Family Hydnoraceae

Hydnoraceae C.A. Agardh

Habit and leaf form. Ectoparasitic herbs. Plants of very peculiar vegetative form; more or less fungoid (the vegetative component consisting of a coarse, rhizomelike ‘pilot root’, from which many slender, unbranched haustorial roots emerge and parasitize the roots of host plants). Leaves absent. Plants with roots; succulent; totally parasitic. Parasitic on roots of the host.

Stem anatomy. Secretory cavities present, or absent; when present, with mucilage. Primary vascular tissue comprising a ring of bundles, or in two or more rings of bundles (normal and/or inversely orientated). Secondary thickening absent. Xylem with vessels.

Root anatomy. Root xylem with vessels (these with simple perforations).

Reproductive type, pollination. Unisexual flowers absent. Plants hermaphrodite. Pollination entomophilous; via beetles.

Inflorescence, floral, fruit and seed morphology. Flowers solitary (arising endogenously from the pilot-roots); medium-sized to large (short stalked, barely emerging above ground, the lower parts often remaining below ground); malodorous; regular; cyclic; tricyclic, or tetracyclic. Free hypanthium present (short).

Perianth sepaline, or petaline (of tepals, sometimes described as a calyx); 3–4(–5); joined (the lobes valvate); 1 whorled; white, or red, or pink, or brown (brown ouside); fleshy (often bearing retrorse bristles).

Androecium 3–4(–5), or 6–8(–10). Androecial members free of the perianth (but inserted on the hypanthium); coherent (united into a thick, sinuose-annular (Hydnora) or ovoid synandrium); 1 whorled. Androecium exclusively of fertile stamens, or including staminodes (Prosopanche). Staminodes of Prosopanche, 3–4(–5) (small, fleshy, below and alternating with the stamens). Stamens 3–4(–5); isomerous with the perianth; oppositisepalous (i.e. opposite the perianth lobes); with sessile anthers (in Hydnora), or filantherous (Prosopanche, where the very short filaments arise from the hypanthium, and the anthers are connate to form a dome or cap with a small central opening). Anthers separate from one another, or cohering (Prosopanche); dehiscing via longitudinal slits, or dehiscing transversely (the thecae longitudinal or transverse); extrorse; many locular (each with numerous pollen sacs). Pollen shed as single grains. Pollen grains aperturate; 1–2(–3) aperturate; sulcate, or sulculate; 2-celled.

Gynoecium 3(–4) carpelled (wholly or partly buried in the soil). Carpels reduced in number relative to the perianth, or isomerous with the perianth. The pistil 1 celled. Gynoecium syncarpous; synstylovarious to eu-syncarpous; inferior. Ovary 1 locular (but becoming occluded by ingrowth of the accrescent placentas). Gynoecium non-stylate (the stigma sessile). Stigmas 1, or 3 (mostly 3-lobed in Hydnora, in Prosopanche constituted by the slightly protruding tips of the placental lamellae); commissural (at least, in Prosopanche). Placentation parietal (in Prosopanche, the lamellar placentas in three parietal groups, covered with ovules, deeply intruded, but not joined in the middle), or apical (in Hydnora, the placental lamellae numerous, branched, suspended from the top of the ovary). Ovules not differentiated; in the single cavity 50–200 (‘very numerous’); orthotropous; unitegmic (this recognisable only around the micropyle); tenuinucellate. Embryo-sac development Allium-type, or Adoxa-type (bisporic in Prosopanche, tetrasporic in Hydnora). Polar nuclei fusing prior to fertilization. Antipodal cells not formed (the nuclei degenerating). Synergids pear-shaped. Endosperm formation cellular. Embryogeny more or less solanad (?).

Fruit fleshy (within, but the pericarp more or less woody); dehiscent (in Prosopanche), or indehiscent (Hydnora?); a capsule, or a berry. Capsules in Prosopanche circumscissile. Fruit 500–2000 seeded (i.e. the seeds very numerous). Seeds endospermic (the endosperm with arabinose polysaccharide). Perisperm present (a thin layer). Seeds minute. Embryo rudimentary at the time of seed release (minute, enclosed in the endosperm).

Geography, cytology. Paleotropical, Neotropical, and Cape. Sub-tropical to tropical. South America, Equatorial and South Africa and Madagascar.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Santaliflorae; Santalales (cf. Rafflesiaceae). Cronquist’s Subclass Rosidae; Rafflesiales. APG 3 core angiosperms; Superorder Magnolianae; Order Piperales.

Species 18. Genera 2; Hydnora, Prosopanche.

Illustrations.
• Hydnora: LS flower.
• Hydnora: LS flower (Lindley).