Habit and leaf form. Small trees, or shrubs. Leaves alternate; petiolate; non-sheathing; simple. Lamina entire; pinnately veined; cross-venulate. Leaves exstipulate. Lamina margins entire; flat.
Leaf anatomy. Mucilaginous epidermis absent. Stomata mainly confined to one surface (abaxial); anomocytic. Hairs absent.
Adaxial hypodermis absent. Lamina dorsiventral. The mesophyll containing calcium oxalate crystals. The mesophyll crystals druses.
Stem anatomy. Cork cambium present; initially superficial (subepidermal). Nodes unilacunar. Primary vascular tissue in a cylinder, without separate bundles. Secondary thickening developing from a conventional cambial ring. Xylem with tracheids; without fibre tracheids (the pits no more than slightly bordered); with libriform fibres; with vessels. Vessel end-walls more or less horizontal; simple. Vessels without vestured pits. Wood diffuse porous; not storied; parenchyma paratracheal (aliform, or confluent and unilateral).
Reproductive type, pollination. Plants dioecious. Female flowers without staminodes. Gynoecium of male flowers pistillodial to vestigial. Pollination anemophilous.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; (male and female) in racemes. The ultimate inflorescence unit racemose. Inflorescences axillary; axillary racemes. Flowers small to medium-sized; regular. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk absent.
Perianth sepaline; 5–9; 1 whorled. Calyx 5–9 (the sepals internally hairy, with multicellular, unbranched hairs); 1 whorled; more or less polysepalous, or partially gamosepalous, or gamosepalous (some members slightly united below); blunt-lobed, or toothed; regular; persistent; non-accrescent; weakly imbricate.
Androecium in male flowers, (8–)10–14(–25). Androecial members free of the perianth; more or less coherent (the filaments of a variable number being united at the base), or free of one another; 1–2 whorled (the members arising as one whorl, but one or more members may be positioned outside the cycle). Androecium exclusively of fertile stamens. Stamens (8–)10–14(–25); isomerous with the perianth to polystemonous; filantherous (the filaments short). Anthers basifixed; non-versatile; dehiscing via longitudinal slits; latrorse; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Pollen shed as single grains. Pollen grains aperturate; 3 aperturate; colpate.
Gynoecium 2 carpelled. Carpels reduced in number relative to the perianth. The pistil 1–2 celled. Gynoecium syncarpous; synovarious (the styles free except at the very base); superior. Ovary plurilocular; 2 locular (basally and above), or 1 locular (in the mid-region, where the septum is lost); sessile. Gynoecium stylate. Styles 2; more or less free; from a depression at the top of the ovary; apical; much longer than the ovary. Stigmas 2. Placentation basal to axile. Ovules differentiated; 2 per locule; funicled; ascending; non-arillate; campylotropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle.
Fruit non-fleshy; indehiscent; capsular-indehiscent (somewhat inflated); 1 seeded. Seeds non-endospermic. Cotyledons 2. Embryo straight.
Physiology, biochemistry. Mustard-oils absent (no myrosin cells). Anatomy non-C4 type.
Geography, cytology. Paleotropical. Tropical. Madagascar.
Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Caryophylliflorae (?); near Caryophyllales (?). Cronquist’s Subclass Hamamelidae; Urticales. APG 3 core angiosperms; core eudicot; Superorder Caryophyllanae; Order Caryophyllales.
Species 2. Genera 1; only genus, Physena.
For a thorough descriptive account of Physena, see Dickison and Miller (1993) who provided ample evidence in support of a monogeneric family. However, its taxonomic affinities remain uncertain. Recent studies (see Gadek et al, 1996) suggest that it might belong near Caryophyllales.