In the "Standard Blossom" at the right, note the ovary. If you should take a razor or a very sharp knife and slice through that ovary the same way you'd cut a tomato in half, you would see that the ovary, like a tomato, is composed of one to several obvious compartments (often called cells, or carpels)..
That's what I've done at the left, with the tiny ovary inside a tomato flower. Our Standard Blossom's super-average ovary has five cells but you can see that this tomato-flower's ovary just has three. Cell number varies in tomatoes, since they are horticultural plants, but in most wild plants the number of cells in an ovary is fixed. In fact, cell number often helps us identify unknown plants.
Inside each cell of the above tomato ovary, lined up so they form shallow Cs, you see several tiny, soft, pale, oval items called ovules. Ovules contain a flowering plant's female sex germs. When they are fertilized by male sex germs, they mature into seeds.
It's worth thinking about the fact that ovules are future seeds. For, it means that the ovary containing the ovules must become... the future fruit.
A blossom's ovary enlarges to become become a fruit, as ovules inside the ovary mature into seeds.
Let's backtrack a bit and come at this topic from a different direction because all this is very important. Let's begin with pollen this time.
At the left you see another cross section of a tomato flower ovary, but this time it's cut from top to bottom, not across the middle. At the right you see a diagram of the same thing, except that the tomato's stigma and style have fallen off, and the tomato ovary has a lot more ovules in it than does the ovary in the diagram.
Pollen grains, two of which you see atop the stigma in the diagram, germinate like two peas germinating in the soil. However, instead of a root, the thing emerging from the germinating pollen grain is a pollen tube. Like a root growing through the soil, this pollen tube grows down through the style, deep into the ovary. Finally the pollen tube's tip reaches an ovule. The male sex germ, which has migrated from the pollen grain on the stigma down through the pollen tube, now penetrates the ovule, and there it combines its genetic material with the female sex germ, which all along has resided in the ovule.
The fusion of the ovule's female sex germ with the pollen grain's male sex germ is known as fertilization. Notice that this is very different from pollination, which is the transfer of pollen from male parts to female parts. Pollination is something that happens before fertilization. We can see pollination, but fertilization happens deep within the ovary at the genetic level.
Let's not forget that not all plant reproduction is accomplished through pollination and fertilization. In some species asexual reproduction (no sex involved) is an important, or even the dominant manner of reproduction.
In late spring often you find whole fields of items such as what you see at the right (picture twice as large as in real life). If you would break off one of those little BB-size objects, squeeze it and smell it, it would smell just like garlic or onion. In fact, that's a head of pea-sized bulbs of the weed known as Field Garlic, Allium vineale. The bulbs developed with no pollination or fertilization involved, yet when they fall to the ground they will sprout just like regular onion bulbs and form new plants.
Field Garlic heads such as the one in the picture can be a mixture of bulbils and flowers, or either all flowers or all bulbils. On our stem page you can see several examples of new plants arising asexually from underground stems -- as when new Nut Grass plants arise from a parent plant's stolons, and when underground tubers such as potatoes sprout new plants from their "eyes." Among algae and other microscopic plants, asexual division is often the main form of reproduction.
At the left you see a Common Blue Violet plant, Viola sororia, just dug from a flower garden near my home. The "open fruit capsule" at the top is open because the plant's blue flowers blossomed several weeks ago, so the fruits already have matured, opened up, and scattered their seeds.
However, many violet species do something very interesting after flowering. On stemlike peduncles or stolons, sometimes concealed underground, they produce cleistogamous flowers, as shown in the picture at the left. Cleistogamous flowers are flowers that to not develop petals and do not expand so that pollinators can visit them. They remain closed and pollinate themselves -- they are self fertilized, and are often very fruitful, producing many seeds. Of course self fertilization does not mingle genetic material from two different plants, but this doesn't seem to bother either violets or Field Garlic!