Why do plants make fruits? What makes a fruit, a fruit? In this lesson, we’ll discuss the purpose of fruit and the structures that make up a fruit, as well as differentiate between types of fruit.
Background on Fruit
Are you a parent, or do you plan to be a parent one day? If so, you’ll probably think a lot about location. Where is the best place for my kids to grow up? Where is the best place for my kids to go to school?
Plants don’t ‘think’ as we know it, but they have to address similar problems. If you’re a plant, you will ‘want’ (if you’ll forgive the anthropomorphizing) your seeds to sprout in a location that is going to be well suited for them to grow. One way to do that is with delicious fruit!
A juicy, fleshy, tasty fruit can be eaten by an animal. Then the seeds pass through its body and land somewhere new. Hopefully, then, the offspring will sprout somewhere where it won’t have to compete with the parent plant.
Not all fruits are juicy and made to be eaten. Some fruits are dry and hard. Some fruits make structures that help seeds catch a passing breeze so that they can travel that way. For example, dandelion seeds, with their fluffy hairlike structures, are actually fruit! But, we’ll focus mostly on the juicy, delicious fruits we enjoy.
By definition, a fruit is a seed plant’s mature ovary, surrounding a simple seed. True fruits include cherries, plums, and peaches. There are multiple types of true fruits. These include:
- Berry, which has a fleshy ovary and one to many seeds. Examples: Blueberry, kiwifruit.
- Hespiridium, a kind of berry with a leathery exterior. Example: Lemon.
- Pepo, a kind of berry with a hard rind. Example: Watermelon.
- Drupe, which has a fleshy ovary and a stone-like center. Example: Cherry.
One fun fact that may surprise people is that blackberries and raspberries aren’t really berries at all! Instead, they are aggregates of drupes.
When a fruit has parts beyond just the ovary, it is termed an accessory fruit or anthocarp. Sometimes older texts call the accessory fruit a ‘false fruit.’ The extra parts may include tissues from the floral tube, petals, sepals, or bracts.
Pears and apples are accessory fruits. The core of an apple or pear is its ovary; next time you eat one, notice the shape. The shape of an apple core is very similar to the shape of the ovary in many flowers. The fleshy part of an apple that you eat is a modified floral tube. There are also multiple types of accessory fruit. These include:
- Pome, in which the ovary is enclosed in a fleshy floral tube. Example: Pear.
- Syconium, in which the whole flower ripens into a hollow fruit. Example: Fig.
- Aggregate accessory fruits. For example, what you may think of as ‘seeds’ on a strawberry are actually individual fruits, and the strawberry itself is accessory tissue.
Fruits exist for the benefits of the seeds they house, right? So what about fruits that don’t have seeds? Even Darwin was baffled by parthenocarpic fruit, or fruit that doesn’t have seeds.
Some parthenocarpic fruit was bred by humans for our human tastes. Seedless watermelons are an example of parthenocarpic fruit that humans created. Horticulturists make seedless watermelons by breeding two sexually incompatible watermelons, resulting in offspring that are sterile and therefore seedless.
Parthenocarpic fruit exists in nature as well, though. For example, as much as twenty percent of wild parsnip fruit is parthenocarpic.
It is believed that parthenocarpic fruit might be a good decoy for herbivores that might pose a problem for plants. For example, caterpillars that destroy parsnip fruit, called parsnip webworms, prefer parthenocarpic fruit over fruit with seeds. The fruit with seeds is more nutritious, but contains a defensive compound that parsnips use to deter pests like webworms. The parthenocarpic fruit has less of the defensive compound. Therefore, when webworms eat parthenocarpic fruit, the fruit with seeds may be more likely to survive.
A fruit is the mature ovary of a flowering plant that contains its seeds. Many fruits are edible to people and animals, and this helps plants to spread their seeds. Some fruits help plants to spread their seeds by making hairlike structures that can catch a passing breeze.
A true fruit is just the developed ovary of a flowering plant. Types of true fruit include the berry, the hesperidium, the pepo, and the drupe.
Accessory fruits, or anthocarps, have other structures besides the ovary. Types of anthocarp include the pome, the syconium, and aggregate accessory fruits.
Parthenocarpic fruit occurs when a fruitlike structure does not contain seeds. Sometimes humans breed for parthenocarpy, as in seedless watermelons. Sometimes parthenocarpy occurs in nature. This is probably for plants to use as a decoy in avoiding herbivory.