The Angiosperm Life Cycle

Angiosperms—the most diverse group of plants on Earth—include flowering and fruiting trees, grasses, vegetables, and other flowering plants. 

The life cycle of angiosperms is dominated by the spore-generating sporophyte stage, rather than the sexual gametophyte stage. 

Like conifers, angiosperms produce two types of spores. 

These male and female gametes are produced in distinct reproductive flower organs. 

Megaspores are produced in carpels. A carpel includes an ovary and its ovules. Each ovule contains a megasporangium, where megaspores are produced.

Within each megasporangium is a megasporocyte—a megaspore mother cell. A megasporocyte produces four megaspores through meiosis; one survives and develops into an embryo sac, which consists of an egg and a small number of other cells. 

Microspores originate in the flower anthers at the tips of stamens. Within the anthers are microsporangia containing microsporocytes. Microsporocytes produce microspores through meiosis. 

A microspore develops into a pollen grain, which contains a tube cell and a generative cell. After the pollen reaches a stigma - the upper part of the carpel - the tube cell becomes a pollen tube. The pollen tube extends down the carpel to the ovule, which contains the embryo sac. The generative cell then divides to form two sperm. 

These sperm are released together into one ovule, in an act of double-fertilization. One fertilizes the egg, and the other fertilizes the central cell of the embryo sac. The fertilized egg then forms a zygote, which will develop into an embryo. The fertilized central cell forms the endosperm - a nutrient storage structure. The embryo and endosperm are packed into a seed. 

In the case of many angiosperms, the ovary then develops into a fruit, which usually contains multiple seeds, or fertilized ovules. The fruit takes many possible forms, typically depending on the species, sometimes looking very similar to the original ovary and other times recruiting additional tissues or joining multiple flowering structures together to create collective fruits.

Finally, a germinated seed develops into a mature sporophyte, which can produce flowers and begin another life cycle.