Rafflesia flowerThe fetid flower of Rafflesia arnoldii, the largest-known flower in the world. The plant is an endoparasite on the vines of Tetrastigma, emerging from its host only to bloom.
rafflesia, (genus Rafflesia), genus of about 42 species of parasitic plants native to Southeast Asia. All Rafflesia species parasitize the roots of Tetrastigma vines (family Vitaceae), and their presence is not made known until the striking flowers emerge from the host vine. One species, Rafflesia arnoldii, boasts the largest single flower of any plant.
Like other members of the family Rafflesiaceae, Rafflesia plants are endoparasitic, 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. Rafflesia plants are thus obligate parasites, which cannot live without the nutrition provided by the host. Despite the dramatic reduction of most of the plant body, the flowers are well developed and can be extremely large.
Rafflesia flowers are sturdy, typically with five substantial tepals (undifferentiated petals and sepals) surrounding the sex organs in a central cup. Interestingly, individual flowers are unisexual, meaning that pollination can occur only if another flower of the opposite sex is simultaneously blooming. The distinctive flowers are sapromyiophilous, meaning that they are pollinated by carrion-feeding flies, and they have a number of adaptations to attract their unconventional pollinators. Most are reddish or purplish brown and have a mottled pattern that resembles rotting flesh. In addition, they emit a fetid carrion odor during the few days they are blooming, and some species even generate heat to simulate decomposition. The unusual pollen is transferred by means of a sticky liquid that dries on the flies. The resultant fruit is a berry containing sticky seeds thought to be disseminated by fruit-eating rodents.
Major species and conservation
Monster flowerAn environmental activist measures the width of a Rafflesia arnoldii flower at Saniangbaka village in Solok regency, West Sumatra, Indonesia.
The genus includes the giant R. arnoldii, sometimes known as the corpse flower or monster 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.
Did You Know?
The unrelated titan arum (Amorphophallus titanum) has the largest unbranched inflorescence of any plant and also smells of rotting flesh to attract carrion insects.
Most Rafflesia species are considered rare and, given their complete dependence on Tetrastigma vines, are extremely difficult to cultivate and very vulnerable to extinction. Major threats include the loss of rainforest habitat, due to logging and other land-use changes, and illegal harvest of the flowers for their purported medicinal properties. As of 2023 only one species, R. magnifica of the Philippines, has been formally evaluated, and it is listed as critically endangered by the IUCN Red List of Threatened Species, though scientists estimate that at least 60 percent of the species are endangered.
parasitic plant, plant that obtains all or part of its nutrition from another plant (the host) without contributing to the benefit of the host and, in some cases, causing extreme damage to the host. The defining structural feature of a parasitic plant is the haustorium, a specialized organ that penetrates the host and forms a vascular union between the plants.
See how parasitic plants form vascular unions with hosts to compensate for an inability to photosynthesizeLearn about several different species of parasitic plants.
Parasitic plants differ from plants such as climbing vines, lianas, epiphytes, and aerophytes; though the latter are supported by other plants, they are not parasitic, because they use other plants simply as a structure on which to grow rather than as a direct source of water or nutrients. Another group of plants that is sometimes confused with parasites is the mycoheterotrophs. Similar to parasitic plants, mycoheterotrophs may lack chlorophyll and photosynthetic capacity, but they live in symbiotic association with fungi that gain nutrition from autotrophic (self-feeding) plants or decaying vegetation. Such plants are not classified as parasitic, because they do not appear to harm the fungi and they lack haustoria.
Parasitic broomrapeBranched broomrape (Orobanche ramosa), also known as hemp broomrape. Frequently attacking agricultural crops, including tomatoes and tobacco, the plant is an obligate parasite and requires a host for its nutritional needs.
All parasitic plant species are angiosperms, among which parasitism has evolved independently about 12 times. Some examples of parasitic angiosperm families include Balanophoraceae, Orobanchaceae, and Rafflesiaceae. Although one species of gymnosperm, Parasitaxus usta, has been proposed to be parasitic, it actually may be a mycoheterotroph as it appears to involve a fungal symbiont.
European mistletoeNumerous European mistletoe plants (Viscum album) parasitizing a tree. Mistletoes are hemiparasites, meaning that they have some photosynthetic ability, and can utilize a variety of host species.
Parasitic plants evolved from nonparasitic plants and thus underwent an evolutionary transition from autotrophy to heterotrophy, which may be partial or complete. Indeed, parasitic plants differ in the extent to which they depend on their hosts for nutrients. Hemiparasites have at least some ability to photosynthesize; they primarily rely on their hosts for water and mineral nutrients. Holoparasites, on the other hand, are nonphotosynthetic and depend on their hosts for all nutrition.
Species of parasitic plants also differ with regard to whether they need the host in order to complete their life cycle. Obligate parasites have an absolute requirement for a host, whereas facultative parasites can live and reproduce in the absence of a host. All holoparasites are, by definition, obligate. Even though hemiparasites are photosynthetically competent, some species nevertheless are obligate in terms of their reliance on a host for their reproduction.
Morphology of parasitic plants
Rafflesia flowerThe fetid flower of Rafflesia arnoldii, the largest known flower in the world. The plant is an endoparasite on the vines of Tetrastigma, emerging from its host only to flower.
Given the different origins of parasitism, it is not surprising that parasitism is manifested in diverse ways. Some species parasitize the roots of their hosts, whereas others attack stems. The haustorium itself may develop from roots or stems, depending on the parasite species, and haustoria display a wide range of morphologies. One prominent distinction is in the proportion of a parasite that grows internally versus externally to the host. For most parasite species, only the haustorium is embedded inside the host, serving to feed the parasite located externally to the host. However, the haustorium of some species proliferates in such a way that all vegetative growth occurs within the host (endophytically), and the parasite emerges only to flower. Examples of that include members of the genus Rafflesia, which grow inside the tropical vineTetrastigma, and stemsuckers (genus Pilostyles), which live within members of the pea family (Fabaceae).
Hydnora flowerThe unusual flower of Hydnora africana, a holoparasitic plant native to southern Africa.
There are many other examples of specialized morphology or life cycles that have evolved either as a necessity for parasitism or as a result of the transition from autotrophy. Holoparasite species that do not need to absorb water from the soil may have root systems that are greatly reduced in size or missing entirely. Similarly, leaves that are not needed for photosynthesis may be reduced to scales, and parasite colour may range from cream to yellow to purple, since chlorophyll production is unnecessary. An example of a holoparasite with such features is dodder (Cuscuta), which during vegetative growth has no roots and only scale leaves and therefore appears to be simply a yellow or orange stem with a network of host connections. Flowers of parasitic species often are similar to those of nonparasitic plants, although notable examples of extreme morphology include the huge Rafflesia flower and the bizarre, fleshy Hydnora inflorescence.
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In order to survive and reproduce, parasitic plants must be able to recognize the presence of a neighboring plant and have mechanisms to ensure that their seeds encounter appropriate hosts. The seeds of generalist parasites (those with a wide range of potential hosts) typically germinate under environmental conditions that are favorable to nonparasites. Once the parasite seeds have developed into seedlings, they then must locate a host. The roots of generalist root parasites are able to recognize and form haustoria with the roots of other plants that they encounter in the soil. Similarly, the threadlike shoot of the generalist dodder seedling, a stem parasite, elongates and uses information about the color of the host and volatile chemicals it produces in order to orient growth toward the host; once the shoot has reached the host plant, it coils and forms haustorial connections.
Root parasite Witchweed (Striga bilabiata), an obligate root parasite. Witchweeds commonly parasitize cereal crops, and their tiny seeds can persist in agricultural fields for many years.
Specialist parasites, many of which are obligates, tend to have additional mechanisms to detect their specific host plants. The best-studied examples are parasites of the family Orobanchaceae (e.g., Orobanche, Phelipanche, and Striga), the seeds of which are extremely small and may sit in the soil for years until the root of an appropriate host has grown nearby (see alsosoil seed bank). At that point the parasite seed detects a chemical signal (generally a strigolactone, a type of plant hormone) exuded from the host root, which triggers germination of the parasite seed. The parasite radicle (embryonic root) then grows a short distance, typically less than 2 mm (0.08 inch), to contact the host root and produce a haustorium.
Impact of parasitic plants
Conifer parasite Dwarf mistletoe (Arceuthobium minutissimum) growing on a pine tree.
Although most parasitic plant species are considered wild flowers or botanical curiosities, several species are weedy and capable of causing substantial losses of agricultural crops. Among the most harmful are members of the Orobanchaceae. Witchweeds (Striga species), for example, generally attack cereal crops, including corn (maize), sorghum, millet, and rice, in Africa. The common name of the plant reflects the dramatic effect it has on affected crops, seeming to magically arrest host growth and devastate yields. The broomrapes (Orobanche and Phelipanche species) attack broadleaf crops in the Mediterranean and Middle East and are a major constraint to the production of legumes, oilseed crops, and solanaceous crops (e.g., tomatoes, peppers, potatoes, and eggplants). Additional weedy genera include dodders, which attack a wide variety of broadleaf crops, and dwarf mistletoes (Arceuthobium), which damage coniferous trees. In all cases, the close physical connection between parasite and host, including location of the parasite belowground or within the host, makes control of parasitic weeds difficult.
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