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.
Further Comments from Members:
All information is included in good faith and has been thoroughly researched prior to printing. The website or the author does not warrant or guarantee the accuracy of any information on these pages, nor does the website or the author accept any responsibility for any loss arising from the use of the information found within. The views and opinions are strictly those of the author or those members who chose to actively participate in the contents herein.
“Hi reader, it seems you use The Bible of Botany a lot. That’s great as we have great pleasure in bringing it to you! It’s a little awkward for us to ask, but our first aim is to purchase land approximately 1,600 hectares to link several parcels of N.P. into one at The Pinnacles NSW Australia, but we need your help. We’re not salespeople. We’re amateur botanists who have dedicated over 30 years to saving the environment in a practical way. We depend on donations to reach our goal. If you donate just $5, the price of your coffee this Sunday, We can help to keep the planet alive in a real way and continue to bring you regular updates and features on Australian plants all in one Botanical Bible. Any support is greatly appreciated. Thank you.”
In the spirit of reconciliation we acknowledge the Bundjalung, Gumbaynggirr and Yaegl and all aboriginal nations throughout Australia and their connections to land, sea and community. We pay our respect to their Elders past, present and future for the pleasures we have gained.