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(Last Modified On 8/12/2013)
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(Last Modified On 8/12/2013)
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Genus
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Citrus L.
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PlaceOfPublication
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Sp. P1. 782. 1753
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Note
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LECTOTYPE: C. medica L. (Wilson, N. Amer. Fl. 25: 221. 1911).
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Description
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Aromatic glabrous shrubs or small trees, usually armed with solitary axillary sharp brown-tipped green spines, the branchlets angled, soon terete, green, glandular punctate, the older branches often thornless. Leaves alternate, 1-folio- late, persistent; leaflets subcoriaceous, usually thin, glandular punctate through- out, entire to serrate, shiny dark green above, paler and duller beneath; petioles usually more or less winged and articulated with the leaflets. Flowers axillary, solitary, paired, or in short corymbose cymes, regular, (4-)5-merous, bisexual or staminate by more or less complete abortion of the gynoecium, 2-5 cm in diameter, often fragrant; plants dioecious or polygamous; calyx shallowly cupular, (4-)5-lobed, persistent, petals (4-)5(-8), free, white, pink, or purplish pink, slightly fleshy, more or less oblong, strongly glandular punctate, imbricate; stamens 20-60, usually 4(6-10) times as many as the petals, polyadelphous or
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Habit
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shrubs or small trees
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Synonym
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Angostura paniculata (Engl.) Elias, comb. nov. basionym: Cusparia paniculata Engl. in Mart., Fl. Bras. 12(11): 120. 1874.
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Description
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free, the filaments linear lanceolate, subulate apically, white, usually variously connate, the anthers oblong to sagittate; intrastaminal disc prominent, annular to cushionlike, supporting the gynoecium; gynoecium syncarpous, the ovary sessile, varying from subglobose and sharply distinct from the much narrower style to truncated, fusiform, or subcylindrical and merging gradually into a style nearly as thick as the upper part of the ovary, glandular punctate, glabrous, (8-)10-14(-18)-loculed, the ovules 4-8 or more per locule in 2 collateral rows, the placentation axile, the style apical on ovary, cylindrical, abruptly expanded into the stigma, deciduous, the stigma more or less capitate, subglobose or oblate spheroidal, sometimes slightly lobed. Fruit a berry (hesperidium), ellipsoidal and often mammillate apically, or pyriform to globose and sometimes depressed apically, the pericarp differentiated into a coriaceous glandular-punctate exocarp (the skin) green to red orange at maturity and dotted with numerous oil glands, a thick spongy white mesocarp (the rind), and a membranaceous endocarp filled with stalked fusiform pulp vescicles containing a watery acid to sweet tissue (the pulp), the thin membranous radial locule walls often loosely coherent and easily separated from one another as well as from the spongy white fruit axis; seeds ellipsoidal to obovoid, plump or flattened, more or less angular, sometimes beaked apically, usually several per locule at the inner angle, the testa coriaceous, the endosperm absent, the embryos 1-many, white or green, the cotyledons fleshy, plano convex, often unequal.
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Distribution
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A genus of southern and southeastern Asia and Malaysia that is widely cultivated in all warmer areas of the world.
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Note
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Swingle and Reece (1967) ascribe 16 species to Citrus, while Tanaka (1954) lists 145 species for the genus. Specific distinctions are complicated not only by hybridization within the genus and with closely related genera, but also by such phenomena as nucellar polyembryony with rejuvenation by nucellar progeny of more or less senescent varieties long propagated asexually, and the spontaneous production of autotetraploids (see Swingle and Reece, op. cit., pp. 359-369, for a discussion of the problems inherent in determining specific limitations in the genus). Failure to take such processes into account has led Tanaka to recognize an overabundance of species in Citrus. The number recognized by Swingle and Reece is a much closer reflection of the actual situation in the genus. Many species have escaped from cultivation and become naturalized. A complication is added where a species is cultivated and later abandoned to become part of the second growth vegetation. Here the presence of a solitary tree in the forest may or may not indicate naturalization. Such is the case in Panama, where only Citrus aurantifolia (Christm.) Swingle unquestionably is naturalized. Another complication in the lowland tropics is that fruit color cannot reliably be used for identification. Whereas elsewhere this can be a useful and reliable character, here fruits often remain green at maturity. Fruit morphology and taste are the most important field characters for positive identification, as many species are difficult to separate vegetatively. Besides the species discussed below, the following also are to be expected in Panama: C. limetta Risso (the sweet line); C. medica L. (the citron); and C. paradisi Macfad. (the grapefruit).
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Reference
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Purseglove, J. W. 1968. Tropical Crops. Dicotyledons 2. pp. 495-522. London. Swingle, W. T. & P. C. Reece. 1967. The botany of Citrus and its wild relatives. Pp. 190-430 in W. Reuther et al. (Editors), The Citrus Industry. Vol. 1. History, World Distribution, Botany, and Varieties. Rev. ed. Berkeley. Standley, P. C. 1928. Rutaceae, in Flora of the Panama Canal Zone. Contr. U.S. Nat. Herb. 27: 222-223. Tanaka, T. 1954. Species problem in Citrus. Tokyo.
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Key
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a. Petioles broadly winged, the wings oblanceolate to apically broadened, 2-32 mm wide. b. Wings oblanceolate, 2-9 mm wide; branchlets usually with numerous axillary spines 5-17 mm long; mature fruits ellipsoidal, mammillate apically, green to yellow, the core solid ...... 1. C. aurantifolia bb. Wings broadened apically, mostly 1-3 cm wide; branchlets spineless, or axillary spines 5 mm long or less; fruits depressed or flattened apically. c. Wings mostly 1.0-1.5 cm wide; branchlets usually spineless; mature fruits globose, orange to red, the core hollow ...... 2. C. aurantium cc. Wings mostly 2-3 cm wide; branchlets with axillary spines 2-3 mm long; mature fruits subglobose, oblate spheroidal, or subpyriform, green to yellow, pink, or red, the core solid ...... 3. C. grandis aa. Petioles narrowly winged or margined, the wings 2 mm wide or less. d. Branchlets usually spineless; mature fruits depressed globose, green to red, the peel easily separating from the segments ...... 5. C. reticulata dd. Branchlets usually with at least a few axillary spines; mature fruits globose or ovoid, green to yellow or orange, the peel not easily separating from the segments. e. Petioles winged; axillary spines few, 5 mm long or less; mature fruits globose, flattened or depressed apically, yellow green to orange, sweet ...... 6. C. sinensis ee. Petioles margined; axillary spines usually many, to 27 mm long; mature fruits ovoid, mammillate apically, light yellow, acid ...... 4. C. limon
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