Endocarp

What do wedges of citrus, hard walnut shells, the white bits inside a pomegranate, and the paper coating around avocado pits have in common?

They are all endocarps.

Grape interior (Courtesy of Carlos Conde's "Biochemical Changes throughout Grape Berry Development and Fruit and Wine Quality")

How can this be? How can structures so very different be the same part? Let’s find out by starting with some basic fruit facts.

Fruit anatomy

The fruits and seeds we eat are plant ovaries. When a flower is pollinated and fertilized, three new structures form: seeds, pericarp, and placentae. Embryonic seeds attach to the placenta, and pericarp begins to grow, to feed and protect the embryonic seed, and to attract seed-spreading herbivores. There are three different types of pericarp tissue: exocarp (outer skin), mesocarp (flesh), and endocarp (inner layer). So, endocarp is the interior fruit that surrounds seeds. But what about all those differences?

​Types of endocarp

Endocarp is generally not fruit in the way you would expect, unless you are talking about peppers or citrus. The fleshy parts of sweet peppers and chili peppers is the endocarp, as are those membranous wedges of fruity goodness found inside lemons, limes, and oranges. If you look inside an apple, the endocarp is the hard clear plate-shaped bits close to the seeds.

Parts of an apple (Eric Guinther) CC BY-SA 3.0

If you take a close look at a stone fruit, such as a nectarine or cherry, the endocarp is very hard and inedible. To us, it looks more like the shell of a nut. And guess what? The hard outer shell of walnuts, pecans, and almonds, that shell is the endocarp, even though, to us, it looks as though it is on the outside. 

Confused? Read on!

Nuts about endocarp

When a nut develops on a tree, the exterior rarely looks like what you see in the grocery store. Many nut species have smooth or furry green exteriors (exocarp). That exocarp coats a hard, familiar shell. That shell is the endocarp of a nut.

Immature almonds

That thready stuff on the inside of your melons and squashes? That’s endocarp, too.

Now you know.