Defining True Fruits

Why isn’t a cashew a nut? Learn the answer to this question and more in the second installment of our Herbarium department, in which we explain the different types of plants and the scientific logic behind their classification.

By Oscar H. Will III

Winter 2016-2017

Fruit Basket

There are fleshy fruits (berries, drupes, pomes, etc.) and dry fruits (achene, legume, samara, and more).

Photo by istock/578footFac

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What exactly is a fruit? For example, piñon nuts are technically seeds, while individual corn kernels are technically fruits. Why is this?

To understand true fruits, it’s useful to review a little general plant classification and anatomy. Gymnosperms (conifers) and angiosperms (flowering plants) are categorized together as spermatophytes in part because they possess highly specialized and developed vascular systems as well as complex and differentiated physical structures, including leaves and other parts related directly to sexual reproduction. The vast majority of spermatophytes produce seeds as the result of that sexual reproduction cycle. More importantly, the female components of that cycle are enclosed in protective structures called ovules, while the male’s sperm is enclosed in the pollen grains.

For gymnosperms, the ovule is not enclosed in an ovary, so when it’s fertilized a more or less “naked” seed is produced — as is the case with conifers, cycads, and technically even the Ginkgo tree (Ginkgo biloba). For this reason, true fruit can only be produced by the larger group of spermatophytes known as angiosperms and, thus, the seed of the piñon pine (a gymnosperm) is not considered a fruit. It is, nonetheless, delicious and nutritious.

The ovule of angiosperms is encased in a closed ovary that generally serves as the base of a structure called the pistil. The tip of the pistil is called the stigma (at the end of the style), and that’s where the pollen grain (which contains the male’s sperm) grows a fertilization tube to the ovary and fertilizes the ovule (see diagram in the photo slideshow).

Whew! OK, so a true fruit is developed from a mature or fully ripe ovary and may contain one or more seeds. In the case of aborted embryos, such as seedless grapes and most bananas, the ovary develops without fertilization through a phenomenon known as parthenocarpy. Exceptions like this are everywhere in the natural world. 

Some flowering plants, such as the strawberry, produce “false” or so-called accessory fruits, where the fleshy tissue we think of as the fruit is not actually ovarian tissue, but rather a swelling at the end of the flower stalk. The true fruits of the strawberry are what we often call the seeds, but in actuality they are achenes — fruits with a close-fitting shell of ovary tissue surrounding the seeds. To better visualize an achene, think of a sunflower seed that’s still encapsulated in its shell.

There are many ways to describe true fruits, and we’ll only cover some of the more prevalent terms here. 

Fleshy Fruits

When fully mature, these fruits are fleshy and ready for seed dispersal.
Berry: The entire ovary wall is fleshy, but the outer layer (skin) can be tough. Berries can contain one to many seeds — tomatoes, grapes, and pomegranates fall into this category. Some botanists classify bananas and avocados as berries as well. 
Pepo: Thick-rinded berry most often seen in the pumpkin (cucurbit) genera. Some botanists argue that bananas are also pepos. 
Hesperidium: Type of berry with fleshy components compartmentalized in sections, notably citrus fruits. 
Drupe: Fleshy fruit that contains a seed encapsulated in a hard, stone-like layer, as seen in members of the cherry family, olives, almonds, and many other common stone fruit.
Pome: These fruits seem to cross the line between true and false fruits; the fleshy material humans generally eat is actually derived from the receptacle, but the seeds are enclosed in a developed ovary called a core. The apple and other members of the rose family fall into this category. 

Dry Fruits

These are fruits that are dry when fully mature and ready for seed dispersal. 
Achene: Most often found in the Compositae family, achenes are indehiscent, dry fruits that consist of a seed that’s encapsulated in an ovary-derived shell that fits closely but is not fused to the seed itself. In the case of a sunflower head, the flower structure (inflorescence) consists of many flowers, each of which might produce a fruit. 
Legume: This is technically a dehiscent (splits open along a seam). In this dry fruit, the mature ovary, such as a bean pod, splits open (often along two seams) to reveal the seeds contained within. Not all legumes are dehiscent. Mesquite and honey locust do not split open and are called indehiscent. 
Silique: A dehiscent fruit that’s typically a member of the mustard family and might be mistaken for a legume. These fruits have two distinct halves that are separated along the longitudinal center. Brassicas and other members of the mustard family fall into this category.
Capsule: This is a dry fruit that splits open along a number of seams at maturity. Poppies, cotton, and catalpa trees are among many genera with this type of dry fruit. 
Follicle: A dry fruit that consists of a single ovary, containing many seeds, that opens along a single seam at maturity. Milkweed fruit is easily recognized as an example of a follicle. 
Grain: Generally a small, dry, single-seeded indehiscent (ovary tissue does not split open at maturity) fruit that’s distinguished by the fusing of the ovary wall to the seed coat. The outer layer is generally known as the bran, the inner embryonic layer is the germ, and the starchy material is the endosperm. Corn kernels, wheat and rye berries, and barleycorns are all grains. 
Samara: Trees such as maples and ash are famous for their winged “helicopter” fruit. Samaras are characteristically single-seeded dry fruit born in clusters. Each fruit has at least one wing. 
Nut: Nuts tend to be one-seeded dry fruits, in which the ovary tissue has developed into a very hard coat within a husk. Acorns, hazelnuts, chestnuts, and beechnuts fit this category well. Interestingly, walnuts and pecans are often categorized as drupes because the fleshy husk is at least partly composed of ovary tissue. Lots of other fruits that we often call nuts aren’t. Peanut fruits are legumes, cashews are drupes, and piñon nuts are seeds. And because scientists discover new stuff every day, this may all change in the future. 

There are other fruit types worth considering, but the previous list includes most of those we usually encounter in our gardening and landscaping activities in North America. Before we leave the topic of fruit for now, however, we need to consider aggregate fruits and multiple fruits.

Raspberries result from the fertilization and development of many ovaries organized in a single flower. Each seed is itself a drupe (sometimes called a drupelet) and surrounded by the fleshy tissue of its own ovary. This type of fruit is called an aggregate fruit because a bunch of individual fruits are born together in a distinct structure. Blackberries also fall into this category. 

The multiple fruit structure is distinct from the aggregate fruit because it arises from a flower-bearing structure or inflorescence that contains many flowers crowded together, usually along a fleshy, stem-like axis. Mulberries (left) and their relative, the Osage orange, produce multiple fruits composed of individual drupes (or drupelets) aligned along a central stem.

Hank booked hundreds of hours in general botany and plant anatomy lab while fulfilling the requirements for his Ph.D. in plant molecular biology and genetics. He finds that writing about plant parts is much more fun than looking at them through the microscope to identify them on laboratory exams. Hank is also the editor-in-chief for both Heirloom Gardener and Mother Earth News.