Rafflesiaceae

Rafflesiaceae, flowering plant family 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. Members of the family are endoparasites, meaning that the vegetative organs 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. 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 can be extremely large.

The family Rafflesiaceae includes the following three genera, mostly in the Old World subtropics: Rafflesia (about 42 species), Rhizanthes (4 species), and Sapria (1 or 2 species). The taxonomy of the family has been contentious, especially given the difficulty in obtaining specimens to study. The group formerly comprised seven genera, based on morphological similarities, but molecular evidence led to a dramatic reorganization by the Angiosperm Phylogeny Group III (APG III) botanical classification system. The genera Bdallophytum and Cytinus were transferred to the family Cytinaceae (order Malvales), and the genera Apodanthes and Pilostyles were moved to the family Apodanthaceae (order Cucurbitales).

The monster flower genus (Rafflesia) consists of about 42 species native to Southeast Asia, all of which are parasitic upon the roots of Tetrastigma vines (family Vitaceae). The genus includes the giant R. arnoldii, sometimes known as the corpse flower, which produces the largest known individual flower of any plant species in the world and is found in the forested mountains of Sumatra and Borneo. Its fully developed flower appears aboveground as a thick fleshy five-lobed structure weighing up to 11 kg (24 pounds) and measuring almost one meter (about one yard) across. It remains open five to seven days, emitting a fetid odor that attracts carrion-feeding flies, which are believed to be the pollinating agents. The flower’s color 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. Other members of the genus have a similar reproductive biology. At least one species (R. magnifica) is listed as critically endangered by the IUCN Red List of Threatened Species.

The flowers of the genus Sapria are similar to those of Rafflesia and also emit a carrion odor. Members of the genus Rhizanthes produce flowers with nectaries, and some species are not malodorous. One species, Rhizanthes lowii, is known to generate heat with its flowers and buds, an adaptation that may aid in attracting pollinators. Species of both Sapria and Rhizanthes are considered rare and are threatened by habitat loss.