What Is a Fruit?

If you have ever been inside a grocery store, you have probably noticed the produce section. Mountains of lemons, piles of tomatoes, rows of cucumbers, several kinds of apples, and more. Some of these items you might know as fruits, and others as vegetables. But what is the difference between a fruit and a vegetable? When you eat an apple, strawberry, or peach, what part of the plant are you eating? This article explores our knowledge of fruits and how to properly identify them.

known together as the stamen and carpel. The carpel is in the center CARPEL Reproductive organ of a flower, which includes the ovary, stigma, and style. After fertilization, the carpel becomes the fruit. of the flower and is composed of three parts, called the ovary, style, and stigma; these parts can all play a role in the development of a fruit ( Figure A). Inside the ovary are ovules, or immature seeds. The carpel is therefore a protective cover, creating an optimal environment for the ovules to transform into seeds. In order to develop into a fruit, the carpel must first receive pollen grains successfully. After this, the carpel is fertilized, and fruit development can begin. The outer parts of the flower, the sepals and petals, are sterile and are usually bright and colorful to attract pollinators, which helps to assure fertilization.
Fruits are the mature and ripened ovaries of flowers [ ]. The first step in fruit growth is fertilization of the carpel. Then, a fruit arises from a series of transformations that occur during the development of the fertilized carpel, resulting in the ovary of the flower maturing and ripening. Throughout this process, the cells in the carpel of the flower change so that the structural layers become the fruit [ ] (Figure ).

THE ROLE OF FRUITS
One of the main functions of a fruit is to spread the seeds and allow the plant to reproduce. Therefore, all flowering plants produce fruit, regardless of whether the fruit is edible, sweet, or soft. This means that, although we call peppers and cucumbers vegetables, they are technically fruits (Figure ). Acorns, maple keys, and the outside of sunflower seeds are also considered fruits. They too develop from a series of transformations of the carpel, and protect, store, and help to scatter the seeds of a flowering plant.

TYPES OF FRUITS
Every single fruit, regardless of whether it looks like the kind of fruit we are used to seeing in the produce section, has three distinct kids.frontiersin.org March | Volume | Article |

FLESHY FRUITS
Fleshy fruits have high water content in the pericarp, and a fleshy mesocarp once they are mature. This means that fleshy fruits are juicier than dry fruits. The group of fleshy fruits includes many of the fruits that you may find in the grocery store, and many sweet fruits, such as peaches and apples. Pomegranates, although we eat the seed and not the fruit, also fall under this category, because their pericarp is soft and fleshy. Avocados, peaches, plums, and other fruits with pits are also fleshy fruits, because they have a thick and fleshy mesocarp (Figure ). Peppers, cucumbers, and tomatoes are also examples of fleshy fruits, even though they are usually referred to as vegetables ( Figure ). In fact, most edible fruits fall into the category of fleshy fruits. These fruits use animals, such as birds or humans, to disperse the seeds.

DRY FRUITS
Dry fruits are hard and dry when they are fully mature (Figures A-C). The pericarp of dry fruits still has three layers-exocarp, mesocarp, and What Is a Fruit? endocarp-but compared with fleshy fruits, they are thinner and do not have as much water. Sometimes the pericarp is in direct contact with the seed, making it hard to tell the fruit apart from the seed. Most dry fruits do not depend on animals to spread their seeds. Instead, they use other mechanisms, such as dehiscence ( Figures A,B) to free the seeds, or using water or the wind to blow their seeds away (think of dandelions: the slightest wind will send the fruits, equipped with parachute-like hairs, flying away).
A strawberry is an example of a dry fruit. If you look closely at a strawberry, you will notice specks on the outside ( Figure D). Each of those specks is a dry fruit, called an achene; the sweet, red flesh of the strawberry is not actually part of the fruit. It is part of the flower that becomes fleshy and edible. Some other examples of dry fruits are corn and pistachios.

SIMPLE, AGGREGATE, AND MULTIPLE FRUITS
In addition to "dry" and "fleshy," fruits can be further described by their structures: simple, aggregate, or multiple. A simple fruit develops from one single ovary, such as a peach or a tomato ( Figure D). An aggregate fruit forms from multiple ovaries in a single flower. Strawberries and blackberries are examples of aggregate fruits ( Figure  D). Lastly, a multiple fruit develops from multiple ovaries of multiple flowers. Pineapples are a good example of aggregate fruits.

VEGETABLES
Fruits are not the only parts of the plants that we eat. The term vegetable can be used to refer to other edible parts of the plant-leaves, stems, shoots, and roots. A carrot, for example, is a vegetable, because the part of the plant that we eat is the root. Broccoli is made of multiple floral buds, and is also traditionally known as a vegetable. Spinach is a vegetable, because it is a leaf.

THE SEEDLESS FRUIT PROBLEM
If a fruit's purpose is to house and disperse seeds, then why do we call seedless fruits like grapes, oranges, and watermelons fruits? Although they do not contain seeds, these structures are still considered fruits, because they develop from the carpel of the flower through a series of transformations. Seedless fruit can come about due to a process called parthenocarpy, in which fruits are still produced, even without successful fertilization [ ].
Some plants have mechanisms in place so that fertilization by their own pollen, or pollen from an extremely similar plant, results in unsuccessful fertilization. This is called self-incompatibility, meaning the plant cannot properly fertilize its flower to create seeds with its own pollen. This allows for fruit development, but without seeds. This is how navel oranges are grown to be naturally seedless [ ].
Another example of a seedless fruit is a banana. Like humans, plants have genes located in structures called chromosomes. Humans have two sets of chromosomes-we get one set from each parent. Many plants follow this pattern, but bananas are di erent: they have an odd number ( to be precise) of chromosomes which makes them unable to produce functional seeds, and therefore after many years of evolution bananas do not have seeds. Instead, bananas have to be grown by planting a piece of an already existing plant to make a new one [ ]. Next time you eat a banana, look for the brown dots in the center which are the remnants of the seed.

TRY IT YOURSELF!
Fruits are all around us, sometimes requiring close observation in order to be discovered. The next time you are eating a salad, look a little bit closer to see which fruits are in the bowl. The next time you hold a strawberry, feel the outside, and see if you can count how many fruits you are about to eat! The next time you go outside, open a maple key and find the seed. When spring comes, check out a flower in your backyard, on your street, or in a park, to see if you can find the carpel. And lastly, the next time someone tells you to eat your vegetables, politely correct them, and tell them you will be happy to eat all of your fruits!

ACKNOWLEDGMENTS
First and foremost, we would like to thank the young reviewers for their comments and helpful feedback which, together with the Editor's comments, helped us improve the final version of this article. We would also like to thank Dr. Lawrence Kelly for helpful discussion during the identification of the di erent parts of the

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
COPYRIGHT © Mendelson, Zumajo-Cardona and Ambrose. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. CECILIA ZUMAJO-CARDONA I am interested in the genetic processes involved in the evolution and development of carpels and fruits across seed plants. I obtained my bachelor's degree at the Universidad de Antioquia in the Pabón-Mora's lab, looking at genes involved in fruit development across flowering plants. I am currently a Ph.D. candidate at the New York Botanical Garden and the City University of New York in Ambrose's lab, mainly working in seed evolution and development in seed plants, in order to better understand the genetic processes underlying the enormous diversity of seeds.