Flower & Seed Structure

Often, flowers are composed of four rings of structure.

While there are endless varieties of flowers and seeds in the botanical world, all seeds have the same structural basics, as do all flowers. Basic biological structure is how plants, animals, and insects are classified. Although internal seed design can vary greatly, every seed has three parts in common will all other seeds. Similarly, although the shape, size, color, and development of one flower varies from another, they all have a certain basic structure.

Seed Structure

In the most basic sense, all seeds are made of three parts: the seed coat called a testa, storage tissue, and the dormant embryo structure. Every seed has a testa to protect it from insects, animals, fungi, and bacteria. The embryo is comprised of a plumule, one to two cotyledons, and a radicle. The radicle is the part of the seed that breaks the testa open to become the root. The plumule will also break the testa, in the other direction, becoming the seed's first plant shoot. Cotyledons are attached to both the radicle and the plumule and will ultimately become the seedling's first leaves; within the seed they contain energy to help fuel the seed's germination. Most seeds also have a microscopic pore in the testa called a micropyle, which allows water to reach the embryo.

Flower Structure

Flowers are symmetrically constructed organisms, meaning their parts are uniform around an axis. Often, flowers are composed of four rings of structure. The outer ring is called the calyx, and it includes modified leaves---called sepals---which surround the flower. The second ring in is comprised of modified leaves, commonly called petals and biologically known as corolla. Within the corolla lies third symmetrical ring, made of the stamens---or the male reproductive parts---which hold the flower's pollen supply. The fourth ring is in fact the flower's center, called the ovary, or female reproductive part.

Female Organ

The complete female organ of a flower is called the gynoecium. Depending on the flower, the female organ may be in a superior (the other parts of the flower are attached to the stem below the ovary) or inferior (the ovary is sunken deep into the stem) position. Some female organs are located somewhere between the two positions, called either half-superior or half-inferior. Within the ovary is a space called a locule, and inside the locule are the ovules (one or more). Inside an ovule is an embryo sac that contains an egg. The larger structure of the ovary leads to the style, which is a tube-like length leading upward to the stigma, which is the part of the gynoecium that collects or traps pollen.

Male Organ

The male complete organ of a flower is called the androecium, which is comprised of one or more stamen. Genetically speaking, a stamen is actually a modified leaf structure. The length of the stamen is called the filament, and its purpose is to lift the anther, or sac containing pollen, high enough to be able to deposit pollen into the stigma of the female organ. The anther actually produces pollen, and pollen produces a plant's sperm cells. The pollen from an anther can be spread by the wind, birds, and most commonly by insects.

Pollination

"Seed initiation by successful union of male and female reproductive elements is the culminating event in flowering. This union depends on 2 key steps: pollination and fertilization," according to Dr. Franklin T. Bonner, author of the USDA Forest Service's publication "Seed Biology". When a pollen grain enters the female stigma and travels down the style, it will fertilize the egg inside the ovule. The result is a seed. Some flowers can self-pollinate, and others receive pollination from other plants, a process called cross-pollination.

References

• Plant Physiology Information: Flower Structure
• "How Flowers Work---A Guide to Plant Biology"; Bob Gibbons; 1984
• National Seed Laboratory---USDA Forest Service: Seed Biology
• Biology Resources: Seed Structure, an Introduction