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.
As with all land plants, the life cycle of an angiosperm alternates between a diploid sporophyte generation (represented here by the flower of the mature sporophyte plant) and a haploid gametophyte generation.
Within the flower's male parts, called the anthers, are millions of diploid cells called microsporocytes. These microsporocytes divide by meiosis to produce haploid microspores.
Meanwhile, a similar process occurs within the flower's female parts, which consist of one or more carpels. In this example, the single carpel consists of a stigma, style, ovary, and ovule. A single diploid megasporocyte exists in the ovule and divides by meiosis to produce four haploid megaspores, only one of which survives.
By producing two different types of spores—the microspores and megaspores—angiosperms and all other seed plants are considered heterosporous.
Each microspore undergoes a mitotic division and differentiation to produce a pollen grain. A pollen grain is the haploid male gametophyte, called a microgametophyte.
The surviving megaspore divides by mitosis to produce seven haploid cells. One large, centrally located cell contains two nuclei, called polar nuclei. Another cell is the egg. The seven-celled structure makes up the female gametophyte, called the megagametophyte.
The pollen grain pollinates the female parts of the flower by landing on the stigma. Here, the pollen grain germinates, and a pollen tube grows down the style until it meets the female gametophyte.
Two sperm from the pollen grain travel through the pollen tube and enter the female gametophyte. One fertilizes the egg, forming a diploid zygote. The other fertilizes two polar nuclei, forming a triploid cell. The fertilization of both the egg cell and the central cell is called double fertilization, a hallmark of the life cycle of angiosperms.
The zygote, which begins the next sporophyte generation, develops into the embryo, while the triploid cell develops into the nutritive endosperm of the seed. The seed germinates, and when the sporophyte matures, the life cycle begins again.
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.