Science Fair Project Encyclopedia
Rhubarb is a perennial plant that grows from thick, short rhizomes, comprising the genus Rheum. The large, somewhat triangular leaf blades are elevated on long, fleshy petioles. The flowers are small, greenish-white, and borne in large compound leafy inflorescences.
The plant is indigenous to Asia. Varieties of rhubarb have a long history as medicinal plants in Traditional Chinese Medicine, but the use of rhubarb as food is a relatively recent innovation, first recorded in 17th century England. Rhubarb is now grown in many areas, primarily for its fleshy petioles, commonly known as rhubarb sticks. In temperate climate rhubarb is one of the first food plants to be ready for harvest, usually in April/May. The petioles can be cooked in a variety of ways. Stewed, they yield a tart sauce that can be eaten with sugar or used as filling for pies, tarts, and crumbles. This common use led to the German slang term for rhubarb, piestengel, or "pie plant." Rhubarb is also used to make wine. In former days, a common and affordable sweet for children in Yorkshire was a tender stick of rhubarb, dipped in copious amounts of sugar.
In warm climates rhubarb will grow all year round, but in colder climates the plant disappears completely during winter, and begins to grow again in early spring. It can be forced, that is, encouraged to grow early, by raising the local temperature. This is commonly done by placing an upturned bucket over the shoots as they come up.
The plant occurs in at least four species. Those most commonly used in cooking are the Garden Rhubarb (R. rhabarbarum) and R. rhaponticum, which though a true rhubarb bears the common name False Rhubarb. The drug rheum is prepared from the rhizomes and roots of another species, R. officinale or Medicinal Rhubarb. This species is also native to Asia, as is the Chinese Rhubarb (R. palmatum).
Rhubarb leaves contain poisonous substances. Rhubarb leaf poisoning is most often caused by oxalic acid, a corrosive and nephrotoxic acid that is abundantly present in a lot of plants. The LD50 for oxalic acid is predicted to be about 375 mg/kg body weight, or about 25 g for a 65 kg human. While the oxalic acid content of rhubarb leaves can vary, it averages about 0.5%, so five kilograms of leaves would be required to reach an LD50 dose. In the petioles, the amount of oxalic acid is much lower, especially when harvested before mid-June (on the northern hemisphere).
The roots and stems are rich in anthraquinones, such as emodin and rhein. These substances are cathartic and laxative, which explains the sporadic abuse of Rhubarb as slimming agent. Anthraquinones are yellow or orange and may colour the urine.
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