Plant Life Cycles and Reproduction

Alternation of Generations

Plant life cycles exhibit alternation of generations, having one multicellular stage with two sets of chromosomes and one multicellular stage with only one set of chromosomes.
Alternation of generations is the production of alternating asexual and sexual reproductive stages, consisting of a multicellular gametophyte generation and a multicellular sporophyte generation. This process occurs in the life cycles of all plants. Plants have different types of multicellular structures. A structure is haploid if it has a single set of chromosomes, which is half the total of the parent; the haploid condition is designated by 1n or n. A structure is diploid if it has a double set of chromosomes in homologous pairs; the diploid condition is designated by 2n. The structures are unlike those of animals in which haploid cells are sex cells that carry half of the chromosomes needed for reproduction. Each sex cell is either a sperm, the male reproductive cell or gamete, or an egg, the female reproductive cell or gamete. In alternation of generations, there are two phases (generations) within one life cycle of a plant.

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

Characteristic Sporophytes 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

Sporophytes make spores and reproduce asexually, while gametophytes produce gametes (sperm and egg) and reproduce sexually. Spores are haploid, having only half the organism's total chromosome number, while gametes are haploid but combine into a diploid organism, having the full complement of chromosomes.

Alternation of generations refers to the fact that many plants (such as the fern shown here) have a diploid stage with two copies of all chromosomes (genetic material) and a haploid stage with one copy of all chromosomes. The mature diploid stage is the sporophyte. Cells from the sporophyte undergo meiosis, which is cell division that reduces the number of chromosomes by half. This forms spores, which are haploid. These undergo mitosis (cell division that does not result in reduction of the number of chromosomes) to form the haploid gametophyte. The gametophyte contains female structures (archegonia) and male structures (antheridia). The joining of the male sex cells (sperm) and female sex cells (ovule) is called fertilization. The genetic material from both sperm and ovule combine to form a diploid zygote, which then grows into the sporophyte.