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Mesocarp |
A fruit in botany refers to a mature ovary. In fleshy fruits, the outer, often edible, layer is the pericarp, which is the tissue that develops from the ovary wall of the flower and surrounds the seeds. If seeds are considered to be akin to eggs developing in the ovary of a fowl, the pericarp would be the female bird's uterus.
However, there are a large number of fruits which are not adequately described by that analogy; for example in most nuts and legumes the edible part is the seed and not the pericarp. Many edible vegetables are actually stems, leaves, and even roots of the plant, but others like the cucumber, squash etc. are the common pericarp and are botanically considered to be fruits. Finally, in some seemingly pericarp fruits the edible portion is actually an aril.
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Fruits come in three main anatomical categories:
In berries and drupes, the pericarp forms the edible tissue around the seeds. In accessory fruits, other tissues develop into the edible portion of the fruit instead, for example the receptacle of the flower in apples and strawberries.
The pericarp itself is typically made up of three distinct layers: the exocarp which is the most outside layer or peel, the mesocarp the middle layer or pith, and the endocarp the inner layer surrounding the hollowed ovary or the containing seeds.
Exocarp (Gr. "outside" + "fruit"), is a botanical term for the outermost layer of the pericarp (or fruit). The exocarp forms the tough outer skin of the fruit which bears oil glands and pigments. The exocarp is sometimes called the epicarp, or, especially in citruses, the flavedo.
Flavedo is mostly composed of cellulosic material but also contains other components, such essential oils, paraffin waxes, steroids and triterpenoids, fatty acids, pigments (carotenoids, chlorophylls, flavonoids), bitter principles (limonene), 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 (Gr. "middle" + "fruit") or Sarcocarp (Gr. "flesh" + "fruit"), is the botanical term for the succulent and fleshy middle layer of the pericarp of drupaceous fruit, between the exocarp and the endocarp; it is usually the part of the fruit that is eaten.
This term may also refer to any fruit which is fleshy throughout. In a hesperidium, the mesocarp is also referred to as albedo or pith because of its soft fiber. It is part of the peel which is commonly removed by hand.
Endocarp (Gr. "inside" + "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 subfamily Prunoideae such as peaches, cherries, plums, and apricots.
In nuts, it is the stony layer that surrounds the kernel of pecans, walnuts etc. and which is removed prior to consumption.
In citrus, the edocarp is separated into sections which are most commonly called segments. The juicy pulp which is filling the segments, is usually referred to as juice vesicles.
The grains of grasses are single-seeded simple fruits where 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.
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