Gymnosperm Sexual Reproduction
Gymnosperm Definition
Gymnosperms are vascular seeded plants that, unlike angiosperms, posses cones instead of flowers. Alteration of Generations An important aspect of the gymnosperm life cycle that influences its sexual reproduction is alteration of generations. This means that gymnosperms exist in both haploid (gametophyte) and diploid (sporophyte) phases. The sporophytes are the green leafy plants while the gametophytes are inside the cones--a defining feature of gymnosperms. Because male gametophytes are located towards the bottom of the tree while the female at the top, and wind is primary agent of pollination for gymnoperms, self-pollination is very unlikely. |
Male Gametophytes
Male cones are smaller than female cones, and the microsporangium at the bottom of the bracts of the cone produce gametophytes. Microsporocytes in the microsporangium undergo meiosis and form four haploid mictospores. Subsequently, mitosis causes each microspore to produce two nuclei, a generative nucleus and a tube nucleus. When this pollen matures, it is released from the cones and blown by wind to female gametophytes. |
Female Gametophytes
Female cones are shaped differently than male cones and contain an area known as the megasporangium within the bracts. In the megasporangium, there are diploid megaspore mother cells that undergo meiosis to form four haploid megaspores. One of these forms a multicellular female gametophyte with an egg while the other three form the rest of the apparatus necessary for reproduction. |
Fertilization
When the pollen lands on a female cone, the tube cell in the pollen forms a pollen tube through which the generative cell travels to another tube known as the micropyle and then the female gametophyte. The generative cell divides into two sperm nuclei. Then, one degenerates while the other fuses with the egg during fertilization forming a diploid zygote.
Seed Development
The zygote undergoes mitosis and eventually forms an embryo. The seed, which contains the embryo, is covered with a seed coat from the sporophyte. During this process of seed development, the scales of the cone are closed. Then, when the seed is mature, the scales open allowing the seeds to disperse. Because, unlike angiosperms, the seeds are not covered with ovaries, no fruit develops.
When the pollen lands on a female cone, the tube cell in the pollen forms a pollen tube through which the generative cell travels to another tube known as the micropyle and then the female gametophyte. The generative cell divides into two sperm nuclei. Then, one degenerates while the other fuses with the egg during fertilization forming a diploid zygote.
Seed Development
The zygote undergoes mitosis and eventually forms an embryo. The seed, which contains the embryo, is covered with a seed coat from the sporophyte. During this process of seed development, the scales of the cone are closed. Then, when the seed is mature, the scales open allowing the seeds to disperse. Because, unlike angiosperms, the seeds are not covered with ovaries, no fruit develops.