The oldest fossil evidence of angiosperms dates back to about 150 million years ago. The angiosperms radiated explosively beginning 65 million years ago and became the dominant plant life on Earth. The name angiosperm ("enclosed seed") is drawn from a distinctive character of these plants: the ovules and seeds are enclosed in a modified leaf called a carpel. The carpel protects the ovules and seeds and often interacts with incoming pollen to prevent self-pollination, thus favoring cross-pollination and increasing genetic diversity.
The life cycle of angiosperms, like all land plants, alternates between a diploid sporophyte generation and a haploid gametophyte generation. Angiosperms represent the extreme end of a trend in the evolution of vascular plants: the sporophyte generation becomes larger and more independent of the gametophyte, while the gametophyte becomes smaller and more dependent on the sporophyte.
In the life cycle of angiosperms, the diploid sporophyte generation is the larger and more conspicuous one. The sporophyte generation produces flowers. The flowers produce spores, which develop into tiny gametophytes that begin and, in the case of the megagametophyte, end their development enclosed by sporophyte tissue. The haploid gametophytes—the gamete-producing generation—are the male pollen grains (the microgametophytes) and the female, 7-celled structure (the megagametophyte) that lies within the ovule.
In a process called double fertilization that is unique to angiosperms, the male gametophyte produces two sperm cells that fertilize two cells within the female gametophyte. One fertilized cell becomes the zygote, which develops into the embryo (new sporophyte), while the other becomes a unique triploid nutritive tissue (endosperm) that feeds the developing embryo.
Textbook Reference: Concept 21.5 Flowers and Fruits Increase the Reproductive Success of Angiosperms