An apple is the round, edible fruit of an apple tree (Malus spp.). Fruit trees of the orchard or domestic apple (Malus domestica), the most widely grown in the genus, are cultivated worldwide. The tree originated from its wild ancestor, Malus sieversii. Apples have been grown for thousands of years in Europe before they were introduced to America by colonists and explorers.

Apples grown from seeds tend to be very different from those of their parents, and the resultant fruit frequently lacks desired characteristics. For commercial purposes, including botanical evaluation, apple cultivars are propagated by clonal grafting onto rootstocks. Apple trees grown without rootstocks tend to be larger and much slower to fruit after planting. Rootstocks are used to control the speed of growth and the size of the resulting tree, allowing for easier harvesting.

There are more than 7,500 cultivars of apples. Different cultivars are bred for various tastes and uses, including cooking, eating raw, and cider or apple juice production. Trees and fruit are prone to fungal, bacterial, and pest problems, which can be controlled by a number of organic and non-organic means. In 2010, the fruit's genome was sequenced as part of research on disease control and selective breeding in apple production.

The word apple is derived from Old English æppel, meaning "fruit", not specifically the apple. 4 That in turn is descended from the Proto-Germanic noun aplaz, descended in turn from Proto-Indo-European h₂ébōl. 5 As late as the 17th century, the word also functioned as a generic term for all fruit including nuts; one example is the pineapple, the swollen accessory fruit outside of the cashew nut known as the "cashew apple", 6 another is a 14th-century Middle English expression appel of paradis, meaning a banana. 4

(a) In General.

1. Tree. The apple tree is deciduous, generally standing from 2 to 4.5 meters (6 to 15 feet) tall in cultivation and up to 15 m (49 ft) in the wild, though more typically 2 to 10 m (6.5 to 33 ft). ⁷ 2 When cultivated, the size, shape and branch density are determined by rootstock selection and trimming method. ⁷ Apple trees may naturally have a rounded to erect crown with a dense canopy of leaves. 8 The bark of the trunk is dark gray or gray-brown, but young branches are reddish or dark-brown with a smooth texture. 29 Young twigs are covered in fine downy hairs; they become hairless when older. 9
2. Bud. The buds are egg-shaped and dark red or purple in color; they range in size from 3 to 5mm, but are usually less than 4mm. The bud scales have very hairy edges. When emerging from the buds, the leaves are convolute, meaning that their edges overlap each other. 2 Leaves can be simple ovals (elliptic), medium or wide in width, somewhat egg-shaped with the wider portion toward their base (ovate), or even with sides that are more parallel to each other instead of curved (oblong) with a narrow pointed end. 9 2 The edges have broadly-angled teeth, but do not have lobes. The top surface of the leaves are glabrescent, almost hairless, while the undersides are densely covered in fine hairs. 2 The leaves are attached alternately by short leaf stems 1-to-3.5 cm (1⁄2-to-1+1⁄2 in) long. 8 2
3. Blossom. Blossoms are produced in spring simultaneously with the budding of the leaves and are produced on spurs and some long shoots. 7 When the flower buds first begin to open the petals are rose-pink and fade to white or light pink when fully open with each flower 3-to-4-centimeter (1-to-1+1⁄2-inch) in diameter. 2 The five-petaled flowers are group in an inflorescence consisting of a cyme with 3–7 flowers. 10 The central flower of the inflorescence is called the "king bloom"; it opens first and can develop a larger fruit. 8 Open apple blossoms are damaged by even brief exposures to temperatures −2 °C (28 °F) or less, although the overwintering wood and buds are hardy down to −40 °C (−40 °F). 10

Apple blossoms

Botanical illustration

(b) Fruit

The fruit is a pome that matures in late summer or autumn. 2 The true fruits or carpels are the harder interior chambers inside the apple's core. There are usually five carpels inside an apple, but there may be as few as three. Each of the chambers contains one or two seeds. 11 The edible flesh is formed from the receptacle at the base of the flower. 12

How apple fruit derives from flower structures

The seeds are egg- to pear-shaped and may be colored from light brown or tan to a very dark brown, often with red shades or even purplish-black. They may have a blunt or sharp point. 1 The five sepals remain attached and stand out from the surface of the apple. 2

The size of the fruit varies widely between cultivars, but generally has a diameter between 2.5 and 12 cm (1 and 5 in). The shape is quite variable and may be nearly round, elongated, conical, or short and wide.

The groundcolor of ripe apples is yellow, green, yellow-green or whitish yellow. The overcolor of ripe apples can be orange-red, pink-red, red, purple-red or brown-red. The overcolor amount can be 0–100%. The skin may be wholly or partly russeted, making it rough and brown. The skin is covered in a protective layer of epicuticular wax. The skin may also be marked with scattered dots. The flesh is generally pale yellowish-white, though it can be pink, yellow or green.

(b) Chemistry

Important volatile compounds in apples that contribute to their scent and flavor include acetaldehyde, ethyl acetate, 1-butanal, ethanol, 2-methylbutanal, 3-methylbutanal, ethyl propionate, ethyl 2-methylpropionate, ethyl butyrate, ethyl 2-methyl butyrate, hexanal, 1-butanol, 3-methylbutyl acetate, 2-methylbutyl acetate, 1-propyl butyrate, ethyl pentanoate, amyl acetate, 2-methyl-1-butanol, trans-2-hexenal, ethyl hexanoate, hexanol. 17 18