Plants exhibit alternation of generations. Gametophytes make gametes by mitosis. (sexual phase) Sporophytes make spores by meiosis. (asexual phase)
Asexual reproduction permits a species to rapidly exploit a uniform and static, or slowly changing environment. Sexual reproduction permits a species, over generations, to respond to and adapt to a changing environment.
Gymnosperms - pines, spruces, firs, cedars, other cone-bearing plants. “Naked-seeded” plants. Seeds develop from ovules. Ovules are on cone scales. Seeds sit on cone scales and are exposed or naked. Seeds are shed directly from open cones.
Angiosperms - flowering plants “Vessel-seeded” plants. Seeds develop from ovules. Fruit develops from ovary. Large plant that we see is the sporophyte. The gametophyte generation is very reduced in size. The male gametophyte is the pollen grain (microgametophyte). The female gametophyte is the embryo sac (megagametophyte) that is found inside the ovule.
Pollen grains develop in the anther within the flower. Pollen is shed and transported to the stigma of another flower. The pollen grain germinates and grows a pollen tube through the stigma and style and into the ovary. The pollen tube grows into the ovule and penetrates the embryo sac. Two sperm nuclei are discharged from the pollen tube. One sperm nucleus fertilizes the egg to form a diploid zygote. The other sperm nucleus fertilizes the central cell containing two polar nuclei to form triploid endosperm. This double fertilization is characteristic of angiosperms.
Pollination may be accomplished by: wind dissemination insect vectors birds bats.
Fruit and seed dispersal may be accomplished by: animal ingestion/defecation animal hoarders/dispersers water dispersal wind dispersal animal carriers. Plants may have different life spans: annuals - complete their life cycle in one year biennials - complete life cycle in two years - grow vegetatively first year, flower second year perennials - live for many years.
Perennials survive winter by: removing sugars and nutrients from leaves, storing food in stems and roots remove excess water from cells, add anti-freeze type compounds to cells cover vulnerable stem tips with winter buds, insulating tip with overlapping bud scales.
Changes are triggered by decreasing temperatures and/or shortened day lengths.