Watermelon (Citrullus lanatus (Thunb.), family Cucurbitaceae) is a vine-like (scrambler and trailer) flowering plant originally from southern Africa. Its fruit, which is also called watermelon, is a special kind referred to by botanists as a pepo, a berry which has a thick rind (exocarp) and fleshy center (mesocarp and endocarp). Pepos are derived from an inferior ovary, and are characteristic of the Cucurbitaceae. The watermelon fruit, loosely considered a type of melon – although not in the genus Cucumis – has a smooth exterior rind (green, yellow and sometimes white) and a juicy, sweet interior flesh (usually pink, but sometimes orange, yellow, red and sometimes green if not ripe). It is also commonly used to make a variety of salads, most notably fruit salad.
Watermelon is thought to have originated in southern Africa, where it is found growing wild, because it reaches maximum genetic diversity there, resulting in sweet, bland and bitter forms. Alphonse de Candolle, in 1882, already considered the evidence sufficient to prove that watermelon was indigenous to tropical Africa. Though Citrullus colocynthis is often considered to be a wild ancestor of watermelon and is now found native in north and west Africa, Fenny Dane and Jiarong Liu suggest on the basis of chloroplast DNA investigations that the cultivated and wild watermelon appear to have diverged independently from a common ancestor, possibly C. ecirrhosus from Namibia.
It is not known when the plant was first cultivated, but Zohary and Hopf note evidence of its cultivation in the Nile Valley from at least as early as the second millennium BC. Although watermelon is not depicted in any Egyptian hieroglyphic text nor does any ancient writer mention it, finds of the characteristically large seed are reported in Twelfth dynasty sites; numerous watermelon seeds were recovered from the tomb of Pharaoh Tutankhamun.
By the 10th century AD, watermelons were being cultivated in China, which is today the world's single largest watermelon producer. By the 13th century, Moorish invaders had introduced the fruit to Europe; according to John Mariani's The Dictionary of American Food and Drink, "watermelon" made its first appearance in an English dictionary in 1615.
Museums Online South Africa list watermelons as having been introduced to Native Americans in the 16th century. Early French explorers found Native Americans cultivating the fruit in the Mississippi Valley. Many sources list the watermelon as being introduced in Massachusetts as early as 1629. Southern food historian John Egerton has said he believes African slaves helped introduce the watermelon to the United States. Texas Agricultural Extension horticulturalist Jerry Parsons lists African slaves and European colonists as having distributed watermelons to many areas of the world. Parsons also mentions the crop being farmed by Native Americans in Florida (by 1664) and the Colorado River area (by 1799). Other early watermelon sightings include the Midwestern states (1673), Connecticut (1747) and the Illiana region (1822).
Charles Fredric Andrus, a horticulturist at the USDA Vegetable Breeding Laboratory in Charleston, South Carolina, set out to produce a disease-resistant and wilt-resistant watermelon. The result, in 1954, was "that gray melon from Charleston." Its oblong shape and hard rind made it easy to stack and ship. Its adaptability meant it could be grown over a wide geographical area. It produced high yields and was resistant to the most serious watermelon diseases: anthracnose and fusarium wilt.
Today, farmers in approximately 44 states in the U.S. grow watermelon commercially, and almost all these varieties have some Charleston Gray in their lineage. Georgia, Florida, Texas, California and Arizona are the USA's largest watermelon producers.
This now-common watermelon is often large enough that groceries often sell half or quarter melons. There are also some smaller, spherical varieties of watermelon, both red- and yellow-fleshed, sometimes called "icebox melons."
In Japan, farmers of the Zentsuji region found a way to grow cubic watermelons, by growing the fruits in glass boxes and letting them naturally assume the shape of the receptacle.The square shape was originally designed to make the melons easier to stack and store, but the square watermelons are often more than double the price of normal ones, and much of their appeal to consumers is in their novelty. Pyramid shaped watermelons have also been developed and any polyhedral shape may potentially also be used.