The gymnosperms are a group of seed-producing plants that includes Conifers, Cycads, Ginkgo and Gnetales. The naked seeds contrast the seeds and ovules of flowering plants the angiosperms, which have enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form cones, or at the end of short stalks as in Ginkgo biloba or the Australian Podocarpus species.
A male cone has a central axis on which bracts known as microsporophylls are attached. These are the sites where the microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Then mitosis of the microspore produces two nuclei, a single generative nucleus and the tube nucleus similar to what is found in Angiosperms. Upon maturity, the male gametophyte or pollen is released from the male cones to be wind distributed to the female cones.
The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone the megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure like the wing. The female gametophyte is contained within a structure called the archegonium.
When the male gametophyte descends onto a female cone, the tube nucleus of the pollen grain begins to grow its pollen tube, through which the generative cell; sperm, migrates towards the female gametophyte through the micropyle. The whole pollen grain enters each ovule through a microscopic gap in the ovule coat ( integument ) which is known as the micropyle. It takes approximately one year for the pollen tube to grow and enter the female gametophyte through the micropyle. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones remain closed throughout the development of the seed . The seed is covered by a seed coat, which is derived from the female sporophyte. The seed development can take several months or up to two years in some specie. Once the seeds are mature the bracts of the female cones open to allow the dispersal of seeds by wind. There is no fruit formation surrounding the seeds as is the case in the angiosperms.
Gymnosperm reproduction differs from that of angiosperms in that the female gametophyte is not enclosed in a fruit like structure. It develops on the leafy like bracts which then develop into the woody like cones. Only single fertilization takes place in gymnosperms instead of the double fertilization which is needed to produce the fruit in Angiosperms. The male and female gametophyte structures are always present on separate male and female cones often on different trees in gymnosperms.
1. The fruits of Gymnosperms are usually woody cones.
2. The microspores develop inside the microsporangium and are known as microsporocytes.
3. The microspores develop inside the megasporangium.
4. The female gametophyte is contained within a structure called the archegonium.
5. Microsporophylls are leaf like bracts that develop along a central axis.
6. It takes at least one full growing season for the pollen tube to reach the female gametophyte.
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