Home » Gardening Information » Garden Botany
The Sexual Plant Life Cycle
Unlike asexual reproduction, sexual reproduction is important in providing genetic variability. All plants that reproduce sexually must go through meiosis. Meiosis is a unique kind of cell division during which the paired sets of chromosomes present in sexually mature plants, called sporophytes, are halved. Because they have the two sets of chromosomes, one from the male parent and the other from the female, the cells of sporophytes are called diploid.
During this process, pairs of homologous, or identical, chromosomes, one from each parent, line up together. Crossing-over, or the exchange of genetic material between these homologous chromosomes, may occur at this time. Crossing-over is critical for producing some of the genetic variability in resulting offspring. Meiosis typically produces four haploid cells, each with one set of chromosomes, from a single diploid cell. These haploid cells become gametes-eggs and sperm. Without meiosis, sexually reproducing organisms would not have a mechanism for reducing the total chromosome number by half so that genetic variability can be introduced via crossing-over. Meiosis is a critical step that must occur prior to the fusion of sperm and egg, which restores the diploid chromosome number.
Alternating Generations
Plants undergo a two-phase cycle of sexual reproduction known as the alternation of generations. This sexual life cycle involves alternation of the diploid sporophyte generation with the haploid gametophyte generation. The sporophyte stage begins with fusion of an egg and sperm, which produces a zygote. The diploid zygote develops into a sporophytic plant. This sexually mature plant eventually produces meiocytes, cells that will undergo meiosis, typically resulting in four haploid cells. These haploid cells, in turn, develop into gametophytes, which eventually form sex organs where eggs and sperm are produced.
The sporophyte and gametophyte stages usually are easy to tell apart; one is typically parasitic or dependent upon the other. Which generation is dominant differs in different plants. Generally, the gametophytes of primitive land plants such as mosses are dominant and larger than the sporophytes. In higher, or more advanced plants, the reverse is true.
In other words, the alternation of generations is the alternation between meiocytes undergoing meiosis and the fusion of gametes that produces a zygote.
The Gametophyte Stage:
The gametophyte generation normally begins with a spore (sexual spore or meiospore) and ends with a gamete. The cells of the gametophyte are usually haploid.
The Sporophyte Stage:
The sporophyte generation usually begins with a zygote and ends with a meiocyte. The cells of the sporophyte are usually diploid.
The Life Cycles of Plants
Following are generalized descriptions of the sexual life cycles of the groups of plants traditionally used by gardeners: bryophytes, including mosses; pteridophytes or ferns; gymnosperms, including conifers; and angiosperms or flowering plants.