The Reproductive Structure of Angiosperm Flowers

Anatomy of the Angiosperm Flower

The flower is attached to the plant by a flower stalk. The stalk widens at its end to form the floral receptacle, from which the other floral parts, or whorls, emerge: the calyx, corolla, androecium, and gynoecium.

The Perianth: Calyx and Corolla

The calyx and corolla together form the perianth.

• Calyx: This is the outermost whorl, composed of modified leaves called sepals. Its primary function is to protect the flower bud.
• Corolla: This whorl is composed of colored leaves called petals. It has a protective function and also serves to attract pollinating insects.

The Androecium: Male Reproductive Organ

The androecium consists of a set of stamens. Each stamen has a filament that widens at its end to form the anther.

Pollen Grain Formation

1. The anther is divided into lobes, each containing pollen sacs (microsporangia), where the pollen mother cells are located.
2. Each pollen mother cell undergoes meiosis, originating four haploid microspores.
3. Each microspore is covered with a double wall: a thick, tough outer layer called the exine, and a thin, delicate inner layer called the intine.
4. The nucleus of the microspore undergoes mitosis, leading to a vegetative nucleus and a generative nucleus. The generative nucleus divides again by mitosis to form two sperm nuclei.
5. The final result is the mature pollen grain, containing three haploid nuclei: one vegetative nucleus and two sperm nuclei.

The Gynoecium: Female Reproductive Organ

The gynoecium is composed of one or more carpels. It typically has a bottle-like shape and consists of an enclosed ovary at the base, a neck-like style, and a stigma at the top where pollen is deposited.

Embryo Sac Formation

1. The ovary contains one or more ovules, which are attached by a short stalk called the funiculus. The region of the ovary where the ovule is attached is the placenta.
2. Each ovule is protected by two integuments that surround it almost completely, except for a small opening called the micropyle.
3. Inside the ovule, the nucellus (the megasporangium) is formed, which contains the embryo sac mother cell.
4. The embryo sac mother cell undergoes meiosis, originating four haploid daughter cells. Three of them degenerate.
5. The remaining cell develops into the embryo sac. Its nucleus undergoes three successive mitotic divisions, resulting in eight haploid nuclei.
6. These eight nuclei arrange themselves: three form the egg apparatus (one oosphere and two synergids) near the micropyle, three form the antipodal cells at the opposite end, and the remaining two fuse in the center to form a diploid secondary nucleus.