Above you can see the story of a fruit. Of course the thing on the right of the picture is a pea pod full of peas, and usually we don't think of peas as fruits. However, here we're using the botanical definition of fruit, given at the right, not the supermarket's. Here's the story:
At "a" in the above picture we have a side view of a pea flower from my garden. At "b" the flower's petals are shriveling up as the newly fertilized ovary in the blossom's center begins enlarging. At "c" the pod has expanded a lot, and you can see newly forming seeds (the peas) bulging inside the pod. The old, dried-up corolla is stuck on the pea's nose. At "d" you see the ripened pea pod, the mature fruit. Now the corolla has fallen off and the calyx is beginning to dry up. In "e" I have removed the side of a pea pod so you can see the peas inside it. All that's needed now is to shell the peas, cook them with some dumplings, and eat them!
Now let's go at the fruit topic from a different angle. At the left you see the immature fruit of the Mayapple, Podophyllum peltatum, a common wildflower in eastern North America forests. In the picture the white-petaled blossom, which was about 2 inches (5 cm) across, has already been pollinated and the petals and sepals have fallen away. The stamens are now brown and dried up, and beginning to fall off. You can see four of the many original stamens in the picture. The flower's pistil is now beginning to expand and eventually it will become a fruit. You can see the dark stigma atop the ovary. Before long the stamens will be gone and the stigma will be hardly noticeable atop the 3-inch-across fruit, and that fruit will be filled with seeds, from which new Mayapples will germinate and grow.
At the right you see images from a very different kind of plant, this time the English Walnut tree, Juglans regia. Walnut trees are monoecious, which means that there are male flowers and female flowers, each being found on every adult Walnut tree. The three fuzzy, roundish items in the picture are female flowers, so you might guess that those dark, curly things are stigmas. The roundish items are immature walnuts immature fruits, according to the botanical definition of "fruit.". Over the summer they will indeed grow and become regular walnut fruits. The caterpillar-like thing shown in the photograph at the top left, with a yellow frame, is a catkin composed of many male flowers, each flower of which is composed of a kind of a calyx and several stamens. Notice how the immature walnuts look a lot like our Standard Blossom, except that all the male parts, the calyx and the corolla are missing!
By now you should see that in order to understand fruits, really you need to know the basics about flowers. If you're lost with those terms like "stamen" and "pistil, before slogging on any further you may want to review our Standard Blossom section .
The picture at the left also might help you get a fix on what fruits are. It shows a Red Delicious Apple just like anyone can find in the local supermarket. Here I've cut the apple down the middle and the neat thing to notice is that you can just barely see the original apple-flower's stamens at the top of the picture. First there was an apple flower, and once it was pollinated its pistil enlarged into an apple fruit, and this picture shows the whole story, for those stamens are part of the old flower. The next time you eat an apple, look for those dried-up stamens!
Admittedly, nowadays most folks accept as "fruits" whatever is displayed in the supermarket's fruit section, and they lump the rest of plant produce into the "vegetable" category. Therefore...
Are strawberries and blackberries, which start out as knotty little green things in a flower's center, "fruits"? And what about tomatoes, pumpkins and squash, ears of corn, and even oak-tree acorns, and those hard, red, pea-sized things that come onto Flowering Dogwood trees in the fall... ?
All these things begin life as pistils in flowers, so, in a technical sense, they are all fruits. Even ears of corn and walnuts!. This is just one of those cases where the popular usage of a word is very different from its more precise scientific usage. If you're speaking with your biology teacher, tomatoes and pumpkins are fruits, but if you're speaking with your grandmother and she says that they're vegetables, then don't debate the issue.
Finally, let's answer the question, "Why bother thinking about fruits?" Of course the answer is that they're so colorful, come in so many colors, odors and tastes, and show so many weird adaptations that they're simply fun to wrap the mind around.
For instance, in the yellow-sided box at the lower right in the picture at the right you see the conelike fruit of a magnolia tree growing near my trailer. That's an aggregate fruit in which the cluster of matured simple pistils have split open and the red, berry-like seeds have tumbled out to be held dangling by very slender threads. In the enlarged picture you can see the thread at the top of the red seed. In real life that seed is about half an inch high (15 mm). What's happening here is that the magnolia fruit is doing two things to help its seeds reach new areas where they might germinate and make new trees.
So, each time you see a fruit, just ask yourself:
What is that fruit doing to help its seeds get where they need to be so they can germinate and make new plants?