The Plant Structure That Contains a Male Reproductive Nucleus

Angiosperms are flowering plants.

The male reproductive organ in an angiosperm is called the stamen. It consists of a two-part structure that contains the pollen grains, or gametophytes. These pollen grains are made of two cells, one that produces two sperm nuclei and another that later produces a pollen tube to reach the female ovule. Most plants cross-pollinate, which means pollen produced in one plant fertilizes another plant, sometimes even far away.

Stamen

Stamen with red anthers and a shorter, thicker stigma in the middle

The stamen is the male reproductive organ of the flower. It is found within the flower and can often be seen with the naked eye. It consists of a single or multiple stick-like filament with a sticky anther attached to the top. Each anther has four lobes, each containing a hollow inner chamber. This chamber -- known as the pollen sac, or microsporangium -- houses the pollen grains. Eventually the anther dries and splits apart, releasing pollen grains. These grains are transported to a compatible stigma, part of the female reproductive organ, and pollination begins.

Pollen Grains

Enlarged picture of a single pollen grain

Pollen grains are single-celled and have a solitary nucleus. The nucleus of the pollen grain is surrounded by two layers: a thin, cellulose-rich inner coating, or intine, surrounded by a hard outer layer known as the exine. The outer layer is covered with a heavily pitted substance called sporopollenin, which protects the grain from damage. A mature pollen grain contains two cells. One cell is a nucleus, which develops into two male gametes. The other cell forms a pollen tube that travels down through the female carpal to the ovary. The male gametes use this tube to reach the female egg for fertilization.

The Travelling Gamete

Once the male gametes reach the female egg in the ovary, fertilization can begin. The pollen tube breaks through the embryonic lining and releases the two male gametes into the ovary. One of the gametes fertilizes the female egg nucleus, which develops into a zygote. The second gamete fertilizes a second nucleus within the ovary. This process, known as double fertilization, is common in angiosperms or flowering plants. The ovary, once fertilized, is known as a seed.

Gymnosperms

Once a cone has spread its pollen, it falls to the ground, its job complete.

Gymnosperms develop naked seeds in cones, unlike angiosperms, which produce a seed in a protective ovary within a flower. Gymnosperms produce two types of cones: pollen cones that house the male pollen grains, and seed cones, which carry the female ovules. The pollen sacs, known as microsporangia, develop on small leaf-like structures called sporophylls within the cone. Once the cone has dried, it splits open, and the wind carries the pollen through the air. When the pollen grains land on a seed cone that contains the ovules, fertilization occurs in much the same way as in angiosperms. Once the ovule has been fertilized, it develops into a seed.

Pollen Facts

Pollen grains come in a multitude of shapes and sizes depending on the plant and its method of travel. The appearance of the exine layer is often used to identify a pollen sample. This layer is known as one of the toughest biological materials; some pollen samples have survived for centuries.

References

• TutorVista: Sexual Reproduction In Flowering Plants
• TutorVista: Sexual Reproduction
• TutorVista: Fertilization
• TutorVista: Structure of a Stamen And A Pollen Grain
• TutorVista: Gymnosperms Explained