Source: Magness et al. 1971
Citrus fruits produced commercially in the U.S. are of 5 major and 3 minor kinds. Major kinds are C. limon, lemon; C. sinensis, sweet orange; C. reticulate, Mandarin orange; C. paradisi, grapefruit; and C. aurantifolia, lime. Minor kinds are C. grandis, pummelo; C. aurantium, sour or Seville orange; and C. medics, citron.
All are produced on relatively small, evergreen trees. All are injured by winter temperatures below about 25°F. Leaves and fruit peel contain oil vesicles. Fruit peel is relatively thick, consisting of a white, spongy endocarp, and with the surface epidermis containing numerous oil vesicles. Interior of fruit consists of 8 to 18 segments, each of which may contain seeds near the inner angle. Segments are composed largely of rather large vesicles filled with juice, which may vary from sweet to very acid.
All citrus fruits will cross with each other, and many such crosses have been made by breeders and some have originated by chance. As a result, we now have in commerce Mandarin orange x grapefruit crosses called tangelos, sweet orange x Mandarin crosses called tangors, and others.