10.2 Flowers, Seeds, and Fruit

10.2 Flowers, Seeds, and Fruit

The vegetative organs keep an angiosperm alive. The flower is the reproductive shoot that builds the next generation. This section is anatomy. Meiosis, pollen tubes, and gametophytes belong to the reproduction chapter. What you need here is the map of the flower, the contents of a seed, and the rule that turns an ovary into a fruit.

Floral whorls

A flower is a short shoot bearing four kinds of organs, often described as modified leaves and often set in whorls. From the outside inward those whorls are sepals, petals, stamens, and carpels. A complete flower has all four whorls. A perfect flower has both stamens and carpels, even if a sepal whorl or a petal whorl is missing.

Sepals form the calyx. They are often green and wrap the bud before it opens. Petals form the corolla. They are often colored or scented, and they attract pollinators. A petal is sterile. It does not contain ovules. The female reproductive organ is the carpel, not the petal.

Stamens and carpels

A stamen is a male reproductive organ. The filament is the stalk. The anther is the sac at the tip, and pollen develops in the anther. A flower may have a few stamens or many. Together they are the androecium. Do not mix the filament up with the neck of the carpel. The filament belongs to the stamen.

A carpel is a female reproductive organ. The stigma is the tip that receives pollen. The style is the neck beneath the stigma. The ovary is the swollen base. Ovules sit inside the ovary, attached to its inner wall. Each ovule can become a seed. The ovary can become a fruit. If several carpels fuse, the flower may show one compound ovary with more than one chamber. The three regions are still stigma, style, and ovary.

From ovule to seed

After fertilization, the ovule develops into a seed. A seed contains an embryo, nutritive tissue, and a seed coat. The embryo is the young plant. The nutritive tissue feeds it. That food may sit in the endosperm, in the cotyledons, or in both. The seed coat develops from the integuments, the outer layers of the ovule. It protects the embryo. The seed coat is not the fruit wall. The fruit wall develops from the ovary.

Cotyledons are the seed leaves of the embryo. A monocot embryo has one cotyledon. In corn, that single cotyledon lies against a large endosperm, and the endosperm holds most of the stored food. A eudicot embryo has two cotyledons. In a bean or a pea, the two cotyledons are thick because they hold most of the food, and little endosperm remains in the mature seed. Some eudicot seeds, such as castor bean, still keep a large endosperm. The exam contrast is the count: one cotyledon in monocots, and two in eudicots. The pantry can be endosperm, cotyledons, or both, depending on the species.

The embryo axis

Inside the seed, position words name parts of the embryo. The radicle is the embryonic root. The hypocotyl is the region below the point where the cotyledons attach. The epicotyl is the region above that point and leads to the young leaves. Those names explain a simple observation: when the seed coat splits, the seedling can send a root down and a shoot up. You will not need every name on every item, but the three-part seed is required: embryo, nutritive tissue, and seed coat.

From ovary to fruit

After fertilization, the ovary matures into a fruit. The fruit wall, or pericarp, develops from the ovary wall. It may be fleshy or dry, and it may split open or stay closed. A fruit protects the seed and helps disperse it. Wind, water, and animals move fruits. A dry pea pod can twist and throw seeds. A fleshy fruit may be eaten, and the seeds may travel with the animal. Through all of that variety, the seed coat still belongs to the seed, and the pericarp belongs to the fruit.

Pods, apples, and grocery names

A pea pod is a fruit because it is a matured ovary. The peas inside are seeds. The pod is the pericarp, and it can split along two seams. An apple is also a fruit because it develops from the ovary of the flower. Much of the sweet flesh is accessory tissue around the ovary, and the papery core traces to the ovary wall. Accessory flesh does not cancel the ovary. The apple remains a fruit. Grocery language is a poor test. A tomato, a cucumber, and a green bean are fruits because each develops from an ovary. A carrot is a taproot. A potato tuber is a stem. A head of lettuce is a cluster of leaves. If the structure develops from an ovary, call it a fruit even when a kitchen calls it a vegetable.

Angiosperms enclose their ovules in an ovary and then package the seeds in a fruit. That enclosure is the feature to emphasize. A pine produces seeds on the scales of a cone. Those seeds are not inside an ovary that ripens into a fruit, and the cone is not a flower. When a diagram shows sepals, an ovary, and a pod, you are in angiosperm territory. Name the carpel, put the ovules inside the ovary, turn the ovule into the seed, and turn the ovary into the fruit.