Citrus

Citrus is a common term and genus of flowering plants in the family Rutaceae, originating in tropical and subtropical southeast Asia. The genus contains three species, and numerous natural and cultivated origin hybrids, including commercially important fruit such as the orange, lemon, grapefruit, lime, and tangerine. The taxonomy of the genus is complex, but recent genetic evidence (see e.g. external link cited below) supports the presence of only three species, C. maxima, C. medica and C. reticulata, with all the other taxa previously accepted as species being of hybrid origin between these three. They are large evergreen shrubs or small trees, reaching 5-15 m tall.

Citrus fruits are notable for their fragrance, and most are juice-laden. They contain a high proportion of citric acid giving them their characteristic astringent odor and flavor. They are also good sources of vitamin C, and apparently flavonoids. In botanical terms, "The fruit of all Citrus trees, in which the true fruit is the peel, [is] made up of an outer layer, brightly colored and rich in glands, a spongey whitish mesocarp, and a membraneous endocarp surrounding the segments. The succulent parts we eat is only a secondary tissue developed as a filler" - Paola Lanzara and Mariella Pizzetti Simon & Schuster's Guide to Trees, pp. 44.

Cultivation

As citrus trees hybridise very readily (e.g., seeds grown from limes can produce fruit similar to grapefruit), all commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness.