By Catherine Kleier, PhD, Regis University
Go to any grocery store’s produce section and you will most likely find tomatoes displayed among their vegetable friends. Yet in the world of botany, they’re considered berries. Why do tomatoes suffer from this produce identity crisis?
Botanically speaking, a berry is a fruit with several carpels, each of which has many seeds. Blueberries and cranberries fit this definition, as do grapes, although many varieties in stores are now seedless. A true berry will have a fleshy inner fruit wall. To understand a true, botanical berry, we need to look inside of a tomato.
The Tantalizing Tomato: Fruit, Vegetable, or Berry?
Picture a tomato. There’s the gooey part with the tiny seeds. There’s a fruit wall on the outside called the pericarp. In a tomato, the outermost part of the pericarp is the exocarp, the middle is the mesocarp, and the very inside is the endocarp. The endocarp is difficult to distinguish in a tomato, but the exocarp is clearly the skin and the mesocarp makes up most of the fruit wall.
The runny parts are called locules, but the locules started as a carpel. There are several different carpels within the ovary and several seeds within each carpel.
This is a transcript from the video series Plant Science: An Introduction to Botany. Watch it now, Wondrium.
Internally, we have several carpels, each with many seeds, and the whole fruit wall is fleshy. This, botanically speaking, means a tomato is a berry. But there are two other variations of berries.
The Astounding Orange: Also Not a Fruit?
If a berry has a leathery rind, it’s called a hesperidium. An example of this would be an orange. Each of the sections of the orange is a carpel. Typically there are many seeds inside, and there is a leathery fruit wall instead of a fleshy wall.
Today, there are many seedless varieties of oranges, but in nature, a fruit should produce seeds, or it won’t be selected because it won’t reproduce without seeds.
How do we get seedless oranges? Citrus fruits generally have a high degree of what botanists call parthenocarpy. This word comes from the Greek words Parthenos for virgin and karpos for fruit. Seedless oranges are literally virgin fruits.
These parthenocarpic fruits develop without seed formation. This can happen because pollination fails due to the genes of the pollen and the stigma being incompatible. Such is the case with oranges. The seedless varieties that farmers grow cannot self-fertilize.
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To make sure the fruit is the same and the oranges continue to be seedless, the orchards are clones—they are all genetically identical. Farmers usually use a graft of the top of the orange tree they want on the rootstalk of another orange tree. Citrus grafts well, which is one reason we have so many seedless varieties.
Parthenocarpic fruits can also form because of chromosome imbalances. When a plant is triploid, meaning it has three copies of the genome, the chromosomes will not match up correctly, and so seeds will not be produced. This is what happens with seedless watermelons. These fruits have to be produced anew each time.
These fruits have to be grown from scratch each time, but even though it’s time-consuming, the seedless variety is coveted by consumers.
Growers have to have a tetraploid plant cross with a regular diploid plant. The sperm and egg of the tetraploid plant will end up having two copies of the genome, and the diploid plant will produce sperm and egg that have only one copy of the genome.
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When these egg and sperm meet, the resulting offspring will be triploid, and it will produce fruit but not seed—the seed won’t come to fruition.
Wonderful Watermelon and Other Cucurbitaceae
The watermelon is also a second type of berry. Although it’s hard to see the carpels when looking at the cross-section of the watermelon, they are there. It’s easier to see them in a related plant, the cucumber.
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It’s easy to spot the fruit of a member of the Cucurbitaceae because it always will be some type of melon. Other members of this family are cantaloupe, squashes, and pumpkins, including zucchini. Botanists don’t call the fruits of this family squashes or melons, but pepos. A pepo is a berry with a hard outer fruit wall.The watermelon is actually a type of berry. Click To Tweet
Perhaps you noticed that there is no mention of blackberries or raspberries. These fruits aren’t botanically berries. They are what botanists call aggregate fruits. These fruits have several carpels aggregated together in one flower so that the carpels are separate, but still together on one fruit.
Think of a blackberry. It’s actually an aggregate of many small balls, each with a seed inside. Those are the seeds that get caught in your teeth when you eat them. Each one of those small round balls with a seed inside came from a carpel. Botanically, they are not berries.
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Common Questions About Distinguishing Fruits From Vegetables
Technically a fruit is a seed-bearing growth that develops from the ovary of the plant. Vegetables are roots, shoots, leaves, and stems of plants that are edible.
Some fruits which are savory instead of sweet get categorized in restaurants as vegetables, and this has carried over into common parlance. However, they are still fruits if they are seed bearing growths from the ovary of the plant, regardless of the sugar content.
Some of the most common fruits mistaken for vegetables are tomatoes, eggplant, olives, and corn.
A coconut is not a vegetable but a drupe. It is a seed, a nut, and a fruit all at once.