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The families of flowering plants

L. Watson and M.J. Dallwitz

Oxalidaceae R. Br.

Excluding Averrhoaceae, Hypseocharitaceae, Lepidobotryaceae.

Habit and leaf form. Herbs. Switch-plants (sometimes), or ‘normal’ plants (mostly); occasionally phyllodineous. Leaves well developed (usually), or much reduced. Plants succulent (sometimes), or non-succulent (mostly). Perennial; with a basal aggregation of leaves, or without conspicuous aggregations of leaves; rhizomatous, or tuberous. Hydrophytic to helophytic, or mesophytic, or xerophytic; when hydrophytic, rooted. Leaves alternate; spiral; ‘herbaceous’ (mostly), or fleshy (in CAM species, e.g. Oxalis megalorrhiza); petiolate; non-sheathing; compound; unifoliolate, or ternate, or pinnate, or palmate; when pinnate, imparipinnate, or paripinnate (Biophytum). Leaflets often pulvinate (exhibiting ‘sleep movements’). Lamina pinnately veined. Leaves exstipulate; leaf development not ‘graminaceous’. Vernation circinnate (usually), or not circinnate (?).

Leaf anatomy. The leaf lamina dorsiventral. Hydathodes present (sometimes), or absent. Stomata paracytic. Lamina with secretory cavities (commonly), or without secretory cavities. Secretory cavities variously with brown, red, or sometimes transparent and crystalline contents; variously interpreted as schizogenous, or lysigenous. The mesophyll containing crystals. The crystals mostly solitary-prismatic. Minor leaf veins without phloem transfer cells (Oxalis).

Axial (stem, wood) anatomy. Cork cambium present, or absent (?); initially superficial. Nodes tri-lacunar. Primary vascular tissues in a cylinder, without separate bundles (in older stems), or comprising a ring of bundles (the bundles collateral, the stems usually with a conspicuous ring of pericyclic sclerenchyma, cf. Geraniaceae); collateral. Internal phloem absent. Cortical bundles absent. Medullary bundles absent. Secondary thickening absent, or developing from a conventional cambial ring.

The vessel end-walls simple. The axial xylem with libriform fibres; at least sometimes including septate fibres, or without septate fibres (?).

Reproductive type, pollination. Plants hermaphrodite (often with additional small cleistogamous flowers); usually heterostylous (mostly tristylous).

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; when aggregated, in cymes, or in panicles, or in umbels. The ultimate inflorescence units cymose. Inflorescences axillary; cymes or ‘sub-umbellate’, often pedunculate. Flowers small to large; regular; 5 merous; cyclic; tetracyclic to pentacyclic. Free hypanthium absent. Hypogynous disk present, or absent.

Perianth of chasmogamous flowers with distinct calyx and corolla; 10; 2 whorled; isomerous. Calyx 5; 1 whorled; polysepalous; regular; persistent; imbricate. Corolla 5 (often lacking in cleistogamous flowers); 1 whorled; polypetalous (usually), or gamopetalous (sometimes slightly connate basally). Corolla lobes markedly longer than the tube. Corolla contorted (usually), or imbricate; regular. Petals sometimes shortly clawed, or sessile.

Androecium 10. Androecial members free of the perianth; all equal, or markedly unequal (the outer whorl usually with shorter filaments); coherent; 1 adelphous (the filaments basally connate); 2 whorled. Androecium exclusively of fertile stamens, or including staminodes. Staminodes when present, 5; external to the fertile stamens. Stamens 5, or 10; diplostemonous, or isomerous with the perianth; alternisepalous (the shorter, outer members opposite the petals), or oppositisepalous (when the outer members lack anthers); alternating with the corolla members, or both alternating with and opposite the corolla members. Anthers dorsifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate. Endothecium developing fibrous thickenings. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or decussate. Anther wall initially with one middle layer, or initially with more than one middle layer (1 or 2). Tapetum glandular. Pollen grains aperturate; 3(–4) aperturate; colpate (to colporoidate?, occasionally 4-rupate); 2-celled (in Oxalis).

Gynoecium 5 carpelled. Carpels isomerous with the perianth. The pistil when syncarpous, 5 celled. Gynoecium apocarpous to syncarpous; semicarpous to synovarious (the carpels more or less free in Eichleria, Biophytum); superior. Carpel (when semicarpous), capitate stylate. Placentation when carpels more or less free, marginal. Ovary usually syncarpous and 5 locular (the lobes opposite the petals). Styles 5; free. Stigmas dry type; papillate; Group II type. Placentation nearly always (i.e. when syncarpous) axile. Ovules 2–15 per locule (to ‘several’); pendulous; epitropous; with ventral raphe; arillate (often); anatropous, or hemianatropous; bitegmic; tenuinucellate. Outer integument contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing only after one has been fertilized, or fusing simultaneously with the male gamete. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids pear-shaped, or hooked. Endosperm formation nuclear. Embryogeny asterad.

Fruit non-fleshy; when semi to apocarpous, an aggregate (e.g. Biophytum), or not an aggregate (usually); when syncarpous (i.e. usually) dehiscent; a capsule. Capsules loculicidal. Fruit often elastically dehiscent (involving the arilliform epidermis of the seeds). Seeds usually copiously endospermic. Endosperm ruminate; oily. Cotyledons 2. Embryo chlorophyllous (1/3); straight.

Seedling. Germination phanerocotylar.

Physiology, phytochemistry. C3 and CAM. C3 physiology recorded directly in Oxalis stricta. CAM recorded directly in Oxalis carnosa. Anatomy non-C4 type (Oxalis). Not cyanogenic. Alkaloids absent (3 species). Iridoids not detected. Saponins/sapogenins present, or absent. Proanthocyanidins present, or absent; when present, cyanidin, or cyanidin and delphinidin. Flavonols mostly absent. Ellagic acid absent (6 Oxalis species). Aluminium accumulation not found. Plants accumulating free oxalates. Sieve-tube plastids P-type (Oxalis), or S-type (Biophytum).

Geography, cytology. Temperate to tropical. Cosmopolitan except for frigid regions, concentrated in the tropics and subtropics. X = (5-)7(-12).

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rutiflorae; Geraniales. Cronquist’s Subclass Rosidae; Geraniales. APG III core angiosperms; core eudicot; Superorder Rosanae; fabid. APG IV Order Oxalidales.

Species 875. Genera 3; Oxalis, Biophytum, Eichleria.

Illustrations. • Le Maout and Decaisne: Oxalis. • Oxalis confertissima: Lindley. • Biophytum sensitivum: Thonner. • Oxalis acetosella, Oxalis corniculata and Oxalis stricta: Eng. Bot. 310–312, 1864. • Oxalis cratensis: Hook. Ic. Pl. 4 (1841). • Oxalis elegans: Bot. Mag. 76 (1850). • Oxalis enneaphylla: Hook. Ic. Pl. 5–6 (1842–3). • Oxalis acetosella (B. Ent.). • Oxalis fruticosa (phyllodineous): Bot. Reg. 41, 1841. • Oxalis hirta: Bot. Reg. 1073, 1827. • Oxalis piottae, cf. O. punctata: Bot. Reg. 1817, 1836. • Oxalis rubrocincta: Bot. Reg. 64, 1842. • Oxalis tortuosa: Bot. Reg. 1249, 1829. • Oxalis magellanica: Hooker, Fl. Novae-Zelandiae (1853).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 15th April 2018.’.