Angiosperms have alternation of generations with the 2n(diploid) sporophyte generation being the dominant generation. The anthers(male), which are the equivalent of microsporangia, produce microspores by meiosis, and the microspores develop into male gametophyte (pollen).
The ovaries(female), which are the megasporangia, produce megaspores which grow into female gametophytes, each of which then produces an egg..
By some means (wind or an animal pollinator), the pollen is transferred to the stigma of the pistil, and a pollen tube grows down into the ovary. Eventually, two sperm nuclei travel down the pollen tube. Pollination is the transfer of the male gametophyte (pollen) to the stigma of the female, while fertilization is when the sperm nucleus and egg nucleus unite.
Angiosperms have an unusual thing called double fertilization. When the sperm nuclei reach the female gametophyte, one sperm nucleus and the egg cell unite to form a new 2n zygote (which grows into an embryo). The other sperm nucleus and two nuclei from the female gametophyte join to form 3n ( triploid: one sperm nucleus and the two nuclei) endosperm which often serves as food for the embryo.
The embryo sporophyte consists of:
- one or two nutrient-storage areas called cotyledons which are in contact with (and absorb nutrients from) the 3n endosperm. Seeds of some species, notably the monocots (think: wheat, corn), store their nutrients primarily in the endosperm, having very small cotyledon(s) which only absorb nutrients from the endosperm as needed, while other seeds, including many of the dicots (think: walnut), store most of their nutrients in their cotyledons and the endosperm is very small, often reduced to a papery coating (like that of a peanut).
- the epicotyl (epi = upon, over), which is the region above the cotyledon(s), and which will become the stem and leaves,
- the hypocotyl (hypo = under, beneath), which is the region under the cotyledon(s). The lower end of the hypocotyl, which becomes the root system, is called the radicle ( root like).