Fruit, Pericarp, Endocarp, Exocarp, Mesocarp

fruit [ froot ] noun: the seed-bearing structure (the ripened ovary and its contents and accessory parts) of an angiosperm

pericarp [ PER-i-kahrp ] noun: the walls of a ripened ovary

endocarp [ EN-duh-kahrp ] noun: the inner layer of the pericarp

exocarp [ EK-soh-kahrp ] noun: the outer layer of the pericarp, also known as the epicarp

mesocarp [ MEZ–uh-kahrp, MES-, MEE-zuh-, -suh– ] noun: the middle layer of the pericarp, also known as the sarcocarp  

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Whether it is the time to plan or plant your vegetable garden or to harvest the fruits of your labor, you may not realize that many of those plants commonly referred to as vegetables, such as beans, peppers, and squash, are actually fruits according to botanical criteria. Likewise, most fruits with berries in their names are not true berries and many culinary nuts are not botanical nuts. As confusing as this might seem to a layperson, if you ask a group of botanists what kind of fruit, for example, a walnut is, each might provide a different answer: a nut, a drupaceous nut, a pseudodrupe, a drupe, a tryma, or an accessory fruit. Even centuries after the first textbook of descriptive systematic botany was published, there are no accepted botanical standards for how to distinguish fruits and name them.

In that 1751 textbook, Philosophia Botanica, Carl Linnaeus identified eight different fruit types (capsula, siliqua, legumen, folliculus, drupa, pomum, bacca, and strobilus). Today, they are among almost 100 identified types of fruit, but not only do scientists disagree over which fruit types to recognize, they do not even agree on the very concept of fruit itself—that is, how broad or narrow a definition should be. [Some do not consider strobilus a true fruit but rather the reproductive structure of conifers and other gymnosperms.] This lack of consensus translates to Flora and other plant data resources, which may ascribe different fruit types to the same species or may avoid naming fruit types altogether and instead rely on physical descriptions.

Understanding a fruit’s composition and type involves examining its relationship to the flower and how it develops. The mature ovary makes up the main part of most fruits. Simple fruits develop from one ovary. Aggregate fruits develop from the merger of several ovaries in the same flower.  Multiple fruits develop from several ovaries in separate flowers within a common receptacle. Using these definitions, blackberry is an aggregate fruit and pineapple is a multiple fruit—common examples cited in English-language textbooks or reference materials. However, Spjut points out that they are the reverse of definitions recognized by authorities in countries like “France, Germany, Russia, and Brazil,” where instead, “a single flower producing multiple fruitlets,” like blackberry, would be considered a multiple fruit.

The ovary may have one (monocarpellary) or more (bicarpellary or bicarpellate, tricarpellary, multicarpellary) carpels, the seed-bearing structures. Fruit may be: apocarpous (having separate carpels like in aggregate fruits); syncarpous (having carpels united in a compound ovary like in some simple fruits); or schizocarpous (having carpels united wholly or partially that separate from each other into distinct seeded-segments—as opposed to sutural separation to release the seeds).

Within the ovary, the chamber in which the seeds develop is called a locule. Depending on the number of locules, a fruit may be unilocular (having one chamber), bilocular (divided into two chambers), trilocular (having three chambers) or multilocular (divided into many chambers).

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The ovary’s position relative to the other floral whorls (androecium, petals, and sepals) must also be considered. When the androecium, petals, and sepals emerge from the receptacle below the gynoecium, the flower is hypogynous and the ovary is superior as in “true” berries, like pepper and tomato. In a perigynous flower, the bases of the androecium, petals, and sepals are fused to hypanthium tissue, which forms an unattached cup around a superior ovary, that falls away as the ovary matures. Fruit examples are the drupes of the genus Prunus, which includes almonds, cherries, olives, peaches, and plums. In an epigynous flower, the other floral whorls are located above an inferior ovary and like the ovary wall are fused to the hypanthium. Besides the ovary, the fruit may include these other flower parts. For example, in a blueberry, an epigynous or “false” berry, accessory tissue surrounds the base of the carpels and the calyx/sepals remain at the tip of the fruit. In pomes, such as apples and pears, fleshy hypanthium tissue surrounds the ovary and there may be “remnants of the flower sepals, petals, stigma and stamen attached at the base of the fruit, on the opposite side from the receptacle.” (Stover-Blackburn, 2024). Some fruits, like the utricle of sugar beet, develop from a semi-inferior ovary.

All fruits have a pericarp, which is the mature ovary wall that surrounds and protects the ovule from which the seed develops. It has three layers: the outermost exocarp, the middle mesocarp, and innermost endocarp.

In fleshy fruits, the pericarp has a high moisture content making all or part of it soft and succulent at maturity. For example, a berry has a succulent mesocarp and endocarp usually embedded with many seeds and is enclosed in a tender, leathery, or tough exocarp, while a drupe has a thin exocarp, fleshy mesocarp and hard endocarp which contains the seed. A pome, formed from an inferior ovary, has a fleshy hypanthium and a tough inner core (the pericarp and carpels wherein lie the seeds). Fleshy fruits do not naturally split open to release their seeds.

In dry fruits, the pericarp lacks moisture and is hard and dry at maturity. Dry fruits are either indehiscent, like achenes, cypselae, samaras, nuts, and most schizocarps or dehiscent, like capsules, follicles, and legumes, which split open at maturity to release their seeds. How these fruits dehisce helps to distinguish them.

Left to right: schizocarp and achene visible in a samara, cypsela, nut, follicles, legumes, capsule (loculicidal), silicles, and siliques.

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References

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Armstrong WP. 2009. Fruits Called Nuts. Wayne’s World: 9 May 2001. (accessed February 15, 2024).

Britannica, The Editors of Encyclopaedia. December 9, 2023. ”fruit”. Encyclopedia Britannica. (accessed February 3, 2024).

Classification of Fruit Types. University of Wisconsin-Madison. (accessed February 3, 2024)

Douglas GE. 1944. The Inferior Ovary. Botanical Review, 10(3): 125–186.

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Nickrent DL. Updated October 2022. Lecture 24 Fruits. Plant Anatomy. Southern Illinois University.

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