Anatomy is a fascinating subject, with so many layers of systems working together to create a whole that’s truly greater the sum of its parts.
So, in a way, when it comes to anatomy, we’re all kind of onions – even when we’re fruits.
Keeping It Simple
But before we break down the breakdown of some of our favorite fruits, it might make sense to take a step back, and examine the wider view of how fruits are officially categorized to begin with.
Fruits are typically grouped into one of three major categories: Aggregate fruits; multiple fruits; and simple fruits.
The first type of fruit is formed from a single compound flower, and they contain many ovaries or fruitlets. (Think, for example, strawberries, raspberries or blueberries.)
Multiple fruits may sound like a similar concept, but they actually start off quite differently. This category is defined by fruits that are formed from the fused ovaries of multiple flowers. (Figs and pineapples are good examples of this type of growth.)
And finally, there’s the simple fruit which, true to name, has the humblest start of all. Simple fruits simply come from a single ovary, and they may contain one seed or many.
Since that covers such a broad swath of fruit, simple fruits are then further classified, into categories of “dry” or “fleshy” fruits. These descriptors typically refer to the quality of the fruit’s pericarp – which we’ll get more into later, but suffice to say for now makes up the fruit’s skin and flesh.
Within the fleshy fruit category, we can drill down even more, into berries, pomes and drupes, all proudly displaying their own unique anatomic oddities.
And the biological mystery goes even further, but in order to get out of it, we must go further in.
As promised, we’re here to revisit the idea of the pericarp.
In a nutshell, this is a catchall term, used to describe the combination of the three distinct layers within a fruit: The exocarp; the mesocarp; and the endocarp.
And since beauty is skin deep, we might as well start on the outside – or, in other words, the exocarp. This is the official scientific word for “fruit skin,” or the outermost layer of a fruit. Which may sound simple, but really only scratches the surface.
In simple fleshy fruits, for example, the entire pericarp will be “fleshy” except for this outermost layer, which truly, in these cases, acts like a skin – and usually a thin one at that.
Furthermore, this can be broken down between simple fleshy fruits whose exocarp is inseparable from their inner layers, called pepos, and simple fleshy fruits whose exocarp can peel completely off, called hesperidiums. You may recognize the concept more readily as cucumbers (a pepo), or lemons (a hesperidium).
On The Inside That Counts
Traveling one layer further into our onion-fruits, we arrive at the mesocarp. Typically, we think of this layer as a fruit’s “flesh,” – whether technically classified as “fleshy” or not. In other words, it’s what we bite into after breaking through their exocarp skin.
This is truly the good stuff, where all the sugars, juices and yumminess lies.
In terms of simple fleshy fruits, this layer helps determine what we consider a “drupe” – AKA peaches, plums and cherries. Aside from what characterizes them as simple fleshy fruits, this group is also known for softer mesocarps that surround a singular seed.
This layer is also of particular importance to simple fleshy fruits in general. The whole reason behind their juicy insides is to trick animals into eating them, and, therefore, going on to spread their seeds through their stool.
It’s the number one way these plants pollinize.
Seeds of Knowledge
Finally, we arrive at the endocarp, the innermost layer of a fruit’s flesh, which often hugs tightly around its seeds.
In our drupe-y examples, this layer is extremely thick and hard, though in other types of fruits – like tomatoes and citrus fruits, the endocarp can be almost like a membrane.
When it comes to simple fleshy fruits, having a skin-like exocarp, fleshy mesocarp and cartilage-like endocarp is what defines a pome, a group that includes apples, pears and quinces.
The endocarp is also of the utmost importance for simple dry fruits. Since these fruits are defined by a flesh that’s typically leathery, papery or woody – including most types of nuts, along with many root vegetables like beets, radishes and carrots – they can’t depend as strongly on an animal’s appetite to help spread their seeds.
Instead, to spread their seeds, dry simple fruits rely more heavily on their endocarp – which can be either indehiscent or dehiscent. In examples of the former, this layer stays intact over the seed throughout the growth of the plant, while in the latter group, the endocarp of ripe fruits will split open, revealing the seeds inside.
Since so much more rests with this innermost layer for simple dry fruits, they’re also more dependent upon outside forces like wind and water to help out with the reproductive heavy lifting. (Though, in extreme cases, some plants have adapted a method called wisteria, by which they explode their seed pods with such force, the seed is thrown into the air – hopefully able to catch the wind and continue its journey in some far-off field.
So it might only take a second to bite into a delicious fruit, but it took an entire planet’s worth of evolution to ensure that fruit was built to please.