The orange (specifically, the sweet orange) is the fruit of the citrus species Citrus × sinensis in the family Rutaceae. The fruit of the Citrus × sinensis is considered a sweet orange, whereas the fruit of the Citrus × aurantium is considered a bitter orange.
The orange is a hybrid, between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It has genes that are ~25% pomelo and ~75% mandarin; however, it is not a simple backcrossed BC1 hybrid, but hybridized over multiple generations. The chloroplast genes, and therefore the maternal line, seem to be pomelo.
The sweet orange has had its full genome sequenced. Earlier estimates of the percentage of pomelo genes varying from ~50% to 6% have been reported.
The sweet orange reproduces asexually (apomixis through nucellar embryony); varieties of sweet orange arise through mutations.
Oranges in Florida
Sweet oranges were mentioned in Chinese literature in 314 BC. As of 1987, orange trees were found to be the most cultivated fruit tree in the world. Orange trees are widely grown in tropical and subtropical climates for their sweet fruit. The fruit of the orange tree can be eaten fresh, or processed for its juice or fragrant peel. As of 2012, sweet oranges accounted for approximately 70% of citrus production. In 2010, 68.3 million metric tons of oranges were grown worldwide, production being particularly prevalent in Brazil and the U.S. states of Florida and California.
Oranges are small flowering trees about 10 m tall with oval leaves that are wavy at the edges and 4–10 cm long.
If you hold an orange leaf up to the light you can see many dots which are actualy glands containing aromatic oils. Orange flowers grow singly or in clusters of up to six, are white and vey fragrant. Oranges are round, and their size and color may vary. The pigments in the skin of the fruit are sensetive to temperature, in areas where ambient temperature does not get cooler than a specific temperature (as in Hawai’i) fruits will appear to be green or blotchy. The fruit (called a hesperidium) always contains eleven segments, and may or may not have hard white seeds.
Citrus sinensis has been cultivated for thousands of years, and is probably a hybrid between other citrus species in Southern and East Asia, although its evolutionary history is hard to pinpoint. Today it can be found literally all over the world.
The orange is now the most widely cultivated fruit on earth. There are three main commercial varieties: blood oranges, navel oranges and normal oranges although many more are certainly grown in small quantities.
Blood oranges, popular in Europe, have orange skin but a deep red flesh. Normal oranges, including the Valencia orange most commonly grown in Florida, are the most common variety used for orange juice. Navel oranges are the most recent variety in commercial cultivation, popular in California. The fruit of this variety actually contains a small orange (the navel) at the tip in addition to its larger more edible counterpart.