What makes a citrus fruit citrus? Do kumquats count?
Citrus is a common term as well as a scientific one for a class of flowering (and thus fruit-bearing) plants in the rue family.
Rue is not well known in the United States but is popular in Central and South America, where its fragrant evergreen branches are often bought in markets for their pleasant scent and belief that they bring good luck. I can’t vouch for its magical properties, but I can tell you it’s a very nice-smelling plant to have in a house.
The name citrus was derived from the Greek word for cedar, perhaps because of similar smells. Citrus fruits are known as agrumes, which roughly means “sour fruits.’’
Most citrus fruits are from a small group of plants that are probably derived from about four species in Southeast Asia. The most common are oranges, grapefruits, tangerines, lemons, and limes.
The largest citrus fruit is the pomelo, sometimes called Chinese grapefruit, thick-skinned with a rind popular for marmalades and cooking.
Many citrus fruits are crosses between more common varieties. The tangelo is a cross between a tangerine and a pomelo. A relatively less-known citrus fruit, the lemandarin or rangpur, is a cross between a mandarin orange and a lemon.
Some caveats are needed for things that look like citrus or have citrus-sounding names but don’t fall into the citrus class. For example, the Spanish lime (also called mamoncillo, ginep, ackee, or limoncillo) is not technically a lime, but rather a relative of the lychee and longan.
The kumquat, despite its resemblance to an olive-shaped orange, is not generally classified as a citrus fruit. That said, it has been crossed with many citrus fruits, producing new ones such as the limequat (cross of lime and kumquat) and sunquat (cross of lemon and kumquat).
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@ globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
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