Hydnoraceae

Genera Included:Hydnora, Prosopanche>

Habit: Leafless, achlorophyllous, terrestrial root parasites. Pilot root tissue pink when sectioned, with vascular bundles containing vessels with simple perforations. Parenchymatous tissues with scattered tanniferous mucilage cells or cavities containing catechin and proanthocyanin (see Coussio et al. 1971).

Parasitism: Root parasites. Some Hydnora are specific for particular Euphorbiaceae. Prosopanche is often found parasitizing Fabaceae (especially Prosopis).

Roots: Forming an extensive horizontal system or two types of roots: pilot and haustorial. The pilot roots are angular (often hexagonal) or terete with vermiform outgrowths on the ridges. The pilot root has a root cap at the apex. From the projections along the angles form clavate haustorial roots that are ca. 1 cm long. When a host root contacts a haustorial root, fusion occurs. Haustorial roots often fuse with each other as well.

Stem: No stem per se exists; the aerial portions of the plant are technically a single-flowered inflorescence.

Leaves: Absent

Inflorescence: Consisting a a single flower.

Plant Sex: Flowers bisexual

Flowers: Forming individually and endogenously within the pilot root tissue, especially at a site where connections to the host root occur. Flower buds tubular, protruding just above the soil surface or flowering subterraneously (in Hydnora triceps). Buds require at least one year to develop. Flowers are large (10-15 cm), fleshy, malodorous, beetle-pollinated (blowflies in H. triceps), perfect, epigynous with a shortly prolonged hypanthium, actinomorphic, and monochlamydous.
Calyx: Tepals 3-4 (5), thick, fleshy, valvate, connate below, the outer surface coarse, cracked, brown, the inner surface showy (white, pink, salmon, orange or red) often bearing retrorse bristles. These bristles detain visiting insects (carrion, scarib, or dermestid beetles) attracted to the putrid odor. In H. africana, the odor eminates from "bait bodies" (specialized hairs) withing the flower. Flowers in H. johannis emerge from the soil following rains. If adequate moisture is present, the perianth lobes are patent and rest on the soil. If moisture is lacking, the lobes do not reflext but open only slightly. Unlike H. africana, the 3-sided perianth lobes in H. johannis are differentiated at the tip into a glabrous, light pink osmophoric cucullus with a strong fetid odor.
Corolla: Absent
Androecium: Highly modified and synandrial, nearly or quite without filaments, consisting of as many stamens as tepals and opposite them but with numerous elongate, extrorse, bisporangiate pollen sacs that open by longitudinal slits. In Hydnora the androecium forms a lobed ring on the hypanthium, the lobes opposite the tepals and sometimes with an upright free tip. In Prosopanche the very short filaments arise from the hypanthium and the extrorse anthers are connate to form a dome or cap with a small central opening.
Staminodia: Small, fleshy staminodes are present below and alternate with the stamens in Prosopanche.
Pollen: Spherical to oblong, binucleate, monosulcate (Hydnora) or bisulcate (Prosopanche), not sculptured. Sticky in Hydnora.
Gynoecium: Composed of 3 (4) carpels united to form a compound, inferior, unilocular ovary that eventually becomes filled by the growth of the accrescent placentas. Stigma sessile and 3-lobed in Hydnora, in Prosopanche consisting of the protruding tips of the placental lamellae. Placentas lamellar, covered with ovules,in Prosopanche numerous in 3 groups, parietal, deeply intruded but not joined in the center. In Hydnora numerous, suspended from the top of the ovary and branched.
Ovule: Very numerous, orthotropous, with a massive single integument, tenuinucellar, in Prosopanche embedded and scarcely differentiated from the placenta except as embryo sacs, the single integument recognizable only in the micropylar area.
Embryo, etc.: Embryo sac bisporic in Prosopanche, tetrasporic in Hydnora.. Endosperm development cellular. The mature embryo is minute, undifferentiated and enclosed in the endosperm.

Fruit: Subgloboser, with a woody or scaly, brown pericarp and a mealy or gelatinous, white, edible mesocarp. The fruit may require up to 2 years to mature. In Hydnora, seeds very numerous (20,000) in the fruit which may measure 8 cm in diameter. The fruit in H. johannis is fragrant, has the texture of an apple, and taste of Annona squamosa (Musselman 1984). The bulk of the fruit is made up of seeds embedded in the white, fibrous placentae. In Hydnora, the dry remnants of the flower break off in a ring around the top of the fruit to expose the inner pulp containing seeds. In Prosopanche the fruit bursts circumscissily when ripe.

Seed: Small, with a hard testa, a thin layer of perisperm and a well developed endosperm with polysaccharide (arabinose) food reserves.

Chromosomes: X =

Link to Family Description in Delta