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October 2, 2008

Plant Anatomy 101: Strawberries

Have you ever wondered why strawberries have their seeds on the outside of their fruits? When I worked at Red Butte Garden, this was a point that we brought up frequently with the first graders. We mostly just pointed this fact out and never really talked about why, other than a quick suggestion of possible seed-distribution benefits. Well today I was thinking about strawberries for whatever reason, and now I want to share what I've discovered!

As we learned in the potato post, there is a big difference between what botanists classify as fruit and what we regular ol' plant eating people define as fruit. Here I will be talking about strawberries as defined by the botanists. Let's back up a second: a true fruit is a fruit from which all tissues are derived from a ripened ovary and its contents. Simple fruits, such as oranges, develop from a single pistil and are true fruits. True berries are also simple and true fruits in that they develop from one ovary. Blueberries, cranberries, grapes, tomatoes, and even bananas fall into this category. It seems that there are a lot of definitions for basically the same thing.

Strawberries, on the other hand, are not true fruits. They are called aggregate fruits which means that the strawberry is formed through many ovaries ripening. The "seeds" on the outside of the strawberry are actually individual little "fruits" that have ripened in their own separate ovaries. These little "seeds" are called achenes (ah-keens) and inside each achene is a little seed. Sunflower seeds are also achenes where the hard shell is the outside of the fruit and the seed is inside the hard shell. Since an aggregate fruit forms from many ovaries, it is also known as a complex fruit. Other complex fruits include blackberries, raspberries, and mulberries. These fruits however, because they lack achenes, are not aggregate fruits.

But what about the bight fleshy part of the strawberry that we eat? Strawberries are not only aggregate, complex fruits with achenes, they are also accessory fruits. Accessory fruits are fruits that contain a significant amount of other tissue in addition to the ripened ovary and mature seeds. Apples and pineapples are accessory fruits. In the case of the strawberry, the extra tissue that we are consuming is the enlarged end of the flower's stamen. Because of this, strawberries must be picked at full ripeness, otherwise they wont ripen after they have been picked.

Random strawberry factoids:
  • Until the mid-nineteenth century, strawberries were considered poisonous in Argentina.
  • Native Americans had already been cultivating strawberries when European colonists arrived. They would crush the berries and bake them into cornmeal. Colonists adapted this practice and made it their own by making the first strawberry shortcakes.
  • Medieval stone masons carved strawberry designs on altars and on the tops of columns to symbolize perfection and righteousness.
  • In Ancient Rome, the strawberry was the symbol for Venus, the God of Love.
Who knew that strawberries were so complex and had such rich histories!

Strawberries Photo Credit


PMG said...

Three other links with interesting strawberry science facts




Nicole said...

Wow! What great links! Thanks for your input. I had no idea that there were 600 different species of strawberry. I also wanted to mention in this post that strawberries are in the same family as roses, Rosaceae.

Ms Viola said...

Nice to see a post to get people thinking about the foods they eat every day. I would like to point out, however, that raspberries and blackberries are indeed aggregate fruits: they are aggregates of drupelets (achenes do not define a fruit as either aggregate or not). You can think of a raspberry as a miniature collection of cherries (which are also drupes or stone fruits and also members of the Rose family). Also, mulberries, pineapples and figs, for example, are multiple fruits: they result from clusters of flowers that have matured into fruit and fused together, appearing to us as a single fruit. Something else that needs correcting is that the red fleshy part of the strawberry isn't a stamen in origin but the swollen bit of the flower called the receptacle, which is the bit of the flower into which the female parts, the pistils, attach. The tiny achenes that dot the surface of the swollen receptacle are, in fact, the former flower's ripened ovaries that contain the seeds.