Reproduction, Fertilization and

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Reproduction, Fertilization and

Gametes in plants—unlike those of animals—are not produced directly by meiotic division of a diploid organism, but by an entirely different haploid plant, in a process known as alternation of generations. In this process, embryos grow into sporophytes , and sporophytes release haploid spores. Spores grow into gametophytes , and gametophytes release gametes. Ga-metes fuse to form embryos.

In mosses and liverworts, the embryo produces a small, but visible, sporophyte in which thousands of spores are produced through meiosis . The sporophyte that we see—the capsule and stalk of the moss—remains dependent on the dominant gametophyte (which is the vegetative moss plant).

In ferns and so-called fern allies, the embryo produces a large sporophyte as the dominant generation (which is recognized as the vegetative plant). Keen observation is needed to see the free-living fern gametophytes, as they rarely reach 1/4 inch, but this is where sexual reproduction occurs in these plants.

In mosses, liverworts, and ferns, the sperm cells have flagella and can swim. Sperm cells are released under moist conditions. Often the sperm are helped by being splashed out of the sperm-producing organ to within swimming distance of the eggs. Just one egg cell is found in each archegonium, which is the female protective organ on the gametophyte. A chemical signal or erotactin may be produced that attracts the sperm cells. When the egg cell is fertilized, it forms the zygote , which divides to form the embryo.

In seed plants, the gamete-producing organs are highly protected and dependent on the sporophyte. Sperm cells form inside male gametophytes, known as pollen. The egg cells form inside female gametophytes, which in turn are located inside ovules. Each pollen grain forms only two sperm cells. The more primitive seed plants—Ginkgo and the cycads—have large sperm cells with hundreds or thousands of flagella. Most seed plants, however, have sperm that lack flagella and are nonmotile . Once pollination occurs, the pollen germinates and forms a tube. Nonmotile sperm cells depend on pollen tube growth for their transportation. Guided by chemical signals, the pollen tube grows over and through protective layers and deposits the sperm cells precisely next to the egg cell.

The egg cell is located within the ovule. In gymnosperms (e.g., conifers), the ovule contains a large female gametophyte with multiple archegonia and eggs. In angiosperms (flowering plants), the female game-tophyte—reduced to one egg and six other cells—is known as an embryo sac. One of the two cells (called synergids) located next to the egg receives the successful pollen tube. Sperm cells are discharged from the pollen tube near the egg cell, and soon fuse with the egg cell to form the embryo. In angiosperms, a second sperm fuses with the central cell to form a nutritive endosperm during double fertilization. Fusion of sperm cells with each egg cell and central cell is required to produce the nutrition-rich endosperm needed for development of the embryo in flowering plants.

During fertilization in plants, male and female gametes: 1) contact one another; 2) adhere; 3) fuse their cells; and 4) fuse their nuclei. Fertilization triggers later embryo development.

see also Bryophytes; Ferns; Flowers; Fruits; Pollination Biology; Reproduction, Alternation of Generations and; Reproduction, Asexual; Reproduction, Sexual; Seeds.

Scott D. Russell