Plant Ovaries

The fruits and seeds that we eat are plant ovaries.

Botanically, an ovary is the enlarged base of a pistil. Pistils are the female reproductive organs of angiosperms, or flowering plants. A flower’s pistil can be made up of one or more carpels.

(Mariana Ruiz) Public Domain

Parts of the ovary

Plant ovaries have walls that surround small eggs, called ovules. Ovaries often have chambers, called locules. Ovules are found inside the locule(s). Some locules contain fruit flesh, while others do not. The number of carpels also determines the internal structure of a fruit. When you cut open a melon, you will see one locule, in the center, and four distinct sections, which were formed by the carpels.

A melon with one locule and four carpels (Foodista) CC BY 2.0

Plant ovaries and pollination

When pollen lands on the style (stalk) and stigma (sticky knob) of a flower, the pollen grain ‘germinates’, sending a pollen tube down to the ovule. This is pollination. When that pollen grain merges with the ovule, fertilization occurs. At this point, three new structures are produced: seeds, pericarp, and the placentae.

    Seeds  Held within the ovary of a seed plant, are ovules. Ovules contain the female reproductive cells. Seeds are fertilized ovules. Fundamentally, ovules are the same thing as animal ovum, or eggs. This is the embryo sac.

    Pericarp Pericarp is the thickened ovary wall that we call fruit. Plants use fruit flesh to protect the seeds, furnish young seedlings with nutrients, and to encourage seed dispersal by herbivores. There are three different types of pericarp tissue: exocarp (outer skin), mesocarp (flesh), and endocarp (inner layer). The dominant pericarp tissue can become hard, as with nuts, or fleshy, as we see in peaches and avocados. In some cases, we eat the pericarp. In others, we eat the seed. When we eat the pericarp, we call it a fruit - but not always.

    Placentae  The place where the ovules and the pericarp connect is called the placentae. If you look inside a tomato, the seeds are growing inside the placental area. The placenta also has an outgrowth, called an obturator, that feeds and guides the pollen tube.

Tomato seeds grow from placental areas of the ovary (FoeNyx) CC BY-SA 3.0

Ovary placement

Plants are classified by where their ovaries are found within the pistil, relative to the attachment of the petals and sepals. This point of attachment is called the insertion point. Ovaries can be superior, half-inferior, or inferior, and their flowers are described as hypogynous, perigynous, or epigynous, respectively.

Ovary placement (a) superior, (b) semi-inferior, (c) inferior (Gustav Hegi) Public Domain

    Inferior ovaries  To say a plant has inferior ovaries is not a genetic slur. Instead, it refers to fruits in which the seeds are located below the other floral parts, within the hypanthium. The hypanthium is a cuplike structure at the base of a flower, that surrounds or is attached to the gynoecium. The gynoecium (‘woman’s house’) is the female part of a flower, or pistil. Pumpkins, squash, cucumbers, pomegranates, bananas, pears, and apples have inferior ovaries.

    Superior ovaries  Superior ovaries are no better than other ovaries, they are simply found above the insertion point. Legumes, such as peas and beans, true berries, and plants that produce drupes, such as blackberries, raspberries, and soapnuts, feature superior ovaries.

    Half-inferior ovaries  Half-inferior ovaries are surrounded by the receptacle, with parts equally above and below the insertion point. Some botanists take this classification to the extreme, by saying a plant has a “two-fifth inferior ovary” but I think that’s taking things a bit far. Peaches, nectarines, and crape myrtles are in this group.

Take a look at the flowers in your garden that are destined to become fruits and nuts. Where is all the action taking place? Can you tell?