8.1 Fruit Morphology – The Science of Plants (2022)

Learning objectives

By the end of this lesson you will be able to:

  • Define “fruit” from a botanical point of view.
  • Describe the differences among simple, aggregate, and multiple fruits.
  • Explain the general characteristics of fleshy and dry fruits.
  • Identify the difference between a true fruit and an accessory fruit based on structure and tissues.
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The graphic to the right shows a cross section of the carpel. Recall that the carpel is the female reproductive structure that is attached to the top whorl of the flower — the gynoecium node. The carpel has three basic parts:

  • The stigma, at the tip, and to which pollen grains adhere.
  • The style, the channel of tissue through which the pollen tube grows.
  • The ovary, at the base, and housing the ovules that contain the plant’s eggs.

The photo below shows a bit more detail about the carpel — in particular, the attachment of the ovules within the ovary via a stalk called the , emerging from the . It also names the chamber in which the ovules hang: the loculus (or locule). Remember the locule, but you won’t need to remember funicule, funiculus, or placenta. If you do come across those names in the future, then recall that they relate to the attachment of the ovule within the ovary.

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A , in the botanical sense, is the ripened ovary together with the within the ovary. People often think of a fruit from the culinary point of view, considering it to be the part of a plant that has seeds and when ripe is ready to eat, and think of vegetables as a savory food that is any edible part of a plant not associated with seeds (these include roots, stems and leaves). But some plant parts that, when we wear our chef’s hat, we think of as vegetables, are really botanical fruits. That green pepper chopped up on your pizza is a botanical fruit, as is the tomato that’s pureed to make the pizza sauce. A squash is another botanical fruit that we treat like a vegetable in the kitchen. Adding to the confusion is the legal decision that tomato fruits are vegetables (Nix v. Hedden 1893; The Washington Post). In Plant Propagation, we’ll define fruits from the botanical standpoint: the ripened ovary of a flower, together with the seeds within that ovary.

Review the diagram below of the tomato flower and fruit to be sure you know exactly which parts of the flower develop into the fruit. In the flower, the ovary wall provides protection to the ovules that contain the egg — the female gametes. The egg cells within the ovules are fertilized by the sperm from the pollen, the ovules develop into the seeds, and the ovary matures into the fruit. For some species, fruits are brown, green, starchy, bitter, proteinaceous, dry, and durable. They may be inedible, unpalatable, or so small as to be terribly inconvenient as a food. The ovary wall doesn’t always become a sweet and fleshy fruit like a peach. In kidney beans, for instance, it becomes a dry and brittle protective pod. Nevertheless, both the peach and the bean pod are botanical fruits.

The illustration shows a cross section of a tomato, with the fleshy ovary wall around the outside, locules (chambers) within the tomato, and seeds forming within the locules. The seeds are attached to the central placenta tissue by the funiculus.

Watch this videos to learn about flowers and fruits (2:45 min);

this video to learn about the structure of the ovary in the flower (3:40);

and this video on the structure of the mature tomato ovary or fruit (2:19).

Review activity

  1. Draw a diagram of a flower showing which parts become the fruit.
  2. Based solely on the botanical definition of a fruit, would the receptacle, petals, sepals or stamens be part of a true botanical fruit?
  3. If you cut open a green pepper where is the locule?

The ripened ovary wall is called the pericarpperi meaning around and carp referring to the carpel. Pericarp = around the carpel. The pericarp can be dry, as with bean pods, or fleshy like a peach, or sometimes both, as in an avocado, where the outer layer is leathery and the inner layer is fleshy.

The pericarp of a fleshy fruit typically has three layers, and each may have distinct characteristics. The photo of a peach below shows the layers making up the pericarp:

  • : the outer layer of the pericarp )also called the epicarp)
  • : the middle layer (fleshy in this example)
  • : the inner layer (hardened with sclerenchyma cells in the coconut example below)

What type of cell makes up this hard shell of a walnut or peach?Sclerenchyma!

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Depending upon the species, the exocarp may be tender, leathery, or hard. It may have oil glands (like in oranges and lemons) or hairs (like in kiwifruit). Similarly, mesocarps and endocarps may have various modifications that make them hard, soft, or leathery depending on the species. Remember that the pericarp is ripened ovary wall tissue and the exo-, meso- and endocarp are layers of that ripened ovary wall.

The coconut is a fruit. The photo below shows that the hard shell around the coconut that you see at the grocery store is actually the endocarp (the inside layer of the ovary wall). The fibrous mesocarp and the leathery exocarp have been removed from the coconut fruit by the time it reaches us. The fibrous mesocarp is used for other purposes like floor coverings in high-foot-traffic areas where a tough, durable fiber is needed.

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A “nut” is a botanical name for a particular type of fruit. From a culinary perspective, there are many foods that we call nuts, but to a botanist a nut is a fruit with a particular structure.

You likely already knew that a peanut isn’t a nut; it’s a type of fruit called a legume, and is related to peas, beans, and locust trees. A cashew isn’t a nut either; it’s a type of fruit called a drupe.If you are interested in seeing other unusual fruits, visit the Fruits called Nuts page posted by W. P. Armstrong at Palomar College.

Review activity

  1. Next time you’re grocery shopping, look around the produce section and try to figure out what parts of the fruits are the exo-, meso-, and endocarp. For instance, what are the parts that make up the banana? Where are those seeds?
  2. Why are fruits important to plant propagation?

Simple fruit

A simple fruit is formed from a flower with one carpel, or multiple carpels fused together so that it looks like just one carpel. The ovary wall surrounding the carpel or carpels ripens into an independent fruit (independent in the sense that it isn’t fused together with other ovaries). The photo below shows a grape, which is a .

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Aggregate fruit

In an aggregate fruit, the fruit is formed from the ripened ovaries present in one flower with numerous simple carpels. The ripened ovaries from that one flower coalesce into one larger unit, but you can still see evidence of the individual carpels. The raspberry, for instance, comes from one flower with many carpels. As the pericarps mature they mature together to form the thimble of the raspberry that we eat. You can still see the mosaic of individual ruby red carpels that fuse together to form the thimble. If you’ve ever harvested raspberries you know that the thimble pulls off a firm white structure, and that structure is the receptacle of the flower. Below is an illustration of the fruit structure.

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Multiple fruit

A multiple fruit is formed from the ripened ovaries present in one flower with numerous simple carpels. The ripened ovaries from that one flower coalesce into one larger unit, but you can still see evidence of the individual carpels. In the photo of the pineapple, below, you can see individual flowers, some of which are still open and showing purple-pink petals. The pericarps of these individual flowers coalesce into one large .

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The distinction between and has to do with the number of flowers involved in the fruit. An aggregate fruit is from one flower with many ovaries, and the multiple fruit is made up of multiple flowers.

Fruit types

Fruits are also categorized according to whether the pericarp at maturity is:

  • Fleshy: accessory parts of the ovary develop into succulent tissues with a high moisture content.
  • Dry: at maturity the fruit has a low moisture content. Dry fruit that opens and releases the seeds from the pericarp is called and dry fruit that remains closed retaining the seed within the pericarp is called .

A dry, dehiscent pericarp may split open along sutures in various ways, and these ways of splitting open are also characteristic of particular types of fruits.

Simple fruits

Simple fruits with fleshy pericarp (exocarp, mesocarp, endocarp):

Drupe

A stone fruit, derived from a single carpel and containing usually one or two seeds. The exocarp is a thin skin, the mesocarp may be fleshy, and the endocarp is hard (i.e., “stony”) as shown in the photo of the peach, below. Examples of drupes include peach, plum, cherry, apricot, and almond.

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Berry

A simple fruit formed from one flower with a superior ovary. The fruit has a fleshy pericarp, one carpel or multiple fused carpels, and many seeds. A tomato (below) is a berry, a grape is a berry, blueberries and cranberries are berries…but a raspberry is not. (Remember that a raspberry is an aggregate fruit where the carpels do not fuse the way they do in multiple-carpel berries).

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Pepo

A simple fruit formed from one flower with an inferior ovary. The fruit has a fleshy mesocarp, a rigid or leathery exocarp, one carpel or multiple fused carpels, and many seeds. The photo of squash below shows the fusion of three carpels to form the fruit, each carpel having many seeds. The fleshy interior that we eat is the mesocarp. Other examples include zucchini, cucumber, summer squash, and winter squash such as acorn and butternut squash.

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Hesperidium

Like a berry, but with a leathery exocarp instead of a fleshy exocarp. Each section of the hesperidium represents one carpel in the flower, but in the mature fruit the exocarp and mesocarp form an uninterrupted cover. The interiors of the carpels are packed with fluid-filled vesicles that are actually specialized trichomes. The exocarp contains volatile oil glands in pits. The orange, below, is an example of a hesperidium. All citrus fruits are this type of fruit.

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Simple fruits with dry pericarp, dehiscent

Legume

Dry fruit made up of a single, folded carpel, multi-seeded, dehiscent along two sutures. It is easy to see the funiculus in peas, When you open the pod to shell out the peas there is a small stalk attaching the pea seed to the pod; that’s the funiculus. Beans are also legumes.

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Capsule

A dry, dehiscent fruit made up of several fused carpels. The photo below shows the exterior of the poppy capsules and a cross section showing the locules with seeds inside. The capsule may split open in several ways depending on the species. In a poppy, the cap pops off to eject the mature seeds.

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Simple fruits, dry pericarp, indehiscent

Caryopsis

A fruit from one carpel containing a single seed. The pericarp is fused to the seed. A corn kernel is a caryopsis. The outside of the corn kernel is the pericarp.

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Achene

Like the caryopsis (one seed per ovary), but the seed can be threshed so that it is free of the pericarp. You can buy sunflower seeds “in the shell.” The “shell” is the pericarp. We also discussed achenes earlier when looking at the actual fruits on a strawberry.

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Nut

A dry, indehiscent, one-seeded fruit with a hard exocarp. The ovaries that produce nuts have more than one carpel, but through abortion, only one seed matures. In the photo below, the pericarp of the acorn is partially encased in a tough covering called the involucre. True nuts will always have a hard exocarp and just one seed, while this is not always the case with culinary nuts. Peanuts, for instance, have a hard exocarp but multiple seeds. Horse chestnuts have a leathery exocarp and hard endocarp. True nuts have a hard exocarp.

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Review questions

  1. What features distinguish simple, aggregate, and multiple fruits?
  2. What is meant by dehiscent and indehiscent? How would you classify the corn caryopsis and the sunflower achene in this regard?

Accessory fruits

Earlier, we sawhow one or more of the other flower parts (androecium, corolla and calyx) can be attached below the ovary (hypogynous parts, superior ovary), above the ovary (epigynous parts, inferior ovary), or around the middle of the ovary (perigynous parts). In the case of epigynous and perigynous parts there are tissues surrounding the ovary to which the other flower parts attach and that can also adhere to the outside of the ovary and become part of the fruit. One example of a fruit from this type of flower where there is (hypanthium) adhering to the outside of the ovary wall is the pome, examples of which are the apple, pear, and quince. Another example is the strawberry, where together the receptacle tissue and ovary wall tissue form the strawberry fruit. Since it isn’t part of the ovary wall, the receptacle tissue is considered an accessory tissue, so the strawberry, instead of being a true fruit, is a type of fruit called an accessory fruit.

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Apple is an example of a species where the culinary fruit part we eat is actually hypanthium tissue rather than ovary wall tissue. In an apple, the ovary is the papery core that encloses the seeds. Since the part we eat isn’t the ripened ovary wall, the apple is called an accessory fruit, signifying that we are really enjoying hypanthium tissue (an accessory tissue), not ovary wall. The hypanthium is also the tissue that makes up the fleshy part of rosehips (fruit of roses) — logical, because apple and rose are in the same taxonomic family: Rosaceae. Plants in the same family have common flower (and therefore fruit) morphologies.

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The situation can be very complex with plants like strawberry where the juicy part we eat is the swollen receptacle of the former flower and the actual botanical fruits are the brown specks sticking to the outside of the receptacle. The strawberry is an accessory fruit because the red fleshy part we eat is made up not of the ovary wall, but primarily of receptacle tissue. The strawberry is also an aggregate fruit formed from multiple ovaries present in one flower. The fruit is an achene that contains the single seed from a single ovary attached to the outside of the receptacle.

Review activity

  1. Cut open an apple and identify where the hypanthium ends and the ovary wall begins.
  2. Next time you are at the grocery store, identify which “fruits” are true fruits, accessory fruits, aggregate fruit, or multiple fruits.

You should know the types of categories, such as simple fruits, multiple, aggregate, dehiscent, indehiscent, and some examples, but you do not need to keep an exhaustive list in your mind. You should also know the definition of an accessory fruit and the relationship of these fruits to epigynous and perigynous flowers.

There are many more types of fruits. If you are interested in some optional reading you can discover more about fruit types. Or do a Google search on “types of fruits” or on any of the specific fruit types mentioned above.

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