מחזור החיים של מכוסי זרע

Plants have a life cycle split between two multicellular stages: a haploid stage—with cells containing one set of chromosomes—and a diploid stage—with cells containing two sets of chromosomes. The haploid stage is the gamete-producing gametophyte, and the diploid stage is the spore-producing sporophyte.

Today, most plants grow from seeds and produce flowers and fruit; such plants are called angiosperms. Angiosperms begin as seeds—structures consisting of a protective seed coat, a nutrient supply, and an embryo. The seed develops into a sporophyte—the familiar, flower-producing plant form.

The reproductive life cycle of angiosperms begins with flowering. Stamens and carpels contain sporangia, structures with spore-producing cells called sporocytes. Sporophytes produce spores as either eggs or sperm, depending on their origin.

For example, male spores—called microspores—are produced within anthers at the tips of stamens. A microspore develops into a pollen grain—the male gametophyte. A pollen grain contains a tube cell and a generative cell, which develops into sperm.

A carpel consists of an ovary and its ovules. Female spores, called megaspores, are produced within ovules. A megaspore develops into an embryo sac—the female gametophyte—which contains the egg.

Pollination allows the sperm-producing pollen grain to reach the egg-containing embryo sac. While the embryo sac is stationary, pollen grains can be carried by wind, water, or animals.

For sperm to fertilize an egg, pollen released from the anthers must reach the sticky stigma at the tip of a carpel. Then, the tube cell of the pollen grain becomes a pollen tube, extending down the carpel to the ovule.

Angiosperms undergo a type of double fertilization that produces an embryo and an endosperm, a nutrient store. The embryo and endosperm are packed into a seed coat, forming a seed. As the ovules become seeds, the ovary typically develops into fruit that helps protect and distribute the seeds.