Fruit anatomy

Categories of fruits

Some plants commonly called vegetables, such as cucumbers, tomatoes, and squash, are actually fruits.

Fruits are found in three main anatomical categories:

Anatomy of simple fruits

In berries and drupes, the pericarp forms the edible tissue around the seeds. In other fruits such as Citrus stone fruits (Prunus) only some layers of the pericarp are eaten. In accessory fruits, other tissues develop into the edible portion of the fruit instead, for example the receptacle of the flower in strawberries.

Pericarp layers

The pericarp is typically made up of three distinct layers: the epicarp, which is the outermost layer; the mesocarp, which is the middle layer; and the endocarp, which is the inner layer surrounding the ovary or the seeds. In a citrus fruit, the epicarp and mesocarp make up the peel.

Epicarp

Epicarp (from Greek: epi-, "on" or "upon" + -carp, "fruit") is a botanical term for the outermost layer of the pericarp (or fruit). The epicarp forms the tough outer skin of the fruit, if there is one. The epicarp is sometimes called the exocarp, or, especially in Citrus, the flavedo.

Flavedo

Flavedo is mostly composed of cellulosic material but also contains other components, such as essential oils, paraffin waxes, steroids and triterpenoids, fatty acids, pigments (carotenoids, chlorophylls, flavonoids), bitter principles (limonin), and enzymes.

In citrus fruits, the flavedo constitutes the peripheral surface of the pericarp. It is composed of several cell layers that become progressively thicker in the internal part; the epidermic layer is covered with wax and contains few stomata, which in many cases are closed when the fruit is ripe.

When ripe, the flavedo cells contain carotenoids (mostly xanthophyll) inside chromoplasts, which, in a previous developmental stage, contained chlorophyll. This hormonally controlled progression in development is responsible for the fruit's change of color from green to yellow upon ripening.

The internal region of the flavedo is rich in multicellular bodies with spherical or pyriform shapes, which are full of essential oils.

Mesocarp

The mesocarp (from Greek: meso-, "middle" + -carp, "fruit") is the fleshy middle layer of the pericarp of a fruit; it is found between the epicarp and the endocarp. It is usually the part of the fruit that is eaten. For example, the mesocarp makes up most of the edible part of a peach, and a considerable part of a tomato. "Mesocarp" may also refer to any fruit that is fleshy throughout.

In a hesperidium, as is found in citrus fruit, the mesocarp is also referred to as albedo or pith. It is the inner part of the peel and is commonly removed before eating. In citron fruit, where the mesocarp is the most prominent part, it is used to produce succade.

Endocarp

Endocarp (from Greek: endo-, "inside" + -carp, "fruit") is a botanical term for the inside layer of the pericarp (or fruit), which directly surrounds the seeds. It may be membranous as in citrus where it is the only part consumed, or thick and hard as in the stone fruits of the family Rosaceae such as peaches, cherries, plums, and apricots.

In nuts, it is the stony layer that surrounds the kernel of pecans, walnuts, etc., and that is removed prior to consumption.

In citrus fruits, the endocarp is separated into sections, which are called segments. These segments are filled with juice vesicles, which contain the juice of the fruit.

Anatomy of grass fruits

The grains of grasses are single-seed simple fruits wherein the pericarp (ovary wall) and seed coat are fused into one layer. This type of fruit is called a caryopsis. Examples include cereal grains, such as wheat, barley, and rice.