Didiereaceae Radlk. ex Drake
Habit and leaf form. Trees, or shrubs (with spiny short-shoots in the leaf axils of the long-shoots). Switch-plants; with the principal photosynthesizing function transferred to stems and ‘cactoid’ (or ‘euphorbioid’). Leaves well developed (but small), or much reduced. Plants succulent (the younger long-shoots succulent, becoming woody with age, the short-shoots spiny). Leaves small; alternate; simple. Lamina entire. Leaves exstipulate.
Leaf anatomy. The mesophyll containing mucilage cells. Minor leaf veins without phloem transfer cells (Alluaudia).
Stem anatomy. Secretory cavities present (in the cortex); with mucilage. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with libriform fibres; with vessels. Vessel end-walls simple. Primary medullary rays wide. Wood parenchyma paratracheal. Sieve-tube plastids P-type; type III (a).
Reproductive type, pollination. Plants dioecious, or gynomonoecious. Female flowers with staminodes, or without staminodes.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in cymes. The ultimate inflorescence unit cymose. Inflorescences terminal (on the long- and short-shoots); more or less dichasial cymes. Flowers cyclic. Hypogynous disk present; intrastaminal; annular.
Perianth with distinct calyx and corolla (though the petaline ‘calyx’ could be interpreted as bracteoles, the ‘corolla’ as sepals); 6; 3 whorled; isomerous. Calyx 2; 1 whorled; polysepalous (petaloid, decussate); persistent. Corolla 4; 2 whorled (2+2, decussate); polypetalous. Petals sessile.
Androecium 8(–10). Androecial members free of the perianth; free of one another (weakly adnate to the outside of the nectary); 2 whorled. Androecium exclusively of fertile stamens. Stamens 8(–10); filantherous (the filaments woolly). Anthers dorsifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate. Pollen grains aperturate; (4–)5–7 aperturate; colpate (but occasionally with one or two pores near the poles in addition to the furrows); spinulose; 3-celled.
Gynoecium (2–)3(–4) carpelled. Carpels isomerous with the perianth, or increased in number relative to the perianth. The pistil (2–)3(–4) celled. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary (2–)3(–4) locular (but only the medial-adaxial one fertile, the others empty). Gynoecium stylate. Styles 1; attenuate from the ovary; apical. Stigmas (2–)3(–4). Placentation basal. Ovules 1 per locule; ascending; apotropous; with ventral raphe; arillate; campylotropous; bitegmic; crassinucellate. Endosperm formation nuclear.
Fruit non-fleshy; indehiscent; capsular-indehiscent (three-angled); 1 seeded (the seed with a small funicular aril). Seeds nearly non-endospermic. Embryo well differentiated. Cotyledons 2 (these fleshy). Embryo curved, or bent.
Seedling. Germination phanerocotylar.
Physiology, biochemistry. Not cyanogenic. Iridoids not detected. Proanthocyanidins present. Betalains present. CAM. CAM recorded directly in Alluaudia, Alluaudiopsis, Decaryia, Didieria.
Geography, cytology. Paleotropical. Tropical. In semi-desert parts of Madagascar. Ploidy levels recorded: seemingly high — 2n=150, 190–200.
Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Caryophylliflorae; Caryophyllales. Cronquist’s Subclass Caryophyllidae; Caryophyllales. APG 3 core angiosperms; core eudicot; Superorder Caryophyllanae; Order Caryophyllales.
Species 11. Genera 4; Didierea, Alluaudia, Alluaudiopsis, Decaryia.