Alternation of generations is a cyclical process involving both a sporophyte generation and a gametophyte generation. A sporophyte is an organism that produces spores in a multicellular diploid form. The diploid cells undergo meiosis (cell division to create gametes), producing haploid cells, or spores. In meiosis, cell division results in four daughter cells from the original parent cell, and each cell has half the number of chromosomes of the parent cell. Haploid spores divide by mitosis, a type of cell division that produces two daughter cells with the same chromosome count as the parent cell. Haploid spores produce two forms of new haploid cells, called gametophytes. A gametophyte is the multicellular haploid form taken by a plant during alternation of generations. Gametophytes give rise to gametes, also called sex cells. Gametes have similar structures, but they develop into separate types of gametophytes that are male and female. Male gametophytes release sperm cells. Female gametophytes are egg cells that release a chemical signal that attracts sperm to the egg. Fertilization, the union of sperm and egg, occurs, the egg becomes a diploid zygote, or fertilized egg, and the cycle begins again.
Comparing Sporophytes and Gametophytes
|Offspring||Spores||Gametes (sperm, egg)|
|Form of reproduction||Asexual||Sexual|
|Form of cell division||Meiosis||Mitosis|
|Cell produced||Haploid spore||Haploid gametes that fuse at fertilization to produce a diploid zygote|