bulbs and many have an onion odor and taste. Plants are perennialized by bulbs that reform annually from the base of the old bulb, or are produced on the ends of rhizomes or, in a few species, at the ends of stolons. A small number of species have tuberous roots. The bulbs' outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation. The inner coats of the bulbs are membranous. 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. Plants produce from one to 12 leaves, most species having linear, channeled or flat leaf blades. The leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum. The leaves are sessile, and very rarely narrowed into a petiole. The flowers are erect or in some species pendent, having six petal-like tepals produced in two whorls. The flowers have one style and six epipetalous stamens; the anthers and pollen can vary in color depending on the species. The ovaries are superior, and three-lobed with three locules. The fruits are capsules that open longitudinally along the capsule wall between the partitions of the locule. The seeds are black, and have a rounded shape. The terete or flattened flowering scapes are normally persistent. The inflorescences are umbels, in which the outside flowers bloom first and flowering progresses to the inside. Some species produce bulbils within the umbels, and in some species, such as Allium paradoxum, the bulbils replace some or all the flowers. The umbels are subtended by noticeable spathe bracts, which are commonly fused and normally have around three veins. Some bulbous alliums increase by forming little bulbs or "offsets" around the old one, as well as by seed. Several species can form many bulbils in the flowerhead; in the so-called "tree onion" or Egyptian onion (A. ×proliferum) the bulbils are few, but large enough to be pickled. Many of the species of Allium have been used as food items throughout their ranges. There are several poisonous species that are somewhat similar in appearance (e.g. in North America, death camas, Toxicoscordion venenosum), but none of these has the distinctive scent of onions or garlic.
2.4- Distribution and habitat The majority of Allium species are native to the Northern Hemisphere, mainly in Asia. A few species are native to Africa and Central and South America. Species grow in various conditions from dry, well-drained mineral-based soils to moist, organic soils; most grow in sunny locations, but a number also grow in forests (e.g., A. ursinum), or even in swamps or water.