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Plant Life Cycles and Reproduction

Life Cycle of Gymnosperms

Gymnosperm life cycles include the production of seeds without coverings.

Gymnosperms are seed plants that do not produce flowers. The name gymnosperm means "naked seed," in contrast with angiosperms, which are flowering plants, in which the seed is enclosed in ovarian tissue. All conifers, cycads, ginkgoes, and gnetophytes are gymnosperms. Conifers include pines, cedars, redwoods, firs, cypress, yews, spruces, and larches. Cycads are primitive compared to conifers and appear to have little in common with them. Cycads grow predominantly in tropical and subtropical zones, such as Central America, Southeast Asia, and eastern Africa. Many species of ginkgoes appear in the fossil record, but today there is only one species, Ginkgo biloba. Gnetophytes include a range of broad-leafed vines, trees, and shrubs, including Ephedra and Welwitschia.

The life cycles of gymnosperms are lengthy processes; some of the trees of this group live a thousand years or longer. An immature conifer sporophyte (seed-producing stage of the organism) grows for several years before it reaches sufficient maturity to produce seeds. Gymnosperms are generally heterosporous, producing two distinct types of strobili for the male and female. A strobilus (plural, strobili) is a structure that has overlapping scales, such as a pine cone. Female strobili are larger than male strobili.

Gymnosperms have a dominant diploid sporophyte generation having two copies of chromosomes, which is the shrub, vine, or tree stage. In the mature sporophyte stage, cones are either egg-bearing (ovulate) or sperm-bearing (produce pollen). Female cones take about two years to mature, depending on the species. During reproduction, both male and female cones produce microspores, which are stored in specially formed leaves called scales. Each female cone carries at least one bare, exposed ovule (a structure in the ovary that contains the female gametophyte and is fertilized by the sperm), and the cones are sticky to hold pollen when it is released. Within the ovules, cells undergo meiosis, producing four haploid cells. One cell survives as a megaspore, which then divides through normal cell division. The result is a female gametophyte. Within the gametophyte, two or three archegonia can be found, each holding one egg cell. Several strobili make up a male cone or catkin, which also produces microspores. In turn, these microspores undergo mitosis and produce pollen grains (male sex cells).

Pollination is the transfer of pollen from the male structures to female structures in a plant. Pollination depends on wind blowing pollen from the male strobili to the female. When pollen fertilizes the egg, a seed forms. Seed germination, the period during which a seed begins to grow, is seasonal; during summer months, the cones open and release their seeds. Going from seedling to maturity is a multiyear process before the life cycle repeats.
In most gymnosperms, the sporophyte grows for several years before reaching reproductive maturity. The sporophyte is diploid, meaning it has two copies of chromosomes. It produces both male and female strobili. The cells of the strobili undergo meiosis, or cell division that reduces the number of chromosomes by half. This forms the gametophyte, the haploid stage. The male sex cells (pollen) then form, and fertilize the female sex cells (ovules). The resulting zygote is diploid.