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.

| Winter 2016-2017

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.

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