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.
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.
Textbook Reference: Concept 27.1 Most Angiosperms Reproduce Sexually