Flowering plants are capable of both asexual and sexual reproduction. In sexual reproduction, which is the subject of the accompanying animation, a haploid sperm cell fuses with a haploid egg cell to form a diploid zygote, which develops into an embryo. In addition to this fertilization event, there is a second event involving another sperm cell and a second cell within the female's reproductive tissue. The product of this second event is a triploid cell that develops into the developing embryo's food supply.
The ovule contains a reproductive cell called the megaspore mother cell or megasporocyte. This cell is diploid and undergoes meiosis to produce four haploid megaspores. In most species, three degenerate, leaving one surviving megaspore.
The one surviving megaspore then undergoes three rounds of mitosis to produce eight haploid nuclei. Cytoplasmic divisions do not immediately follow the nuclear divisions, so the eight nuclei initially share the same cytoplasm. This multinucleate structure is called the embryo sac.
Within the embryo sac, cell walls form between most of the nuclei. Three cells, called antipodal cells, form opposite the opening of the ovule. Three also form near the opening; two are called synergids, and the other is the egg. Finally, two nuclei, called polar nuclei, remain together in one large central cell.
The egg cell and the single cell with two polar nuclei will eventually take part in a double fertilization event. Before this can happen, the male gametes—the sperm—must travel to these cells within the female's reproductive organs.
To reach the embryo sac, a pollen grain that has landed on the stigma begins to germinate, sending a long pollen tube through the style and ovary.
A haploid cell, called a generative cell, travels down the pollen tube. The generative cell travels behind the tube nucleus, which is the nucleus of the large cell that makes up the bulk of the pollen tube and grain. The generative cell divides by mitosis to produce two haploid sperm cells.
The pollen tube reaches the opening of the ovule and digests its way into one of the synergids. The synergid degeneratess, and one of the two sperm cells fertilizes the egg cell. The synergid degenerates as one of the two sperm cells fertilizes the egg cell, producing a diploid zygote.
The second sperm cell fuses with both of the polar nuclei, fertilizing them to produce an unusual triploid cell. As the zygote develops into an embryo, the triploid cell develops into the endosperm, which serves as the embryo's food supply.
Double fertilization, in which a diploid zygote and a triploid endosperm form, is a characteristic unique to angiosperms (flowering plants). Gymnosperms (such as pines), nonseed tracheophytes (such as ferns), and nontracheophytes (such as mosses) lack this double fertilization event.
After the endosperm is formed, large amounts of nutrients are moved from other parts of the plant to this tissue, allowing the endosperm to stockpile starch, proteins, and lipids. The endosperm provides the embryo with these nutrients during early development.
In many flowering plants, the nutrients of the endosperm are transferred to the developing plant's embryonic leaves, called cotyledons. Either the endosperm or the cotyledons then provide the necessary nutrients to the young plant when it first germinates into a seedling. After germination, the plant's leaves begin photosynthesis, providing the seedling with its own energy source.