The most dominant form of plant on Earth. There are 250,000 species. Fertilization by pollen and pollen tube; water is not required for fertilization, as sperm are non-motile. Cross-pollination can occur over distances by wind or insects. Heterosporous. The endosperm provides embryo nutrition. Dormant seeds form in a developed ovary (fruit). Seed dormancy allows for seed dispersal mechanisms and development of a seed bank. Seeds germinate when conditions are right. Seeds can survive for years, and can be transported to colonize new areas.

1. Microspore mother cells form in microsporangia, and undergo meiosis to produce four microspores each. The first mitotic division gives rise to vegetative and generative cells, divided by a thin wall. The generative cell divides to produce two sperm cells, before or after pollination.

2. Megaspore mother cells form in megasporangia, and undergo meiosis to produce four megaspores each, three of which die. The surviving megaspore undergoes three rounds of mitosis, producing eight nuclei. Four nuclei move to each pole, then one from each end moves to the centre, and thin cell walls form around the remaining six, forming the embryo sac.

3. Upon pollination, a pollen tube forms from the vegetative cell, and grows through the style to the ovule, through the micropyle, and into the embryo sac where it discharges the two sperm cells into a synergid.

4. Double fertilization: one sperm fuses with the egg to form a diploid zygote, and one sperm fuses with the central cell's two polar nuclei to form a triploid endosperm. Synergids and antipodals disintegrate.

5. The embryo and endosperm both divide mitotically and, with parts of the ovule, develop into the seed.

Gametophytic apomixis

Apomixis where the megaspore mother cell divides metiotically, but all resulting cells die instead of just three. A somatic cell of the ovule then divides mitotically to form a diploid embryo sac that becomes the embryo. The endosperm may develop with or without fertilization from pollen, depending on the species, but the latter is rare. Common in Kentucky bluegrass.

Gametophytic apomixis

Apomixis where the megaspore mother cell does not undergo meiosis, but proceeds directly to mitosis to produce an embryo sac within a diploid egg cell. The diploid egg divides mitotically to produce a diploid embryo. The endosperm may develop with or without fertilization from pollen, depending on species, but the latter is rare. Common in perennial grasses.

Apis mellifera

They are attracted to soybean flowers, and will cross-pollinate soybeans. They usually work their way down rows of plants while pollinating, occasionally jumping rows. On cool days they will not emerge from their nest.

Megasporocyte

A diploid cell in which mieosis occurs, forming four megaspores.

Microsporocyte

A diploid cell in which meiosis will occur, forming four microspores.

Selaginella

Lycopodiophyta

The oldest living vascular plant division. A primitive, seedless, vascular plant. Heterosporous, with motile sperm.

1. The sporophyte produces microsporangium and megasporangium in the axils of sporophylls.

2. The microspore mother cells undergo meiosis, producing four haploid microspores each, which are released onto the soil.

3. The megaspore mother cells undergo meiosis, producing four haploid megaspores each, but three die, and only one is released onto the soil.

4. Microspores develop into microgametophytes. The cell divides within the microspore wall, becoming multicellular. An outer jacket of cells form, as well as an antheridium which produces biflagellate sperm. The microgametophyte cracks open and releases the sperm.

5. Megaspores develop into a multicellular megagametophyte, which has an archegonium that produces eggs and ruptures through the outer wall.

6. Sperm swim to the archegonium in a film of water and fertilize the eggs, forming a diploid zygote.

7. The zygote develops into the sporophyte which remains embedded in the megagametophyte.

Adventitious embryony

Apomixis where one or more diploid cells of the ovule integuments or ovary wall undergo mitosis to form a diploid embryo, or embryos. The megaspore mother cell undergoes meiosis normally to form a haploid megaspore, which undergoes mitosis normally to form an embryo sac. Fertilization is required for endosperm development, and also fertilizes the haploid egg. Depending on the species, multiple viable embryos may be produced in one seed. Sometimes the sexually produced embryo fails to mature or germinate. Common in citrus.