Angiosperms are plants that reproduce sexually, by producing flowers. Flowers can be either male or female, with one flower fertilizing the other, or they may be self-pollinating, with flowers that possess both male and female organs within the same flower. Pollen is produced within the flower to provide the male portion of the reproductive process.
The Male Reproductive Organ
The entire male portion of the plant is referred to as the androecium. The primary parts of the androecium are the stamen and the anthers. Some stamens are long filaments, protruding conspicuously from the center of the flower, as is the case with lily flowers. Other stamens are very short and are attached to modified leaves within the flower, as in magnolia flowers. Stamens support the anthers within the flower.
Each anther is at the end of a stamen filament. Within the anther is sporogenic tissue that produces the actual pollen grains. Another portion of the anther is called the tapetum; it provides nutrition for the pollen that will develop within the anther.
In the beginning of the process, pollen mothercells form. These will eventually become spores that reproduce by cell division. The tapetum will eventually enclose each spore in a cellular casing called an exine. Exines are often sticky, allowing the pollen grains to attach to pollinators such as insects.
After the pollen matures, it will contain the male gametophyte, or sperm cells, that are necessary for fertilization of the flower. The grains will dry out on the surface of the anther and remain there until moved by some physical force, such as wind, rain or animal interactivity.
The pollen grains are then transported by this action to the stigma, the tip of the female reproductive portion of the flower. When each pollen grain contacts the stigma, it will fuse to it and release the male gametophyte, which will then move through the stigma, then into the pistil and finally into the ovary by means of a tunnel called a pollen tube. Germination then takes place.