RAFFLESIALES

Monster flower (Rafflesia arnoldii). order of flowering plants constituting three families (Rafflesiaceae, Mitrastemonaceae, and Hydnoraceae) and up to 60 species, although Mitrastemonaceae is not included in some classifications. The order is notable for being strictly parasitic upon the roots or stems of other plants and for the remarkable growth forms exhibited as adaptations to this mode of nutrition. The vegetative organs of most plants of the order are so reduced and modified that the plant body exists only as a network of threadlike cellular strands living almost wholly within the tissues of the host plant (in Rafflesiaceae), or as a thick, creeping, underground organ called a rhizomatoid (in Hydnoraceae). There are no green photosynthetic tissues, leaves, roots, or stems in the generally accepted sense, although vestiges of leaves exist in some species as scales. The flowers are well developed, however, and range in size from minute to extremely large. The monster flower (Rafflesia arnoldii), with the largest known flower, is parasitic upon the roots of Tetrastigma species, large vines of the grape family (Vitaceae) found in the forested mountains of Malaysia. Its fully developed flower appears above ground as a thick, fleshy, five-lobed structure weighing up to 11 kg (24 pounds) and measuring almost one metre (about one yard) across. It remains open five to seven days, emitting a fetid odour that attracts carrion-feeding flies, which are believed to be the pollinating agents. The flower's colour is reddish or purplish brown, sometimes in a mottled pattern, with the sex organs in a central cup. The fruit is a berry containing sticky seeds thought to be disseminated by fruit-eating rodents. The family Rafflesiaceae includes the following genera, mostly in the Old World subtropics: Pilostyles (22 species), Bdallophytum (4 species), Apodanthes (5 species), Rafflesia (12 species), Cytinus (6 species), Rhizanthes (1 or 2 species), and Sapria (1 or 2 species). In contrast to the giant flower of Rafflesia is Pilostyles thurberi of southwestern North American deserts. A parasite on the stems of Dalea species and other pea family (Fabaceae) shrubs, its length outside the host plant is only 5 or 6 mm (about 0.25 inch). The family Hydnoraceae contains two genera: Hydnora (12 species), of tropical and southern Africa and Madagascar, and Prosopanche (6 species), of Paraguay and Argentina in South America. The family differs from the Rafflesiaceae in having bisexual flowers and no leaves whatever. Hydnora is parasitic on the roots of Acacia species and Euphorbia species. Prosopanche is parasitic on the roots of Prosopis species. Flowers of the Hydnoraceae are beetle-pollinated. The family Mitrastemonaceae contains a single genus, Mitrastemon, with two species. They also lack chlorophyll and are endoparasites in the roots of plants.