Ricinus communis, the castor bean or castor oil plant, is a species of perennial flowering plant in the spurge family, Euphorbiaceae. It is the sole species in the monotypic genus, Ricinus, and subtribe, Ricininae. The evolution of castor and its relation to other species are currently being studied using modern genetic tools. It reproduces with a mixed pollination system which favors selfing by geitonogamy but at the same time can be an out-crosser by anemophily (wind pollination) or entomophily (insect pollination).

Its seed is the castor bean, which, despite its name, is not a true bean. Castor is indigenous to the southeastern Mediterranean Basin, Eastern Africa, and India, but is widespread throughout tropical regions (and widely grown elsewhere as an ornamental plant).

Castor seed is the source of castor oil, which has a wide variety of uses. The seeds contain between 40% and 60% oil that is rich in triglycerides, mainly ricinolein. The seed also contains ricin, a water-soluble toxin, which is also present in lower concentrations throughout the plant.

The common name "castor oil" probably comes from its use as a replacement for castoreum, a perfume base made from the dried perineal glands of the beaver (castor in Latin). It has another common name, palm of Christ, or Palma Christi, that derives from castor oil's reputed ability to heal wounds and cure ailments.

The glossy leaves are 15–45 cm (6–18 in) long, long-stalked, alternate, and palmate with five to twelve deep lobes with coarsely toothed segments. In some varieties they start off dark reddish purple or bronze when young, gradually changing to dark green, sometimes with a reddish tinge, as they mature. The leaves of some other varieties are green practically from the start, whereas in yet others a pigment masks the green color of all the chlorophyll-bearing parts, leaves, stems, and young fruit so that they remain a dramatic purple-to-reddish-brown throughout the life of the plant. Plants with the dark leaves can be found growing next to those with green leaves, so there is most likely only a single gene controlling the production of the pigment in some varieties. The stems and the spherical, spiny seed capsules also vary in pigmentation. The fruit capsules of some varieties are more showy than the flowers.

The flowers lack petals and are unisexual (male and female) where both types are borne on the same plant (monoecious) in terminal panicle-like inflorescences of green or, in some varieties, shades of red. The male flowers are numerous, yellowish-green with prominent creamy stamens; the female flowers, borne at the tips of the spikes, lie within the immature spiny capsules, are relatively few in number and have prominent red stigmas.

The fruit is a spiny, greenish (to reddish-purple) capsule containing large, oval, shiny, bean-like, highly poisonous seeds with variable brownish mottling. Castor seeds have a warty appendage called the caruncle, which is a type of elaiosome. The caruncle promotes the dispersal of the seed by ants (myrmecochory).

Castor oil has many uses in medicine and other applications.

Extract of Ricinus communis exhibited acaricidal and insecticidal activities against the adult of Haemaphysalis bispinosa Neumann (Acarina: Ixodidae) and hematophagous fly Hippobosca maculata Leach (Diptera: Hippoboscidae).

Castor oil is an effective motor lubricant and has been used in internal combustion engines, including those of World War I airplanes, some racing cars, and some model airplanes. It has historically been popular for lubricating two-stroke engines due to high resistance to heat compared to petroleum-based oils. It does not mix well with petroleum products, particularly at low temperatures, but mixes better with the methanol-based fuels used in glow model engines. In total-loss-lubrication applications, it tends to leave carbon deposits and varnish within the engine. It has been largely replaced by synthetic oils that are more stable and less toxic.

Ricinus communis is the host plant of the common castor butterfly (Ariadne merione), the eri silkmoth (Samia cynthia ricini), and the castor semi-looper moth (Achaea janata). It is also used as a food plant by the larvae of some other species of Lepidoptera, including Hypercompe hambletoni and the nutmeg (Discestra trifolii).

Ricinus is extremely allergenic and has an OPALS allergy scale rating of 10 out of 10. The plant is also a very strong trigger for asthma, and allergies to Ricinus are commonplace and severe.

The toxicity of raw castor beans is due to the presence of ricin. Although the lethal dose in adults is considered to be four to eight seeds, reports of actual poisoning are relatively rare. According to the Guinness World Records, this is the world's most poisonous common plant. Symptoms of overdosing on ricin, which can include nausea, diarrhea, tachycardia, hypotension, and seizures, persist for up to a week. However, the poison can be extracted from castor by concentrating it with a fairly complicated process similar to that used for extracting cyanide from almonds.

If ricin is ingested, symptoms commonly begin within 2–4 hours, but may be delayed by up to 36 hours. These include a burning sensation in mouth and throat, abdominal pain, purging, and bloody diarrhea. Within several days there is severe dehydration, a drop in blood pressure, and a decrease in urine. Unless treated, death can be expected to occur within 3–5 days; however, in most cases, a full recovery can be made.

Poisoning occurs when animals, including humans, ingest broken castor beans or break the seed by chewing: intact seeds may pass through the digestive tract without releasing the toxin. The toxin provides the castor oil plant with some degree of natural protection from insect pests such as aphids. Ricin has been investigated for its potential use as an insecticide. The castor oil plant is also the source of undecylenic acid, a natural fungicide.

Commercially available cold-pressed castor oil is not toxic to humans in normal doses, either internal or externally.

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