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The domesticated apple was derived from M. sieversii, widely known by the synonym M. pumila. It is often referred to as orchard apple or, formerly, table apple. Other species and subspecies are generally known as "wild apples," "crab apples," "crabapples," or "crabs."
Apple trees are typically 4–12 m tall at maturity, with a dense, twiggy crown. The leaves are 3–10 cm long, alternate, simple, with a serrated margin. The flowers are borne in corymbs, and have five petals, which may be white, pink or red, and are perfect, with usually red stamens that produce copious pollen, and a half-inferior ovary; flowering occurs in the spring after 50–80 growing degree days (varying greatly according to subspecies and cultivar). Crabapples are widely grown as ornamental trees for their beautiful flowers or colorful fruit, with numerous cultivars selected for these qualities and for resistance to disease.
The fruit is a globose pome, varying in size from 1–4 cm diameter in most of the wild species, to 6 cm in M. sylvestris sieversii, 8 cm in M. sylvestris domestica, and even larger in certain cultivated orchard apples; among the largest-fruited cultivars (all of which originate in North America) are 'Wolf River' and 'Stark Jumbo.' The center of the fruit contains five carpels arranged star-like, each containing one to two (rarely three) seeds. Fruits from species other than M. domestica are not an important crop in most areas, being extremely sour and (in some species) woody, but they are used as a natural source of pectin for preserves, to flavor cider, and in Asian condiments.
Apples require cross-pollination between individuals by insects (typically bees, which freely visit the flowers for both nectar and pollen); all are self-sterile, and (with the exception of a few specially developed cultivars) self-pollination is impossible, making pollinating insects essential. Malus species, including domestic apples, hybridize freely.
Malus species are used as food plants by the larvae of a large number of Lepidoptera species.